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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 June 28, 2013

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?

If you do install an exhaust-only ventilation system, don’t bother installing passive fresh air inlets in the walls. Fresh air will find its way into the home through random cracks.

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..”

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 for ventilation purposes. 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.

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. However, builders who need to troubleshoot problems may be interested in several low-cost methods of measuring airflow:

  • The August 2002 issue of Energy Design Update describes how to build a homemade flow hood using a cardboard box and a $90 digital anemometer.
  • Two Lawrence Berkeley National Laboratory engineers, Iain Walker and Craig Wray, have written a paper describing a method of measuring airflow with a “calibrated” laundry basket and a manometer.
  • Terry Brennan promotes a method of measuring bath exhaust fan airflow with a cardboard box and a credit card. For more information on this method, click here. The essential details of the method are explained in the caption to Image #2, below.
  • The easiest way to measure airflow at a supply register is the garbage bag technique developed by Don Fugler of the Canada Mortgage and Housing Corporation.

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

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  1. Renewaire

Wed, 11/17/2010 - 18:11

ERV vs Wave Home Solutions vs. Dehumidifier .
by Eric

Helpful? 0

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.

Thu, 11/18/2010 - 03:44

Response to Eric
by Martin Holladay, GBA Advisor

Helpful? 0

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.

Mon, 07/18/2011 - 15:13

Cheap Ventilation Solution...
by michael isaacs

Helpful? 0

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

Mon, 07/18/2011 - 16:04

Edited Mon, 07/18/2011 - 16:05.

Response to Michael Isaacs
by Martin Holladay, GBA Advisor

Helpful? 0

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.

Tue, 08/02/2011 - 12:46

Edited Tue, 08/02/2011 - 20:56.

Need reno advice please
by Jill Buffie

Helpful? 0

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?

Wed, 08/03/2011 - 04:09

Edited Wed, 08/03/2011 - 04:10.

Response to Jill Buffie
by Martin Holladay, GBA Advisor

Helpful? 0

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.

Sun, 08/07/2011 - 13:10

HRV debate
by Jill Buffie

Helpful? 0

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?

Sun, 08/07/2011 - 15:45

Edited Sun, 08/07/2011 - 15:46.

Response to Jill Buffie
by Martin Holladay, GBA Advisor

Helpful? 0

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

Sun, 09/25/2011 - 16:16

Edited Sun, 09/25/2011 - 17:31.

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

Helpful? 1

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!)

Mon, 09/26/2011 - 06:29

Response to David Butler
by Martin Holladay, GBA Advisor

Helpful? 0

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.

Tue, 09/27/2011 - 13:33

equipment models, price tiers
by David Butler

Helpful? 0

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.

Fri, 04/13/2012 - 13:38

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

Helpful? 0

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....

Fri, 04/13/2012 - 13:51

Response to Todd Oskin
by Martin Holladay, GBA Advisor

Helpful? 0

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.

Tue, 09/25/2012 - 20:59

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

Helpful? 0

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.

Wed, 09/26/2012 - 03:25

Response to L. Buser
by Martin Holladay, GBA Advisor

Helpful? 0

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.

Sat, 11/03/2012 - 18:20

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

Helpful? 0

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.

Sun, 11/04/2012 - 06:49

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

Helpful? 0

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.

Wed, 07/03/2013 - 19:34

by Phil Lawson

Helpful? 0

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.....

Thu, 07/04/2013 - 04:49

Response to Phil Lawson
by Martin Holladay, GBA Advisor

Helpful? 0

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.

Wed, 01/01/2014 - 17:20

exhaust only ventilation
by rick Martelli

Helpful? 0

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

Thu, 01/02/2014 - 07:27

Response to Rick Martelli
by Martin Holladay, GBA Advisor

Helpful? 0

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.

Sun, 01/19/2014 - 09:15

venting into the duct system
by john bailey

Helpful? 0

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.

Sun, 01/19/2014 - 10:04

Response to John Bailey
by Martin Holladay, GBA Advisor

Helpful? 0

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?

Sun, 01/19/2014 - 11:41

more venting into the duct system
by john bailey

Helpful? 0

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.

Sun, 01/19/2014 - 11:46

Edited Sun, 01/19/2014 - 12:15.

Response to John Bailey
by Martin Holladay, GBA Advisor

Helpful? 0

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.

Thu, 08/28/2014 - 17:38

Expelling of indoor contaminant with supply ventilation
by Venkat Y

Helpful? 0

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.

Thu, 08/28/2014 - 18:05

Response to Venkat Y
by Martin Holladay, GBA Advisor

Helpful? 1

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.

Thu, 08/28/2014 - 21:09

Martin, Thanks for the
by Venkat Y

Helpful? 0


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.

Fri, 08/29/2014 - 03:37

Response to Venkat Y
by Martin Holladay, GBA Advisor

Helpful? 0

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.

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