Heat Recovery Ventilation
Being a retired architect, I did not learn about heat recovery ventilators (HRV) until 5 years ago. The first installation I tried on a client’s house was three years ago. Planning to install more, I asked my former client about his experience with the unit. He told me it didn’t work very well, since in the winter it blew cold air into his bedroom. So, when I called the manufacturer’s tech assistance number yesterday, they asked about my experience with their product. I said fine, except for the problem with cold air. They replied that if the outside air is 0 degrees, one shouldn’t expect the incoming air to be more than 40-50 degrees, which would feel cold. However, they had a new engineering fix for this. They have a sensor to shut the unit off when outside temperatures fall below a certain point. I asked them why they didn’t just connect the outside intake to the furnace cold air return so the incoming air would be tempered. They replied, “You, could do that.”
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Replies
William,
You didn't post a question. However, if you want more information on the issues you raise, you might want to read this article: Ducting HRVs and ERVs.
William,
I, like you, am scratching my head wondering how to deal with the problem of ventilators dumping cold air into our home. A vendor and I were having this very conservation today as I prepare to make a ventilator choice for our small cabin in zone 6 North Central Minnesota at 46 degrees north latitude.
Essentially, my understanding is that the more efficient the ventilator is, the more heat from exhausted air will be exchanged with the incoming air. The hitch of course is that as ventilator efficiency increase so too does the likely cost. .
For example, my vendor's efficiency calculations below represent theoretical results from two popular ventilators. My layman's understanding is that efficiency decreases as temperatures drop.
If it is 0 F outside, 68 F, the delta is 68 F
•ERV at 60% recovery - the fresh air will be at 41 F
•ERV 95% recovery - the fresh air will be at 65 F
The latter is comfortably within a tolerable range. The former is not.
Ultimate Air is one of the most efficient ventilators I have found. But I am told even it would require a pre-heater.
For more on this quandary see:
https://www.greenbuildingadvisor.com/community/forum/mechanicals/22910/duct-heater-required-hrv-cold-climate
Randy,
Your examples show the advantage of choosing an HRV that is efficient. You're right.
That said, it usually isn't too hard to locate the fresh air grilles in places where the incoming air doesn't cause comfort complaints. If the house has a heating system, the air is soon warmed.
we sometimes install bedroom fresh air ducts in closets with the idea that the air will not be blowing on anyone and will be tempered as it enters and mixes with room air. It's obviously helpful if the door is a bypass or bifold and not particularly tight. We've had no complaints. In general, the ducts should be mounted high on the wall in a location where the colder air will not be felt.
If the supply ducts are metal (better conduction than pvc) , in conditioned space, and more than a few feet long, shouldn't the air be near ambient temperature by the time it exits the duct at such a low cfm?