Had an inspection yesterday (brand new townhouse) and the place had a Heat Recovery and Ventilation (HRV) unit. It’s basically a big box with dampers that change position depending on season, humidity and other factors, to either exhaust warm, stale air from the house or to bring in outside cooler air. I don’t believe it has any moving parts, except for the dampers. I’m not at all familiar with these units and I was unable to find any vents (return or supply). Is it possible that this unit was taking exhaust from the bathrooms and/or hooked up to individual room hydronic space heaters (that can also be used to cool rooms in the summer with cold water or possibly outside air from this unit). Anyone know much about these units?

James, HRV’s and their cousin ERV’s are great little devices. There is alot of information on them that a simple google search will provide. In a nut shell their primary purpose is to replace interior stale air with exterior fresh air and heat (or cool depending on the season) the incoming air stream as much as possible by passing the two air streams through either a cross-flow or counter-flow heat exchanger.

Humidity control is an additional side benefit.

The air flow of the incoming and out going air streams should be balanced so as not to create pressure differential from the outside.

The units can be hooked up to run independently usually to remove moisture from high moisture generating areas such as bath rooms and kitchens or they can be hooked up to use the central duct system. Units are often installed to activated using a humidistat.

Units should also have a filter installed on the incoming air stream. Some models even use a hepa filter

basic 3D image of an HRV.jpg

HRV instalation.jpg

exchanger.pdf (51.9 KB)

search2.pdf (93.3 KB)

HRV instalation.jpg

Thanks again, Paul. I had found a few things through a search, but I still don’t get how the thing works without supply vents. The place has no central heating or cooling, just individual room hydronic units. Could there be ducts hooked up to them? I did take the cover off of one but wasn’t looking for a duct. I can understand the exhaust units (bathroom, kitchen) being hooked up to it, but where is the supply to the townhouse from the unit?

There should be four (4) ducts coming from the unit.

Two (2) of them will terminate somewhere inside the house. One supply and one exhaust. The vents in the bathroom and kitchen would be the exhaust pulling the moist air from those areas. The supply side is typically located centrally. Often (and I don’t personally like this) there is no supply duct leading from the HRV unit. So the the supply is located in the room where the HRV is located. If you had one of the “ports” that didn’t lead anywhere then that is likely the case. If you had two of the “ports” that didn’t lead anywhere they might as well turn the unit off as the unit’s air stream is ‘short circuiting’ itself and not doing any good.

The other two (2) will terminate out side. One supply and one return. Having said that it is also possible to than these one wye into a special connection called a concentric or co-linear vent. It is basically one pipe inside of another which allows for only hole in the wall. It may look like only one pipe to the outside but its really two. Typically this ‘wye’ will occur close to the exterior wall just before going out.

Does that help?

Yes it does. Thank you