Why is a well air sealed house with exhaust ventilation more energy efficient than a average house?
user-974877
| Posted in Energy Efficiency and Durability on
I believe it but its hard to explain. Are there studies and reports on the subject? I live in zone 4 NW California not real cold but lots of cool nights.
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Replies
Larry,
In most older homes, random air leaks account for a large percentage of the home's heat loss. Every house is different, of course, so it's impossible to generalize about this percentage. However, energy modeling programs exist which can do a fairly good job of estimating the percentage.
Blower-door tests can be used to measure the rate of air leakage in an existing home, and this data can be used to improve the accuracy of energy modeling results.
In most older homes, plugging air leaks saves a lot of energy. It's one of the most cost-effective measures available for lowering energy bills.
A very well sealed house with no ventilation system uses less energy than a very well sealed house with a ventilation system. However, such a house isn't healthy. We include ventilation systems to be sure that indoor air isn't stuffy, smelly, or too humid.
A very well sealed house with a mechanical ventilation system that ventilates at the rate specified by ASHRAE 62.2 will usually use less energy than an old, leaky house. Here's why: the rate of air exchange attributable to the ventilation equipment is less than the rate of air exchange through unsealed cracks in an old, leaky house.
So... fewer air changes means less lost energy, even taking the fan energy into account. Makes sense, as long as fan energy is minimal.
Unfortunately, I think we're still at a point where exhaust-only ventilation is used quite a bit, and it's a blunt instrument. The control is typically a timer. The owner may or may not actually use the fan. When in use, it draws air through leaky spots, which may or may not be in places where new air is needed (bedrooms, for example).
I see situations where there's a fan installed, and the timer is active, but the house doesn't need it... a blower door test says it's not tight enough to require mechanical ventilation. I see other situations where a HRV is installed, and running, and the occupants are sitting there in their mittens and hats with the cold air blowing through the house (which probably isn't tight enough to need mechanical venting).
We need much tighter houses and much better ventilation systems, with smart controls that sense humidity and CO2 and deliver air when/where it's needed... simple, packaged systems that builders can understansd and homeowners will use and appreciate. Hopefully we'll get there someday soon.