Furnace air filters - bad idea?

May 19, 2023 8:48 AM

My spouse and I are having a discussion about my putting a filter on the air intake floor vent for our furnace/AC. It looks a bit like this.

I noticed that a lot of debris was falling into the vent as it is in a high traffic area, so I put a filter underneath it. Like this.

I did not put filters on the vents that push out the air.

Her concern is that a filter on the cool air intake will affect the performance of the furnace/AC. My thoughts are that filters on the air outputs would do that – but a filter on the intake is fine.
Am I wrong? – I accept I may be and if so, are we better off with a mesh debris catcher? Or is the solution doing more regular duct cleanings?

posted by Lescha to Home & Garden (6 answers total)

Best answer: It is important to understand that that is not the air intake, but the return air duct. Air blows out of the other vents, and a return air duct is necessary to balance out the system. Possibly there is also an "air intake" that mixes some outside air into the system, and that definitely should have a filter, to keep out bugs, pollen, etc.

For the return duct, you do not need a high-performance filter, and the one you linked to probably does reduce the air flow enough to affect the system's performance. They will do that whether you put them on the intake or the output ducts. If the main goal is to reduce "debris" (ie., larger, visible stuff up to and including small toys) from getting in there, the debris catcher is the right solution. You could also just go to the hardware store, buy some cheesecloth, cut a piece to size and tape it to the back of the return register. You should not need duct cleaning except once in a blue moon.
posted by beagle at 9:00 AM on May 19, 2023 [5 favorites]

It's quite likely that your furnace already has a filter on the intake air side (the blue things with the metal screen with a bunch of circular holes that you see in hardware stores) that should be switched out every once in a while. Those are mostly to deal with particulate matter (dust and airborne stuff) but would prevent any debris from entering the furnace. If you're worried about collecting a bunch of junk in the duct leading to the furnace, that's a different thing.

Whether it has any effect kind of depends - if you have other return air grilles in the house, I'd think that if you restrict airflow on this one with a filter those other grilles will just suck a bit harder. If this is the only return air grille, then you'll be restricting air flow to some degree into the system, or pulling more outside (unconditioned/unheated) air if there's an exterior intake.
posted by LionIndex at 9:18 AM on May 19, 2023

I think there’s a downside to this plan because your furnace could be reliant on the flow of cooler air to keep the hot parts from getting too hot and potentially damaging themselves.

My ancient furnace, which I haven't turned on in years, has a heating element that consists of a coil of thick nichrome wire on a porcelain armature which is sitting right in front of a giant squirrel cage fan. If those wires don’t have airflow, they will get much hotter, draw more current, and could burn themselves out or cause other problems such as a burned dust smell.
posted by jamjam at 9:37 AM on May 19, 2023

Is that the only return air duct in your whole place? Putting closed-cell foam that dense on one vent will just block it, and more air will be drawn into the other ducts. And like beagle said, you're going to make the system work harder to keep air moving.
posted by JoeZydeco at 9:50 AM on May 19, 2023

Could you make something out of window screening that would protect it from debris, but allow more air flow?
posted by amtho at 1:42 PM on May 19, 2023

You almost certainly have a filter in the system somewhere - in our case it’s in the basement right before the furnace proper. You shouldn’t need double. That one should be changed about every 3 months though most people do closer to 6.
posted by amaire at 9:53 AM on May 21, 2023