How do I find my furnace filter dimensions? - Around Town Plumbing and Heating

How do I find my furnace filter dimensions?

You had one chore to do: grab a new filter for the furnace. So you hop in the car, speed off to the home repair store and wander the aisles for forty minutes before realizing you’ll have to guess at the actual size filter needed. If your sense of proportions is better than mine, maybe you’ll get lucky and pick the right one! Another option would be to scoot back home and find the information you need.

Head right to the source. Most HVAC systems have the air filter housing as close to the blower unit as possible in the return air duct. Air filtration happens as the blower draws air through the filter and into the furnace for heating and circulation. That way as much as possible (dust, pet hair, microbes, pollen) is filtered out before the air enters your furnace. The grill or box cover over the opening that contains the air filter should be located on the wall nearest to wherever the furnace is mounted, on the opposite side of the wall as the furnace. If not there, the filter may be located inside, or on the air handler itself. You are looking for tall thin slot which hopefully has a filter in it.

Illustration of where your furnace filter goes

Finding Furnace Filter Size: Even Without A Filter

If there is a filter in the slot that looks to fit well, mark down the measurement listed in bold type along the leading edge of the filter. Your filter should fit the opening closely in all three dimensions (length, height and thickness) while still being able to slide it in and out. If you have to jam the thing in or rip it apart removing it, you probably should remeasure the size.

If there is no filter in the slot, it is a good thing you went to look at it! Running without one recirculates dirty air through your house and can mess up your blower among other things. Measure the slot. Shop for a filter marked with dimensions as close as possible to the dimensions of the slot. The filter you want will actually be a little smaller than the slot, about 1/4 inch in each dimension, but most furnace filters are marked with rounded off numbers (nominal measurement instead of actual measurement). In other words, if you are looking for a filter that should actually measure 15½” x 24¾” x ¾”, you would look for a filter marked 16″ x 25″ x 1″.

By the way, when measuring your furnace filter, height and width are interchangeable: 16″ x 25″ x 1″ is the same as 25″ x 16″ x 1″. Think of it this way, tipping the filter on its side doesn’t effect how well it works.

However, it is important to pay attention to which way the filter is facing. There is an arrow printed on filters that should point toward your furnace. The arrow indicates the intended direction of air flow through the filter. Nothing will explode if you do it wrong but your filter will work better and more efficiently if you install it facing the proper way.
Filters are easy to swap with a short trip to the hardware store. However, it is always advisable to have an HVAC expert from Around Town do routine inspection on your system to make sure it stays safe and in good repair. Schedule a visit today!

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