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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 Martin Holladay

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

Nov 17, 2010 5:07 AM ET

Some more questions about ERV
by Patrick


Thanks for your information. When we shop for ERV, we find some only have one continuous mode and some have two mode: intermittent and continuous.

Could you give us some explanation about these two mode and tell me which ERV having only one continuous mode or having two modes is better for operation and energy efficiency?



Nov 17, 2010 6:31 AM ET

Latest response to Patrick
by Martin Holladay

You should probably direct your questions to the ERV manufacturers. Every manufacturer has a tech help hotline to answer questions.

Different ERVs come with different control options. If the ERV lacks a control for intermittent operation, I see no reason why it shouldn't be possible to use an external control -- basically, a timer or a humidistat -- to control the operation of the ERV.

Every ERV needs to be commissioned so that its ventilation rate is confirmed. If you want 40 cfm of ventilation, you don't want an ERV to ventilate at 60 cfm. If there is no way to dial down the ventilation rate, intermittent operation will give you the ventilation rate you need. For example, if a 60-cfm ERV is operated for 40 minutes out of every hour, the ventilation rate is 40 cfm.

All of these control options should be clearly understood by the ERV manufacturers. You can go online and read their installation manuals, all of which contain instructions for commissioning and operation.

Nov 18, 2010 1:03 AM ET

dedicated ventilation ductwork
by Patrick

hi, Martin:

If the fresh air does not go into HVAC, do you think we lose some energy efficiency in Winter since the fresh air temperature from ERV could be lower than the supply air? Eg, the temperate of fresh air from ERV coming in the house is 40F, however, the temperate of the supply air is 70F, then fresh heated air needs to go into return duct work to heat again to 70F or it will mixed with the supply air, if so, it will drop down the room temperate a little bit?

Nov 18, 2010 5:29 AM ET

Response to Patrick
by Martin Holladay

No, there shouldn't be any energy penalty due to the fact that fresh outdoor air needs to be heated. The air needs to be heated no matter where it is introduced -- whether it is introduced through a grille in your bedroom at the end of a dedicated ventilation duct, or whether it is introduced into your furnace ductwork. Either way, it's still 40 degree air.

However, there is a major energy penalty you don't mention: the penalty that arises from using an inefficient furnace fan to distribute ventilation air instead of an efficient ERV fan. Furnace fans often move 900 to 1200 cfm; a ventilation fan may only move 80 cfm. Running a furnace fan to distribute ventilation air wastes a lot of electrical energy during the swing seasons -- spring and fall, when your furnace fan would ordinarily be idle.

Nov 24, 2010 7:11 AM ET

ERV to prevent indoor dryness in CA winter
by Bruce C

Hi all,

We are building a 3500 sq ft house near Palo Alto, California, which has mild dry sunny summers, but in winter gets into the low 30s F with spotty to heavy rain. For A/C, which is only needed a few weeks in the year, we are specifying Fujitsu ASU9RLS mini-split units, 26 SEER. For heating, we are installing hydronic radiant floor heat with solar and gas sources. We tend to get indoor dryness in the winter heating season.

Question: Will an ERV help keep the indoor humidity at higher, more comfortable levels? Right now we have specified an HRV (Broan HRV90HT), but a Broan tech support rep told us the HRV would dry the house out. We tend to keep windows cracked open year round, but on the hottest or coldest days we plan to close the windows and turn on the HRV or ERV.

By the way, does anyone know if the Broan HRV90HT has any noise or other issues?

Nov 24, 2010 7:37 AM ET

Response to Bruce
by Martin Holladay

Considering your climate, I can't imagine that you'll have problems with indoor dryness, no matter which method of ventilation you use, as long as your envelope is relatively airtight.

Homes that have very dry indoor air during the winter either (1) are located in a very cold climate, or (2) have very leaky envelopes. If you are building a new home, do your best to reduce air leakage, and you'll be fine.

During the winter, the supply air from an HRV will be somewhat dryer than the supply air from an ERV, so choose an ERV if you're worried. But I don't think an HRV will cause any problems at all in your climate.

Jan 17, 2011 4:37 PM ET

wrt how do you define large vs small house
by Jay Hersh

Good article, just a few questions:

How large is a large house?
And is that only conditioned living area, or overall volume?

The house we're planning is just under 2100sf heated (hydronic baseboard is looking like the winner but I haven't ruled out mini-split ductless) living space with about 1400sf unheated basement about 1/2 of which may someday get finished.

The house will be a vacation home for the next 10-15 years. That means when occupied it will have 2-6 people on average however it will probably only be occupied 30-45 days of the year at the outset.

So is this a small house or a large one with respect to deciding between an HRV or an ERV? There's a bit of cost difference between these units so I'd like to make the right choice.



Jan 17, 2011 5:46 PM ET

Response to Jay Hersh
by Martin Holladay

I would call your house average in size (for the U.S.), neither big nor small.

Don't sweat the decision. Buy an HRV or an ERV -- whatever you want. Either one will work fine. As I wrote in the blog, the energy consumption of the appliance (and its efficiency) matter more than whether it is an ERV or HRV.

Jan 21, 2011 2:36 PM ET

HRV and RH during a Seattle Spring
by Frank

Martin, my 2.14 ach50 house is ventilated with a venmar HRV set on low. My RH is currently 45-55. The bedrooms seem to have a higher RH at 50-55. The temp here in winter Seattle is typically 40 with the RH at 90+.
As it warms up here in the coming months, it will still be humid outside...should I expect a corresponding rise in my inside rh and plan to run a dehumidifier if I want to maintain inside rh ~50? In other words, what's the best way to control home rh when it's less than 20 degrees colder outside (with high outside rh)? thank you

Jan 21, 2011 2:44 PM ET

Response to Frank
by Martin Holladay

If you want to lower your indoor relative humidity during warm humid weather, your best bet is a stand-alone dehumidifier.

May 31, 2011 3:03 PM ET

Edited May 31, 2011 3:04 PM ET.

Retrofit in NC
by Bradley Yoder

We are getting ready to tighten-up a roughly 6000 sqft home here near Raleigh NC, sealing and insulating the crawl space and open-cell spray-foaming the roof deck and gable ends in the attic. Two things: 1) I don't see any conclusive recommendations of one over the other (HRV/ERV) for this mixed humid climate; 2) do you feel a blower door is a critical part of the process of determining the exact ventilation recommendations, or can we be reasonably comfortable, with communication with our HVAC pro, that an RV install will provide sufficient pre-conditioned fresh air?

May 31, 2011 3:10 PM ET

Response to Bradley
by Martin Holladay

You wrote, "I don't see any conclusive recommendations of one over the other (HRV/ERV) for this mixed humid climate."

I agree. That's why I wrote, "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."

Q. "Do you feel a blower door is a critical part of the process of determining the exact ventilation recommendations?"

A. No. The most important step is to follow ASHRAE 62.2 recommendations.

Neither an HRV nor an ERV provides "pre-conditioned fresh air." Instead, these appliances provide fresh exterior air, with some heat recovery to lower the energy penalty of ventilation and to slightly temper the temperature of the ventilation air.

By the way, I don't know what "an RV install" is.

Jun 1, 2011 3:59 PM ET

Response to Martin
by Bradley Yoder

10-4, Martin. Thank you. Owner leaning toward an ERV, so that just might be the kicker, eh? By RV install I meant any recovery ventilator installation.

Jul 13, 2011 8:01 AM ET

ERV/HRV recomendations
by Kurt Samson

HI Martin, GREAT ARTICLE. I am in need of an ERV/HRV in a mixed climate (central NJ). Are your top recommendations still the UltimateAir and Venmar? Anything new we should know about? I have contacted both companies direclty and price wise both seem to be within $100 of each other.

If it helps any, our house is a tightly sealed house, 3600 sq feet colonial, 2 story, 2 systems (one in attic, one in basement). I am leaning towards the ERV only because it seems it might be a little less costly to operate overall...but I am a gadget man and do like the LCD on the Venmar. Your thoughts based on my current situation.

Thanks much Martin.

Jul 14, 2011 7:39 PM ET

Response to Kurt
by Martin Holladay

Q. "Are your top recommendations still the UltimateAir and Venmar? Anything new we should know about?"

A. Yes, I still like the UltimateAir and the Venmar. However, if you can afford to spend more, the Zehnder HRV from Europe is getting great reviews -- it's very quiet and very efficient. Just expect to pay more.

Jul 22, 2011 1:52 PM ET

Response to Paul DeMent
by John Mattson

Sorry to be so late to the party. Paul ref aux water heater. While a starving student I noticed that I could turn OFF the gas in the water heater and that the PILOT light alone would heat enough in 24 hours for a comfortable shower and dishes. Years later I learned that you could get bacterial growth in such a system and you should occasionally heat it up enough to pasteurize it. Much of conservation depends on trading time for energy.

Oct 6, 2011 11:57 AM ET

very small CFM needs
by Mojave Disaster, 3B

It's possible this detail is in the comments somewhere, but this might be worth adding to your list of manufacturers for small applications--we've ordered one of these Panasonic ERVs for an approx 1200 cf (120sf x 10'h) space. Can be set to 40/20 or 20/10 cfm at installation. Should arrive in the next couple days--we'll see how it goes. Price online has been in the mid $300 range.

Nov 23, 2011 11:33 PM ET

Using existing ductwork
by Mike Strecker

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.

Still a little confused on this comment?
I understand that using the furnace blower to distribute the incoming air from a HRV would be unwise. But why cant the duct work be used in a passive manner and have the HRV blower push the air through it. I understand that a direct return to the living and bedrooms would better circulate the air in the house. But would not the existing duct work be almost has good at distribution of the ventilation air. In a separate discussion on ventilation it is even mentioned that 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. if this is true then a HRV that supplies the return air into the duct system should be more then adequate to meet the ventilation needs and you still get the benefit of some energy recovery with the HRV vs a exhaust only system or is there something I'm missing? Thanks for all your great advice. It's incredible how much you have help me over the years better understand all this.

Nov 24, 2011 12:59 PM ET

Edited Nov 24, 2011 1:01 PM ET.

Response to Mike Strecker
by Martin Holladay

One of the great benefits of an HRV connected to dedicated ventilation ductwork is that good distribution of ventilation air is provided, especially to remote bedrooms. The same cannot be said for an exhaust-only ventilation system.

If you hook up an HRV to existing furnace ductwork, the fresh air is much less likely to reach remote bedrooms, because such ductwork is oversized and usually leaky. The fresh air will end up somewhere in the house, but remote bedrooms won't get the air exchange.

Nov 24, 2011 10:49 PM ET

Thanks Martin for working on Thanksgiving!!!
by Mike Strecker

Thanks for the response. Cant believe you took the time to answer on Thanksgiving. I'm remodeling an older home so running a dedicated return duct may be a little difficult but I'll try to find a way to do it.
HAPPY THANKSGIVING! you are a great example of what Thanksgiving is all about.

Nov 28, 2011 9:50 PM ET

HRV installers in northern VT
by Jay Hersh

Can anyone (Martin?) recommend any HRV installers someplace in the vicinity of Stowe, VT?

Memphramagog was recommended to me by one of my contractors but despite having spoken to him several times about hiring him as a design consultant and having him install the HW I've purchased (a Fantech SHR1505R which my consultations with my contacts Efficiency VT indicate should be sufficient for my home's needs) for some reason he just hasn't followed up with me. So I need to find someone else to review the design as well as help me get a handle on controlling the humidity in the interim (more on that below) but I haven't had much luck with online searches.

I sort of need to do something about this soon because we've been having some humidity control issues. The house is a SIP over timber frame design. The frame went up in mid August as did the SIP walls. The house was protected from rain intrusion but open to the air for a couple of months since the windows and doors installation didn't get completed until late October. Unfortunately VT had an unusually wet summer and fall. Since late October the house has been very tightly sealed though I haven't had the blower door test done yet to quantify it. Even though the building materials had a couple of months with the house not being sealed they still seem to be putting out a lot of moisture and we've been having condensation problems which have at times been very bad.

So I even went so far as to install the HRV unit myself in a temporary manner with the exhaust drawing air out of the house from the basement and the intake air coming in to the house on the upper floors in the hopes that this would pull the most dense and moist air out and thereby lowering the humidity. While at this seems to be doing so with the humidity dropping below 60% at other times it rises up into the mid 70s. The house doesn't have a heating system installed yet. The only heat is from the passive solar component of the design and the operation of the Vermont Bun Baker wood stove (which BTW we like a lot so far) that we had installed at the times we're there working on the house.

Unfortunately I can't do a final installation on the HRV because the interior wall framing build out is only about 1/2 way done and none of the finish work on it has even started. However I would like to try to get someone in that has experience with HRV installations in the hope that they might be able to determine if more can be done to control the humidity and, until the house is ready for the final duct installation to also review the plan for that (which I've previously gone over with the rep from Efficiency VT) to try to make sure the intake and exhaust ducts will get installed in suitable locations.



Nov 29, 2011 4:44 PM ET

Response to Jay Hersh
by Martin Holladay

I just got off the phone with David Hansen at Memphremagog Heat Exchangers. You can reach him at 802-229-2722. He's the best installer of HRVs in the state, and the only one I would recommend.

He says he's already talked with you, and hasn't charged you anything to answer your questions. He says he is available to do work at your house in Stowe whenever you're ready for him.

Dec 27, 2011 5:49 AM ET

Asthma & Allergies - Los Angeles
by Josh Rader

What is the best ventilation system to purify indoor air for Los Angeles? And Might the answer be different for the San Gabriel Valley; Encino/Sherman Oaks compared to the Westside; Santa Monica/Mar Vista? I ask about the two areas because it's 15-20 degrees hotter and extra smoggy in the valley during spring and summer compared to the cooler and "seemingly" less smoggy Westside.

Assuming it's as air tight a home as possible with in reason for a 1940/1950 retrofit here in L.A what do you recommend?

Also, how does your recommendation handle VOC's, moisture and dryness?

Thank you for your time.



Considering LA has the WORST air quality in America and both the Westside and the Valley are next to busy airports both Van Nuys and Santa Monica airport....Major freeways and conjested roads

Dec 27, 2011 7:23 AM ET

Response to Josh Rader
by Martin Holladay

Outdoor air can be filtered; if you live in an urban area, filtering your air is probably a good idea. But (as far as I know) there is no magic device to "purify" air that is fouled with car exhaust and industrial pollution.

Ultimately, preventing poisoning from these sources is a political problem, and I suggest you become an environmental activist.

Jan 16, 2012 12:58 PM ET

Edited Jan 16, 2012 12:59 PM ET.

Question: DUCT HEATER required in cold climate house?
by John Scime

Hi there - I'm in the beginning stages of building a super-insulated house near Ottawa, Canada. One of the last details / sub-contracts is for the HRV system. One of the firms providing a quote insists a 1500-2000w duct heater is required.

This seems silly given that the house has R50 walls, R80 attic, tripled-glazed fibreglass windows and will be tight at less than 1.0 ACH50.


Jan 17, 2012 7:43 AM ET

Response to John Scime
by Martin Holladay

In your climate, an HRV or an ERV needs controls that handle low outdoor temperatures to prevent the heat-exchanger core from icing up and to maintain comfort. Every HRV or ERV that I know of includes such controls. Usually, a temperature sensor will shut down the HRV periodically in cold weather to allow the core to warm up; some manufacturers circulate interior air through the core periodically to keep the core warm. These controls work, and no additional electric resistance heater is needed.

In general, the need for mechanical ventilation decreases slightly in very cold weather, because the stack effect increases. Homes generally leak more air in very cold weather than in mild weather.

So, don't install the heater -- unless there is an unusual technical reason for continuous mechanical ventilation in your home.

Jun 13, 2012 11:49 AM ET

Humid air less dense than dry air
by Chris Johnson

I just wanted to correct one very common minor misconception. Jay Hersh wrote " the hopes that this would pull the most dense and moist air out and thereby lowering the humidity."

Moist air is lighter per volume (less dense), not denser, than dry air -- as pilots, meteorologists and engine tuners will know. However, the temperature of the air affects its density much more than moisture content, so, for example, the stack effect is going to far override any stratification caused by moisture, I'd think.

Oct 29, 2013 6:12 PM ET

help for advice
by Joe Pa

I dont know if anyone will read this so long after the thread. But I am wondering something and the builder isn't helping a whole lot.

I just moved into a 2 year old 3000 square foot house in the Sacramento, CA area. As you know, the summers are hot and dry.. and the winters can get chilly (but not that bad).

The ventilation system the builder installed has seperate duct system - but it uses the AC blower to move the air. I'm not sure what kind of system it is - but I do know it is a Honeywell.

The problem I am experiencing (and it was kind of alluded to in the posts) is that the darn thing runs constantly... probably 12 hours a day and uses a ton of energy. No lie - about half my energy use is from this system.

My wife, me and my 5 month old son live in this house... 2 bedrooms are vacant...

Anyway - question... do you think the system is running too much?


Oct 29, 2013 8:29 PM ET

Response to Joe Pa
by Martin Holladay

Q. "Do you think the system is running too much?"

A. Yes. It's almost always a bad idea to use a furnace fan or the air-handler fan of an air-conditioning system to move ventilation air.

If you have a Honeywell ERV, it comes with its own fan or fans. If the ventilation contractor installed ventilation ductwork, you shouldn't need to use the fan on your air handler.

Perhaps you have ventilation ductwork for the exhaust system but not the supply system.

In any case, you can call Honeywell for advice. (Try this number: 800-468-1502.) You may need to install some new ventilation ductwork to separate your ventilation system from your cooling & heating system.

Nov 17, 2013 1:20 AM ET

Edited Nov 17, 2013 1:54 AM ET.

Installing an ERV in a windy area, etc.
by David Evererst

Hi Martin,

I have a few questions for you. Any thoughts you have would be appreciated!

1) I live on the top of hill (in the midwest), and it is quite windy most of the time. Am I just wasting my time trying to add in any kind of mechanical ventilation? Is wind infiltration going to overpower any kind of mechanical system that I install?

I had planned on installing some kind a balanced system, probably an ERV. Would I be better off using a supply system to try to obtain positive pressure? My intuition says, however, that there is no way we are going to overpower the wind effect with positive pressure, so this would not be helpful.

Just FYI, we have no choice but to connect it our existing furnace system; installing dedicated ducts for the ventilation system is not a possibility for us right now. I understand your logic why this is not advisable, but due to some complexities of our particular situation, our options are limited. But we normally keep the fan for the furnace system on a lot anyway (my wife has some health problems where moving air is important), so we will not be wasting any energy (at least not any more than we would otherwise) by connecting the ERV to our existing system.

2) The plan is to just install the ERV in a loop on the return side of the furnace system. Is that the best way (assuming, as I said, that we have no choice but to connect it to the existing system)?

3) I REALLY would like the incoming fresh air to be filtered with full HEPA filtration. Ideally, I would like the entire furnace system to have a HEPA filter on it, but my contractor insists that this would increase the static pressure of the system too much. So we are just putting a MERV 13 filter on the furnace system itself. That's OK, as long as we are just filtering the air that is coming from inside the house to begin with. But when we are filtering the air that is coming from the OUTSIDE, which might be full of pollen, smoke, etc., then due to my wife's health issues we want to have full HEPA filtration on it. And I assume that we can put a HEPA filter within the ERV loop without increasing the static pressure too much.

But how can this be done? Do you know of any ERVs that incorporate HEPA filters into their design? And if we use one that does not incorporate HEPA filtration into it, and just have a standalone HEPA filter after the air passes through the ERV, how do we assure that we are maintaining a balanced system?

4) Is there a concern about condensation in the ducts in the summer (in the intake duct before the ERV and the exhaust duct after the ERV)? These are just ducts that are open and exposed to hot humid air, so it seems that condensation inside these would be a problem when the ducts enter conditioned space.

Thank you so much for your input!

Nov 17, 2013 6:26 AM ET

Edited Nov 17, 2013 6:33 AM ET.

Response to David Everest
by Martin Holladay

David, Q. "I live on the top of hill (in the Midwest), and it is quite windy most of the time. Am I just wasting my time trying to add in any kind of mechanical ventilation? Is wind infiltration going to overpower any kind of mechanical system that I install?"

A. No, you are not wasting your time. However, if your house experiences high levels of infiltration and exfiltration, to the point where you notice the air leaks, it would make sense for you to hire a contractor to perform blower-door-directed air sealing work. Sealing your envelope will make you more comfortable.

If the wind at your house blows from just one direction (primarily), it makes sense for the fresh air intake to be located on the leeward side of your house, not the windward side.

Q. "Would I be better off using a supply system to try to obtain positive pressure? My intuition says, however, that there is no way we are going to overpower the wind effect with positive pressure."

A. By definition, if one side of your house is under positive pressure due to the wind, then the opposite side of your house is under negative pressure. Air exchange is always balanced. If the wind is forcing air into your house, it is also sucking air out of your house. So your house experiences both positive and negative pressure due to the wind.

Q. "The plan is to just install the ERV in a loop on the return side of the furnace system. Is that the best way (assuming, as I said, that we have no choice but to connect it to the existing system)?"

A. No. There are many ways to duct an HRV or ERV, as the installation manual from the manufacturer will show you. Some include an exhaust duct; some include one or two supply ducts. Looping to the return air duct isn't particularly sophisticated. If you can find a way to extend an exhaust duct to a bathroom, or to extend a fresh air duct to a bedroom, it's always a good idea. But if you have no choice, you have no choice.

Here is an example of an installation manual with diagrams of different ducting options:

The problem with running your furnace fan to distribute your ventilation air is that most furnace fans are energy hogs. I certainly hope that your furnace fan is equipped with an ECM blower -- that is, an energy-efficient blower -- for the sake of your energy bill.

Q. "We normally keep the fan for the furnace system on a lot anyway (my wife has some health problems where moving air is important), so we will not be wasting any energy (at least not any more than we would otherwise) by connecting the ERV to our existing system."

A. Well, there are actually a lot of ways such a system can waste energy. In addition to the problem of having a furnace with an inefficient fan, there is a potential problem with overventilation. A furnace fan can easily pull more air through the ERV than intended by the ERV manufacturer. Make sure that your installer understands these problems and tests the airflow through the ERV when commissioning the system..

Q. "Do you know of any ERVs that incorporate HEPA filters into their design?"

A. Yes. Here is a link to one of them:

Q. "Is there a concern about condensation in the ducts in the summer (in the intake duct before the ERV and the exhaust duct after the ERV)?"

A. I have not heard of any problems with condensation. HRVs and ERVs are designed with condensate drains where necessary. Consult the installation manual for the unit you are considering to see if a condensate drain is required.

Sep 22, 2014 10:27 AM ET

An Energy Recovery Ventilator Is NOT a Dehumidifier by Allison B
by Richard Beyer

Apparently people think ERV's are dehumidifiers, even today. (2014)

Feb 25, 2016 2:53 AM ET

Energy Recovery Ventilation System
by Redfred Garett

Exhaust ventilation systems can also contribute to higher heating and cooling costs compared with energy recovery ventilation systems because exhaust systems do not temper or remove moisture from the make-up air before it enters the house. Energy recovery ventilation systems provide a controlled way of ventilating a home while minimizing energy loss. They reduce the costs of heating ventilated air in the winter by transferring heat from the warm inside exhaust air to the fresh outside supply air. In the summer, the inside air cools the warmer supply air to reduce cooling costs.

Thanks !!

Feb 15, 2017 4:39 PM ET

Is ERV needed in central europe climate?
by davor radman

I'm trying to spec out a ventilation system for a 2200sqft house in central Europe (Croatia). Summers are humid where I live, and winters are also not dry (50-90%).

Is there a need for me to go with a more expensive ERV solution?

It's also not just the price, but enthalpy ventilation units (as we call ERV here) are much fewer in numbers, looking at phi certified components.

Feb 15, 2017 4:45 PM ET

Response to Davor Radman
by Martin Holladay

I'm not sure how cold your climate is. Nor do I know how tight your house will be.

Here in the U.S., we recommend some type of mechanical ventilation system for all new homes that are built with attention to airtightness. Leaky homes generally don't need a ventilation system.

In warm climates, a simple ventilation system (like an exhaust-only system) usually makes more sense than an expensive HRV or ERV. For information on simpler approaches to ventilation, see Designing a Good Ventilation System.

-- Martin Holladay

Feb 16, 2017 2:54 AM ET

It will be a new house,
by davor radman

It will be a new house, airtight and with good insulation. Planned thermal needs are ~8000 btu/ft2.

Average temp nov-mar is 35f, year around 55f.
Humidity is >40% at all times, except on the very rare times the temp drops below -10 Celsius (a week or two in a year at most).

I have read the article you linked, which led me to conclude that an HRV is sufficient. But as a complete noob, I'm still looking for confirnation :)

Feb 16, 2017 5:27 AM ET

Response to Davor Radman
by Martin Holladay

I agree with you than an HRV will be fine.

-- Martin Holladay

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