Mill Creek NetZero Home Heat Recovery Ventilator
I heard a story once about a man who built a house using insulated concrete forms (ICFs). While I don't advocate their use in general, ICFs have some distinct advantages (certainly over conventional construction). The primary of these advantages is supreme air tightness. ICF homes (those that are built with ICFs from top to bottom) can achieve hourly air change rates of 0.2. In contrast, the Mill Creek NetZero Home has an airtightness measurement of 0.36 ACH, which is really amazing for a wood-framed house.
So back to this guy who built the ICF house. He apparently wasn't aware of how air tight his house was, or at least the consequence of that air tightness. Six months after he moved in, he had to rip all of the drywall out because it had rotted. He had built a house as tight as a plastic bag, and forgotten a critical aspect: ventilation.
Since we don't have any furnace ducting (no furnace means no hot air to move around), we installed 6" piping throughout our house to bring fresh air in and out of the living spaces. The ventilation outlets and inlets have diffusers on them that can be adjusted to balance the air flow.
fresh air is delivered to the kitchen of the Mill Creek NetZero Home via a diffuser (the white circle in the upper right corner)
The outbound and inbound pipes meet in basement, where they enter (and exit) the Heat Recovery Ventilator (HRV).
The system exhausts stale air from the high moisture producing areas of the house: the kitchen bathrooms and laundry room. In the HRV, this stale air gives up most of its heat to the incoming fresh air which is delivered to the other rooms in the house. The ductwork is carefully size so the the air can be picked up and delivered in equal amounts and with the least possible fan power.
Heat Recovery Ventilator (HRV)
Lifebreath HRV, MCNZH
An HRV is a device that draws fresh, cold air from the outside while at the same time heating it up with warm, stale air from the inside. It does so by having the two streams of air pass by each other separated by a thin membrane (Lifebreath HRV's double core is made of conductive aluminium).
We chose the the Lifebreath ECM 195 (with an ECM motor) because it gave us the best combination of low electrical usage and high efficiency heat recovery. A fan powered by an ECM motor maintains its efficiency over a wide range of speeds, whereas that same fan with an AC motor draws the same amount of power no matter what speed it is at. VanEE and Venmar make some HRVs with even higher heat recovery efficiency than the Lifebreath, but these models are not available with ECM motors. The models they do make with ECM motors are less efficient at recovering heat from the outgoing stale so more reheating is needed.
As long as the ventilation system is keeping the house's humidity under about 40%, we are comfortable with ventilating at a rate lower than the recommended 1/3 Air Changes per Hour (ACH), thereby saving significantly on electrical energy for the fan. Our confidence comes from the fact that the house was built with materials that don't offgas (or barely do), making the air healthy by default. Note also that here in Edmonton, Alberta our air is very dry compared to other parts of Canada, so we need to exchange less air to keep our humidity levels down. Today I lowered the ventilation rate to its lowest setting, the 1st out of 5 speeds, and set it to run only 20 minutes out of every hour. The dry winter air should will keep the house humidity level under 40%.
At zero degrees Centigrade, the the Lifebreath runs at about 81% efficiency (recovers 81% of the heat that is in the indoor air). At minus 25 degrees, its efficiency is only 77%, because it needs to run in reverse every so often to rid itself of ice that occurs when the indoor air encounters the much colder outdoor air (numbers from page 3 of the manual). The low speed that we're running it at should increase the efficiency, because the outdoor air is in contact with the indoor air for longer since it is moving more slowly.
Ventilation is essential to an airtight home. If we didn't recover the heat from the outgoing air, we would be negating much of the benefit gained by building such an airtight house. We are hoping to push the envelope by finding a ventilation rate that is healthy, but that requires the least energy possible to achieve its benefits.