HRV or ERV in cold climate?

Most authorities recommend HRVs in all but Mixed-Humid and Hot-Humid climate zones. In a house as tight as yours is likely to be, high winter humidity is going to be the problem. An ERV will cycle the unwanted humidity back into the house rather than eliminate it.

The primary advantage of an ERV is not the minor latent heat savings in cold climates but the dehumidification energy savings in a humid summer climate.

See: http://www.fantech.net/iaq23.gif

Hi There!

I have to answer this post due to it's complete lack of insight into the maths and physics of the ERV technology. When people state that "most energy researchers have concluded that the results are not very significant", for some reason they always fail to provide a source for this information.

That air conditioners work fine is not surprising. The question we want to raise is the use of energy. Considering the fact that evaporating 1 liter of water requires about the same energy as keeping a 60Watt lightbulb lit for 11 hours (approx 2500KJ), and adding to this the observation of how much water actually drips from your AC (measure it!), the conclusion is impossible to misinterpret - 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 (even taking COP into consideration). This is not fiction or guesswork, but maths and physics at its simplest.

Furthermore, Martin states that "There's just too much moisture involved for the ERV to make much of a difference", which is a complete misconception. Since an ERV works on the principle of vapor pressure difference, the higher the difference in humidity and temperature between the air streams, the larger the driving force of humidity through the membrane, which consequently increases efficiency. The truth is, the more humid the climate, the more energy is saved by an ERV.

A common misunderstanding though, is that an ERV works as a standalone unit, which is only partly true. If there is nothing keeping the temperature and humidity down indoors, there is no differential in vapor pressure between the air streams, and consequently, no exchange of humidity between the air flows. In short, an ERV should work in concert with a properly sized air conditioner.

I sure hope this advisory forum will be better technologically updated in the future, since with this kind of advice, there is simply no way we will reach the climate goals required to maintain our standard of living in the future. We need to cut down energy use, and that is not accomplished by advising people to just use air conditioners and telling them that technologies like the ERV does not work.
For reference:
http://www.ahridirectory.org/ahridirectory/pages/erv/defaultSearch.aspx
(This is an independent test organisation that exists in order to give you, the consumer, a fair chance of comparing performance between eg HRV and ERV models. just choose "Packaged" and a producer and you will see the test results and efficiencies for that model or brand.)

To conclude, ERV is the future of most heat exchange. There is no point in leaving more than 50% of the energy for the birds, no matter where the technology is used. ERVs DOES work very neatly, ESPECIALLY in humid areas. In winter though, one does well to note that a bypass function may be needed for ventilating excess indoor humidity in extreme cases.

Regards,
Johan

The study that Martin refers to was a simulation, not a field study, used just a single set of operating parameters for the theoretical HRV and ERV equipment, and considered only one hot/humid climate - that of Houston (acknowledging that conditions in south Florida, for instance, would be significantly different). However, it did show that an ERV in Houston would reduce the number of hours of peak indoor humidity (over 70%).

Other studies by equally reputable researchers, such as Stephen C. Turner, P.E. - a mechanical engineer at Brown University, Providence, R.I., a voting member of SSPC 55, Chairman of ISO TC 205 Building Environment Design, and on the R.I. State Building Code Board - publishing in the ASHRAE Journal, suggest that with the proper mix of HVAC equipment, an ERV can lower indoor RH and overall operating costs (though he was describing commercial applications).

Every study has its own limitations and no single study can be used as "gospel". Another review of ERV problems, which included a field study, by Bruce Davis - a building science specialist with Advanced Energy of Raleigh, North Carolina - indicated that most ERV deficiencies are system design and installation problems. For instance, he recommends that an ERV never be integrated with the HVAC system, never include the bathroom or kitchen (separate exhaust fans), be designed for continuous operation (most efficient), have their airflows checked for consistency with specs, and separate the outside intake and exhaust ports to prevent crossover.

Another report indicated that the airflow of an ERV can be reduced as much as 50% after the first month of operation by dust in the filters and media.

It still remains true that the industry universally recommends HRVs for northern climates and ERVs for mixed/humid and hot/humid climates, with appropriate defrost mechanisms for freezing weather. But, the technology and sensible and total efficiency of any particular piece of equipment and how it's integrated with the rest of the heating/cooling equipment and ductwork is what matters most.

It’s the air conditioner and/or the dehumidifier that is lowering the indoor RH, not the ERV. The ERV is making the situation worse.

Not according to the study you reference. In the Houston case, the ERV resulted in fewer hours of peak relative humidity (over 70%). This is, in part, because (as the study states) "Even in many hot, humid climates indoor air has a higher humidity than outdoor for a considerable part of the year."

Remember, moisture transfer in an ERV is dependent upon vapor pressure differential, which is a function of absolute, not relative, humidity.

Note, too, that the leaky Houston house with no mechanical ventilation had considerably fewer hours of peak indoor humidity than the "standard" house without mechanical ventilation. There seems to be a net benefit to letting in outside air.

The cases in which Bruce Davis found the ERV contributing moisture to the house was because it was integrated into the HVAC ductwork with the air-handler fan running whenever the ERV would run. But, when the heat pump cycled off and condensate pooled in the catchment pan, the moving air would pick up this moisture and make indoor conditions worse. Compounding the problem was the bathroom suction duct adding considerable moisture to the ERV outflow, which was partially returned to the incoming air. To make matters even worse, a dramatically unbalanced air flow in one of these "integrated" systems dramatically lowered the moisture transfer efficiency of the ERV.

Martin,
as stated earlier, an ERV was never meant to be a Standalone unit. 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. It is after all an exchanger..

Along the same working principle, if you have problems keeping humidity down indoors, you can connect an ERV in series with your AC, which then PRE TREATS the air by using the dry and cool (compared to outside) air inside to move away moisture from the incoming air . Thus, the air that your air conditioner needs to cool (and consequently condense water from) will have a lower temperature AND moisture content, which means your air conditioner won't have to condense such vast amounts of water. It is easy to check. Just put a bucket under the draining tube from your AC, and then connect an ERV and see how much less water you get at the same temperature/humidity.

The only time the situation may become as you describe is in winter, if moisture is not vented out but recycled in the ERV. This is the situation when you'd need a bypass.

Martin,

Twice you said, "The source of my information is Max Sherman from Lawrence Berkeley National Laboratory." I assumed you were referring to the report, "Humidity Implications for Meeting Residential Ventilation Requirements", by Iain S. Walker and Max H. Sherman, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, April 2007.

That is the report in which he concluded, as you noted, "The use of an ERV did not change the humidity distribution in a hot, humid climate compared to a continuous exhaust system." And it is also the report which I quoted that indicates that an ERV does make a measurable difference in peak humidity events.

Sherman also concluded that "For the hot humid climate of Houston [ASHRAE] 62.2 ventilation increased the average RH by about 5% to 10% RH but reduced the occurrence of high humidity levels above 70% RH.

So, while you choose to emphasize the increase in average RH determined by this single simulation study using a single set of parameters, you seem to ignore the more important finding that the ERV reduced incidents of peak RH, which is what causes moisture problems, occupant discomfort and increased AC or dehumidification energy use.

since an ERV is inherently a passive unit. It CANNOT lower the humidity by itself

Johan,

An ERV is hardly a passive device, as it requires energy input and maintenance, has moving parts, requires an internal logic unit to control the defrost or bypass cycle...

And ERV manufacturers often tout the dehumidification benefit of their units, such as this from American Standard AccuExchange™ Energy Recovery Ventilator (ERV): "Saves energy during hot, humid weather, by pre-cooling and dehumidifying incoming air."

And, using an ERV to pre-condition the air supplied to an HVAC system is highly discouraged and can lead to both increased indoor humidity and mold in the ductwork.

This, for instance, from American Aldes:

"We do not recommend ducting the supply air from the HRV to the ducting of the HVAC. During summer months, the outdoor humidity supplied through the HRV may cause condensation on interior surfaces of the HVAC equipment and the supply plenum and ducting. These surfaces will be around 60F, and outdoor air dew points in the summer in much of the US are well above this temperature. These are the very conditions that promote mold growth. How ironic it would be that the very system tasked to provide good indoor air quality by means of ventilation can be the very means of promoting mold growth, by lack of attention to basic psychrometric principles."

This situation is also one of the problem findings in "Use and Misuse of Energy Recovery Ventilators (ERVs) in Hot, Humid Climates" by Bruce Davis which I cited earlier.

Martin,

In the report, "Residential Dehumidification Systems Research for Hot-Humid Climates", Feb-2005 by Armin Rudd, Joseph Lstiburek, P.Eng., and Kohta Ueno, the conclusion was:

System 5: Energy Recovery Ventilator (ERV) System (Three Homes Tested)
This system will not dehumidify house air, but will lessen the need for dehumidification.

The ERV system did not show good humidity control performance. Its first cost was high, but operating cost was low. The lack of humidity control resulted because, while this system has the capability to lessen the latent load of ventilation air, it cannot dehumidify the conditioned space.

This can be thought of as dehumidification in “ventilation mode” as opposed to dehumidification in “recirculation mode.” For energy-efficient houses with controlled mechanical ventilation, the reduction of sensible heat gain and interior moisture generation were the dominant factors in increasing indoor relative humidity above 60%. The ERV systems were rated to reject about 60% of the latent load from ventilation air and would have shown more improvement in humidity control if ventilation air was a dominant factor.

In other words, while an ERV may be highly efficient at removing moisture from the incoming air stream, and certainly far surpasses an HRV or conventional exhaust/supply fan for that purpose, it can only dehumidify the incoming air and not the air within the building, which requires recirculation.

Another study, Assessing Six Residential Ventilation Techniques in Hot/Humid Climates, published by the Florida Solar Energy Center, and originally published in Proceedings of ACEEE 2004, states:

The best performing system, case 4 (10/20 cycle with dehumidifier), was able to maintain the humidity nearly constant for almost 80% of the test period. The next best performers were case 2 (spot ventilation) and case 6b (energy recovery ventilation).

And that study measured more total condensate from one of the tested ERVs than from either the the dehumidifier or the AC unit. In other words, an ERV is a very effective dehumidifier, perhaps more so than some AC units or stand-alone dehumidifiers.

But you can't dehumidify a house by only removing moisture from ventilation supply air - it requires moisture removal from ambient indoor air (recirculation).

Thank you guys for this very informative discussion. I work with developing ERV cores and have for many years, which means that ultimately, I move in labs and test facilities rather than real houses and buildings.

When I state an ERV is a passive unit, I am referring to membrane type ERVs, which are passive devices, except that they use fans and sometimes some controlling equipment, especially in the sense that they will not actively control the humidity. Or, as Robert stated:

"In other words, while an ERV may be highly efficient at removing moisture from the incoming air stream, and certainly far surpasses an HRV or conventional exhaust/supply fan for that purpose, it can only dehumidify the incoming air and not the air within the building, which requires recirculation."

In European houses, the house envelope is sealed. With mechanical ventilation, we mostly use HRVs, either metal or plastic, here to recycle energy. The only sensible input we have, is waste heat from electrical applicances, sun inlet and humans. The only moisture source inside the house are humans (100ml/hr). If the house is built properly (according to our standards) the air exchange is high, approx 0.35 litres/s and m2 floor area. This means that if the air conditioner has a correct setting, and the ERV is properly installed, the incoming air should always be a little less humid to compensate for the added humidity indoors, exactly like a chiller is set a little below room temperature to compensate for heat leaking in. As I have mentioned earlier though, the trick is to install a by pass. This by pass is there to make sure that when humidity inside is too high (above 70-75%) the air is vented out without recycling in the ERV core. This efficiently lowers the humidity inside since the incoming air is drier. This of course, only works in buildings with a SEALED ENVELOPE. I know this isn't very common in Australia, and perhaps the kind of houses you've seen in the reports have been self draught houses (i don't know the term in US-english). If there is no sealed envelope, I wouldn't expect much result from an ERV..

I would very much like to read that article you mention. Is it available online somewhere?

Correction in the above: The other humidity sources are of course washing, kitchen and bathroom, but as Robert stated earlier, these should have their own outlets, separated from the ERV ducting.

I might also add that the bypass should quite often be applicable in night-time, when humidity and temperature levels may very well go below indoor temperatures, and thus the ERV would heat and humidify the incoming air instead of the other way around..

Martin,

I don't know what Johan had in mind, but all those moisture sources you've listed are anthropogenic - caused by human occupancy. The only moisture source, assuming no mechanical leak or wet crawl space, that is not anthropogenic is natural air leakage or moisture diffusion through the envelope - which should be insignificant in a well-built house.

In Houston, the outdoor air during the summer is generally much more humid than the indoor air. Ventilating with an ERV or an HRV during the summer in Houston raises the indoor relative humidity, it doesn't reduce it.

Depends on what the indoor RH is. Normal July outdoor RH for Houston is 90% at 7:30 AM, 58% at 1:30 PM and 66% at 7:30 PM. Depending on efficiency of indoor dehumidification, the outdoor RH may very well be either higher or lower than indoors at different times of day. During the hottest part of the day, outdoor RH may be lower than indoor RH.

And, whether an ERV would raise the RH depends on what it's being compared to. Compared to conventional ventilation (to meed ASHRAE 62.2) or natural leakage, it would reduce it.

We don't agree on this issue. You apparently continue to rely on a single source of theoretical data which is not supported by some of the field data and research of other experts. Consequently, you continue to make blanket statements about ERV's, when the experience with them is quite mixed and mostly dependent upon installation variables.

I am interested in hearing more about the original poster's question... Does an ERV make sense in very cold climates such as northern Michigan or here in the Canadian prairies where it is very difficult to maintain comfortable humidity levels even in very well sealed homes? The difference between the absolute humidity levels of outside and inside air can be significant in these climates during the dead of winter, so it seems that an ERV might make sense here. I have read elsewh ere that there can be problems with the rather fragile cores if the unit does not have an effective defrost cycle.

Garth,

If your house is tight enough to require mechanical ventilation, then excess indoor humidity and not dryness should be the problem in winter. If a house is properly sized for its occupants, and very air-tight, then the anthropogenic humidity (4-5 gallons per day for a family of four) should be more than enough to keep the house at the 30%-40% RH range.

Even cold climates typically have higher outdoor RH in winter than in summer, though that equates to lower winter absolute humidity.

Of course, if the house is too big for the number of occupants, or if the occupants never cook food or do laundry at home, then the house will be dry. Lifestyle and appropriate size matters, in many ways.

HRVs are recommended for norther climates and ERVs for southern climes.

http://www.houseneeds.com/shop/images/fantech_selectsizemapmed.gif

Robert
My neighbor recently built a new modestly sized home to R2000 standards and currently operates an HRV to provide fresh air. His house is very dry in winter...20 to 25% RH indoors. His hardwood floors develop gaps and he complains of dryness to the point that he operates a humidifier for several months. I know this is hard to believe but I have heard of many others who experience the same problem. Perhaps he operates his HRV to much for the size of house he has...

Garth,

A "modest sized house" has become decidedly immodest. In the 50's and 60's, a 1200 SF 3-bedroom house was the middle class dream. I've designed a 900 SF 3-bedroom house for a Farmer's Home self-help building program and I've built a 768 SF (exterior dimensions, 8" double-wall) 3-bedroom home for a community land trust in rural Tennessee. Those are modest homes.

Most new homes are excessive in size, cost, material consumption and energy consumption. And one of the prices paid for such excess is winter dryness if the old ASHRAE 0.35 ACH standard is followed. Partly for that reason, the 62.2 standard has been modified to a per bedroom and per square foot standard, which typically reduces the ACH.

I build healthy houses and never plan more than 0.25 ACH - less than that is often sufficient. One option is to control whole-house ventilation with a dehumidistat instead of a timer.

Martin
I agree with your two possibilities by I also believe that there may be a third. I am assuming that your two possibilities have been ruled out.
3. It is extremely difficult to maintain a comfortable indoor RH when outside air has an absolute RH of close to zero for much of the winter.

Garth,

I think you meant to say "absolute humidity" rather than "absolute RH".

But no where on earth is absolute humidity "close to zero". In the most extreme desert environments, RH may drop to as low as10%, but at 120°F, that's more absolute humidity than in a house at 70° and 40% RH.

Calgary, Alberta, which is considered a semi-arid continental climate, has an average Janurary temperature of 16°F and an average winter RH of 55% (summer is 45%).

A 2,000 SF house, with 0.25 ACH ventilation, would lose about 4 gallons of water per day in a Calgary January. The typical family puts 3-5 gallons of water per day into the indoor environment, so that should allow the house to maintain a comfortable RH above 30%.

Increase that house to 3,000 SF at the same ventilation rate, and the house will lose 6 gallons per day and normal activity may not be able to replenish that volume. Just one more reason to build houses sized appropriately to the occupancy load.

With the risk of repeating the previous post, but in other words, I believe the point Garth is trying to make is that in winter, outside absolute humidity can easily be as low as 2 grams per kg air and less (-10C and 100% RH is equivalent to AbsH of 1.6 grams/kg air) which means that if you take that air and heat it, the absolute humidity will be the same but the relative humidity will drop drastically when heated to 20C. For example: if outdoor temp is -5C and 85%RH, and that air is heated to 20C, the resulting indoor RH is only 15%. One has to keep in mind though, that as stated a few times above, there are several anthropogenic sources of moisture indoors that vary greatly over the hours of the day that may affect indoor air quality negatively. The problems arise if the ERV is "too efficient", or rather, that the ventilation rate is so low that excess moisture (indoor RH of 70% and above) is not vented but returned with ingoing air from the ERV to add to the indoor air. This is a typical situation where a by-pass should kick in.

Ultimately though, the key is (as Robert has stated a few times above) to balance air flow, regeneration and HVAC equipment to optimally suit not only your house, but your way of living. An ERV may increase comfort in a cold climate in winter, but it may also provide too high RH indoors which will promote fungal, bacterial and mite growth (above 70-75% RH). Too low RH though (typically less than 10-15%), will result in material issues (eg wood cracks) and health problems such as decreased resistivity to airborne pathogens, dried out humus membranes in throat and nose, itchy skin, irritated eyes and tiredness etc..

Ultimately, I guess my own best recommendation would be to get a couple of loggers for temp and RH (eg Hobo) and do some monitoring of the actual indoor conditions over time. If coupled with weather data, I'm sure you can deduce what's best for you and your house. In the long run, knowing the house will be your best bet to increase comfort and lower energy bills..

I would also recommend you to read all the posts above since they contain a lot of very useful information and a few good references..

Johan
Thank you for restating my question in a much more understandable way. I do agree that monitoring of indoor RH would be not only prudent, but necessary. My only remaining question regards the durability of an ERV core when outside temps reach down to -10C and stay there for several weeks at a time...I am assuming that a reliable defrost mechanism must be built in.

I believe that varies greatly from brand to brand. Something to have in mind though, is to connect an ERV in series with an HRV. The HRV is put on the "cold side" (outdoor) and takes care of pre-heating incoming air and the ERV sits on the "hot side" (indoor) and takes care of pre-drying outgoing air, which means there won't be so much condensation in the HRV. I haven't tried it myself, but I believe this may be a viable solution for cold climates. It would bring the best of both worlds, although it will probably be a little more expensive as well.. If Robert or Martin has heard anything or know of anyone who has used such a unit, I'd be very interested to hear about their experience.

Johan
Your idea of using an ERV and an HRV in series makes sense to me. Perhaps someday, some manufacturer will design a ventilator that would have both types of cores in series, in one unit....best of both worlds at minimal additional cost....

Yohan and Garth,

The problem with the ERV/HRV combination is that, in the winter moisture must be evacuated in order to keep indoor RH below the danger zone. So it is counterproductive to recycle indoor humidity with an ERV unless the house is too large for its occupancy or too leaky, both of which are design flaws that cannot be corrected by additional mechanical equipment.

Martin,

The fact that "every HRV manufacturer" recommends integrating bath and laundry exhaust into their centralized systems is no more guarantee of function than spray foam manufacturers recommending less than code R-values in order to keep installed cost down sufficiently to sell their product.

Again, if a primary purpose of a whole-house ventilation system is to remove excess humidity, then it's counterproductive to use an ERV to recycle this humidity from point sources back into the conditioned space and counterproductive to impose a high-moisture burden on an HRV which would have to run its energy-wasting defrost cycle to compensate for the excessive condensation.

Integrating a bathroom exhaust fan into an ERV is like running gutter downspouts into a foundation perimeter drain - it defeats the purpose of both systems.

Robert,
So I understand how an ERV venting bathrooms would recycle the humidity back into the house but would that also happen with an HRV? And, if separate bathroom and kitchen fans are needed to eliminate moisture, how does one balance the airflow in the HRV to account for the fans?

Donald,

An HRV does not recycle moisture, only heat or coolth (sensible energy), unless the exterior exhaust and intake ports are too close together (should be 10' apart) or there is internal bypass leakage.

Unless an HRV is a pressure-balancing unit, greater interior negative pressure from independent exhaust fans will increase the inflow through the HRV and decrease the outflow. However, in the heating season, interior negative pressure prevents the exfiltration through the thermal envelope which results in condensation and moisture damage.

In a cooling climate or season, it's important for the same reason to maintain a slight positive interior pressure, so some method of increasing supply air when exhaust fans are running is necessary, or an HRV can be used to exhaust moisture point sources and an ERV used for whole-house ventilation.

Robert,
In the cooling season, since I'm building without air conditioning, I'll probably have the windows open so I am mostly worried about the heating season. If I am understanding your last post correctly, I should be able to use an HRV to exhaust moisture point sources and ventilate the whole house? Also, the intake and exhaust motors could be adjusted to create a slight negative pressure.

Donald,

Yes, many designers and builders rely on an HRV for whole-house ventilation, but it will never be as effective at eliminating source moisture (and other pollutants) from such areas as showers and laundry room as will dedicated exhaust fans (assuming they're used and controlled by a delayed-off timer so that they run for at least 10-15 minutes after the room is vacated.

An HRV can often run at a constant low speed with a boost speed controlled by a bathroom timer switch, but that simply increases the outflow from all exhaust ports and draws bathroom moisture throughout the house.

Those who, like myself, believe in the importance of eliminating pollution and moisture at the source, rather than relying on "dilution as the solution to pollution" (which is the function of a whole-house ventilator), use dedicated exhaust fans in shower rooms and kitchen range hood (and laundry room if chlorine bleach is commonly used).

And, since it's redundant to use three or four independently ducted exhaust fans in addition to a centrally-ducted whole house ventilator, I use high-quality bath fans as the exhaust-only whole house ventilator with strategically-located passive make-up air inlets (Aldes Airlet 100s), and both short-term timer switches in the bathrooms and parallel 24-hour programmable timers to control daily ventilation rates.

While this system does not recover the heat in exhaust air, it is less expensive to install and operate and maintain, and offers the advantage - in a cold climate - of providing negative pressure.

An HRV does not "draw bathroom moisture throughout the house".

It certainly can, depending on the proximity of other exhaust ports and their relative position to the supply ports. If the bathroom door is left open following a long shower, then competing negative pressure zones will draw bathroom moisture (likely 100% RH) in various air streams in various directions.

If you want to evacuate a humid bathroom, you don't turn on the bath fan and also the kitchen hood. You create negative pressure only in the bathroom. A central HRV system creates multiple negative pressure zones (around each exhaust grill).

If there is any air leakage around a bathroom window or the tub trap hole in the subfloor, then air will flow out of the bathroom towards all exhaust ports in proportion to the relative negative pressures. Even just with the bathroom door open, thermosiphon and forced convention currents will be created with cooler dry air moving along the floor and warmer moist air moving along the ceiling (dry air is more dense than moist air at the same temperature and a lot more dense when it's cooler).

In regards to the article about ERV's Getting a Yellow Flag, by Max Sherman, and Martin's post:
"Max Sherman is pointing out that some ERVs allow formaldehyde to be transferred from the exhaust air stream to the incoming fresh air stream."
I'd like to note that Max Sherman does not have test data regarding ERV's and their transfer of formaldehyde. He states that ERV's "can" transfer formaldehyde, but this is speculation at best, based on chemical simularities between water vapor and formaldehyde (i suspect... as he makes clear reference to that). The article is so cleverly written... i am amazed at it as i read it again... It is very misleading in my opinion. There is no testing of ERV's and formaldehyde transfer. There isn't even a written standardized protocal for such testing. I traded emails with Max on this topic to confirm. The fact is -there has been no testing regarding ERV's and Formaldehyde transfer. I do not know the purpose of this article or any scientific backing for it.
In light of that... Max (and LBNL) will be conducting some testing on this topic. They are (and myself included) discussing the protocal for such a test right now. I hope to report on this testing at a later date.... and just for record - i do think there will be some formaldehyde transfer. But nothing to worry about. Like breathing the windex you just used to clean a window.
Now, just as a note on this topic. Formaldehyde is what i would call a decaying chemical. Concentrations will be highest when the formaldehyde off gassing item is recently made (carpet?).. and will continually off gas less amounts until it is gone... (negligible). Ventilation (in general) will dilute the formaldehyde concentrations in a given volume (house). There are many factors which influence this concentration in any given house to begin with, such as - how tight the house is, how much formaldehyde is being off gassed into it, how big is the house, does it have operable exhaust strategies (bath fans, kitchen fans,..?) how much are they used... and so on. Then we can discuss - does the house have a mechanical ventilator... is it an hrv or an erv? If its an ERV, how much moisture does it recover (which is different summer and winter condition...). So here I lay my opinion out for scrutiny: if a protocal can be written, and all the priror assumptions standardized (or every explicit case tested)... i do not believe that a diluting ERV will be much more "hazardous" to ones health with regard to the formaldehyde concentration in that exact case study... than if just a hrv were used in the same exact installation. If it can be measured... i'm guessing on a scale of 1-10... 10 being very bad... that the ERV is a 2.26 and the HRV was a 2.24. I'm glad we spent all the time, effort, and cost to determine that... Lets do it again for the other 25 different house conditions that could be present.
Anyhow... please look me up in 4-6 months for this answer. I do hope to have it.... but no promises.

In a recent article in Home Energy Magazine, Rob Dumont describes the ventilating system in the "Factor 9" home built in Regina SK in 2007. The home is highly insulated, has an ACH50 of 1.2, and the local climate has 10500 HDD.
http://www.homeenergy.org/article_full.php?id=747
The HRV draws stale air from the bathrooms, laundry, and kitchen areas. He writes, "Two different heat-exchange cores are used, one with plastic plates and one with treated paper plates. The latter will allow moisture in the exhaust air to be recycled back into the home in the winter , when the indoor air tens to be too dry"
It would be interesting to hear more from Rob Dumont on this issue...

I, too, would be interested to hear from Dumont. Regina summertime humidity hovers around 50%, while the winter RH is 80%-90%. Is the air too dry or is he exchanging too much air volume or is the house too big for its occupancy load?

A tight house should not be dry in the winter. That was indicative of a leaky house with too much air exchange. In fact, a primary function of whole house ventilation in a cold climate is to rid the house of excess humidity (which is a precursor of mold, dust mites, and formaldehyde outgassing). This is why an ERV rarely makes sense in the north country.

I think that the reason for Dumont's approach lies in the answer to this question. What is the RH of the incoming fresh air (say @ 10 degrees F and 85% RH) when warmed to room temperature? If I knew how to read a psychometric chart, I would answer it myself. I am betting it is less than 10% RH...

Garth,

That outside air warmed to 68°F would result in less than 9% RH. But that's not the point.

A 2000 SF house with 8' ceilings and continuous ventilation at 0.25 ACH with baseline interior humidity at 40% would lose 3.84 gallons per day by air exchange. A typical family of four contributes 3-5 gallons per day to the interior environment, so this house should easily be able to maintain a healthy interior relative humidity.

If it's drying out, either there's too much air exchange, or the house is too large for its occupancy load, or the occupants eat only take-out food, shower at the health club and have a laundry service.

Robert
A detailed description of the Factor 9 home can be found here
http://www.cmhc-schl.gc.ca/en/inpr/bude/himu/inbu/upload/65664EnW.pdf
By my calculations, the ventilation rate is .145 ACH. The house is larger than 2000 SF(3239 SF including the basement area) but does house a family of four.

Garth,

Then that house, even on a 0°/90%RH winter day should be losing only 4 gallons per day through ventilation, and 0.3 gallons per day through diffusion (with a 1 perm wall), with a steady-state interior RH of 35%.

If the house is too dry, perhaps there is more stack-effect leakage than they imagine, or perhaps their strategy of using two HRV cores was baseless.

It would be interesting to get some actual performance data.