Can I use a 1" reusable filter in a furnace designed for a 5" filter?

Question

My furnace calls for a 20x25x5 inch replaceable filter. I want to buy a permanent/reusable filter but can only find a 20x25x1 inch size.

Is that ok to use?

Answer 1

The correct person to ask is the person who designed and installed your system.

Generally, for the same level of filtration, a one inch filter will create substantially higher static pressure levels than a five inch filters.

This means that a one inch filter will either have to be a type that does a much worse job of filtering, or it will risk causing issues such as AC icing, overheating, or blower motor failure. To know if this will actually be an issue with your system will require measuring the current static pressure and knowing the design limits of the system. We don't have that information.

I strongly suggest using the filter that the system is designed for. Trying to hack something else here is false economy.

Answer 2

Can I use

sure, but

while the other answer was generalized, the specific reasons are this

• you mentioned furnace
• you did not mention AC or go into any more detail, so jumping to a conclusion of AC icing is a bit of a stretch... first might not even be applicable, and generally a 1" [clean] filter will likely operate better than a dirty 5", so you really can't say.
  • i'd have to look to be sure but i'm pretty sure a new 5" will outflow a new 1" due to more surface filter surface area and less flow restriction; these numbers are published; so in regards to AC specifically is it ok to use probably but would it ever be better probably not. You for the most part want maximum airflow across your coil for AC, and it's highly unlikely you have an oversized blower pushing more than the 500 cfm per ton to where you'd ever be on that side of the numbers.
• the problem with a furnace is the temperature sensor and with a new 1" paper definitely restricting flow more than a new 5" paper, you might get furnace short cycling as you trip the high limit temperature sensor due to not enough airflow, more so with a dirty 1" vs 5". that is what would happen. was the furnace designed with no forgiveness to allow any amount of restriction due to a slightly dirty 5", usually not. how much, try it and find out. and with a permanent/reusable style 1" which are generally not paper they don't restrict enough to where it would be a problem when it's new and clean. if you can fit it with good judgement (which I assume that would be a given but for people asking here who knows) and the filter is not expensive to where if it doesn't work it's a big loss, try it.
• as for asking the person who designed the system, that would not be the installer, and asking either makes me think of that scene in ocean's 11 where the guy asks for another room, the girl is playing solitaire on the computer and says give me one minute, um no. That's the answer you will get if you call the designer or installer of your furnace.