Have you ever lived in or visited a home and noticed curtains moving on a windy day? Perhaps you have unplugged something from a receptacle on an outside wall and found the plug to be cold. These are signs of a leaky house. In older (and some newer) homes, this is how fresh air is introduced to the home. So what’s the problem with a bit of fresh air?! We need to think about this from two distinct but related angles: Indoor Air Quality (IAQ) and Energy Efficiency.
We spend quite a bit of time in our homes – particularly during the heating season. As occupancy increases, more oxygen is removed, more carbon-dioxide is produced, and exposure to potentially toxic air is at yearly highs because we don’t have windows open. Our cabinets, furniture, flooring and many other household goods, are constantly off-gassing; further degrading the quality of the air we breathe. You get the picture… not pretty. In a leaky home, fresh air is introduced around doors, windows, lights, receptacles, rim-joists and many other avenues. Bathroom fans and clothes dryers remove moisture-laden air from our homes, creating a negative-pressure which also draws fresh air in. Air is opportunistic that way and from a health perspective, that’s a good thing. Even on a still day, the stack-effect will help keep air moving, but it’s not consistent and unless you have expensive monitoring equipment, you have no way of knowing what your IAQ is.
Fresh air is either forced in (wind) or drawn in and although good for our health, it presents a significant problem from an energy efficiency perspective. That fresh air is either displacing or replacing conditioned air; that is, air you have already paid to heat or cool. What we need is a means of regulating/controlling fresh air while capturing the energy from outgoing stale air. Heat-Recovery-Ventilators (HRVs) serve that purpose and have been around for many years. Most HRVs are installed so they draw air from smelly/humid locations (kitchen, bath, laundry) and deposit the fresh air into the cold air return plenum on the furnace. In a Passive House we don’t have a furnace or an air handler (typically) so the fresh air is delivered to the bedrooms, family room, office etc. The HRV in a Passive House also runs 24 hours a day, not just when you hit the button before your shower. This is an important distinction: Consistent, balanced introduction of fresh air is the secret to a much healthier home.
The HRV runs ’round the clock, it harvests heat energy from the out-going stale air, and the air quality is great; so all is good, yes? Not all HRV’s are created equal. A typical builder grade HRV is (at best) 50% efficient; only half of the energy in that stale air is recovered – the other half is lost. If our goal is energy-efficiency, this is unacceptable. Typical North-American made HRVs have a cross-flow heat exchanger, and a small one at that. The fresh and stale air-streams pass through channels that are perpendicular to each other, so there just isn’t enough surface area to recover significant amounts of heat:
Cross-Flow Heat Exchanger in a basic HRV.
Increasing the heat exchanger size will help a bit, but to achieve > 80% efficiency requires a different approach: Passive House HRVs have a counter-flow heat exchanger and they’re large. The parallel air-streams and increased surface are make all the difference in the world. Throw in EC (Electronically Commutated) blower motors and they won’t cost a fortune in electricity to operate:
More Efficient Counter-Flow Heat Exchanger.
As I write this post, I am aware of only one Canadian manufacturer of HRVs that incorporates a sizable counter-flow heat exchanger. We were glad to find it, but it would be nice to have options, and going with a European model is not only costly, but not Canadian eh! I’ll spare you the details of modelling an HRV in the PHPP software. Just know that it entails gathering all the performance specs, and that’s not a simple task considering North-American testing methods (www.hvi.org) are different from those in Europe.
There is one more rather unique feature of many Passive Houses that dramatically increases the efficiency of an HRV. That’s all I’m saying for now, you’ll have to wait until my next post to find out what it is…