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.


4 Answers 4


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.


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.


Put your hand on the duct work to see what ducting is worm and what is cold when the furnace is running. That will tell you which way the air is flowing. Cold comes in and the hot goes out.


Simple check. Have a light weight small piece of paper close to the duct. If the paper is pulling in, then it is inlet. If the paper is pushed away from the duct then outlet.

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