Build it tight AND ventilate right! With tighter building envelopes, getting ventilation right is critical to indoor air quality (IAQ) and occupant health.  In a recent editorial in Home Energy, Max Sherman, a senior scientist at Lawrence Berkeley National Lab, wrote about energy recovery ventilators (ERV):

"A heat recovery ventilator (HRV) recovers sensible heat by exchanging heat between incoming and outgoing airstreams. An energy recovery ventilator (ERV) is similar to an HRV but also allows moisture to be exchanged.  ERVs make a lot of sense both in hot-humid climates and in cold climates. In the former they reduce the dehumidification load by keeping excess moisture out; in the latter they reduce the humidification load by keeping moisture in. Nevertheless, there are reasons why one may wish to be cautious in using ERVs." 

Some contractors use ERVs in place of bathroom and kitchen fans that are meant to exhaust moisture, but most ERV's capture and return the moisture--and other unwanted vapors like formaldehyde--back to the living space. 

What's your experience with ERV's?


(credit: KyotoCooling)

Tags: ERV, IAQ, ventilation

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If ERVs capture and return the moisture to the building, why do they make a lot of sense in hot-humid climates?
Hugh, the ERVs keep moisture out while letting fresh air in. It's assumed that there is also active dehumidification in the conditoned space.

Hugh Stearns said:
If ERVs capture and return the moisture to the building, why do they make a lot of sense in hot-humid climates?
The statement "Some contractors use ERVs in place of bathroom and kitchen fans that are meant to exhaust moisture, but most ERV's capture and return the moisture--and other unwanted vapors like formaldehyde--back to the living space." Is what has me a bit confused. If an ERV is returning moisture and toxins, in what sense is it a ventilator.

I am also curious about the energy used and cost of these systems. At what point does it just make more sense to provide well controlled fresh air ports or build less air tight buildings? While these systems make sense in a diagram from a physical perspective, do they make sense at any cost, both in terms of ROI and energy consumption? And how do we create that equation?


Hugh Stearns said:
The statement "Some contractors use ERVs in place of bathroom and kitchen fans that are meant to exhaust moisture, but most ERV's capture and return the moisture--and other unwanted vapors like formaldehyde--back to the living space." Is what has me a bit confused. If an ERV is returning moisture and toxins, in what sense is it a ventilator.

I am also curious about the energy used and cost of these systems. At what point does it just make more sense to provide well controlled fresh air ports or build less air tight buildings? While these systems make sense in a diagram from a physical perspective, do they make sense at any cost, both in terms of ROI and energy consumption? And how do we create that equation?

There's the rub, or yellow flags, as it were. I gather it's only a ventilator in the sense that it brings in fresh air and exhausts conditioned air, but your other concerns about moisture and toxins are shared by Max Sherman, the author of the Home Energy editorial. I'll forward your questions about energy use and cost to our editor, who may be able to get more detail or answers from Max.
I would suggest that building tight, means 2 things: A) Energy Efficiency B) Indoor Air Quality. Ventilating Right also means the same two things.

Houses breathe. The question is how much and where? Does this house breathe like an insect, through many openings in the body? Many do! Does this house breathe like a mammal, through a controlled opening, with various filters? Some do? Which would you rather live in? Now imagine a picture of an insect with the Spiracles, Tracheae, and Tracheoles pointed out and outlined, along with the respiratory system of a mammal, with the nose, throat, lungs? Which would get the attention of the average family?

The above I should credit to my friend Ben Stallings. And as Sean is always pointing out -- doing the right thing is not always about ROI. Having a safer house is worth something.

If you wish to leave some building leakage, you should adhere to the old .35 level of safety. Many houses are built tighter than that over the past 10 years or so. Do we just ignore them?
I would consider a safer house an aspect of ROI. I think that it is incumbent upon us to be able to quantify the various components of ROI. In this case we are looking at at least three variables (up front cost, monthly cost, % removed toxins.) The second and third variables are the ones that I don't understand well. One of the goals of an ERV is to reduce the cost of conditioning outside air by preconditioning it. There is a certain amount of energy that goes into running the ERV. What is the net energy savings? In terms of health and probably cost of conditioning, what is the quality of air brought in by an ERV compared to the quality of air in building with a calculated amount of passive leakage? What happens if we provide strait fresh air supply at vents and dryers like we do with fireplaces? What if we provide that fresh air supply directly at the air handler? I suspect that these are not one size fits all answers but depend on other variables such as temperature differential and humidity.
How about ERV's in cold climates?

In the cold climate of coastal Maine (7500HDD) I specifically chose ERV against conventional widsom because I did'nt want to have to re-humidify indoor air dried out by using an HRV in cold/dry winter conditions (plus no complicated de-icing cycle or drain lines). Both systems draw just from 2 Baths and a kitchen and have dedicated ducting. One system is 6 yrs old and indoor winter humidity is stable about 35% and indoor air quality is very good!

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