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Follow-up: Properly sizing mechanical systems

November 22nd, 2006

Follow-up to: Book: The Natural House

Alex was just showing me an article called Smart and Cool ($) that appeared in a 2005 issue of Home Energy magazine.

In summary: Over-sized air conditioners aren’t just inefficient, they can make your house uncomfortable to live in. As I hypothesized about furnaces in the previous post: AC units cool and dehumidify more efficiently, the longer they run.

A heavy-duty AC unit will run in short bursts; not long enough to dehumidify at all because the water won’t have time to run off the cooling unit in the ventilation system. When the AC turns off, but the blower keeps going, the air picks up the small amount of water that had started to collect, and sends it back into the house making it uncomfortably humid.

In contrast, an AC unit that runs all the time eventually adds so much condensation to the cooling coils, that the water drips off and is drained away out of the ventilation system and therefore, out of the air.

When planning your mechanical systems it seems to me that it is actually worse to wind up with a heating or cooling system that is too strong than one that isn’t strong enough.

We aren’t planning to install AC at this point. We think we can keep the house cool by opening windows in the evening to collect cool air, and relying on good insulation and air circulation to keep the house cool through the day. In Toronto’s climate, we find there is only about 1 week every year where our current home (even as badly insulated as it is) is uncomfortably warm. In our new house, we’re hoping we find that to never be the case.

I think people would be amazed at how comfortable they could make their homes if they stopped relying on AC and simply opened their windows in the cool evenings, and sealed their house during the day. In our climate, residential AC really seems like an enormous waste for its limited benefit.

Posted by Colin