How Do I Determine My HVAC System’s Airflow Direction?


How can I tell which way the air flows in my furnace and duct work? My filter has an arrow, but which way does the air travel in the duct work? I can’t tell you how many times I’ve encountered a filter in backwards when servicing my customers equipment.

Because, it actually is difficult for the lay person to determine the correct direction of the air flow in a furnace or HVAC system. It’s a fact that even I, yes me, with over 40 years of experience, has had to stand back and do a little study before understanding a systems air flow direction.

But alas, and to your recue, there are some simple things to look for so that you can understand your HVAC system better and the air flow it produces. Specifically, which direction the air flows or even way the the arrow on your filter points.

Every furnace has an air inlet and an air outlet. The inlet is referred as the return air and the outlet is the supply air or conditioned air into the home. The direction of the air flow is from the return air to the supply side. Remove the doors of the furnace. The direction of the airflow is from the blower section to the burner or elements.

Conditioned air means filtered, heated, cooled, humidified or purified or what ever you want to pay for to make your home comfortable.

So logically speaking we want the filter to be in the return of the air handler (furnace) to keep all the components of the furnace clean and the air that the furnace delivers to your home. The image below shows an electric furnace with an AC coil. Not all installations will have this coil.

Notice just under the tip of the arrow that says “Filter Goes Here” there is about a 2″ wide strip of metal at the top of the furnace. That’s a removable door that the factory put there for a flimsy 1″ filter. Manufactures put that slot there because a lot of installers cut corner or purchasers want cheap installs. A good filter will be exterior to the furnace and be 2 inches thick or more.

The filter can on top of this furnace is a Honeywell and takes a 4″ filter. Don’t balk at the cost. They will last much longer and do a superior job compare to the even pleated 1″ filters. Check out how they look on Amazon here.

Typical Down Flow Installation In A Garage

If you are so inclined to remove the doors on your furnace then you can make this image a comparison to your particular installation. The blower has to blow into the electric strip heaters which are in the control section. So your air flow would be in that direction. Electric furnaces are multidirectional. They can be installed in an up, down, or horizontal position.

What’s The Airflow Direction On An Up Flow Gas Furnace?

Using the information from the content of this article, you should be able to tell which direction the air flows through this furnace, further enabling you to know which direction that silly arrow on your air filer is supposed to point.

Notice in the image below the Honeywell air filter. It’s to the left of the bottom of the furnace. If you recognize that the air flow is from the filter to the furnace then you’ve nailed it. The bottom compartment or door on the furnace is the blower or fan compartment. The air from the blower goes through the burners on the top of the furnace.

Airflow it up and you filter arrow would point to the right

So your filter arrow is going to point towards the furnace. Don’t worry too much if you have discovered that the filter has been in backwards. Just leave it until it’s ready to be changed. If you are not sure when a filter is ready to be changed go here and read all about knowing, before you throw your money away.

Horizontal Furnace Airflow Direction

It never hurts to learn more about these furnaces and how they are laid out. In the HVAC business we run into a lot of horizontal installations in attics and sometimes in crawlspaces. Either one is not an ideal place for ductwork or the equipment to heat and cool an home.

The extreme temperatures of an attic are a hard barrier to overcome when looking for good efficiency. But, nevertheless contractors and designers keep putting them in some of the most undesirable places. So with what you know now, look at the image below and determine what direction the air flows.

Airflow is left to right and your filter arrow points to the right

I’ll bet you got it right. Filter on the left of the furnace, blower in the middle and controls on the right. Air is flowing from the left to the right.

Airflow Direction For A Ceiling Or Wall Return Grille

The same rule applies for anything in the return side of the furnace. The airflow is from the return grill toward the furnace or inward of the return grill.

Up or inward for ceiling and wall return grills

It doesn’t help matters when the 3M is upside down and you’re on a ladder trying to decipher the dang thing. Just concentrate on the arrow and you will find success. Bottom line in this whole airflow direction discussion is that the airflow goes from the filter to the furnace.

By the way, if you want to improve your filter for these filter grills there is a great product out that will decrease the number of times you have to change your filter and help increase the air flow over using one of those 1″ pleated filters.

These filters are available on Amazon. They work pretty slick. The only modification you may have to make is cutting some of the insulation off inside the filter box just beyond the filet. Looking at the image of the filter you can see a 1″ lip around one side of the filter. This lip fits into the filter grill just like your old filter but the bulk of the filter fits up inside and you close the filter grill door and there you go.

The detail is not too good on the image above so you can get a better look at the filter by clicking the image and viewing it on Amazon’s website. I wish I had one in stock at the time of this writing or I would have included a really good image.

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Chad Peterson

Chad Peterson is near 40 year veteran of the HVAC industry. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

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