Wednesday, June 4, 2014


The plumbers have installed the heat recovery ventilator (HRV) and it's associated ductwork. This mechanical air exchange system works together with the exhaust fans (located in each bathroom) to maintain the indoor air quality.

HRV vents out of the attic wall in the back of the house

 Typical residential construction didn't used to have systems like this. It used to be, the building envelope (dividing line between inside, heated space and outside) was leaky enough to provide plenty of fresh air. Todays tighter building envelopes (look at previous posts talking about CI - continuous insulation- for more info) don't allow for a free air exchange (which means we use less fuel to heat and cool our homes- yay!) so we need to provide a mechanical exchange.

This is done quite simply here at the New House Next Door. The two bathroom fans exhaust warm, moist air out. On it's way out, it passes through the HRV, where the heat from that air is removed before the air is expelled to the outdoors. The HRV pulls fresh outside air in, transfers that recovered heat to warm the outside air, and then deposits the fresh air into the house.

The other part of this system that is important to mention is that the attic, where the ductwork and the HRV are located, is conditioned space. Ductwork should always be located within the building envelope for the best efficiency. (see the spray foam insulation applied directly to the underside of the roof sheathing in the picture below)

Ductwork in a conditioned attic - as it should be

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