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The air inlet for my electric forced air heating is a small vent on the duct in the crawl space.

Although the crawl space is sealed with a concrete floor and heated (ie insulation is around the outside) it seems wrong to be taking the 'fresh' air from a dusty unused crawl space and blowing that around the house.

Does this make any sense? Should I block this vent, if so do I need to fit another and where?

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You might want to consider that this return is drawing moist air out of the crawl space. It could get pretty funky if air isn't exchanged at all. – pboin Apr 22 '11 at 9:43
up vote 1 down vote accepted

For heating I like to have an intake at the lowest part of the house to pull out the cold air. This may very well be in or near that crawlspace. If it has a filter, then I wouldn't worry about dust. Just be sure you're replacing the filter regularly. However, I would want to know how fresh air gets into that crawlspace. If it's sealed off, then you are hurting the performance of your heating system. Otherwise, your intake is effectively where ever the opening to the crawlspace is.

When possible, it's nice to have a short path between the return vent and the blower unit. In addition to less cost of materials, you have less distance you are forcing the blower to suck the air through, and therefore more efficiency.

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The inlet vent is on the duct just before the filter, next to the heater+fan so that makes sense – mgb Apr 24 '11 at 16:18

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