HVAC Air Filters Type and Locations | Forced Air Systems


We notoriously forget to replace air filters in our forced air heating / air conditioning systems… Air flow starts deteriorating through the house, system seams to be operating, but it takes significantly longer to achieve desired temperature …

The solution might be simple – clogged air filter, or missing air filter, and in that second case, you might have a contaminated evaporator coil (part of air conditioning system). Air filters, usually come in two groups: electronic/electric and passive (reusable, electrostatic or disposable).

Electronic/electric – reusable filters, click on link for detailed information and maintenance instructions (this one is for American Standard Inc. brand, but principal is the same for most of them) – use caution when performing all maintenance tasks, parts of those filters require extremely delicate handling.

Operation of some units might be noisy and resembles sounds from mosquito / bug zapper (device with fluorescent, blue light that you hang on your back-yard)… which some people don’t like and replace electronic cells with regular, disposable filters.

Passive (reusable, electrostatic or disposable) – depending on type, should be cleaned or replaced anywhere between 1-4 months Location of filter varies significantly, but common areas to look for one are:

  • furnace blower compartment (depending on setup, it might be top, bottom or any side section of the furnace) – access requires compartment service cover to be removed – be very careful! – by removing blower compartment cover, safety switch located underneath should be activated and turn off the furnace / disable blower.

However, some older furnaces have no such device installed, or it could be broken / removed, compromised by taping it in constantly closed position, bypassing, relocating from designated position – in such case furnace / blower will not stop when you remove the cover, or might start turning during filter replacement.

Therefore, for your safety, before changing filter (s), turn the furnace off by switching thermostat from “on” to “off” position, and / or by turning off service switch, usually located in close proximity to the furnace (if you don’t feel safe, you can also disconnect power to the furnace by turning off the breaker or unscrewing fuse in the main electrical panel – assuming it’s been properly marked).

Large air filter installed in compartment located next to the furnaceAir filter located behind the main air return port grill compartment located next to the furnace enclosure

narrow slot installed next to blower compartment, sometimes with a cover

  • house air return grill – usually installed on one of the walls or ceiling, might contain filter behind its door (usually secured with screws or small latches)
  • in some of the attic installed heating / AC systems you might actually have to claim up to the attic in order to replace air filter – be careful!

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