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Choosing Equipment for a Whole-House Ventilation System

What's right for a cold climate: a heat-recovery ventilator or an energy-recovery ventilator?

Posted on Sep 20 2010 by Scott Gibson

Tight, super-insulated houses need some kind of mechanical ventilation to keep indoor air healthy. Installing exhaust fans in the kitchen and bathrooms is the simplest option. More often, energy-efficient builders install 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.).

Both systems incorporate a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. that moderates the temperature of incoming air, which helps ease the energy penalty in both summer and winter. But ERVs also are designed to transfer some of the humidity in the air stream. And that's what's puzzling Donald Lintner.

Lintner is building a house in upper Michigan with R-40 walls and an R-60 roof. Winters are certainly chilly (9,500 heating degree days per year), but Lintner doesn't expect to need air conditioning.

"Most sources seem to suggest an HRV in this situation," he writes, "but I have had several people recommend an ERV as the better choice. What would I gain with an ERV in my situation? I thought ERVs were for air-conditioning climates?"

Either one will work
ERVs may help a little, says senior editor Martin Holladay, but it probably won't make a lot of difference. Either option should work just fine.

"Some cold-climate homes get too dry during the winter," Holladay says. "For such homes, an ERV may help a little — by transferring a small amount of moisture from the outgoing stale air to the incoming fresh air.

"Although ERV manufacturers are strong believers in the value of humidity transfer between air streams, most energy researchers have concluded that the results are not very significant," he adds. "That said, most purchasers of HRVs and ERVs are happy with their equipment."

Still, says Robert Riversong, the drying potential during humid weather shouldn't be overlooked. "The primary advantage of an ERV is not the minor latent heat savings in cold climates," he says, "but the dehumidification energy savings in a humid summer climate."

Maybe in theory, Holladay replies, but actual performance is another story. He cites research by Max Sherman at the Lawrence Berkeley National Laboratory suggesting the dehumidification potential of ERVs in hot, humid weather is minor — so minor it's "hard to measure."

"The only problems with the idea are math and physics," Holladay adds. "In areas of the country where air conditioning is common, one of two situations applies. Either (1) Air conditioners work pretty well at keeping indoor air dry, or (2) air conditioners can't keep up with latent removal, leading to periods of high indoor humidity.

"Situation 2 occurs in Houston. If you are somewhere where the outdoor air is so humid that you have this problem, an ERV ain't going to solve it. There's just too much moisture involved for the ERV to make much of a difference."

It's really about energy
You're missing the whole point, Johan tells Holladay.

Yes, air conditioners can reduce indoor humidity, he says, but the question becomes one of energy consumption. Evaporating one liter of water with AC requires the same amount of energy as leaving a 60-watt light bulb on for 11 hours.

"The conclusion is impossible to misinterpret," Johan says. "If your AC condenses 2 liters in a night, you could have had your outdoor lights on for this time instead if you used ERV technology... This is not fiction or guesswork, but math and physics at its simplest."

Johan adds that ERVs work on the "principle of vapor pressure difference," meaning the higher the difference in humidity and temperature in the two air streams, the more efficient ERV becomes. "The truth is, the more humid the climate the more energy is saved by an ERV."

One caveat: ERVs aren't designed to work as stand-alone units. They should be paired with a properly sized air conditioner to keep temperature and humidity levels down indoors.

"An ERV was never meant to be a stand-alone unit," Johan writes. "It is a typical case of synergy between two technologies, since an ERV is inherently a passive unit. It CANNOT lower the humidity by itself, because it needs both a humid AND a dry air stream. Thus, whereas you'd spend $50 on air conditioning without an ERV, you would have spent about $25 if you had used an ERV WITH the air conditioner. That's the whole point of the ERV."

He directs readers to the Web site of the Air-Conditioning, Heating, and Refrigeration Institute, where the performance differences between ERVs and HRVs can be compared.

It may be a case where two sides just can't agree.

"In a hot, humid climate, either an air conditioner or a dehumidifier can be used to lower indoor humidity levels," Holladay says. "However, an ERV will not lower indoor humidity levels. In fact, ventilating with an ERV will raise indoor humidity levels. The more you ventilate, the worse the effect. About the best thing you can say about ventilating with an ERV in a hot, humid climate is that (in terms of helping indoor humidity levels) it isn't quite as bad as an HRV. "

Which type of ERV to choose?
Assuming, for the sake of argument, there's some merit to an ERV in a hot/humid climate, there's still the question of whether they are affected by pollen and mildew, says GBA advisor Michael Chandler.

Chandler says he never liked the type of ERV that uses a dessicant wheel, such as models from Honeywell. He was more impressed with the technology from Venmar, which uses a moisture permeable separator.

But, he asks, "Have you heard of any adverse issues with the moist membrane and imperfect air filters leading to dust accumulation and mildew growing inside the permeable membrane ERVs? I've seen mildew growing on my stainless-steel potting bench and even vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). soffits down here."

Not a problem with Venmar ERVs, Holladay says. Just make sure to follow the maintenance schedule from the manufacturer and change or clean the filters when called for.

Expert opinion
We asked GBA advisor Peter Yost for an expert opinion. Here's what he had to say:

● Martin is spot-on with this one.

● ERVs and HRVs work great for sensible heat exchange between outgoing and incoming air, and ERVs can boost performance with some supplemental latent exchange.

● ERVs are NOT active dehumidification systems; they work to help maintain the moisture content of indoor air (or reduce the drying out of indoor air) in cold climates but can accomplish very little in hot-humid climates where both the indoor and outdoor air streams are damp, especially during shoulder seasons when you might need dehumidification and your AC system is not accomplishing any because it's not running.

● Donald Lintner is unlikely to be unhappy if he selects a good HRV or ERV for his climate. But I would put my dollars into the most reliable HRV or ERV with the highest cfm/watt rating. With mechanical ventilation systems such as these running 24/7, their energy efficiency is pretty important.

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Sep 20, 2010 11:41 AM ET

Another option
by Armando Cobo

I would propose another option where instead of ERVs and HRVs in southern hot-humid and mix-humid climates install a dehumidifier AND a high efficient/variable speed HVAC equipment with fresh air make-up controlled by a mechanized damper and an AirCycler or an IAQ thermostat. This system must be commissioned and balanced too. You can control your indoor humidity and have IAQ at a low speed for energy savings.

Sep 20, 2010 12:19 PM ET

Response to Armando
by Martin Holladay

You are describing Armin Rudd's recommended system for hot humid climates -- a central-fan-integrated supply ventilation system coupled with a stand-alone dehumidifier. For those unfamiliar with the system, you can read more here:
Designing a Good Ventilation System

Sep 20, 2010 1:28 PM ET

HRV w/ Bath Exhaust Fans?
by Jill Neubauer Architects

I am installing a LifeBreath 155ECM HRV in a new home. It will be exhausting air from 3 bathrooms spread across 2 stories. The HVAC sub is recommending installing separate bathroom exhaust fans for local exhaust (shower moisture, odor, etc.).
An energy star consultant says this is not necessary and redundant. It certainly seems redundant to me, but the HVAC sub feels that at max speed on the Lifebreath you are pulling around 150cfm @ .2" static pressure that means 50cfm at each bathroom location. For 2 of the 3 bathrooms, I would typically use a 110cfm fan because of the size. So the HVAC sub feels like it will take too long running the HRV at full speed to adequately vent the space.

A 50cfm Panasonic bath fan uses about 5.9watts at 0.1" static pressure. While the Lifebreath uses 95watts at high speed. (not sure if those numbers are comparable, but all I could find on the specs.)

Any thoughts...additional bath fan or jsut wire the HRV to be switched to high speed for local exhaust?

Sep 20, 2010 1:48 PM ET

Addendum - 95watts at high
by Jill Neubauer Architects

Addendum - 95watts at high speed (150cfm) = 1.57watts/ cfm?
So maybe I answered my own question....running the Panasonic for 20mins. =118watts
Running the Lifebreath at full for even 40mins. =60watts

Am I interpreting these specs. correctly?

Sep 20, 2010 2:19 PM ET

by Jill Neubauer Architects

So there is some bad math above...but bottom line. It seems the Lifebreath has a much lower watt/cfm rating than the Panasonic.

Sep 20, 2010 4:06 PM ET

Response to Chris
by Martin Holladay

I agree with your Energy Star consultant and I disagree with your HVAC contractor.

The entire point of buying an HRV is that it provides an exhaust fan and a supply fan -- balanced ventilation. It will be perfectly adequate to exhaust the air from your bathrooms.

Sep 20, 2010 11:15 PM ET

Ventilation vs. Dehumidification
by Christopher Peck

Thank you for this article. I'm still struggling with the ventilation vs. dehumidification issue for our 1600 sqft home built in 1973 in Sonoma County, California. Our summers are very dry and warm, with easily 6 months of drought and enough warmth to leave windows open half the year. Our winters on the other hand are cool and very wet. On rare occasions it can reach 20 degrees, but mostly our winters hover around 50 degrees with near 100% humidity.
My issue is that it seems like drawing in cool air with very high humidity to replace warm air at 100% humidity in a bathroom isn't much of an improvement. I'm aware that warm air holds more water than cool air, but does a 20 degree air temp difference represent much actual water? We need to reduce RH because it's easy for towels to get musty and definitely the old windows (remainder will be replaced this fall) build up a lot of moisture. And that's with running the bathroom vent for 30 minutes during and after a shower.
My conjecture is that dehumidifying makes better sense in the bathroom and we can avoid ventilation there altogether. (Obviously the kitchen still needs venting for combustion by-products.) All we need in the bathroom is RH reduction, and we'd benefit from retaining the heat, and a little heat given off by the dehumidifier.
But I'm not an engineer, just an amateur remodeling his home, so I don't know if this conjecture holds water, so to speak.
Any thoughts are greatly appreciated in advance, thanks!

Sep 21, 2010 3:58 AM ET

Response to Christopher
by Martin Holladay

Twenty degrees makes a huge difference; look at a pyschrometric chart. At 50 degrees, the air can only hold half as much moisture as it can at 70 degrees.

The bottom line: even if the air is 100% saturated at 50 degrees F, it's useful to ventilate with outdoor air, because the air will help lower the indoor humidity at 70 degrees F. Outdoor air will help dry your towels.

So don't forget to install a bathroom exhaust fan, and don't forget to use it.

Sep 21, 2010 11:33 AM ET

Response to Chris Harris (and Martin)
by Michael Wentz

There are two reasons I question Martin's response about whether or not to have separate bath fans with the HRV.

- First, while Chris did not give geographical or climate information, he is choosing an HRV. Am I wrong to think that likely means cooler climate, open windows in the summer, and therefor the HRV is off? If so, wouldn't you want to have separate bath exhaust fans?
- Second, for the relatively low cost of putting in bath exhaust fans, is it worth it to put your bathrooms at risk of potentially not moving enough of the humid air?
- Looks like I have a third point, though it's more of a question since I truly don't know whether or not it is relevant. Since you tune HRVs, would you tune it differently if you did have exhaust fans than if you did not? If you do, that might mean in the case without separate bath exhaust fans you would be moving more air than necessary most of the time, and potentially losing some of your wintertime heat.

Also, separately, to the author: did I miss something or was "Johan" added to the article without an introduction (or full name)?

Sep 21, 2010 11:46 AM ET

Response to Michael
by Martin Holladay

1. I don't know Chris's climate either, but I wouldn't jump to conclusions. HRVs can be used in a wide variety of climates.

2. Any HRV manufacturer can help you choose which model HRV to buy to adequately ventilate your house. That includes adequately handling exhaust from your bathroom. This is a routine part of HRV specification and installation. If your system is specified and installed correctly, of course your bathroom will be adequately exhausted. If you choose the wrong HRV and/or you fail to properly install and commission your system, of course it's possible to screw things up.

3. If I understand your second question correctly, I agree that every bathroom should either have an exhaust fan or be connected to an HRV. Either one will work -- but you need one of the two systems.

4. I don't advocate -- nor do HRV manufacturers advocate -- connecting an HRV to a grille in your bathroom ceiling, and then also installing a separate exhaust fan. You don't need both.

5. The reference to "Johan" is a reference to a GBA reader who posted an answer to the original question posted on our Q&A page:

This page -- Scott Gibson's "Q&A Spotlight" -- is a summary of the discussion that arose from a question on our Q&A page (the link I just gave).

Sep 21, 2010 8:42 PM ET

ERV and moisture
by Jamie Kaye

The most recent Home Energy Magazine has an article by Max Sherman titled "ERV's Get The Yellow Flag".

I live in a hot-humid climate and have a RenewAire ERV which runs about 40% of the time. I have a right-sized HVAC, so I am not relying at all on the ERV for dehumidification, but from the attached lind above it seems that when I am showering, I am just efficiently moving that moisture around my house instead of evacuating it. This seems to make sense and so I am just putting a greater latent load on my air conditioner.

I live down the street from Bryant Park on Hilton Head where Armin Rudd did the HVAC design work and used a central-fan-integrated supply ventilation system coupled with a stand-alone dehumidifier approach. It seems more effective in doing what is most important simply and cost-effectively than my ERV. In the mild months, since I don't have a dehumidifier within my system, I almost need to add a one. My house reaches a high in the low 60% RH range in those mild months, so I am not sure how well the ERV is doing in those regards. With what I have read and what I understand, these ERV's need to get one step better to be worth it next time.

Sep 22, 2010 5:22 AM ET

Response to Jamie
by Martin Holladay

I'm not sure whether you realize that ERVs were never intended to act as dehumidifiers. They can't dehumidify. An ERV is a whole-house ventilation system. Its main purpose is to exhaust stale air from your house and bring in fresh air.

If the outdoor air is humid, the ERV won't dehumidify. It will simply slightly lower the moisture load attributable to ventilation -- if you are running an air conditioner or a dehumidifer in your home.

Sep 22, 2010 4:32 PM ET

Thanks for the feedback Martin
by Christopher Peck

Delayed but sincere, I wanted to thank you Martin for the feedback on my moisture questions and confusions. I tracked down a psychometric chart, and indeed, the moisture difference is significant.
It's counter-intuitive for me though, as it can really take a long time to dry out a bathroom here in the winter. Perhaps I need better fans, they might be simply noisy and not that effective at remove the moist air. I think we need to improve our sones/cfm ratio!

Sep 22, 2010 6:11 PM ET

Christopher it may not be your fans fault
by gavin

Hey Christopher,

The largest problem we find with all ducted systems in California, poor instilation and high static pressure. Installing a new fan with the same duct issues may get you a fan thats more effective at removing moisture, if your fan is acctually ducted outside the building, but it may be an added energy cost to run a varible speed fan if the fan duct system is typically installed rather than installed for low static pressure. I think that if if ever becomes common practice in CA to test installed quality we would find very disappointing results. At least that's what we have found for the last five years worth of test results on existing home regardless of age. In short the performance of any ducted ventiation system, weather bathroom exhaust, HRV, ERV will be directly impacted by it's install quality and at least in California we don't have a good track record with that. It's one thing to say you are going to get a CFM based on a chart, it's another to actually get a CFM for a particular watt draw.

Sep 23, 2010 7:49 AM ET

ERV does not stand for Energy-Recovery-Ventilator
by Karl K

ERV stands for Enthalpy Recovery Ventilator, not "Energy" Recovery Ventilator.

Sep 23, 2010 8:51 AM ET

Response to Karl
by Martin Holladay

You are technically correct -- or at least you might have been technically correct, 15 or 20 years ago. However, ERV manufacturers gave up fighting this vocabulary battle years ago. They've all thrown in the towel. These days, "ERV" stands for "energy-recovery ventilator."

Sep 23, 2010 5:03 PM ET

make up air
by Bob Taylor

we recently had a BPI audit done on our home- a triple wide manufactured unit, located in Poulsbo, Washington. Blower door tests indicate the house is tight enough that some make up air is recommended, but not required. we need 130 cfm make up air -- what options might we consider to supply this make up air?

Sep 24, 2010 4:50 AM ET

Response to Bob Taylor
by Martin Holladay

Bob Taylor,
You may be confusing two issues -- mechanical ventilation and makeup air.

1. Every tight house needs a ventilation system. You can install an HRV, an ERV, an exhaust-only system, or a supply-only system. Here is more information on ventilation systems:
Designing a Good Ventilation System

2. The term "makeup air" is used to refer to a source of outdoor air that is introduced into a house that is at negative pressure (that is, the house is depressurized). There are a variety of reasons that a house can be depressurized; among the appliances causing such a situation are a clothes dryer, a range hood, a furnace, a water heater, a fireplace, or a wood stove. In most cases, these appliances don't need a dedicated source of makeup air, but sometimes they do. The biggest problem is usually the range hood.

If a BPI home-performance rater told you that your house needs makeup air, ask why. Which appliance is depressurizing your house?

Sep 24, 2010 9:38 AM ET

response to Martin Holladay
by Jamie Kaye


I did know that ERV's were not dehumidifier's, but was under the impression that they help extract some of the humidity that was being brought in from the fresh air intake on the system. Just as the system recovers some of the conditioned air and exchanges it with the incoming unconditioned supply air, I understood the technology would also exchange some of the humidity prior to entering into the conditioned space which would help to not elevate the RH level if the outside humidity levels were higher than indoors. Is this correct?

What is your take on Max Sherman's article stating that having the ERV used in bathrooms causes the humidity to be exchanged back into the system and other parts of the structure?

Sep 24, 2010 9:49 AM ET

Response to Jamie Kaye
by Martin Holladay

You've got it right: an ERV brings two air streams -- the exhaust air stream and the supply (fresh) air stream -- into close proximity so that some of the moisture in the more humid air stream can be transferred to the less humid air stream.

During the summer, an ERV only makes sense if your house has air conditioning. Assuming the air conditioner is on, your indoor air will be less humid than the outdoor air (at least in a hot, humid climate). Running the ERV won't help lower your indoor humidity -- that's what the air conditioner is for -- but it will be "less bad" than ventilating with a fan that doesn't permit moisture transfer.

Max Sherman is right: if the main thing you want to do is to remove humid air from a bathroom, you don't want an ERV. Using an HRV (or an ordinary exhaust fan) makes more sense in that situation than using an ERV.

Sep 25, 2010 8:37 PM ET

Martin, If the discussion is
by Kurt


If the discussion is about erv's and hrv's aren't we assuming that there is a need for ventilation because the house is so "tight"? Isn't their purpose to not lose all of the energy spent on conditioning the indoor air as it is on it's way out of the house? If so doesn't it make sense in an air conditioned home in a hot humid climate to use an erv which also reduces the rh of the incoming air, even if only slightly? The Armin Rudd method sounds simpler and less expensive for lowering humidity, but it doesn't recapture any spent energy does it?. It seems to me the only question is; Does an erv (in hot humid) by virtue of helping lose less energy while ventilating the house pay for itself or not? Otherwise one may as well just use bath and kitchen exhaust fans in conjunction with equivalent sized dumb air intakes (for a lot less money)??
Thanks for running this discussion because there doesn't seem to be much consensus. I work in North Central Florida.
Kurt Johnsen

Sep 27, 2010 6:48 AM ET

Response to Kurt
by Martin Holladay

1. You're right that any tight house needs a mechanical ventilation system.

2. There are many ways to ventilate a house. The most common method in hot, humid climates is to install a central-fan-integrated supply ventilation system. Most builders think such a system gives the best bang for the buck.

3. Calculating whether the added cost of installing an ERV with dedicated ventilation ductwork makes sense is complicated, and every house will be different. Payback calculations are notoriously complicated, and depend on your assumptions. I think that installing an HRV or ERV with dedicated ventilation ductwork is the best possible ventilation system, but such systems are expensive to install. The choice is yours.

4. I think it's fairly clear that the cost to operate an ERV with dedicated ventilation ductwork will be less than the cost associated with operating a central-fan-integrated supply ventilation system.

5. Joe Lstiburek and Armin Rudd advocate that homeowners in hot, humid climates should ventilate their houses at a lower rate than recommended by ASHRAE 62.2. This recommendation is controversial, but many builders and homeowners follow it. If you ventilate at a lower rate, it lengthens the payback period associated with the investment in an ERV.

Sep 27, 2010 7:19 AM ET

Radon and air exchangers
by L. Martin

What about using an air exchanger to reduce Radon levels in a house? I have a house that is not particularly tight, but it has hot water heat & radon just below the acceptable federal levels at 2.4 pCi/L. This may be a whole different conversation to have.

Sep 27, 2010 8:04 AM ET

Response to L. Martin
by Martin Holladay

L. Martin,
Indeed, radon mitigation is a whole separate topic. You should consult a knowledgeable radon mitigation contractor.

In some homes, mechanical ventilation systems (dilution) are indeed used as part of a radon mitigation strategy. In other homes, the best solution is sub-slab depressurization. It's impossible to generalize about the best strategy without a site visit.

Sep 28, 2010 9:06 AM ET

Southern California ERV/HRV
by Matt Weaver

I read all these comments with great interest as we are building a new house in the inland area of Southern Californi i.e. dry and hot.
We are planning spray foam for roof and the interior insulation, modern AC with unit on both floors, good window placement, modern whole house fans etc.
I am concerned about a stale feeling of lack of fresh air as well as the potential to save energy in all ways possible. I also do not prefer the feeling of a closed up house with air conditioning except on only the very hottest days.
Would an HRV or ERV work in Southern California to cool a house with fresh outside air and minimize the need for AC?
I keep wondering if we are bringing in hot air from outside am i just going to put more load on the A.C?
Also, if we are building new what if we ran some PVC pipe under the slab to draw the incoming air through, could that help bring in cooler outside air?
Any suggestions for a new house in Southern California would be very appreciated

Sep 28, 2010 9:19 AM ET

Response to Matt Weaver
by Martin Holladay

Matt Weaver,
The purpose of an HRV, ERV, or any other ventilation system (including an exhaust-only ventilation system or a supply-only ventilation system) is to provide fresh outdoor air, not cooling.

If you have times of the year when it's hot during the daytime, but temperatures are cool (50s or 60s) at night, you may want to use a different type of equipment: a whole-house fan. My grandfather, an air-conditioning engineer who was president of ASHRAE, never installed air conditioning in his Altadena, California home. Instead he used a whole-house fan to cool his house at night. Here is more information on whole-house fans:

If you are operating an air conditioner, you are correct that the outdoor temperature is probably higher than the indoor air temperature, so introducing more outdoor air (through an ERV, HRV, whole-house fan, or any other type of fan) just places a larger load on your air conditioner.

I don't recommend that you install buried ductwork in hopes that you can draw outdoor air through the tubes to cool incoming air. Such systems are expensive to install and have limited benefit. (The outdoor air soon changes the temperature of the soil around the buried ducts, erasing any benefit. And some systems have had mold problems.)

Oct 18, 2010 8:07 PM ET

Looking to reduce moisture in my home
by Ron

I have a very tight 1700 Sq ft home in Coastal Maine, poly vapor barrier and high insulation values with radiant & base board heat . I have issues with moisture on windows on most days & a few mold spots in one of the frequently used baths, but no other signs of mold in the home. We are diligent about fans in the baths during and after bathing and even crack the bath window on the coldest winter day to flush out the moisture. The air in the home doesn't seem to be stale but moist..... Any moisture created in the dwelling, stays in the dwelling. I have no other moisture entering the dwelling. We use a small dehumidifier daily, but it can't always keep up. I have read the posts above and determined that ERV is not for me as I don't want ant moisture returned to the dwelling. The option of installing an HRV with ductwork in the baths to remove moisture and recover some or most of my heat $$ seems to be the best option..... Am I correct that the HRV will not return any of the moisture rich air that I am trying to exhaust? Or should I look at whole house Dehumidifiers only? That seems to be the only question left or am I missing a large benefit from the fresh air that I am unaware of? By the way, no issues in the open.

Oct 19, 2010 3:09 AM ET

Response to Ron
by Martin Holladay

First, you need to verify that your bathroom exhaust fan is working. Start with a simple test like the toilet-paper test: turn the fan and see if it will hold a piece of toilet paper against the grille. Then go outside and see if the fan is blowing the outdoor flappers open. Get a ladder and put your hand near the exterior termination: is the fan working?

Then you need to buy a hygrometer at Radio Shack to determine the relative humidity of your indoor air. How humid is the air in your house? It's important to know the number.

Then you need to investigate whether you have a damp basement or crawl space. Even if it seems dry, it may be damp. An experienced home performance contractor can help you with this investigation.

Finally, any tight house needs a mechanical ventilation system. By adding the right controls to your bathroom exhaust fan(s), the fan or fans can be used. However, an HRV would be even better -- if you can afford it.

Oct 19, 2010 7:52 AM ET

Fans work good....No moisture
by Ron

Fans work good....No moisture from crawl space or Basement, on an Alaskan Slab (Ledge issues)...well insulated and very clean soils under slab. Am I correct that the HRV will not return any of the moisture rich air that I am trying to exhaust? Also can you provide typical heat recovery #'s for a quality unit so I know the appropriate bench mark. The HRV's are approx $600-$800 that I am looking at.

Oct 19, 2010 2:31 PM ET

Second response to Ron
by Martin Holladay

Is there polyethylene under your slab?

An HRV does not transfer any moisture from the exhaust air to the supply air.

An HRV will probably recover 60% to 85% of the heat in the exhaust air stream.

Oct 25, 2010 2:17 PM ET

Our HRV was the best investment I've made for my house
by John

I live in a small, 1500 sq. foot house in northern New Jersey.

My wife and I have a high-end Fantech HRV (not ERV) with a expensive, multifunction controller and we love it. Basically, it lets us get fresh air year round. It has paid for itself many times over, I suspect because it saves us money on both heating and air conditioning.. (It allows us to use AC much less in the summer and in the winter, it saves us money because otherwise, we'd have to leave a window cracked open..

Few Americans realize how dangerous the cumulative burden of chemicals that we breathe in from household items is.. most newer houses are filled with items made of fiberboard - which emits formaldehyde.. for example..

I'm also acuetly aware of mold - it makes my breathing difficult..

Around eight years ago, I was heavily exposed to mold in a rented, actually- rent controlled apartment. While there, I got really sick. I'm still sick. One of the impacted aspects of my health has basically been that now I am hypersensitive to mold and chemical fumes of any kind.

Year round any mold exposure, can (and in the fall, when its unavoidable, it invariably does) make me very ill..

I would suspect that many contractors end up this way too.. (actually, I know so from reading statistics, contractors, farmers, teachers and librarians are at high occupational risk for many mold-related illnesses.)

For that reason, especially, our HRV has been very helpful for me. Its allowed me to gradually get better- by keeping the indoor air fresh - even as its warm or clod inside.. Its allowed me to have a level of control over the air that without it would be impossible.

I wish that I had known what I do now and installed a small HRV in the window, even, of our rented apartment, if I had, I would npt be sick today..

One thing that few people realize is that mold's ability to trigger an immune system response can persist in a house for decades..if the house isn't cleaned down to the studs and all moldy anything completely removed and replaced...

In many buildings that would be very hard if not impossible to do without spending a huge amount. So an HRV is essential.

The same thing goes for newer houses -because of the formaldehyde.

Almost no older houses have been completely free of mold for their whole lifespans.. As I have read in the scientific literature, a completely mold free environment is what it takes to recover from heavy exposure. Otherwise, mold exposure is cumulative.

Oct 25, 2010 2:31 PM ET

Exhaust fans and depressurization
by John

I just wanted to comment that exhaust fans - because of the fact that they pull in dusty air from inside of the walls and have the potential to backdraft combustion appliances as well.. are bad news for me.. they invariably make me ill. I'm considering installing some kind of interlock that will activate a makeup air fan blowing in whenever any fan is blowing out. Otherwise, especially if the shower fan and the cooking fan are on at the same time and no window is open, a bit later, I start feeling ill.

Read my earlier letter for the context (of why.)

Two identical fans in a window - one going in, one going out, would give you air exchange without depressurizing your house.

Also, there is the stack effect.. a window upstairs in the winter will let out warm air, causing your basement to be depressurized.. hot air rises..

You can see this effect very clearly with a smoke pencil or a very thin- the very thinnest and cheapest- painters dropcloth - for example, to see if your attic is leaking air outside- duct tape it over a door or other point leading to your attic. (This is also a good trick for finding leaks)

Somewhere around the middle of your house's height there will be an equilibrium point.. opening a window a bit there wont cause stack-effect driven- leaks..

Oct 25, 2010 2:45 PM ET

Usually running at 1/4 duty cycle
by John

We usually run our HRV at 1/4 duty cycle on its medium speed. The lowest speed is so silent as to be inaudible but I found that the HRV would not balance on the lowest speed for some reason. On "low" it blows out more than it blows in. You can see the curves for the various units at (I think that is the URL.)

For us, the middle speed works the best. So we run it either continuously, or 15 min out of every hour. Continuously whenever we want more fresh air..(showering, cooking, etc.) We have separate kitchen and bathroom fans too.. they were here before the HRV was.

They need to be cleaned every few months, as they have filters on the intake. But it only takes a few minutes. Twice a year I also clean out the ducts with a long brush and vacuum cleaner.

Oct 29, 2010 10:45 PM ET

Choosing the best brand
by Boulder

Like John, I also have severe mold and chemical sensitivities. We are building an extremely tight AAC (autoclaved aerated concrete) 2900 sf, 2 story home in the mountains of Boulder, CO. While it isn't too humid outside, we have concrete slab floors on both levels and concrete walls so there will be some dampness indoors for a while. I've talked to a lot of installers of HRV/ERVs and everyone recommends a different brand. I liked Venmar Eko but apparently it isn't big enough nor can we hook up to air filter later if needed. They treat their Hepa filters (so not good for chemical sensitivities). Stirling is good but do we need an ERV - from above, I have concern about adding any moisture back to house. Seeking recommendations for efficient and (if people know) least toxic units (e.g. insulation in unit is enclosed, can hook up to air filtration if we find it's needed, no treated filters, least mold/moisture issues) and advice whether we should go ERV or HRV.

Oct 30, 2010 3:49 PM ET

ERV / HRV cores
by Joe - ecoENERGY advisor

My 40 year old home (in Canada) is pretty tight with 2.5 ACH at 50Pa

I use a Venmar model "Constructo 1.0" HRV with programmable controller to reduce indoor humidity in the winter and also ensure enough fresh air is brought into the house.
For spring to fall seasons, I swap out the HRV core with an ERV core.
This unit is the only one I have seen that allows this core swap (although there may be others)

Based on the readings from my Current Cost Envi whole house power monitor, the combination
of ECM motor in my air handler (low speed) and HRV/ERV (set and balanced at 80CFM) running consumes 100W

Nov 12, 2010 6:27 PM ET

by Anonymous

I have a small bugalow in Canada and in the fall and winter there is excessive moisture on the newer windows.
Do you have a remedy?

Nov 13, 2010 6:29 AM ET

Response to Anonymous
by Martin Holladay

There are only two remedies for this problem:
1. Lower the indoor humidity level. In winter, this is usually done by increasing the rate of mechanical ventilation. One easy way to do that is to leave your bungalow's bath exhaust fan running for 24 hours a day.

2. Increase the temperature of the interior window pane. This is usually done by adding storm windows or swapping the windows with new windows with more layers of glass (for example, exchanging double-pane windows for triple-pane windows).

Nov 30, 2010 9:54 PM ET

by Jay

We have updated our 1800sf bungalow in upstate NY resulting in a tight house. Probably a similar climate to the gentleman in MI in the article (80's-90's and humid in the summer, 10 above to 10 below in the winter) . We currently have a properly sized bathroom fan and range hood. We are looking to exchange air. We are installing central air in the spring. A local contractor has reccomended an ERV.

1: My house's humidity in the winter seems to naturally run around 45% - 55%. I not want it any more humid than that. Is an ERV going to raise that level or just reduce that level less than an HRV?

2: In terms of finding the most energy efficient unit, I see a wide variation in sensible heat loss between models and manufacturers. How standardized is the testing for sensible heat loss? Should I be more concerned about these numbers or the cfm/watt ratio?

3: Living in a city, I am concerned about bringing in odors from outside... smoke, paper mill, my grilling food. I am wondering how effective a carbon filter would be at eliminating these odors... such as on the Venmar duo unit.

Dec 1, 2010 5:29 AM ET

Response to Jay
by Martin Holladay

Q. "Is an ERV going to raise that [indoor moisture] level or just reduce that level less than an HRV?"

A. The latter. Ventilating during the winter brings dry outdoor air indoors, lowering the indoor RH.

Q. "I see a wide variation in sensible heat loss between models and manufacturers. How standardized is the testing for sensible heat loss?"

A. For standardized testing data, refer to information published by the Home Ventilating Institute. Here is a link to their published data:

Q. "Should I be more concerned about these [thermal heat-recovery efficiency] numbers or the cfm/watt ratio?"

A. Both are important. The best HRVs or ERVs will have high thermal efficiency and will move a high number of cfm per watt.

Q. "I am wondering how effective a carbon filter would be at eliminating these [outdoor] odors."

A. I'm not sure. Any other readers want to comment?

Dec 1, 2010 9:02 PM ET

Sensible Recovery Efficiency
by Jay

Thank you for the HVI link Martin. It is exactly what I was looking for. If I am reading the explanation of what Sensible Recovery Efficiency (SRE) is, that is the number I am looking for during the heating months and Total Recovery Efficiency (TRE) is what I am looking for during the cooling months.

Please correct me if I am wrong, but I think that these values, SRE and TRE, take into consideration the cost (watts) to run the motors as well as the sensible energy recovered from the exchanged air. The higher the SRE or TRE the more efficient your unit overall... right?

This HVI link seems critical to consumers looking for an efficient model that suits their needs.

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