I just brought a new house. In the new house, the furnace filter is 1 inches; whereas, my old house is 5 inches. I am just wondering should I change the 1" into 5" at my new home? All the research online said that 5" is better than 1". Is that true?

  • 4
    Is the furnace exactly the same? Furnaces are made to work with certain filters. Using the wrong filter will probably not work as well.
    – crip659
    Jul 18, 2023 at 15:05
  • 2
    A big furnace filter is semi-effective way to get cleaner indoor air. a dedicated air filter unit will work better, but takes up space and makes extra noise.
    – dandavis
    Jul 18, 2023 at 17:45
  • 2
    Whatever you do, don't stack filters. That creates too much resistance.
    – isherwood
    Jul 18, 2023 at 18:09

3 Answers 3


The bigger filter is better, but it would necessitate modifications... AND the furnace blower is designed to pull through a specific filter.

Just stick with the filter the system calls for.

  • 2
    Thicker filters typically have less restriction due to larger surface area.
    – KMJ
    Jul 18, 2023 at 15:35
  • Yes, should have stopped at filter.
    – STS1SS
    Jul 18, 2023 at 15:39
  • my 5" filter blocks a lot less flow than a ultra-fine 1" hepa filter.
    – dandavis
    Jul 18, 2023 at 17:46
  • 1
    The furnace manufacturer should have technical information indicating what amount of flow restriction/back pressure is allowable.
    – Armand
    Jul 19, 2023 at 4:05
  • 1
    @JimStewart yes indeed, if system restriction is particularly bad the motor can end up with insufficient cooling which causes long term problems. It has to be really bad though, like a filter unchanged for years, evaporator freezing, or wildly insufficient return area. Or all three.
    – KMJ
    Jul 19, 2023 at 14:34

If a five inch fits, use it. I've seen people use one inch filters in the big five inch housings before because the 1 inch filters are individually cheaper. The five inch filters are indeed better, but they don't fit in every unit.

  • Our Space-Gard filter unit takes Aprilaire 201 pleated paper filter media which come folded. They are expanded in installation to 6 inches thick fold-to-fold. There is a set of plastic combs which separate the pleats. It is truly a PITA to replace, but I believe it is a superior filter that allows me to change it only once a year. My HVAC service has said that they can retrofit a rack which would allow using pre-expanded filters, but which have less total area of paper filter media. I am sticking with the Aprilaire 201. Jul 19, 2023 at 18:17
  • Nice! Our Honewell housing holds anything up to about 5 inches. We could do the filter once a year but the payback based on increased static pressure is about six months in, since the heat pump runs so many hours in the winter.
    – KMJ
    Jul 19, 2023 at 19:03

Is your air handler/furnace/ac evap coil oriented vertically or is it oriented horizontally? Is it in the attic or is it in a closet off a hallway?

A 5 inch thick filter is better, but usually the extra height filter unit must be installed when the air handler is installed. It would be excessively troublesome and costly to retrofit a 5" filter unit unless the the unit is on its side in the attic. If that were the case, it would still be costly but it would be doable, otherwise it does not really make sense for the benefit you'd gain.

For over 30 years we have had a Space-Gard High Efficiency Air Filter unit at the base of our air handler, oriented vertically, in a tight closet off a hallway. The outside height of the filter unit is 9.5 inches and it takes a $35 pleated paper filter which is about 6 inches in height.

I change the filter once a year and it is a PITA to change. A relative nearby has the same filter arrangement and they cannot change the filter. I do it for them.

Three years ago we replaced our entire unit and the HVAC contractor had to source a special low height evaporator coil to be able to get the new unit into the closet.

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