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Trying to figure out which way my air filter is supposed to go- is airflow in the direction of the red arrow or the blue? Thank you!!!!


3 Answers 3


It's the red arrow. The black arrow next to the filter size also shows this. Furnaces generally draw air in from the bottom.


The air flow will be rightward in this picture. Here are several ways to tell:

  1. The air filter is always the first thing the return air flows through when it enters the furnace. That way, the machinery doesn't get clogged with dust. Since the furnace is on the right and a duct is on the left, the air must be flowing left to right (red arrow) in this picture.

  2. If you remove the filter, or the access panel on the furnace, and start the furnace, you'll be able to tell that the air is moving rightward (in addition to being sucked in through the opening) and then blown upward by the blower. Of course, you should not leave it running for very long like this, since it is then taking in unfiltered air.

  3. The air will flow through the blower (fan) before it flows through the heat exchanger (tubes that get hot), so that the blower motor is not exposed to unnecessary heat. So, if you look inside the furnace and see a duct connected to the blower, that will be a duct where air is entering the furnace.

  4. Since hot air prefers to rise, the cold return air is often, but not always, supplied at the bottom of the furnace, so a duct connection at the bottom will be an opening where air flows into the furnace, which is rightward in this picture. (But some furnaces don't follow this principle — the blower is much more powerful convection — so this doesn't necessarily tell you anything with certainty.)

  • Buoyancy ("hot air prefers to rise") isn't really applicable - counter-flow furnaces have the burner at the bottom, blower at the top, and blow air downward. They work just fine. The real clue is the positioning of the burner and blower. Blower always pushes air across the burner/heat exchanger. In this photo the louvers in the front cover indicate burner is on top so we know air flow is upward.
    – Greg Hill
    Commented Aug 8, 2023 at 17:53
  • @GregHill Fair enough. I thought about mentioning blower positioning but didn't, and I wasn't aware that counter-flow furnaces specifically existed. Edited.
    – Kevin Reid
    Commented Aug 8, 2023 at 19:13

Others have noted the red arrow is correct, but it's worth noting how to tell what's what in your picture.

There's a fairly large insulated flex duct on the left side of your picture. If you follow it, it will run to wherever your intake filter is from inside your house. I know this is an intake because

  1. It's flexible (lower pressure)
  2. It's enormous. Probably 16".

A line running to a room off the exhaust side will likely not exceed 8" (the vast majority I've seen are 6"). And you wouldn't typically connect a flex line directly to the exhaust side of a blower that big. Normally you hook it to something more rigid, like a fiberglass manifold. That main trunk will then reduce over distance, and your individual rooms will get their own flex line from that trunk. Throwing your discharged HVAC air into a trunk would be a rather odd decision, since you would lose a lot of pressure by doing that, and air movement is key in HVAC efficiency.

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