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Which way does air typically flow in a furnace? Put another way, does the filter usually sit before the incoming cool air reaches the burners or after?

I'm trying to figure out which way the air flow arrow should point on my air filters. There are a lot of web pages that almost answer this question but not quite.

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Here's a good cut-away diagram of a typical gas furnace that shows the airflow (source) –  gregmac Jan 4 '13 at 1:19
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3 Answers 3

up vote 7 down vote accepted

The filter sits between the air return duct and the heat chamber, BEFORE the cool air enters the furnace.

The air flow arrow almost certainly will point at the furnace, not away from it.

If the filter sat after the furnace, there would be potential hazards associated with it, like possibility of fire (hot air hitting a cardboard frame) and release of nasty substances due to overheating.

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Simple question, simple answer. Many thanks. –  duozmo Jan 4 '13 at 1:21
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Typically your air filters are at the inlet of your furnace. The air should be moving into the furnace through your filters.

Point the arrow into the wall/floor.

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I needed to determine the AIR FLOW DIRECTION of the heat system so I could install the air filter with its arrow pointing in the right direction. I asked but never got the same answer all the times. So I reasoned this way: the air enters cold and goes out hot, then I touched (very carefully!! to not get burnt) the pipes and could learn which was the inlet pipe(the cold one) and the outlet(or hot) one. The best way to ensure that you don't get burnt is to make the touching a few minutes after the furnace stops functioning; that way the hot pipe is not very hot.

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