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Designing a Good Ventilation System

Ventilating is easy — it’s ventilating right that’s hard

Posted on Jun 15 2009 by Martin Holladay, GBA Advisor

UPDATED on April 15, 2016

Most green builders include some type of mechanical ventilation system in every home they build. That’s good. Since green buildings usually have very low levels of air leakage, mechanical ventilation is usually essential.

Unfortunately, several research studies have shown that a high number of mechanical ventilation systems are poorly designed or installed. Among the common problems:

  • Ventilation fans with low airflow because of ducts that are undersized, crimped, convoluted, or excessively long.
  • Ventilation systems that ventilate at too high a rate, or for too many hours per day, resulting in a severe energy penalty.
  • Ventilation systems that waste energy because they depend on inappropriate fans (for example, 800-watt furnace blowers).

It’s disheartening to learn that many green homes waste energy because of poorly designed ventilation systems that were improperly commissioned.

If you’re unfamiliar with residential ventilation systems, it’s a good idea to review the ventilation information in the GreenBuildingAdvisor encyclopedia.

The ASHRAE standard

ASHRAE’s residential ventilation standard (Standard 62.2) sets the minimum ventilation rate at 7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area.

Systems complying with ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. have ventilation rates that are relatively low; for example, a 2,000-square-foot house with three occupants requires 83 cfm of mechanical ventilation. That’s about as much airflow as is provided by a typical bath exhaust fan.

Since ventilation airflows are typically quite low, ventilation ductwork needs to be impeccably sealed. If ventilation ductwork is leaky, fresh air won’t reach its intended destination.

Prominent building scientists are now debating the merits of the ASHRAE 62.2 ventilation rate. Max Sherman, former chairman of the ASHRAE 62.2 committee, defends the existing ASHRAE formula. On the other hand, Joseph Lstiburek, the well-known building scientist and gadfly, argues that the existing ASHRAE ventilation rate is too high, resulting in unnecessarily high energy costs — especially in hot humid climates, where the introduction of high volumes of outdoor air increases the need for cooling and dehumidification.

Lstiburek and Armin Rudd, a fellow engineer at the Building Science Corporation, advise designers of Building America houses to ventilate at a lower rate. “These [Building America] homes have roughly 50 to 60 percent of the ventilation rate required by ASHRAE standard 62.2,” Rudd has written. “The lack of complaints by occupants indicates that the systems are working to provide indoor air quality acceptable to the occupants.”

The “great rate debate” is far from settled; stay tuned. (For more information on this topic, see two articles: Ventilation Rates and Human Health and How Much Fresh Air Does Your Home Need? On August 7, 2013, Joseph Lstiburek released a new proposed ventilation standard, "Ventilation for New Low-­Rise Residential Buildings.")

Do we really need mechanical ventilation?

As more and more local building codes include ventilation requirements, fewer builders are able to get away with building new homes without mechanical ventilation. However, a few die-hard holdouts defend homes without mechanical ventilation.

One reason why homes without mechanical ventilation systems work better than expected is that many common household appliances act just like exhaust-only ventilation systems. Such appliances include:

  • Power-vented water heaters (50 cfm),
  • Clothes dryers (100 to 225 cfm),
  • Central vacuum cleaners (100 to 200 cfm), and
  • Wood stoves (30 to 50 cfm).

When these appliances are operating, fresh outdoor air enters a house through random cracks to replace the air that is exhausted.

However, homes without ventilation systems are homes of the past. The building science community has reached a consensus: build tight and ventilate right.

What are my choices?

After two decades of experimentation, builders have narrowed ventilation options down to four main options:

  • The simplest system is an exhaust-only ventilation system based on one or more bath exhaust fans.
  • For better fresh air distribution, choose a central-fan-integrated supply ventilation system.
  • For the lowest operating cost, choose a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV) connected to a dedicated duct system.
  • If you don't relish the thought of installing complicated ventilation ductwork, consider installing one or more pairs of innovative Lunos fans from Europe.

Can I install a supply-only ventilation system in a cold climate?

Some builders worry that a supply-only ventilation system (for example, central-fan-integrated supply ventilation) won’t work in a cold climate, because the ventilation fan will drive interior air into building cavities where moisture can condense.

This worry is needless. As energy expert Bruce Harley explains, “The upper portions (walls and ceilings) of every home — typically most of the second floor in two-story homes — already operate under positive air pressure in cold weather, due to the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.. The relatively small airflow of most supply-only ventilation systems (75 cfm to 150 cfm) will have little effect on this situation other than to shift the neutral pressure plane down slightly, in all but the very tightest of homes. … In cold climates, I believe that distributed, supply-only ventilation such as that supplied by a ducted distribution system controlled by an AirCycler, or other ducted low-flow supply ventilation, is vastly preferable to single or multi-port exhaust-only systems, except in extremely tight homes (in which case balanced supply and exhaust ventilation is the best choice).”

What’s wrong with exhaust-only systems?

As Harley’s comments make clear, many energy experts (including Lstiburek) disparage exhaust-only ventilation systems. The main argument against exhaust-only ventilation systems — for example, a Panasonic bath exhaust fan controlled by a timer — is that they don’t provide adequate distribution of fresh air. As a result, some rooms have plenty of fresh air while other rooms remain stuffy.

According to some ventilation experts, ASHRAE 62.2 — which currently lacks any provision requiring fresh-air distribution — should be revised to include a distribution requirement. Armin Rudd has written, “I think distribution of ventilation air is an important issue. Bringing in ventilation air and hoping that it will provide adequate indoor air quality throughout the whole house is just a hope and a prayer.”

Research shows, however, that in some homes — especially small homes with an open floor plan — exhaust-only ventilation systems work well. If the exhaust fan is well chosen — my own favorite is the Panasonic Whisper Green fan, which uses only 11.3 watts to move 80 cfm — exhaust-only ventilation systems have very low installation and operating costs.

If you choose this type of ventilation system, it’s important to remember to undercut the bathroom door.

Do I need passive air inlets?

Most homes with exhaust-only ventilation systems don’t require any passive fresh air inlets in the walls. Unless the house is unusually airtight, fresh air will find its way into the home through random cracks. (For more information on this issue, see Passive Air Inlets Usually Don’t Work.)

A 2000 Vermont study (“A Field Study of Exhaust-Only Ventilation System Performance in Residential New Construction In Vermont”) by Andy Shapiro, David Cawley, and Jeremy King, investigated whether passive fresh air inlets make any sense. The researchers studied 43 new homes (22 of which had passive fresh air vents) with exhaust-only ventilation systems. They wrote, “When the EOV [exhaust-only ventilation] fan was operating, 35% of the vents were exhausting inside air, 48% were supplying outside air, and 17% of the vents were not moving air.” The explanation? “The pressures induced by fans in these [studied homes] … were low relative to pressures induced on a house by natural forces, including wind and temperature-driven stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season..”

Note that there is an exception to this guideline: if your house approaches PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. levels of airtightness (aiming for 1 air change per hour at 50 Pascals or less), an exhaust-only ventilation system may be starved for makeup air. Most Passivhaus homes have a balanced ventilation system (an HRV or an ERV). Builders of very tight homes who prefer to install an exhaust-only ventilation system should consider the installation of one or two passive air inlets.

There's an easy way to check whether your exhaust fan is starved for makeup air: simply measure the exhaust air flow. If you are aiming for 50 cfm of exhaust air flow, and that's what you're getting, then everything is fine. If 50 cfm of air is leaving your house, that means that 50 cfm of outdoor air is simultaneously entering your house.

Central-fan-integrated supply ventilation

For years, the engineers at the Building Science Corporation have been singing the praises of central-fan-integrated supply ventilation systems. These systems can only be used in homes with forced-air heating or cooling systems. The systems include three important components:

  • A duct that introduces outdoor air to the furnace’s return-air plenum;
  • A motorized damper in the fresh air duct;
  • An AirCycler control to monitor the run-time of the furnace blower and to control the motorized damper.

The AirCycler control (also known as a FanCycler) prevents both underventilation and overventilation. When the AirCycler notices that the furnace fan hasn’t operated for a long time, the control turns on the fan to prevent underventilation. When the control notices that the fan has been operating continuously for a long time, the control closes the motorized damper to prevent overventilation.

During the swing seasons — spring and fall — the furnace blower will need to operate occasionally for ventilation purposes, even when there is no call for heat or cooling. In most climates, about 15% of the annual blower run time for such systems will be devoted to ventilation only. If the system is properly commissioned, the furnace will supply a 7% outside air fraction during ventilation mode.

The big downside to central-fan-integrated supply ventilation is that the installer needs to understand how to design and commission the system. HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors capable of this task are rare. Unless the designer of a central-fan-integrated ventilation system takes great care when specifying the furnace and programming blower operation, such a system can have unreasonably high operating costs.

A well-designed central-fan-integrated supply ventilation system needs a furnace with an energy-efficient ECM blower. Such furnaces cost between $1,000 and $1,500 more than conventional furnaces. If you end up using a furnace with a conventional blower motor — that is, one that draws 700 to 800 watts — the ventilation system will incur a big energy penalty. (For purposes of comparison, a Panasonic exhaust fan draws 11.3 watts, and most HRVs draw 100 watts or less).

Duct systems and fans designed for heating and cooling are not optimized for ventilation. While ventilation airflow is typically in the range of 50 to 100 cfm, furnace fans move as much as 1,200 to 1,400 cfm. One study (Robb Aldrich, Chicago, 2005) found that a poorly designed central-fan-integrated supply ventilation system in a house with an 800-watt furnace fan used 347 kWh of electricity for ventilation during a swing-season month. During the same month, an identical home with an exhaust-only ventilation system used only 6% as much electricity for ventilation. Although the researchers were somewhat worried that the exhaust-only ventilation system might be ineffective, the data were reassuring: all of the rooms had very acceptable CO2 readings.

Will cold outdoor air damage my furnace?

Some builders worry that central-fan-integrated supply ventilation systems won’t work in a cold climate, where cold outdoor air might damage the furnace. According to Armin Rudd, such concerns are baseless — as long as the ventilation system is well designed.

Assuming a high outdoor air fraction (15%) and a low outdoor temperature (-30°F), a furnace equipped with a supply-only ventilation system will experience mixed return-air temperatures no colder than 55°F, as long as the thermostat is set to 70°F. Even in Chicago, such systems work well.

Do I really need the AirCycler and motorized damper?

To reduce costs, some builders install the lazy man’s version of a central-fan-integrated supply ventilation system — one that includes a passive fresh air duct to the return-air plenum, but without a motorized damper or AirCycler control.

What’s wrong with this approach?

  • During the swing seasons, when the furnace fan isn’t operating, the house won’t get enough fresh outdoor air, and homeowners may complain of stuffiness.
  • During the rest of the year, when the furnace fan is operating regularly, the house will be overventilated, resulting an a severe energy penalty. During the winter, all that unnecessary cold air will need to be heated; during the summer, all that unnecessary hot air will need to be cooled and dehumidified.

An HRV with dedicated ventilation ductwork

The best ventilation performance and lowest operating cost comes from an HRV or ERV with dedicated ventilation ductwork. Such a “gold standard” system should be designed to pull stale air from bathrooms and laundry rooms, while introducing fresh air to the living room and bedrooms. [Author's postscript: After this article was written, a new type of energy-efficient balanced ventilation system, the CERV, became available in North America. For more information on the CERV, see A Balanced Ventilation System With a Built-In Heat Pump.]

Although HRVs and ERVs save energy compared to exhaust-only or supply-only ventilation systems, they are expensive to install. The high cost of these systems raises questions about their cost-effectiveness, especially in mild climates. To learn more about this issue, see Are HRVs Cost-Effective?

For ventilation purposes, either an HRV or an ERV can work well in any climate. The presumed advantage of ERVs over HRVs in hot, humid climates is not based on research or field data. As Max Sherman has written, “Almost all hot, humid climates have hours when it is dryer outside than inside, and then ERVs actually make the [indoor] moisture problem worse. The net effect this that ERVs are about a wash [compared to HRVs] for humidity control in those climates.” (For more information on this topic, see "HRV or ERV?")

Lunos fans

The Lunos fan is a new type of ventilation fan from Germany. Installed in pairs, the wall-mounted ventilation fans automatically alternate between exhaust mode and supply mode. Because each fan includes a ceramic core, they are able to recover heat from the exhaust air stream.

These fans are particularly useful for retrofit applications, or for any situation where the installation of ductwork would be awkward. For more information, see European Products for Building Tight Homes.

To commission a ventilation system, you need to measure airflow

Anyone who commissions a ventilation system needs to learn how to measure airflow. Manufacturers offer an array of accurate (and expensive) instruments to measure airflow, including $2,000 flow hoods. Builders who need to troubleshoot problems may be interested in several lower-cost methods of measuring airflow, including the use of a home-made flow hood, a method requiring a cardboard box and an old credit card, the garbage-bag method, and a method using a laundry basket or wastebasket.

For more information on these test methods, see these two articles:

Last week’s blog: “Farewell to the Chimney?”

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  1. Renewaire

Nov 17, 2010 6:11 PM ET

ERV vs Wave Home Solutions vs. Dehumidifier .
by Eric

My objectives are improved indoor air quality and lower summer humidity. I am in the Washington, DC area which is hot and humid in the summer and cold in the winter. I have a townhouse with storm windows and recently added energy efficient windows and doors. I have a heat pump heating and AC system and a heat pump hot water heater. I currently run a standalone small dehumidifier in the summer. If I'm away for 2 weeks in the summer with the AC and dehumidifier off, the basement will smell musty. Since I have asthma and allergies (especially dust mites), I'm concerned about indoor air quality. I also have standalone air purifiers on each floor. The basement is borderline on radon at 3-4.

I'm finding it difficult to find objective information and am wondering whether it would be advisable to have an ERV, Wave Home Solutions (or competing product), and/or a whole house dehumidifier.

Nov 18, 2010 3:44 AM ET

Response to Eric
by Martin Holladay, GBA Advisor

Every tight home needs a mechanical ventilation system to bring fresh air into the house. It sounds like your house does not have one.

If you read my article, you'll find that I survey several choices for providing fresh air to a home.

Dehumidification is a separate issue from mechanical ventilation. If your air conditioner is incapable of controlling indoor humidity levels, then a stand-alone dehumidifier is a good choice.

Radon control is also a separate issue. If you have radon concerns, you should contact a radon mitigation contractor.

Finally, if you have special medical issues that made you nervous about your home's air quality after you return from a vacation, it may be worthwhile to operate your air conditioner and dehumidifier while you are gone. The price of two weeks' cooling may be worth it to avoid medical problems.

Jul 18, 2011 3:13 PM ET

Cheap Ventilation Solution...
by michael isaacs

I've been trying to come up with a cost-effective and yet sensible ventilation solution as part of 1904 home remodel in Portland, OR. Been reading this and other stuff online and hope for some suggestions ... 2-levels, 1200sqft each. An unfinished basement(1200sqft) with a natural gas dryer venting out. Kitchen on main level includes a 700cfm hood. House came with 2 Rinnai NatGas Wall furnaces which i mean to keep. Top floor has a 48,000BTU output furnace which i will isolate to top floor vents, closing off 1st floor vents from this furnace.

Main level is open with living, family, dining , kitchen space all in a continuum: one bedroom and attached bath are the closed off rooms with doors. Top floor has a large central space with doors leading to one bath and 3 bedrooms.

My thoughts: one Panasonic ERV on each of 1st, 2nd stories in the open areas connected with a short duct run(<3ft) directly to outside air. Assuming the top floor erv counters the stacking effect. Add insulation to the attic. Panasonic Whispergreen 80cfm fan in 1st floor bathroom.

Makeup air for kitchen hood: since the hood will be used intermittently (&very rarely at 700cfm) will a 18cfm passive inlet work? open a window? should i counter the basement dryer's effect to fight stacking? do i have to do something with the attic space... vague ideas of condensation in attic....
thanks for any help

Jul 18, 2011 4:04 PM ET

Edited Jul 18, 2011 4:05 PM ET.

Response to Michael Isaacs
by Martin Holladay, GBA Advisor

1. The only way to reduce the stack effect is to seal the leaks in your home's thermal envelope. An ERV will not affect the stack effect, since it is a balanced ventilation system (air in = air out).

2. Your home does not need 2 ERVs. One is plenty. Calculate your ventilation rate using the ASHRAE 62.2 formula.

3. Yes, you can crack a window open to provide makeup air for your range hood, although a better solution is to replace your existing range hood with a much smaller model with a lower cfm rating. For more information on range hoods, see Makeup Air for Range Hoods.

Aug 2, 2011 12:46 PM ET

Edited Aug 2, 2011 8:56 PM ET.

Need reno advice please
by Jill Buffie

Wow, what a lot of great info! I am looking at my HVAC choices for our 1972 house in the BC interior. (mild winters, hot summers) It's 1300 sq ft, open concept and soon to be airtight with all new windows, doors and insulation. We are a small family of 3. My question is this: what are everyone's opinions about the HRV/ ERV debate where the heating/cooling is not supplied by furnace, but by an air to air heat pump exchanger? Can I go with the central fan integrated supply option?

Aug 3, 2011 4:09 AM ET

Edited Aug 3, 2011 4:10 AM ET.

Response to Jill Buffie
by Martin Holladay, GBA Advisor

If you have an air-to-air heat exchanger, I assume that your space heat is delivered through ductwork. That means you can use a central-fan-integrated supply ventilation system if you want.

If you prefer an HRV or an ERV, I recommend the installation of dedicated ventilation ductwork. If you are willing to compromise on energy efficiency, you can also distribute the ventilation air from an HRV or ERV through your space-heating ductwork, although such systems aren't as good.

Aug 7, 2011 1:10 PM ET

HRV debate
by Jill Buffie

I will have ducts installed for the air exchanger. When deciding between the HRV or ERV, do things like floor plan (mine's open), family size (3) and sq ft-age (mine is 2700) make a difference?

Aug 7, 2011 3:45 PM ET

Edited Aug 7, 2011 3:46 PM ET.

Response to Jill Buffie
by Martin Holladay, GBA Advisor

To find the answers to your questions, see HRV or ERV?

Sep 25, 2011 4:16 PM ET

Edited Sep 25, 2011 5:31 PM ET.

Great article, but not so fast dis'ing central-fan-integrated...
by David Butler

Late to the party, but someone just pointed me to this discussion. Really great article Martin. You covered a lot of bases that are not well understood by home performance practitioners.

However, do do take exception to your characterization of the additional cost of ECM ($1000 to $1500). I'm surprised no HVAC dealers called you on that. My Lennox Elite ECM air handler costs about the same as the equivalent Elite model with PSC motor. In the case of a furnace, it's not the ECM motor that adds so much cost, it's that darned variable speed drive logic and multi-stage burner. Unfortunately, only a handful of manufacturers make single-stage ECM furnaces. Carrier's Boost 90 is one example. Write it down.

OTOH, ECM air handlers are available from all the majors at very little upcharge (single speed, 5-tap X13 motor), and these models have identical efficiency as the equivalent variable speed models. As long as the duct system isn't restricted (e.g., blower operates in low-to-mid range of the cfm-watt curve), ECM blowers are far more efficient than their PSC counterparts. Unfortunately, too many contractors use these systems as a band-aide for undersized or otherwise restricted ducts, pushing the blower into the least efficient part of its performance curve.

When operating and ECM blower for ventilation with a cycle controller, it should be set to operate on low. For example, my 3-ton AHU only consumes 105 watts on tap 1 (low).

Bottom line, central-fan-integrated supply ventilation with cycle control is often a good choice. But as you point out, it must be properly designed and installed. But that's true for any ventilation system (don't get me started on poorly installed energy recovered ventilators!)

Sep 26, 2011 6:29 AM ET

Response to David Butler
by Martin Holladay, GBA Advisor

I wasn't disrespecting central-fan-integrated supply ventilation systems; I was merely alerting readers to the need to understand how they work and to be sure they are properly commissioned.

I never wrote that an ECM blower costs $1,000 to $1,500 more than a PSC blower; rather, the problem is that furnace manufacturers (until recently, perhaps) have chosen to spec ECM blowers only on their high-end furnaces, not their entry-level models, so than anyone looking for a furnace with an ECM blower had to spend $1,000 to $1,500 more than they needed to. This was certainly the case when I wrote the article; if the situation has changed -- (and I do take note of your information on the Lennox Elite, although I'm not sure whether the Lennox Elite is affordable or pricey) -- then that's a good thing.

Of course I agree with you that, as long as you keep your filters clean and design your ductwork properly, ECM blowers will save you energy and provide the best performance -- especially if you intend to use your furnace to distribute ventilation air during the swing seasons.

Finally, you are 100% correct that any ventilation system -- including a system with an ERV or an HRV -- can be (and often is) poorly installed. (The worst systems I've seen are ERVs or HRVs hooked up to space-heating ductwork.) That's one reason I wrote the article -- to guide GBA readers and help them avoid problems.

Sep 27, 2011 1:33 PM ET

equipment models, price tiers
by David Butler

Elite is Lennox's mid-tier product line (Merit is base, Signature is high). The model number for the ECM air handler is CBX27 (R410a). Dealer cost is about the same as the Elite CBX32 with PSC (CBX27 is actually slightly less for some sizes). Compared to the CBX26 in the Merit line, the CBX27 costs about $150 to $250 more, depending on size. I tend to specify mid-tier products in general.

Carrier's Boost ECM furnaces are also mid-tier (Performance series) -- the Boost 90 58MEB (40k Btu/hr, single-stage) and Boost 90 58MEC (60k Btu and up, 2-stage). Carrier just introduced the Performance Series Boost 96 furnace -- the 59SP5 (40k Btu/hr and up, single-stage).

I don't have ready access to Carrier prices, but I do know the Boost models cost substantially less than variable speed models, especially the 58MEB-040-12, which is large enough for most high performance homes. One thing I like about this model is it can handle up to 3 tons of cooling even though it's got the 40k burner. Getting enough blower capacity for a/c is often an issue with furnaces, often forcing an oversized model.

Apr 13, 2012 1:38 PM ET

'Occupant-Sensing Ventilation' by conservation technology
by Todd Oskin

Any thoughts? Looks like the cost is getting up to be pretty close to a HRV/ERV...depending on the duct-work.

Appears to be an exhaust only system, but with humidity and/or motion sensing monitors.. and with optional "air inlets" (see FRESH-AIR GRILLES) for super-tight buildings....

Apr 13, 2012 1:51 PM ET

Response to Todd Oskin
by Martin Holladay, GBA Advisor

The supplier is a reputable company, and I'm sure the system works well. However, I would withhold judgment on the energy-efficiency claim -- that this exhaust ventilation system operates at "the efficiency of central heat-recovery ventilation" -- until I read the same conclusion in a report from a third-party researcher. I don't know of any researcher who has performed such a study.

Sep 25, 2012 8:59 PM ET

House and crawl space ventilation in coastal marine climate
by L Buser

Great forum. I enjoyed reading the article and letters. But I am left wondering about my situation.

Here is where we are at right now:

I have a 40-year-old split level house in the coastal marine climate of B.C., similar to Vancouver or Victoria. Winters rarely see snow but there is lots of rain and wind (up to 50 mph +), temps about 30-50 degrees F. Summer day times are about 70-85 degrees F and 60-70 percent humidity.

The house is 1700 sq ft (excluding the crawl space) and about 17,000 cubic feet total, with the crawl space. There are two occupants. It is fairly well sealed.

The foundation wall is concrete block about 30 inches high, with 24 inches below grade.

The floor in the crawl space (front of house) and basement (rear) are at the same level and are concrete. Moisture isn't a problem. (As a precaution, I used a penetrating sealer on the crawl space floor and the basement floor varies from epoxy paint to ceramic tile to vinyl flooring).

The basement wall was finished using standard framing, vapor barrier and fiberglass insulation (I may change it to xps and gyproc). The concrete block in the crawl space is exposed.

What I want to achieve:

My wife is sensitive to smells. The crawl space air gets drawn up into the house by the stack effect and the heating ducts (which aren't sealed) and I would like to eliminate that and add fresh air to the house with mechanical ventilation. At the moment we ventilate by opening windows. The high winter winds also add fresh air though air leakage.

I have been considering a continuous-operation Panasonic fan, such as the variable speed 80 model that allows settings of 30-70 cfm. Would this be a good idea? I could also set it up with a programmable timer. It would draw air from the rest of the house to provide ventilation, remove the stale air from the crawl space and prevent the stale air from rising into the house.

I was concerned that the fan could pull more vapor through the concrete block walls or through cracks in the 24 inch wood wall above the foundation, and if this could be a problem. I could cover the concrete block wall in the crawl space with eps (higher vapor transmission than xps).

I considered adding a passive air intake in the crawl space to go along with the fan but this seems like it might not be a good idea from what I have read. Also, it would reduce the draw of air from the floors above.

Other ventilation information:

We have a new Lennox high efficiency furnace with ECM. I also had the Lennox Ventilation Control System installed ( It has a motorized damper and 6 inch diameter fresh air intake to the cold air return. It seemed like a good idea but I have been frustrated with it as the control is on the cold air return in the crawl space so it is not easy to change the settings. If you set it to run 15 minutes, it will run 15 minutes every hour, even if the furnace is not on. This results in cool air blowing through the floor ducts and causes a draft feeling of discomfort. As a result we don't use it at all.

The heating ducts run through the crawl space and keep it at about 55 degrees F in the winter and 65-70% humidity. A hot air supply duct from the furnace could be added to raise that temperature but that would also increase the pressure in the crawl space so I don't know if that is the right thing to do.

We have a new Panasonic 80 cfm in the upper floor bathroom and an older fan in the basement bathroom. There is a kitchen on the front (middle) floor and in the basement (but it is not used at this time). No stove fans (we don't fry food and open a window if necessary). No replacement air intake for the dryers (in the kitchen and basement kitchen).

I hope I haven't overwhelmed you with details but I wanted you to have enough details to best assess the situation. Thanks.

Sep 26, 2012 3:25 AM ET

Response to L. Buser
by Martin Holladay, GBA Advisor

L. Buser,
First of all, the best place to ask your question is on GBA’s Q&A page.

If you post your question there, more people will see it, and you’ll be able to get answers from a wide number of people (including me).

Briefly, however, here's what I suggest:

1. The crawl space smell that bothers your wife is not a ventilation problem. It is a humidity problem. You need to address this problem in the crawl space. For more information, see: Building an Unvented Crawl Space.

2. You need to seal air leaks between your crawl space and the conditioned space above.

3. It is essential that you seal the seams of your duct system to make your duct system as airtight as possible. Here is more information: Sealing Ducts.

4. You already have a ventilation system; however, it lacks an AirCycler control (also known as a FanCycler). Get one installed.

Nov 3, 2012 6:20 PM ET

What about highly-filtered ventilation systems?
by Barbara A. Smith

Everybody here seems to be assuming that the outdoor air is not polluted with chemical sprays, wood smoke, exhaust fumes, or whatever. I knew a family that had to have a balanced ventilation system (no A/C necessary in the marine west-coast climate of the Bay Area) and the heating system was hydronically heated coils buried in the slab..

What if a person is chemically sensitive and needs not only a HEPA filter but several hundred pounds of activated charcoal as well to deal with the aforementioned outdoor air pollutants? This family had a HUGE amount of ductwork for an approximately 3,000 square foot home and ran their ventilation-only system 12 hours a day. They had a pre-filter, 1,000 pounds of activated charcoal, and a HEPA filter. They did not (in my mind) live in a particularly polluted area, but they could not predict when their neighbors might decide to use their fireplaces.

John Bower, who wrote the book, Healthy House Building for the New Millineum, was able to get away with a only a HEPA filter a a few pounds of activated charcoal. They lived in bedroom community where their neighbors were gone during the day and thus not pollute the outdoor air with wood smoke. But if you can't predict your neighbors' pollution, or if you live in a polluted almost-all-the-time area as I do, and are chemically sensitive to boot, what's the most energy efficient way to ventilate? Obviously, a dedicated ventilation system is the only way to go, and unfortunately, most builders are not familiar with them.

Nov 4, 2012 6:49 AM ET

Response to Barbara A. Smith
by Martin Holladay, GBA Advisor

Thanks for your comments. Here's my reaction:

1. Many studies have confirmed that, on average, the indoor air in U.S. homes is much more polluted than outdoor air. This is true even in urban areas that suffer from air pollution. There are a huge number of possible pollutants in most U.S. homes; these include humans (who have viruses, who give off odors, and who produce water vapor), dogs, cleaning products, the byproducts of cooking, tobacco, flame retardants from sofas and armchairs, and chemicals in carpeting. I don't doubt that you have attempted to minimize these emitters, but nevertheless, indoor air is usually more polluted than outdoor air.

2. If HEPA filters and charcoal filters help your symptoms, then you should continue to use them, of course. However, no one should install these devices on their ventilation systems unless someone in their home has medical issues, because HEPA filters and charcoal filters introduce static pressure in the ventilation system, and require the use of much more powerful fans to push air through the thick filters. As a result, these filters incur a major energy penalty, and your electricity bill is higher than it would be if you didn't have these medical symptoms.

Jul 3, 2013 7:34 PM ET

by Phil Lawson

In a really tight house (< 1 ACH @ 50), or a passive house, you can reduce your heating load to next to nothing with a high efficiency HRV/ ERV (ie Zehnder or Ultimate Air) but at higher cost (1500-2500 for base unit). A new system coming out (CERV from ) solves a number of problems with one unit that includes:
- the heat (no more minisplit in your wall) as it integrates a heat pump similar to a minisplit and has a back up electric coil heat for very cold periods
- the HRV component
and a few features that outperform efficiency wise: demand ventilation by remote switches; recirculation of air or bringing in fresh outside air with high efficiency depending on VOC or CO2 levels; and automatic on and off depending on interior CO2 and VOC levels. Instead of constant operation (like most high efficiency HRV's) the unit runs when needed and balances air to preset (via a central control unit) levels that can be adjusted (ie VOC or CO2 levels in the range of 800-1000 ppm).
The outcome is HRV, central heat and central AC in one ducting system with no real need for supplemental heat in a really tight well designed passive or other high performance house. The cost is about the same as a minisplit and high end HRV/ERV..... and this is the first US made available high end system of this sort at a reasonable cost that I am aware of.....

Jul 4, 2013 4:49 AM ET

Response to Phil Lawson
by Martin Holladay, GBA Advisor

You wrote, "In a really tight house or a passive house, you can reduce your heating load to next to nothing with a high efficiency HRV/ ERV (ie Zehnder or Ultimate Air)."

I'm sorry, but you are wrong. An HRV or ERV will not reduce your heating load to next to nothing. Running an HRV or ERV always increases your heating load -- it never reduces your heating load. The more you ventilate, the higher your heating bill.

What reduces your heating bill is the thick insulation and the air-tightening measures. Ventilation always increases your energy cost.

Jan 1, 2014 5:20 PM ET

exhaust only ventilation
by rick Martelli

Hi Martin and Everyone,

I know you may have stated that exhaust only ventilation may be a decent option for smaller homes with open floor plans. I have a single level home with a walk out basement. Each floor has roughly 800 to 900 sqft. Each level is very open with the exception of the bedrooms obviously. I was planning on running 2 Panasonic fans in the bathrooms in the lower level and a Panasonic fan in the bathroom in the main level. You had stated that it may not be necessary for the installation of passive air which would be great. However, the home is getting spray foamed on all perimeter walls and up to the peak of the roof due to the cathedral ceiling. There will be no ridge or soffit venting. Given the complete sealing of spray foam, do you still feel it would be unnecessary for me to install passive venting? I was considering those Lunos fans but they are quite pricey

Jan 2, 2014 7:27 AM ET

Response to Rick Martelli
by Martin Holladay, GBA Advisor

First of all, you don't want to run three Panasonic fans continuously, because they will probably overventilate your house. But you probably know that.

As long as your fans are controlled by timers to prevent overventilation, I think you'll be fine.

There's a simple way to determine whether your house has enough air leakage for an exhaust-only ventilation system: measure the exhaust fan flow when the fans are installed. This process, called "commissioning," should be a routine part of any fan installation, as I explained in the last section of my article.

If your fan is rated at 60 cfm, and you can measure that it is exhausting air at that rate (or near enough to that rate to meet your expectations), then clearly there is enough makeup air. Air in = air out.

Jan 19, 2014 9:15 AM ET

venting into the duct system
by john bailey

Hi folks, having read the discussion I am left thinking about providing outside air to the duct system. This could be for example a 100cfm fan supplying the ducts through a backflow damper. I realize that the flow through the ducts would be very slow, but the volume would have to travel through somewhere. So I wonder why this would not solve the problem of the 800 Watt motor and provide pretty good distribution.

Jan 19, 2014 10:04 AM ET

Response to John Bailey
by Martin Holladay, GBA Advisor

The usual method for the type of ventilation system you describe is explained in the article. This type of ventilation system is called a "central-fan-integrated supply ventilation system." It requires a motorized damper, a FanCycler control, and a furnace or air handler with an ECM blower.

Adding another fan (as you propose) doesn't simplify things; it complicates things. You need to prevent the air handler fan from pulling outdoor air through your proposed outdoor air duct -- how will you do that? You need a control that manages the operation of the ventilation fan so that it doesn't run at the same time as the furnace fan -- how will you do that? You need to come up with a way to ventilate the house when the furnace fan is running for 12 hours straight -- how will you do that?

Jan 19, 2014 11:41 AM ET

more venting into the duct system
by john bailey

Oh, I see there are issues I hadn't thought of.

Maybe my own situation is peculiar. I have a FHA system I do not use because I have a lovely radiant floor for winter. So my ducts are idle. It seemed to me venting into those for the eleven months of the year we do not use AC could be a decent venting solution.

But it does seem that anyone should be able to vent to the high pressure side of the ducting as long as they have a damper.

Jan 19, 2014 11:46 AM ET

Edited Jan 19, 2014 12:15 PM ET.

Response to John Bailey
by Martin Holladay, GBA Advisor

You wrote, "It does seem that anyone should be able to vent to the high pressure side of the ducting as long as they have a damper."

Reread the article. For this system to work, you need more than a duct from the exterior and a (motorized) damper.

The FanCycler control (also called an AirCycler control) is essential; it shuts the motorized damper when necessary to prevent overventilation, and energizes the furnace fan when necessary to prevent underventilation.

The outside air duct should be connected to the return air plenum, not the supply air plenum.

The above comments are in response to your statement that "anyone should be able to vent to the high pressure side of the ducting as long as they have a damper."

If we are talking about your specific case -- rather than "anyone" -- then you can rig up any kind of ventilation system that works for you. If you want continuous ventilation at a steady rate, you probably don't need any control more complicated than a toggle switch, as long as (a) you know how to measure airflow so that you can verify that your fan isn't overventilating, and (b) you have a good way of shutting the outdoor air duct tightly during the air conditioning season, and (c) you have another strategy for ventilation when you are operating your air conditioning system.

Aug 28, 2014 5:38 PM ET

Expelling of indoor contaminant with supply ventilation
by Venkat Y

I have condensation on windows in my ICF home in the winter, which I guess means I have high humidity. If I employ supply ventilation using an AirCycler to bring fresh dry air from the outside, I am wondering where will the moisture-laden air already in the home go? TIA.

Aug 28, 2014 6:05 PM ET

Response to Venkat Y
by Martin Holladay, GBA Advisor

All homes have leaks in their thermal envelopes. Air is always entering your home through cracks (this is infiltration) and also leaving your home through cracks (this is exfiltration).

If you install a supply-only ventilation system, fresh air is delivered through your HVAC system, slightly pressurizing your house. Air leaves your house the same way it always does -- through cracks and holes in the thermal envelope.

Aug 28, 2014 9:09 PM ET

Martin, Thanks for the
by Venkat Y


Thanks for the response. When you say all homes have leaks in their thermal envelopes, do those include tight ICF homes like mine that normally don't allow moist air to escape? Even then I take it the moist air in my home quite won't exit in amounts anywhere comparable to that taken out by a balanced ventilator?

Thanks again.

Aug 29, 2014 3:37 AM ET

Response to Venkat Y
by Martin Holladay, GBA Advisor

Q. "When you say all homes have leaks in their thermal envelopes, do those include tight ICF homes like mine?"

A. Yes. All homes have measurable rates of air leakage. That's why we test homes with a blower door -- so that we can measure the rate of air leakage.

Q. "I take it the moist air in my home quite won't exit in amounts anywhere comparable to that taken out by a balanced ventilator?"

A. The volume of air exiting your home is always exactly the same as the volume of air entering your home. (Otherwise, your house would blow up like a balloon and explode.) If your supply ventilation system is introducing 100 cfm into your home, then 100 cfm is escaping through envelope cracks.

Jan 6, 2015 8:14 PM ET

random cracks vs. passive inlets & summertime humidity
by Timothy Godshall

I am designing a house with an exhaust-only ventilation system (2 Panasonic bath fans on timers) and have read the debate between relying on random cracks and passive inlets to supply make-up air. I like the simplicity of not having to install passive inlets, but I wonder about the effect of drawing humid summer air through a wall into a cool house. Isn't that a recipe for condensation on the cool, inside surface of the walls? Even if you install passive inlets, wouldn't moisture condense in them, too?

Jan 7, 2015 6:28 AM ET

Edited Jan 7, 2015 6:29 AM ET.

Response to Timothy Godshall
by Martin Holladay, GBA Advisor

The short answer to your question is that you shouldn't worry.

Good building envelope design goes a long way to avoiding problems.

The most likely surfaces to experience summertime condensation are metal air-conditioning ducts and metal ceiling registers or duct boots. If you can keep outdoor air away from these surfaces, you should be OK.

The fact is that your walls experience infiltration of exterior air whether you want them to or not. The main driving force for this infiltration is wind. Wind finds your wall's cracks, and wind pressures greatly exceed the pressures exerted on a wall by a ventilation system. Almost all of the humidity introduced by these wind pressures is harmlessly absorbed and harmlessly evaporates.

If exterior air enters passive air inlets during the summer, the temperature of the incoming air raises the temperature of the metal or plastic grille, so condensation is unlikely.

Jan 8, 2015 8:52 PM ET

response to Martin
by Timothy Godshall

Thanks, Martin. I'm glad to check that off my list of worries!

I am curious to know, though, why the infiltration of warm humid air in the summer is of little concern but the exfiltration of warm humid air in the winter is. (You probably recognize my name as I have been posing questions about that problem on another discussion thread.) The only thing I can think of is that the indoor vs. outdoor temperature differential in climate zone 4 is not typically as great in the summer as it is in the winter, so there's not as much likelihood of condensation. Is that it, or are there other reasons?

Jan 9, 2015 4:19 AM ET

Edited Jan 9, 2015 7:38 AM ET.

Response to Timothy Godshall
by Martin Holladay, GBA Advisor

Q. "Why is the infiltration of warm humid air in the summer of little concern, but the exfiltration of warm humid air in the winter is?"

A. First of all, in winter the temperature of cold surfaces can be much lower than the temperature of surfaces in summer. In the area of Vermont where I live, it's possible for wall sheathing or roof sheathing to be at 0°F or even -20°F. Air conditioning ducts never get that cold. And gypsum wallboard never gets much colder than 70°F in the summer.

Second, the stack effect is a very powerful driver, and the stack effect depends on delta-T. Wintertime delta-Ts are much higher than summertime delta-Ts.

It's possible to get summertime condensation problems (for example, when inward solar vapor drive is the driving force, and a builder chose to install a reservoir cladding, a vapor-permeable wall sheathing, and interior poly or vinyl wallpaper) -- but you have to make more mistakes for these things to happen than you do to get wintertime condensation problems.

Feb 9, 2016 10:27 AM ET

Plusaire product question
by Ryan Hagerty

Martin,I have found a Canadian ventilation product called Plusaire. It looks like a passive ventilator as well as being attached to the powered furnace. Is this a viable option in the field of ventilation? I am considering ventilation for my 1970s era house. Thanks

Feb 9, 2016 10:59 AM ET

Response to Ryan Hagerty
by Martin Holladay, GBA Advisor

This ventilation box seems to be designed to serve a central-fan-integrated supply ventilation system. However, it seems to lack a motorized damper. Without a motorized damper, it appears that the Plusaire box can allow a lot of exterior air to enter your home -- in other words, it can over-ventilate.

If you want a central-fan-integrated supply ventilation system, I suggest that you use the components mentioned in my article (a motorized damper and an AirCycler control), not a Plusaire appliance.



Feb 9, 2016 11:30 AM ET

Response to Martin
by Ryan Hagerty

Thanks for the comments Martin
Can a damper and AirCycler be used stand alone with an existing central air system? The videos I have seen seem to have dedicated fresh air input ducts. Would a system such as this solve back drafting problems in an oil furnace? I had a blower door test yesterday and some low levels of CO was detected.

Feb 9, 2016 12:00 PM ET

Response to Ryan Hagerty
by Martin Holladay, GBA Advisor

Q. "Can a damper and AirCycler be used stand alone with an existing central air system?"

A. I'm not sure what you mean. All central-fan-integrated supply ventilation systems are installed in homes with a forced-air HVAC system (ductwork connected to a furnace or an air conditioner). If you have forced air ductwork, then your house is a candidate for a central-fan-integrated supply ventilation system.

Q. "The videos I have seen seem to have dedicated fresh air input ducts."

A. I don't know what video you are talking about. But it is certainly true that a central-fan-integrated supply ventilation system requires an outdoor air duct. The duct conveys outdoor air to the return plenum of the furnace or air handler. Somewhere in that duct run, you need to install a motorized damper controlled by an AirCycler.

Q. "Would a system such as this solve back drafting problems in an oil furnace?"

A. No. Backdrafting is usually caused by powerful exhaust appliances (for example, range hood fans). For more information on backdrafting, see Makeup Air for Range Hoods.

The two most common solutions to backdrafting problems are: (a) replace the backdrafting appliance with a sealed-combustion appliance, or (b) introduce outdoor makeup air to a location near the backdrafting appliance, via a passive outdoor air duct or a makeup air appliance.

A ventilation system is different from a makeup air system. So far, we've been talking about ventilation systems, not makeup air systems.

Feb 9, 2016 2:21 PM ET

Response to Martin
by Ryan Hagerty

Thanks for your patience Martin,

The video I looked at was from the AirCycler webpage. Now that I look back, it was for a "G2 whole house ventilation system" not just the AirCycler. I am looking for something easily retrofitted into an existing forced air HVAC system.

As to my oil furnace backdrafting comment, I was confused as to the difference between make up air and ventilation. Since both ventilation and make up air are introducing fresh air, can one solve the other at times?

I had an energy audit recently and the auditor found low CO levels near the oil furnace. I didnt have any fans running, or any powered air usage going on at the time of the readings. The auditor found a substantial air leak behind the chimney which I have filled. My other suspicion is the chimney itself being very cold and larger than the pipe going into it. So whether it be make up air, ventilation or a chimney liner, I am hoping to solve the CO problem.

Thanks for your help!

Feb 9, 2016 2:32 PM ET

Edited Feb 9, 2016 2:34 PM ET.

Response to Ryan Hagerty
by Martin Holladay, GBA Advisor

Q. "I was confused as to the difference between make up air and ventilation. Since both ventilation and make up air are introducing fresh air, can one solve the other at times?"

A. No. A ventilation system is designed to introduce fresh outdoor air for occupant health (and sometimes, to lower indoor humidity levels). In most cases, ventilation systems exhaust stale air while introducing fresh air.

If you have a backdrafting problem that requires the installation of a makeup air system, that's what you should install. A makeup air system has different design requirements and details from a ventilation system. Moreover, almost all ventilation equipment manufacturers note that their equipment is not intended to provide makeup air.

Q. "I had an energy audit recently and the auditor found low CO levels near the oil furnace. ... My other suspicion is the chimney itself being very cold and larger than the pipe going into it."

A. This venting problem is a potential safety hazard. Since your energy auditor evidently failed to offer you useful advice on how to solve the problem, you need to hire a combustion expert to inspect your venting arrangement and recommend a solution. A gas company or a contractor who specializes in oil furnaces or oil boilers should have enough expertise in venting issues to provide a solution. Don't delay -- fix this promptly.

Mar 28, 2016 9:30 AM ET

inadequate fresh air ventilation for power vented gas HW heater
by Marion Solen

We recently purchased a property that is just under two years old - it is located on 3rd and 4th floor of a structure that was built from scratch and has power vented hot water heater on the 4th floor supplying our property. I noticed that it seemed to be running constantly and shutting on and off and that it took a long time for hot water to regenerate. I am a Realtor and have a small amount of experience and I was concerned that the ventilation was inadequate. My plumber came and followed the Bradford White flow chart and confirmed that my suspicions were correct. In fact, when we disconnected it from the fresh air intake and just used room air - the hw heater operates just fine.
Problemis what to do now? We think the width of the pipe is too narrow - it goes a fairly long distance and has a couple of tight turns as well. Also, it is in a finished ceiling in a bedroom before it goes outside (we also see stains on the ceiling there that I thought was a roof leak -however, ever since disconnecting this pipe - the ceiling stains have not worsened despite heavy rains - we think there may have been actual leaks and/or condensation dripping into the ceiling - no way to know. The builder (is also a neighbor) says there was a one year warranty which is over (however- do not think this should apply to improper construction). At any rate, is there a way to increase ventilation or completely avoid using that pipe. We have a very open floor plan although that hot water heater is in a closet - we can get additional air in there by opening a hole where there is already a grill for fresh air return into the HVAC which is installed in the same closet.
Bottom line - is it OK to use this hot water heater without connecting to the fresh air intake?

Mar 28, 2016 9:46 AM ET

Edited Mar 28, 2016 9:49 AM ET.

Response to Marion Solen
by Martin Holladay, GBA Advisor

This question has nothing to do with mechanical ventilation systems -- the subject of this article. Rather, your question has to do with providing combustion air for your water heater.

I don't know what type of water heater you have. It may be a sealed-combustion water heater (one that requires ducted outdoor air) or it may be a power-vented water heater (one that can be supplied by unducted room air).

In any case, it seems clear that the outdoor air duct is defective. Either the diameter is too small; or there are too many elbows; or the seams are leaky; or all of the above.

If diagnosing and fixing this problem is beyond the capabilities of your plumber, you should contact a technical representative from Bradford White.

Apr 14, 2016 11:23 AM ET

Dedicated HRV design question
by Adam W

New subscriber here - is there a GBA article on how to layout the supply and returns of an HRV system? As in - bedrooms all get supply vents, laundry and kitchen get returns, etc...

Apr 14, 2016 11:38 AM ET

Edited Apr 14, 2016 11:41 AM ET.

Response to Adam Wride
by Martin Holladay, GBA Advisor

Briefly, the living room and bedrooms get the supply registers, and the bathrooms, laundry, and kitchen get exhaust grilles. (Note that the ceiling-mounted kitchen grille should be located far from the range.)

Your should follow the instructions of the HRV manufacturer, of course. Note that installation methods that share ducting with a forced-air heating system are inferior to installation methods using dedicated ventilation ductwork.

For more information, see Installing a Heat-Recovery Ventilator.

I also recommend these two articles:

Ducting HRVs and ERVs

Does a Home with an HRV Also Need Bath Fans?

Jun 14, 2016 4:01 PM ET

Edited Jun 14, 2016 4:03 PM ET.

Exhaust only with 2 bathrooms
by Timothy Tucker

Considering going with an exhaust only approach on the mid-90's home that we just purchased. It's a long way to go to get to the point where we likely need to be too worried about exhaust (lots of sub-par sealing and insulating, including a gaping hole for the whole house fan), but any feedback to help with planning would be appreciated.

There's a larger first floor bathroom (bath 1), a 1/2 bath on the opposite side of the house (bath 2), and an upstairs full bath (bath 3). (Brief view of the floor plan attached).

The fan for bath 2 has flex duct running from the fan into the attic area above -- unsure how difficult the run between the floors would be to replace, or if it'll even be possible to replace.

Bath 1 would have its own fan on a switch / timer.

My current thought is to use a Panasonic inline fan to serve baths 2 & 3, connected to a switch / timer in bath 3, with no switch in bath 2. (So the exhaust would pull from both rooms whenever the upstairs bath was in use or when the timer indicates recirculation is needed).

Note that bath 3 also has a laundry chute connecting to the laundry room below, so option 2 to exhaust stale air from the downstairs would be to change to a louvered door for the chute and possibly add a jump duct between the bathroom and laundry room. At which point, I might just leave the existing cheap exhaust fan in bath 2 alone.

house floor plan.png

Jun 14, 2016 4:24 PM ET

Response to Timothy Tucker
by Martin Holladay, GBA Advisor

If you are going to be installing bathroom exhaust fans in three bathrooms, I think that each bathroom should have a separate fan, and each fan should be controlled by a switch located in the bathroom it serves. Of course, some of these fans can also be on timers; a typical approach is to have continuous operation at a low fan speed, with the option of a higher fan speed when desired (often by flipping a toggle switch). The low-speed operation can either be continuous or timed (20 minutes on and 40 minutes off, for instance).

If you haven't read them yet, I urge you to read these two articles:

Bathroom Exhaust Fans

Is Your Ventilation System Working?

Jun 15, 2016 8:38 AM ET

At this point, there are fans
by Timothy Tucker

At this point, there are fans already present in each bathroom, the debate is what to change.

The consensus that I saw from the Bathroom Exhaust Fans article seemed to be that there wasn't a need for an exhaust fan in a small bathroom with no shower (which is the case for bath 2).

Here's a clearer idea of the options that I'm looking at:
1. Remove the fan from bath 2 to eliminate an extra hole in the ceiling that isn't really needed. Bath 3 gets an upgraded to an quieter inline fan connected to a timer for exhaust. (Given that the ceiling is textured and would be difficult to patch without looking odd, this option is probably a non-starter).

2. Leave the setup for bath 2 as-is, with a cheap but noisy fan that isn't likely to see much use. Bath 3 gets an upgraded to an quieter inline fan connected to a timer for exhaust. Door to the laundry chute gets replaced with something that will allow some air from the laundry room below to be exhausted when venting.

3. Bath 3 gets upgraded to an quieter inline fan connected to a timer for exhaust. Bath 2 is also connected to the same inline fan for exhaust. Likely requires a higher CFM fan than option 2, since more duct work would be connected and increase static pressure, but would possibly give better ventilation for the first floor areas near the bathroom.

Would you agree that option 2 sounds like the best bet?

Aug 21, 2016 3:06 PM ET

Response to Timothy Tucker
by Martin Holladay, GBA Advisor

Half baths (bathrooms without a shower or tub) need an exhaust fan for odor control. So whatever you do, don't leave that room without a fan.

Inline fans are fine, but you don't need to switch to an inline fan to get a quiet fan. Panasonic makes lots of ceiling-mounted fans that are extremely quiet -- so quiet that many people don't even realize they are on.

Dec 11, 2016 11:07 AM ET

Our ventilation system
by Danny O

Our ventilation system suffers from most of the issues you mentioned, and I'm trying to figure out how to improve the situation. We have two ERVs (basement and unconditioned attic) running at ~ 120 cfm each (big house that requires about 240 cfm according to ASHRAE calculations). They are mounted to our home return ducts. Because the ERVs are small, we run them at their maximum flow rate 24 hours a day in winter and summer, which entails a large energy penalty because the two furnace fans are also running constantly to propel the ventilation air. The efficiency of these ERVs is not great (heat and moisture transfer), and in the winter, the air coming out of the registers is pretty cool during the part of the cycle when the furnace isn't heating the air. When we were designing the system with the HVAC contractor, we were cautioned to avoid adding a dedicated duct system for the basement ERV because it would require an expensive retrofit. We could have more easily put a dedicated ERV duct system in the attic, but because the attic is unconditioned, we were told that this would be problematic. So we stuck with using the existing furnace ductwork. I'm wondering if we could at least partially address the comfort and energy efficiency issues we're experiencing by 1) stopping use of the attic ERV altogether and 2) replacing the basement ERV with a commercial ERV running at around 600 cfm mounted to the return plenum that would only run when the furnace heat was also running (so we'd still be getting close to 240 cfm of ventilation, on average, but not continuously)? I don't know if the combined flow of the furnace fan (~1000 cfm) and such an ERV (600 cfm) would cause problems. The lower duct system (which provides air to the basement and main level of the house, but not the upstairs) is pretty leaky (but only leaks inside the house, I'm told), so I don't know if air resistance would be an issue. I also don't know if that large volume of ventilation air would be heated sufficiently by the furnace. I'd be interested in your thoughts on this idea and any other suggestions you might have. Thanks.

Dec 12, 2016 7:37 AM ET

Response to Danny O
by Martin Holladay, GBA Advisor

I don't recommend the use of an ERV if the ERV uses forced-air ductwork connected to a furnace with an energy-hog fan. This is a bad system.

Before making any decisions, the best step would be to hire a home-performance contractor or energy rater capable of measuring ventilation system air flow. (This article explains various methods of measuring air flow: Is Your Ventilation System Working?)

When the furnace fan is running, the furnace fan may be helping pull extra air through your ERV. As a result, you may be ventilating at a much higher rate than you think.

240 cfm is a lot of ventilation (and of course your current setup may be delivering much more). Even though that's what ASHRAE 62.2 recommends, I wonder whether it's necessary. How many people live in the house?

Dec 12, 2016 9:54 AM ET

Our Ventilation System
by Danny O

Martin, thanks for your response and recommending the article, which I found interesting.

When we had the ERVs installed, we used a flow hood outdoors to measure the rate of fresh air being sucked through each intake mounted on the outside of the house. The flow into each intake was about 120 cfm. Is it correct to assume that this is the amount of fresh air entering the house? The contractor also tested the flow for one system by inserting a small probe in the fresh air duct just before it entered the return plenum—this value was close to the flow rate measured using the flow hoods. Are these the correct ways to measure the ventilation system airflows?

Four people live in the house. The ASHRAE calculation is based on a home energy auditor’s testing (including blower door test and home volume calculations). The house was pretty tight to begin with, and the auditor sealed the attic floor (among other things) to make the house even tighter.

Your recommendation to avoid using forced-air ductwork makes sense to me. So, if we had the opportunity to start over, would you recommend adding a dedicated ERV duct system from the basement, even though the retrofit would be expensive? Would you put a dedicated ERV duct system in the unconditioned attic (or first seal the attic walls/ceiling to make it a conditioned attic)? Alternatively, we could have installed a motorized damper controlled by an AirCycler, but wouldn’t that also require use of the forced-air ductwork and have an even higher energy penalty than our current system? Plus, the AirCycler (or our existing ERVs) wouldn’t be able to get close to 240 (or 120) cfm if running discontinuously and limited to 7% of total furnace air flow. I’d appreciate any advice you have. Thanks!

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