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How to choose the right mechanical equipment for a balanced ventilation system in your home

Posted on Jan 22 2010 by user-756436

After investigating various ventilation options, many residential designers conclude that they want either a heat-recovery ventilator (HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ) or an energy-recovery ventilator (ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.). They often remain confused, however, about which of the two devices to choose.

Every tight home needs a mechanical ventilation system.

Most builders choose one of three ventilation options:

A balanced ventilation system with an HRV or an ERV is the preferred ventilation system for a Passivhaus building. Although balanced ventilation systems are expensive to install, they have the lowest operating cost of any ventilation option — assuming, of course, that the designer or installer hasn’t made any blunders. (Sadly, this can be an optimistic and risky assumption.)

The purpose of an HRV or an ERV is to deliver fresh air to a home’s interior. Neither appliance is designed to provide makeup air for combustion appliances or kitchen exhaust fans. HRVs and ERVs are not space-heating devices, heat-delivery devices, or energy-saving devices. The more hours that an HRV or ERV operates, the more energy it uses — electrical energy to operate its fans, as well as heating or cooling energy to make up for the conditioned air that these devices expel from a home.

What they do and how they work
An HRV’s fans pull fresh air into a home while simultaneously exhausting stale air from the home. In most installations, the fresh air is delivered to the living room and bedrooms, while the stale air is removed from bathrooms, laundry rooms, and sometimes the kitchen.

Both the fresh air stream and the stale air stream flow through the HRV. The core of the appliance allows some of the heat from the warmer air stream (the stale air in winter, the fresh air in summer) to be transferred to the cooler air stream. In winter, in other words, the appliance “recovers” some of the heat that would have otherwise been exhausted. This heat transfer occurs without any mixing of the two air streams.

An ERV does everything that an HRV does. In addition, an ERV allows some of the moisture in the more humid air stream (usually the stale air in winter and the fresh air in summer) to be transferred to the air stream which is dryer. This transfer of moisture — called enthalpy transfer — occurs with very little mixing of the two air streams. (The cross contamination rate for one well-regarded ERV, the UltimateAir RecoupAerator, is 9.6%.)

Why ventilate a house?
Before we can clarify the choice between an HRV and an ERV, we have to consider the question, “Why should a house be ventilated?” As it turns out, the question has several answers, including:

  • To provide enough fresh air to keep the occupants healthy;
  • To remove odors;
  • To dilute indoor pollutants; and
  • To lower the indoor relative humidity.

Most of these goals are easy to understand. (Even so, establishing an optimal ventilation rate to achieve these goals is a contentious issue.) However, using ventilation to achieve the last of these four goals — lowering the indoor relative humidity — gets problematic.

To prevent moisture damage to a house, lower humidity levels are always preferable to higher humidity levels. In other words, dry is always better than damp. However, some people begin to complain if the indoor relative humidity is too dry — say, 20% or below. (Of course, people have lived healthy lives for thousands of years in climates where the relative humidity is often below 20%, so it’s not at all clear that low humidity levels are unhealthy.)

Ventilation can only reduce the indoor relative humidity if the outdoor air is dryer than the indoor air. Since cold air can’t hold as much moisture as warm air, ventilating a house helps lower the indoor relative humidity only when it’s cold outside (or on dry days during the spring and fall). In most parts of the U.S., ventilation during hot weather actually introduces more moisture into the house — that is, it tends to raise rather than lower the indoor relative humidity.

What do manufacturers recommend?
Unfortunately, you can’t depend on HRV and ERV manufacturers to tell you whether your home is better off with an HRV or an ERV. Many manufacturers’ Web sites include misstatements:

  • The Fantech Web site and Lennox advise readers that the only relevant criterion is climate.
  • The Broan Web site falsely claims, “An HRV … is used only in the cold months of the year to resolve high moisture problems in the home. An ERV … can be used all year round to provide fresh air for your home.”

Other myths
These myths — that the choice between an HRV and an ERV depends only on climate, and that HRVs can’t be used during the summer — are only two of the many red herrings encountered by builders in search of accurate information on HRVs and ERVs. Other commonly repeated myths include:

  • ERVs can’t be used in cold climates because their cores will freeze.
  • In a humid climate, an ERV can act as a dehumidifier or can help address high indoor humidity.

Both of the above statements are false. (Freeze-up problems were solved years ago by the development of controls with a defrost cycle.)

Ventilating in hot, humid climates
When an HRV or ERV ventilates an air-conditioned house during the summer, the cool exhaust stream absorbs heat from the incoming fresh air. In other words, the incoming outdoor air is cooled by the outgoing exhaust air. This is only possible in an air-conditioned house. If there’s no air conditioning, the exhaust air won’t be cool, so there is little opportunity for heat exchange to occur.

If the house has an ERV, some of the moisture from the incoming outdoor air is transferred to the exhaust air. This reduces, but does not eliminate, an undesirable moisture source. (In hot, humid climates, the increased moisture load caused by summer ventilation is an unavoidable drawback to any type of ventilation.)

In a hot, humid climate, it makes more sense to install an ERV than an HRV — but not for the reason that many people assume. “ERVs are not intended to reduce indoor relative humidity during the summer,” explains Daniel Forest, the vice president of R&D for Venmar, a manufacturer of HRVs and ERVs.

Operating an ERV during the summer in Houston, Texas, doesn’t lower the indoor relative humidity; rather, it makes the situation worse. The best that can be said is that, from a moisture-load perspective, operating an ERV is less bad than operating an HRV — assuming, of course, that the home is equipped with a dehumidifier.

The main reason to prefer ERVs over HRVs in Houston is that the additional moisture introduced by the ERV — a latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates. that the air conditioner must contend with — is less than the moisture that would have been introduced by an HRV. While HRVs and ERVs both cause increased energy use, the energy attributable to ERV operation is less.

High humidity in tight houses
In Houston, a tight, well insulated house is more likely to have problems with high indoor humidity than an older leaky house. In a tight home, an air conditioner doesn’t run anywhere near as often as in a leaky home, especially during the swing seasons (spring and fall). If the air conditioner is rarely on, there are fewer opportunities for the HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment to dehumidify the interior air. Of course, adding mechanical ventilation only makes the situation worse.

The solution to this problem is not an ERV. The solution is a stand-alone dehumidifier.

Two researchers from the Lawrence Berkeley National Laboratory, Iain Walker and Max Sherman, wrote a paper, “Humidity Implications for Meeting Residential Ventilation Requirements,” that includes a discussion of the effects of residential ventilation in Houston. They wrote, “The use of an ERV did not change the humidity distribution in a hot, humid climate compared to a continuous exhaust system.”

Elaborating on this finding in an e-mail, Sherman wrote, “It is true our results show little value in ERVs in hot, humid climates, but it is important to understand why. … Almost all hot, humid climates have hours when it is dryer outside than inside and then ERVs actually make the moisture problem worse. The net effect is that ERVs are about a wash for humidity control in those climates. … On the other hand, if there were independent humidity control (such as … a stand-alone dehumidifier) then ERVs pay big dividends in terms of energy savings in hot, humid climates. To say it another way, the whole idea of an ERV is not to change the indoor humidity (and temperature) with ventilation. So if the indoor humidity is good already, the ERV reduces energy costs. If it is bad already, the ERV is not likely to help.”

What about cold climates?
Many people assume that HRVs make more sense than ERVs in cold climates. However, the situation isn’t that simple.

Whether or not the interior of a cold-climate home is humid or dry during the winter depends on several factors, including:

  • The leakiness of the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.;
  • The ventilation rate of the ventilation equipment;
  • The number of square feet per occupant; and
  • The behavior of the occupants.

At one extreme would be a large, leaky, Victorian house occupied by two elderly people who rarely cook and have few houseplants. At the opposite extreme would be a small, tight home occupied by a six-person family that eats home-cooked meals and takes frequent showers.

Is the house dry or damp?
In most cases, an old leaky Victorian home doesn’t need an HRV or an ERV. (It makes little sense to install a $3,000 ventilation system in a home that already has a high air-exchange rate.)

But even if we narrow our focus to new homes with tight envelopes, we find that winter humidity levels vary widely. Big homes with few occupants tend to be dry during the winter. Small homes with many occupants tend to be humid during the winter. These two types of homes may need different ventilation systems (or may need to be ventilated at different rates).

According to Don Fugler, a senior researcher at the Canada Mortgage and Housing Corporation, some Canadian houses can benefit from an ERV. “Although I have never promoted ERVs, we’ve started to see situations where an ERV may make more sense,” said Fugler. “In a new, energy-efficient house with no major moisture sources in a very cold climate — a prairie climate — the code-required level of ventilation will dry out your house way too much.”

Another building scientist, Terry Brennan of Camroden Associates in Westmoreland, New York, agrees with Fugler that the answer to the question, “HRV or ERV?” is, “It depends.” Brennan said, “Where I have monitored indoor relative humidity — usually in houses that are typically 2,400 square feet or smaller — HRVs maintain 30% or 35% RH, so I would say they didn’t need enthalpy. But in a bigger house with only two people, it might be different. Of course, it depends on how airtight the house is.”

Other ways to address humidity and dryness
It’s possible to overthink the choice between an HRV and an ERV. After all, there are other ways to address humidity problems in houses. In fact, these other factors tend to overwhelm performance differences between HRVs and ERVs.

For example, regardless of the type of ventilation equipment in your home, you can adjust your interior relative humidity during the winter by adjusting your ventilation rate. If your house is too humid — usually indicated by the presence of condensation or frost on your windows — just increase the ventilation rate. In other words, run your fan for more hours per day.

If your house is too dry — usually indicated by dry skin or static electricity problems — just reduce your ventilation rate. (Be careful, however — if you reduce the ventilation rate too much, you risk undermining other important ventilation functions like odor removal.)

If you live in a hot, humid climate, and you’re worried about high indoor humidity during the summer, reduce your ventilation rate. (Building scientist Joseph Lstiburek has made the somewhat controversial recommendation that homeowners in hot, humid climates should ventilate at a lower rate than the level recommended by 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.).

Finally, if your indoor relative humidity is too high during the summer, you probably need a dehumidifier. Ventilation won’t solve this problem.

The bottom line
If you want to install an HRV or an ERV, which should you choose? Here are some guidelines:

  • For a small, tight house in a cold climate — especially a house with a large family — choose an HRV.
  • For a large house in a cold climate — especially a house with few occupants — choose an ERV.
  • In a hot, humid, climate, an ERV will cost a little less to operate during the summer than an HRV.
  • In mixed climates, choose either appliance.

However, the above guidelines aren’t set in stone. For example, Paul Raymer, a ventilation expert and former member of the ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. 62.2 committee, is skeptical of the idea that large homes with few occupants could benefit from an ERV. “For big homes, an ERV might have little or no impact [on indoor relative humidity] — unless it was a big ERV,” said Raymer. “And you generally don’t need as much ventilation air in a big house with few occupants.”

In fact, the difference in performance between these two types of appliances is so slight that many builders ignore climate, house size, and occupancy, and instead make their choice based on energy efficiency.

If energy efficiency is your most important criterion — and I believe it should be — choose either the UltimateAir RecoupAerator 200DX ERV (which draws 40 watts to deliver 70 cfm, or 1.75 cfm/watt) or the Venmar EKO 1.5 HRV (which draws 24 watts to deliver 49 cfm, or 2.04 cfm/watt).

Remember, stupid installation details will undermine the efficiency of even the best equipment. It makes little sense to install a high-efficiency HRV or ERV by connecting it to the plenums of a furnace with an inefficient blower motor. If your furnace blower comes on every time your HRV operates, then the efficiency of the HRV motor is irrelevant. That’s why anyone who goes to the expense of purchasing an HRV or ERV should insist on an installation with dedicated ventilation ductwork — not an installation that tries to use existing furnace ductwork.

Maybe you don’t really need an HRV
Finally, it’s worth mentioning that it’s possible to have a well-ventilated home without an HRV or an ERV. It’s much cheaper to install a central-fan-integrated supply ventilation system controlled by a FanCycler.

If you choose this route, be sure that your furnace has an energy-efficient ECM blower.

Manufacturers of HRVs and ERVs
American Aldes Ventilation
HRVs and ERVs


HRVs and ERVs

Carrier Corp.
A relabeler of HRVs and ERVs manufactured by others

HRVs and ERVs

HRVs and ERVs

Lennox International
A relabeler of HRVs and ERVs manufactured by others




Venmar Ventilation
HRVs and ERVs

Last week's blog: "Home Dashboards Help Reduce Energy Use."

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Image Credits:

  1. Venmar Ventilation

Jan 22, 2010 8:34 AM ET

Heat Exchanger Cores
by Interested Onlooker

The North American HRVs seem to favour cross-flow heat exchangers rather than the counterflow heat exchangers which are more common in Europe. Why is this ?

Jan 22, 2010 6:50 PM ET

Poor installation
by user-774062

Thanks for the patient, nuanced discussion. I wanted to make the case for simplifying. All the HRV's I've tested have shockingly low flow rates where it counts, and categorically low flow rates overall. To make matters worse, I've measured the power draw of the Venmar Eko 1.5 (with ECM) and found it to use twice as much electricity as rated, both at high and low speeds.
Both of these problems are symptoms of poor installation. They lead me to ask, what exactly are you getting for this $2000 doodad?
For nearly all homes I advocate an exhaust-only strategy. Because exhaust-only ventilation unbalances house pressures, this strategy can result in good distribution of ventilation air when randomly-scattered holes in the envelope leak inward. Exhaust ventilation is usually inexpensive to install, and has a high likelihood of good-enough installation such that 60-80% of rated flow is common. Finally, the motors use about 20% of the electricity of a typical residential HRV, making for low operating costs.
High flow rates, ok distribution, low operating cost: simple is beautiful.

Jan 22, 2010 7:59 PM ET

Exhaust-only ventilation systems
by user-756436

Li Ling Young,
I agree that exhaust-only ventilation systems can work well, especially in small homes with simple, open floor plans. Their great strength is their simplicity and low energy use — especially if a Panasonic fan is chosen.

The downside to exhaust-only systems is uneven fresh air distribution. Whether this is a problem or not depends on the characteristics of the house where the system is installed. I don't believe that the use of passive air inlets, which are often overwhelmed by the stack effect and wind, solves the problem of uneven fresh air distribution in houses with exhaust-only ventilation systems. Nevertheless, in many homes exhaust-only systems work very well indeed.

You haven't really explained what you mean by the examples of "poor installation" that caused the low flow rates in the HRVs you tested. But I can imagine the problems included convoluted duct systems, undersized ducts, or clogged filters. Every ventilation system needs to be well designed, properly installed, and commissioned. Commissioning must include flow rate verification. If the systems were badly designed, poorly installed, and never commissioned, the problem is not the HRV. However, the examples you cite are arguments in favor of simplicity.

Jan 22, 2010 8:28 PM ET

Exhaust only
by DancingDan

I worry about where you're sucking the air thru, esp. on a tight house. Why not just install some inlets if you want to avoid an ERV?

Jan 23, 2010 10:44 AM ET

Clarification and question
by CarlSeville

Great piece, Martin. This sentence confused me a little: "If the house has an ERV, some of the moisture from the incoming outdoor air is transferred to the dryer exhaust air." Are you referring to the clothes dryer exhaust air or the drier exhaust air in the house?

On this point "In Houston, a tight, well insulated house is more likely to have problems with high indoor humidity than an older leaky house.", it seems to me that it depends on the season. A tight house is less likely to have high indoor RH in the summer as the outdoor moisture won't work its way in through infiltration.

I agree with Joe L's point that 62.2 can be excessive. It seems to me that the standard was developed for worst case situations. If the house is well built, has minimal pollutants and toxins in construction materials, the occupants don't use nasty smelling cleaners and personal products, take their shoes off, and don't have pets, 62.2 is very likely overkill. If they smoke, have dogs, and the house is full of VOCs they may need more.

Finally, Broan has come out with an interesting new system (smart sense) that coordinates all the bath fans and the kitchen vent hood with an intake damper and turns fans on and off automatically to provide a set amount of ventilation. The switches communicate with each other and the damper control through the house wiring to manage the total ventilation CFM, making sure that it reaches the desired minimum. Looks good at first glance, I'd be interested in your take on it.

Jan 23, 2010 11:09 AM ET

Response to Carl
by user-756436

1. The intended meaning of the phrase, "the dryer exhaust air" was "the exhaust air, which happens to be more dry." Because my phrasing was confusing, I have corrected the text above. Thanks for pointing out the confusion.

2. Concerning moisture problems in tight houses in Houston: Armin Rudd of the Building Science Corporation did a lot of great research on this issue. I reported on his findings in the January 2003 issue of Energy Design Update. In that article, I wrote in part:

"For several years, Pulte Homes has been building new houses in Houston with a package of energy-efficiency features developed by the Building America program. Before Pulte’s involvement with Building America, their Houston homes had no noteworthy problems with high indoor humidity. But to the surprise of Building America consultants, Pulte’s first generation of Building America homes in Houston had unacceptably high levels of indoor humidity.

"To probe the origins of the humidity problem and to develop solutions, Building America sponsored a study examining dehumidification options for production builders in hot, humid climates like Houston’s. The study, directed by Armin Rudd, principal engineer at the Building Science Corporation, compared equipment designed to lower indoor humidity in 20 Pulte homes in Houston. Rudd presented his findings on October 10, 2002 at the Energy and Environmental Building Association (EEBA) conference in Phoenix, Arizona.

"Rudd’s data convinced him that the humidity problems in the Houston Building America houses were not due to the increased ventilation rate, but were an indirect result of the improvements in window performance and envelope tightness. These improvements lowered the buildings’ sensible cooling loads to the point that existing air conditioners are unable to handle the latent load. The research confirmed that in these houses, the only way to keep the indoor relative humidity dependably below 60% is to install a dehumidifier."

To learn more about of Rudd's findings, read his entire report, "Dehumidification Systems: Research Results," posted online here:
or here: .

3. I agree about the Broan SmartSense fans. I reviewed the product in the April 2007 issue of EDU, when I wrote:

"The fans come with controls that are able to communicate with other fans in the home, keep track of the minutes of manual operation of all the home’s exhaust fans, calculate the amount of additional ventilation required each hour to meet ASHRAE 62.2 requirements, and automatically operate all of the home’s fans when necessary to meet ASHRAE 62.2 minimum ventilation rates. Moreover, the fans are quiet and energy-efficient.

"The fans are designed to be installed in homes requiring two or more exhaust fans. Each fan includes an integral control module. Using a table that comes with the installation instructions, the installer programs one fan -- the mas-ter unit -- with the desired ventilation rate for the entire home, according to the ASHRAE 62.2 formula; the factory default rate is 90 cfm.

"Like most bathroom exhaust fans, Broan SmartSense fans can be operated with a manual wall switch. Once each hour, the SmartSense control calculates the total manual run time of each fan during the previous hour. (The fans communicate with each other over the home’s power lines; no communication wiring is required.) Before initiating a cycle of automatic ventilation, the SmartSense control gives credit for any operation time resulting from use of the manual wall switches. Automatic ventilation run times are distributed evenly among all the fans in the house. If manual fan operation during the previous hour exceeds ASHRAE 62.2 requirements, no additional automatic ventilation occurs.

"For example, assume that a house has a four-fan system and an ASHRAE 62.2 ventilation requirement of 60 cfm (3,600 cubic feet per hour). The ventilation rate is programmed into the control when the system is commissioned. If none of the fans has been operated manually for a full hour, then the control will direct each of the fans to operate long enough to exhaust 900 cubic feet (one fourth of the ASHRAE 62.2 requirement). Ventilating from four fans provides a better distribution of fresh air than ventilating from a single fan.

"The control can be set to “vacation mode,” suspending all automatic ventilation until the next time the master fan is manually operated, at which point automatic ventilation resumes."

Jan 23, 2010 12:55 PM ET

More discussion of tight houses in Houston
by user-756436

Just to be clear: tight houses are always more energy efficient than leaky houses, even in Houston. I strongly believe that houses should be built as tight as possible.

Although I pointed out that tight houses in Houston often have high levels of indoor humidity, there's a simple solution to the problem: install a stand-alone dehumidifier. If you do that, your tight Houston home will be a low-energy home — and a dry one.

Jan 23, 2010 2:45 PM ET

Exhaust Only Systems
by Brett Moyer

I must take issue with Li LIng Young's stance on exhaust only systems; "For nearly all homes I advocate an exhaust-only strategy... Simple is beautiful" I understand that he clarified his advocacy of this type of ventilation strategy with "for nearly" but comments like these (intentionally or not) scare builders out of tight home construction.
They begin to think, "this ventilation stuff is too complicated" and its back to business as usual- building leaky, inefficient homes.
Or perhaps the builder decides to build a tight home, and puts serious effort into air sealing and tight construction. But because of his perception home ventilation, his ventilation strategy remains the same; exhaust only. Perhaps this strategy consists of 4 bathroom fans. But what about the other exhausting appliances in the home... clothes drier, 300 cfm range hood, and atmospheric-venting water heater... and why not throw in a wood-burning fireplace?
This really isn't all that atypical. Doesn't anyone see a problem in this?

Jan 23, 2010 10:36 PM ET

Great Article
by Allan Edwards

Thanks Martin for a great article and for references to Houston's climate.

Jan 24, 2010 3:43 PM ET

by Riversong


Good article, but some corrections are necessary.

You said twice that, with an ERV, no mixing of the two air streams occurs. While it's hard to find manufacturer's specs on cross-contamination, ASHRAE Standard 62-2004 allows 5%-10% ERV cross leakage (depending on whether it's exhausting from the kitchen or just bathrooms). I suspect there is some leakage in every unit.

You said that "people have lived healthy lives for thousands of years in climates where the relative humidity is often below 20%, so it’s not at all clear that low humidity levels are unhealthy." Sure, but they evolved in those climates and didn't have expensive hardwood flooring or overly-sensitive mucous membranes. Low humidity not only causes wooden furniture to loosen up, but can cause respiratory problems and irritation of nose, throat and skin. Houses like RH between 20% and 40%, while humans like RH between 30% and 70%. So the appropriate indoor compromise is 30%-70% RH (at the low end in winter).

You say that "freeze-up problems were solved years ago by the development of controls with a defrost cycle." But that solution can involve an energy-intensive electric resistance defroster or dampers which recirculate indoor air, thereby bypassing the fresh air function. Those dampers are also likely to increase cross-contamination when they're closed.

You claim that " if we narrow our focus to new homes with tight envelopes...big homes with few occupants tend to be dry during the winter." I would like to see evidence of that. A big empty heated house might be dry, but almost any occupant load in a tight house (unless it's an enormous house) will drive moisture levels upwards. A big house with the same level of tightness has much smaller ACH because of the larger volume, and it's winter air exchange which dries out a house.

Don Fugler's commnet that “In a new, energy-efficient house with no major moisture sources in a very cold climate — a prairie climate — the code-required level of ventilation will dry out your house way too much” is largely irrelevant to the rest of us. Everywhere in the US, typical winter RH levels are higher (12% avg.) than in the summer (though absolute humidity is lower).

And I thought the most interesting finding of the Lawrence Berkeley study was "The use of an ERV did not change the humidity distribution in a hot, humid climate compared to a continuous exhaust system." Which suggests that we shouldn't simply gravitate to the latest technology without understanding what it really does.

Jan 24, 2010 3:46 PM ET

I disagree
by Riversong

with Brett Moyer's comment about the elegant simplicity and high functionality of an exhaust-only system.

Brett said, "comments like these (intentionally or not) scare builders out of tight home construction."

On the contrary, if builders understood that they could build tight homes without the added expense of a centrally-ducted heat-recovery ventilation system, they would be more likely to concentrate on envelope improvements.

Jan 24, 2010 3:50 PM ET

Right On!
by Riversong

And I'm right with Li Ling Young's advocacy of simple and functional exhaust-only ventilation. More complex technology invariably creates more potential failure modes and requires more maintenance for reliable operation, often underperforming from ratings or expectations.

There are many ways to mess up the design and installation of a central ventilation system.

Martin continues to base his concern about passive make-up air inlets on one flawed study which demonstrated stack effect exfiltration from inlets in leaky homes. In a reasonably tight home, passive fresh air inlets combined with efficient bath exhaust fans work beautifully. That's the only system I've ever used.

Jan 24, 2010 4:05 PM ET

HRV/ERV advice we recieved
by 9QPHeSBktH

We just finished a tight house (0.6 nach) with no AC (solar assisted radiant floor heat only) and our testing trade contractor told us we would be wasting our money if we went with an ERV over an HRV. Their thinking was that without AC we would see no dehumidification benefit in the cooling season. I agree with this but I was more concerned with moderating humidity levels during the winter. The cost was negligible between the two and I felt like this was cheap insurance as I think homes this tight have a lot of unknowns as far as how they will behave with respect to humidity etc. I'd like to get others input on my thought that an ERV may help moderate humidity levels during the heating season. BTW we are in northwestern NC at an elevation of about 4,000 feet with 5480 HDD and 1033 CDD.

Jan 24, 2010 5:02 PM ET

About the "errata" noted by Robert Riversong
by user-756436

Thanks for your comments. Since you apparently agree with me on many points, I'm not sure why your comments refer to "errata."

1. ASHRAE 62.2 is a ventilation standard. The standard's maximum limits to air-stream cross-contamination are not measurements of actual cross-contamination. Any HRV or ERV manufacturers care to chime in with actual measurements?

2. You appear to agree with me about the health of people living in low-humidity climates. No argument there.

3. I never claimed that low indoor humidity was good for wooden furniture, so again, we are in agreement.

4. The defrost cycles with which I am familiar do not use electric resistance heat. They involve shutting down the ventilation function for a certain number of minutes to allow recirculation of indoor air through the core to raise the temperature of the core before resuming ventilation. Again, since I never described the mechanism of the defrost function, this fact is something we agree on and does not represent an error.

5. Although you assume that the experience of residents of Alberta and Saskatchewan is irrelevant, I always write with Canadian as well as U.S. readers in mind. Although the information I provided may not apply to your climate, it is still of interest to some readers. Obviously, the information I provided on Houston conditions similarly does not apply to you -- but it is of interest to Texans.

6. My article attempts to provide guidance to those trying to decide between an HRV and an ERV. I never derided exhaust-only ventilation systems or central-fan-integrated supply ventilation systems. For many homes, these two other ventilation methods are entirely appropriate and in fact preferred. On that fact I agree with you.

7. This is not the first time you have disagreed with my observation that wind and the stack effect often overwhelm passive air inlets, undermining their utility. It seems we have a clear disagreement on this one issue. Since we have debated it in the past, there's no need to reopen the issue here.

Jan 25, 2010 6:24 AM ET

Bypassing the HRV
by homedesign

Passivhaus mentions bypassing the HRV during "night flushing".
There are times when conditions outside are "better" than inside.
Precooling or preheating or "drying" with increased volume of outside air makes sense sometimes.
Of course occupants can open their windows.....
Are there mechanical systems that allow for bypass and high volume air change?

Also... I have noticed that the microclimate around my house can easily vary by 15 degrees (in the shade)depending on North or South

Do any HRV's take advantage by switching between warm and cool intakes?

Jan 25, 2010 7:58 AM ET

Night flushing
by user-756436

As far as I know, the best available equipment for night flushing -- air exchange at night for the purpose of cooling or drying interior air — is the NightBreeze developed by the Davis Energy Group. More info here:

Apparent differences in microclimates on the sunny and shady sides of the house cannot be used to good effect for cooling. That's because the air temperature is the same on both sides of the house. As soon as you start sucking in outdoor air, you're basically mixing the air near the wall and pulling air from a large volume. If you are drawing 80 cfm of ventilation air, that's 4,800 cubic feet per hour. You're not just pulling the "shaded air" right near your siding — your pulling in outdoor air from the great outdoors.

That's why there are no energy savings that accrue from placing the outdoor unit of your air conditioner in the shade compared to placing the unit in the sun.

Jan 25, 2010 9:02 AM ET

for Travis Thompson
by John Semmelhack


If you have a very tight house (I'm assuming you meant 0.6ACH50, not ACHn), the ERV will lead to higher indoor relative humidity (RH) levels during the heating season when compared to an HRV or an exhaust-only system.

In the heating season, your indoor air generally has a much higher moisture content than the outdoor air. The ERV will recover some of this moisture, leading to higher RH levels (when compared to an HRV or exhaust-only system).

Jan 25, 2010 9:03 AM ET

Cross-contamination in ERVs
by John Semmelhack

The cross contamination in the Ultimate Air 200DX is about 4%.

Jan 25, 2010 9:12 AM ET

For John Brooks
by John Semmelhack

Some ERVs and HRVs have an "economizer" function that allows heat recovery to be bypassed when outdoor conditions are favorable. I use the Ultimate Air 200DX in my house. It has an economizer that, when activated, turns off the energy recovery wheel when the outdoor air is in the range of 55-70F.

Jan 25, 2010 9:17 AM ET

Cross contamination
by user-756436

Thanks for the cross-contamination information on the RecoupAerator.

Jan 25, 2010 11:57 PM ET

4% Recoupaerator cross-leakage??
by furniturefarmer

John, where did you get the 4%? The Home Ventilating Institute ( has a certified products directory that lists the exhaust air transfer ratio (cross leakage) for each of its certified products. For the Ultimate Air Recoupaerator this ratio is .096, or 9.6%. Am I interpreting this incorrectly?

And to put this in perspective, ASHRAE standard 62-2004 states that up to 10% cross leakage is acceptable when exhaust air is from a restroom....

Jan 26, 2010 10:45 AM ET

RecoupAerator and more
by Jason Morosko

This is a great conversation happening here. I hope no one minds if I write a little... yes, we manufacture the RecoupAerator... I am a primary design engineer responsible for the RecoupAerator..... but I do try to stay neutral on facts. Cross leakage has been a passion (thorn) for me for the last 3 years or more. Mr. Brach is correct.. HVI lists our unit at 9.6%. I was at Bodycote (testing facility in Canada) for three weeks while they were doing all of the performance testing (including cross leak). The 9.6% was a number that was not repeatable... same day, time, test... the numbers were not consistant, so we settled with them on that number. I believe that the cross leak will vary depending on flow and duct pressures, anywhere from 2-10%... so i cannot tell you a firm number. The next question is.. does it matter? My opinion is... well, no. Reasons: First, most competitors will start recycling leaving stale air, back into incoming fresh air for defrost. This is 'cross leak'... and a lot! Second, I've never had major complaint concerning odors from leaving air getting back into incoming air.. Third... If you live in a house without mechanical ventilation, you re-breath the same air "all the time"... and I haven't heard of terrible results thereof... So, i'm just not sure there is a horrible consequence from cross leak. Further, the performance testing has penalized us for the leak... Meaning: our performance would be even higher without the leak. I had our unit testing in the EU, with some effort spent to limit the cross leak.. and with less cross leak, we had apparent sensibles of 98%+, because less air is leaking past the heat exchange medium when you have increased cross leak. OK.. enough on that one for now.

Next, yes, our unit has the night flush... "econocool", built in, standard. It has limitations.. you can read about it on our site.

The Venmar Eco... i've looked at this unit a few times... bravo for the EC motors (finally)... we've been using them since 1999... and the defrost is 28% recirc at 23 F, and 45% recirc below -17 F.
Performance for it and ours can be seen here, page 103 and 120 i think:

The recoupAerator: our theory is that you wanted fresh air all the time.. or at least when you choose... in which case below 12 F, we need active defrost of some fashion. Basically, we have a pulse modulated duct heater... keeping air coming to the unit at 12 F if it is below.. or we have a water coil, which you can do a ground loop.. 50 F water... or what not... for defrost. When its cold outside.. there is a penalty.. no getting around it. Some recylce inside air.. some warm up the incoming air.. Both take energy, or a loss of fresh air. The only way around it is to move to a warmer climate.

OK.. i'm out of time right now.. Humidity and HRV vs ERV... my goodness.. i'll have to dedicate another novel to that discussion.. but later..

Jan 28, 2010 11:18 AM ET

More discussion of these issues
by user-756436

Anyone interested in this discussion will probably also be interested in a parallel discussion inspired by this blog over at the JLC Web site:

Of course, if this topic interests our readers, I hope further comments to be made here at the GBA site so GBA readers can benefit from the discussion.

Jan 29, 2010 7:09 AM ET

for John Semmelhack
by 9QPHeSBktH

Thanks for your reply. Yes we are at 0.6 ACHn. It appears as if we made the right decision on the ERV over HRV.

Mar 25, 2010 4:58 PM ET

by Aandrew

Hello Martin
We are building a new house just north of Dawson Creek, BC. as we are building a tight, well insulated building we are considering installing a Venmar HRV 2600 to deal with indoor air quality issues.
I was wondering if anyone is familiar with this unit? As it gets down to minus 40 during the winter is there any possibility of the unit freezing up. I couldn't find any info on their website indicating whether there is a deicing feature. The unit would be in a heated, utility room located in the center of the lower portion of the house. So it would only be subjected to extreme cold air through the fresh air intake of approx ten feet.
Also does anyone know how noisy the unit is?
Different question... regarding thermostats. We are installing baseboard heat and i am considering electronic proportional control thermostats. Do you have any info on them including: make, models and operation?

thanx in advance

Mar 26, 2010 5:15 AM ET

Venmar 2600
by user-756436

The Venmar 2600 has an "Auto" operation mode that senses the outdoor temperature. When the outdoor temperature is too low, it prevents air exchange to protect the core from freezing or icing up. For more information on this feature, contact a technical representative at Venmar. You might want to speak to John Pothier ( pothierj [at] ) or just call 800-567-3855.

Concerning the thermostat question: you might want to post your question on our Q&A page here:

Apr 20, 2010 4:59 AM ET

HRV or ERV ?
by Charles123

HRV or ERV ?


I am debating on installing either a HRV or ERV for my house in Toronto,


about 2000 sq feet,
a new high efficiency gas forced air furnace and AC
upgraded wall insulation has made this old house much tighter !!

Yes i understand that i'm in the HRV zone but from what i can understand an

ERV will work all year round with a little less heat transfer efficiency

than an HRV in the winter but much better that an HRV in the summer, ie

remove heat and humidity from the outside air instead of using the HRV in

summer as basically an air exchanger without removing heat or humidity.

Some people say HRV and some say ERV. I called a few installers and most

say HRV, but then when i did my own internet research about 1/2 say ERV ?

To save money i was thinking of installing a HRV kit myself, but i can't

find any ERV kits mainly because they say they must be balanced etc.

But I'd pay the extra to get the most energy efficient and useful one i can

use comfortably all year round.

The HRV2600 DIY install kit i was looking at was this one about $790 CND +

another $200 for the duct install kit and control unit=!venmar_shop

I have a new high efficiency gas forced air furnace and AC and would be

doing a simplified return to return install.

My only fear with this HRV is that in the summer it will basically be

sucking in fresh warm moist air into the house forcing me to run my AC and

dehumidifier more?

It says = "Ideal for humidity problems and odours" but is this winter or

summer humidity ?

Or should i mail order an ERV online and try installing it myself.

How's this one?

or any others ?

Thanks in Advance

Apr 26, 2010 7:31 AM ET

Advice for Charles
by user-756436

Many of your questions are answered in this blog, so I suggest you re-read it.

To minimize the negative effects of introducing hot humid air into your house during the summer, be careful that you don't overventilate during the summer months. Reduce the ventilation rate and see if your house is still comfortable. You are correct to worry about raising the indoor humidity by overventilating.

Either an ERV or an HRV will work for you.

I think you are making a big mistake to use ductwork designed for your forced-air heating system as ventilation ductwork. Your ventilation system will perform better with dedicated ventilation ductwork.

Jul 1, 2010 7:40 PM ET

ERV vs. HRV in Hot Dry Climate
by Tony

Hi Martin,

Interesting article you wrote. However, I think I have a situation not covered by it. What do you recommend for a hot and dry climate (4,000 ft. elevation) such as that found in south-central New Mexico's Mesilla Valley? An ERV or an HRV? The house will also be a farily tight one.


Jul 2, 2010 4:47 AM ET

Response to Tony
by user-756436

My advice to you can be found in the article: "The difference in performance between these two types of appliances is so slight that many builders ... make their choice based on energy efficiency. If energy efficiency is your most important criterion — and I believe it should be —choose either the UltimateAir RecoupAerator 200DX ERV (which draws 40 watts to deliver 70 cfm) or the Venmar EKO 1.5 HRV (which draws 13.5 watts to deliver 40 to 80 cfm)."

Jul 2, 2010 10:23 AM ET

by Tony

Thanks Martin, guess I should have realized that's what you were going to say.

Jul 22, 2010 1:31 PM ET

ERV and bath fans
by jwyman

Hi Martin,

2,800 sf double-walled (partially vaulted) house with triple glazed windows in Western Massachusetts - Climate zone 5 - HDD = 7928 with two (plus occasional guest) occupants. Warmboard radiant floor system.

A Renuwaire EV300 is specified to run at 60% per hour to provide 8 air changes per day (.35 air changes per hour) based on ASHRE 62-1999. 6" oval ducting will run up to 85 cfm for all exhaust points with 8" trunk and adjustable volume dampers at merger points.

My question has to do with the ERV and bathrooms/laundry rooms and kitchen exhaust locations. Would it be preferable to install separate exhaust (only) fans in these locations? One bath has a walk-in shower with no enclosure. There will be an exhaust fan over a commercial range. I'm concerned with the low cfm of the ERV and these specific spot locations.


Jul 22, 2010 1:43 PM ET

Response to Jon
by user-756436

It sounds as if you have a separately ducted range hood exhaust fan in your kitchen. That's good. (I hope it isn't too powerful; low-speed range hood fans are better than high-speed fans.)

Your ERV should be perfectly adequate to handle the moisture in your other locations (bathrooms and laundry rooms). Most ERVs can be operated at high speed with a booster switch located in bathrooms; this allows you to increase the exhaust air flow when desired for odor removal or moisture removal.

If you have further questions about your RenewAire ERV, call the tech help line at RenewAire at 800-627-4499 or 608-221-4499.

Jul 22, 2010 3:14 PM ET

Thanks, Martin foe the
by jwyman

Thanks, Martin for the response.

The RenewAire ERV is a single speed unit, operated on a percent of hour run time with 20-40-60 minute timers at each exhaust point to insure planned airflow from each location when needed for spot ventilation needs. My concern is with 80 cfm in a master bath with an open shower and this type of unit.

Jul 22, 2010 3:29 PM ET

ERV issue resolved
by jwyman


I spoke to tech support at RenewAire, who are extremely helpful. All bathrooms will be handled by the ERV. In addition a single spot exhaust on a timer will be placed directly over the open shower area.

Thanks again for the advice.

Jul 30, 2010 3:10 PM ET

Whole House Design: Mini-Splits, ERV's & De-hunidifiers

Your article and the ensuing discussion have been very informative to me, for a variety of reasons. My wife and I are currently in the design phase of our retirement home and, based upon the information and opinions expressed in this thread, I think I am now prepared to finalize some major design considerations regarding HVAC. We will be living in a relatively hot-humid climate and, as you know (as well as many of the contributors to this thread) the economics of weather tight construction with affordable “cooling” is a delicate balance in my location. I would value input on my current design elements. First, some design background.
We have made a decision to use off-grid PV power generation and solar heated water. This decision was driven by several factors: The site is remote and we had to deal with the cost to install U/G power from the grid, about $20K; the current tax credits help with the cost, and; we have accepted the fact that PV power is a life style change. Also, we propose to use propane for cooking and back-up water heating.
Our house will be located about 35 miles SW (as the crow flies) of downtown Atlanta, Ga. which puts me slightly on the north side of Zone 3. We anticipate this to be our “last” house, thus the design will incorporate amenities and spatial considerations for when we get really old. The “heated/cooled” area is about 2,300 sf above a 1,680 sf (unheated) basement that is fully underground with the only openings to be for two vehicles. We anticipate all PV equipment (batteries, inverter, etc.) and mechanical equipment to be located in the basement. Our short-term goals are to construct a PV powered house as economically as possible with a long-term goal aimed at simple and low maintenance operation.
I have spent several months researching and studying a HVAC design that would incorporate earth tubes. I was (and still remain) intrigued with the simplicity of the concept, that is, pre-cooled or pre-heated outside supply air from an unending source. I was always concerned about the mold issues, but believed it could be overcome with a design that facilitated access to the piping for cleaning and disinfecting, as may be required. I am impressed with the web site that advocates a design that deals with the mold issues by a full under-drain system that captures and wicks the condensation away from the pipes. Apparently this design has been installed several times and thus, offers empirical knowledge. This was much more “real-life” information that I have been able to find on the subject, anywhere. When I made a rough cost estimate for me to install a similar system at my location, my decision was made not to proceed with the earth tubes. Although my estimate of $6.5K may be high, I reconciled that the earth tube system did not offer any financial advantages, and given the uncertainties in the air quality issues, engineering snafus, and potential maintenance issues, I focused on other systems to meet the heating/cooling needs for my house.
In addition to this one, I’ve read most of your articles (and accompanying threads and links) in GreenBuildingAdvisor, including on “Hot Climate Design”, “Heating a Tight, Well-Insulated House”, “Choosing Triple Glazed Windows”, “Using Ceiling Fans to Keep Cool Without AC”, as well as, many of the other contributors. The collected information provided in GreenBuildingAdvisor, tons of research, other web resources, and reviews of the various manufacturers, has led me to a conceptive HVAC design. The design consists of: Mini-splits for both heating and cooling, de-humidifier, and an ERV ventilating system. Because a heat/cooling load has not been calculated for the house, I know equipment selections can not be finalized at this point. But hopefully equipment model selection will just be fine tuning and some equipment (such as ERV’s and de-humidifier) can be determined with the info currently available. Even if this overall design concept seems like a reasonable approach to solve my cooling needs, the devil could be in the detail.
• The mini-splits I’ve researched are mostly 208/220V systems that consume lots of amps. Are there any manufacturers offering any low energy models?
• Is the wall mounted vs. ducted unit (hidden in wall) an aesthetic or a functional issue?
• Do multi-split mini’s (one compressor-multi fan coils) consume less energy? Which is more efficient: two (2) single units operating at the same time, or two (2) fan coils operating on a (4) fan coil compressor? This is probably confusing, but given that energy consumption is a major consideration in my circumstance, I hope you can figure-out what I’m trying to say. I may be able to use a single compressor at my house with multi fan coils, but only if the energy consumption is more efficient. If in the final design it is determined that I will need 4 fan coils, I believe it would be rare event for me to operate them all at the same time
• Do I understand correctly in the research report “Conditioning Air in the Humid South” by the Building Science Corporation, that it was concluded that the most cost effective de-humidifier system (in their study) was the stand-alone installed in the attic (or basement?) and vented to the main living? (page 20)
• Would the above installation be an acceptable part of the HVAC design in my house that uses mini-splits and an ERV?
• Should the ERV and de-humidifier be separate duct systems?
ERV (or HRV):
• I’ve reviewed the UltimateAir ERV technical and it seems to be a very good system for my application. I like the options and the way they interface into the control unit. I especially like the EconCool, the heating coil options. If the option performs as stated, would the EconCool be a useful option for my application?
• Maybe I missed it on the UltimateAir website, but one aspect of the heating coil I did not see were controls and pumps for the water circulation. Do you know if UltimateAir has any info on that?
• If the final installation price is reasonable, would it make sense to install the heating coil even with the split-mini’s.
• Although the max 8K btu’s is not much, could it suffice for most of the heating days in my house?
• Are there any other manufacturers that have the heating coil option?
• What are some other energy design/construction be considered.
• What are your thoughts on wall and roof R-values with this proposed system?
• Would a propane tankless water heater be an effective booster for the solar heated water and used with the heating coil in the ERV?

Jul 31, 2010 4:43 AM ET

Response to Paul DeMent
by user-756436

Paul DeMent,
As someone who has lived off-grid for 35 years, and who has advised several people on off-grid equipment choices, I think you are making several fundamental errors.

1. There are only two heating systems I would recommend for off-grid homes: wood stoves and propane space heaters (sealed combustion units with through-wall venting) that do not require electricity. All other equipment uses far too much electrical power to consider.

2. Using PV for air conditioning is an option only for millionaires. Air conditioners and ductless minisplits use far too much power for an off-grid system. Use ceiling fans and put a few ice cubes in your lemonade.

3. Mechanical ventilation systems are on for several hours a day, and therefore quickly become energy hogs. I would stick with a system that uses a Panasonic WhisperGreen exhaust fan (11.3 watts to move 80 cfm). And I would use it only when necessary. (In other words, open the windows whenever possible.)

4. Tankless (instantaneous) water heaters work fine as a booster for a solar hot water system. That's what I have in my house. However, if I were to do it again, I'd stick with a simple tank-style propane heater, which costs less and is more dependable.

Good luck. You have to stop thinking like a grid-connected person. You will have far less electricity if you live off-grid than you are used to. In winter, when your heating needs are highest, your electricity production will be very low.

Jul 31, 2010 7:26 AM ET

Thanks for the Off-Grid advice


Well, It's gonna be real hard to give up the AC (mini-splits or whatever), but over the last few years as off-grid reality sank-in, I've become accustom to the change in mind-set.

I ain't no millionaire, but if I were to find just a little PV investment cash for some mechanical cooling, what would you recommend as the best-bang-for-the-buck.

Thanks Again

Jul 31, 2010 11:30 AM ET

Response to Paul
by user-756436

Instead of spending $100,000 on enough PV panels and a huge battery system to air-condition your house, you could use the $100,000 to buy another piece of land -- one that is on the utility grid.

Aug 2, 2010 7:28 AM ET

got I!


Aug 16, 2010 2:11 PM ET

Heating/Cooling off grid
by w5mRKW6vhr

Have you considered a geothermal heat pump? The efficiencies are much higher than the options you've suggested, and with a larger lot the PEX lines are inexpensive to bury horizontally.

Aug 16, 2010 2:18 PM ET

Off-grid GSHP?
by user-756436

There is no way that an off-grid PV system can power a ground-source heat pump. In winter, when you need heating, a PV system in a northern climate just isn't producing much electricity.

Nobody who lives off-grid chooses electricity for their heating system.

Sep 22, 2010 4:08 PM ET

HRV watts
by Andy Shapiro

I measured my Venmar Eko 1.5 on low speed at 22 watts. Still fantastic even if not as low as advertised. Note that these EC motors are noisier. I bought a (<$100) silencer from EFI for the supply which totally solved the problem. some installations get it on exhaust also, but I don't mind the noise in the bathroom.

I think ducted ventilation is the price of living in a box: you need air where you live. I have measured air flows at trickle vents and the direction of flow depended on the wind direction. We don't build tight enough to make these work. Look at the blower door curve for the house and see how much pressure the Panasonic or whatever exhaust fan's flow will exert on house pressure. It is VERY little -- much lower than low speed wind. So the wind direction overwhelms the exhaust fan system. You also don't know from what basement crack your outdoor air is coming from -- rather get it straight from outdoors to breathing area.

Oct 28, 2010 2:52 PM ET

ERV/HRV options
by Solar powered house

Firstly, thanks a bunch for posting very information stuff online ‘ERV or HRV’. I am in a plan to design a net zero 1000sq-ft passive solar house for my school in eastern region. Task is to that it has to be powered completely by solar energy with no grid absolutely. In regards to HVAC system, after considering few things I decided to have ERV/HRV, Desiccant dehumidifier operated by solar hot water source and ducted heat pump. However, after reading though the article, I concluded with two options:
Option-1 : ERV (Model# UltimateAir RecoupAerator 200DX) and Mr.Slim ducted heat pumps
Option-2: HRV (Model# Venmar EKO 1.5), NovelAire Comfort Plus 300 Electric Whole House Desiccant Dehumidifiers and Mr.Slim ducted heat pumps
Can you help to provide any comments?

Oct 28, 2010 3:11 PM ET

Response to Srilu
by user-756436

You are in the eastern region of what country -- the U.S.?

It's very difficult to power the electrical appliances you are talking about -- heat pumps, ERVs, etc -- in an off-grid building. If you are talking about an off-grid building with a PV array, it's certainly impossible to heat it with a heat pump -- because there won't be much sun during the cold months.

Of course, if you have a large wind turbine in a very windy location, or a dependable source of hydroelectricity, that changes everything.

Oct 29, 2010 11:43 AM ET

by Srilu

Yes, I am in eastern region of USA. I completely agree to what you have mentioned but, it still is part of our test to design house ONLY powered by solar and to run all calculations based in that. May be, they check our design capability rather than practical feasibility of it in the market. So sorry if that confuse you but I am glad to read your article as it helped me to proceed further on my research.
Thanks again for your swift response!!

Nov 15, 2010 4:12 AM ET

ERV installation option
by Patrick

We want to add ERV system. However, the house was built and already has HVAC duct.

After we did some research, we find now we can do separate return air from the bath room and fresh air goes to supply dect of HVAC. However, our contractor told us he will exhaust stale air from return duct and put a new duct for separate fresh air to the living room cutting a grill on the floor. Which way is better? thanks!

Nov 15, 2010 5:24 AM ET

Response to Patrick
by user-756436

The best way to duct an HRV or an ERV is always to use dedicated ventilation ductwork, separate from the forced-air ductwork. Ideally the exhaust air is pulled from the bathrooms or laundry room and the fresh air is supplied to your bedrooms and living room. Any other ducting method is inferior.

Be sure you understand which fan is being used to distribute your ventilation air. If the contractor expects you to run your furnace fan every time you need to ventilate, just to distribute your ventilation air, you will be using a tremendous amount of electricity compared to using the ERV fan and dedicated ductwork.

Nov 16, 2010 1:42 AM ET

thanks! Martin. Now there are
by Patrick

thanks! Martin. Now there are two contracts proposal, both are partially dedicated: one is separate, additional ductwork to be used to transfer stale air from the kitchen area to the ERV while the fresh air is ducted with HVAC return air.

The other is in opposite. A separate, additional ductwork will be ducted directly to a new fresh air grill in the living room while the stale air will be connected to the HVAC return duct.

Both are partially dedicated ventilation. One is stale air using separate ductwork, the other is fresh air using separate ductwork, which way is most efficient? Please help!


Nov 16, 2010 5:34 AM ET

Second response to Patrick
by user-756436

How do these contractors propose to wire the fan controls? Will either proposal depend on the furnace fan to distribute ventilation air? Or do your contractors think that the ERV fan can be used?

If they depend on the ERV fan, how will they commission the system to be sure that the correct airflow (CFM) is reaching each room with ducted fresh air, and to be sure that the correct airflow is being pulled out of each room than is being exhausted? After all, they can't use a damper on each duct to adjust airflow if they are using heating/cooling ducts.

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