Should I replace this 4" furnace filter with a 5" one?

Question

I got a new furnace this year, and want to replace the filter. It's 4" (actually 3 5/8), but the cabinet looks like it would easily take 5.

Would a 5" replacement be better? Or should I just get another 4"?

Answer 1

The thing to make sure of is that the rating of the filter allows the proper air flow and filter level. If the 5" filter will fit and it has a similar density it would allow the system to move the air with less resistance. I would caution about increasing the density of the filter as this may affect the efficiency of the unit. I have repaired a blower that the owners thought if 1 filter was good 2 would be better and got some really expensive heppa filters this caused the blower motor to run very hot and it baked the grease out of the bearings in just a few years. I added an electrostatic element and the went back to the standard filter after this the owners said the house was more evenly heated and the furnace was running less.

Answer 2

The whole point of a nice filter cabinet like that is to have a tight fit so very little air bypasses the filter otherwise you might as well just stick a cheap filter in there. Those usually take a manufacturer specific filter to fit properly. Most universal filters are under sized so they can fit most cabinets, just bit well. Manufacturer specific filters are usually more money but should fit like a glove.

Answer 3

Call the furnace installer and ask what size, type, and specs for the filter to fit the rack he installed. It looks as if a certain type filter should be installed since it has a cover door. Usually these filter racks handle a certain type of filter and not a standard 4" or 5" filter.

Answer 4

It certainly appears that the filter is undersized. Ideally, it matters only a small amount as long as the existing filter seals against the outflow as noted by the arrow.

You can ensure that a larger filter would work/fit by checking the Actual Size for the 5" filter you are considering.

According to AirFiltersDelivered, the 16 x 25 x 5 filter is 15 3/8" x 25 1/2" x 5 1/4", while your filter shows 15 1/2" x 24 1/2" x 3 5/8".

The five inch filter would be 1/8" smaller in the 16" direction, not too bad, although larger might be a better direction to have a mismatch. Is there enough flange on the outflow window to allow another eighth inch smaller filter casing?

The longer measurement for the five inch filter is one inch. It's tough to be certain from the photo, but you might make that okay.

The thickness is another uncertain-from-the-image measurement. Can you fit another inch and five-eighths? I'm going to say no, as that's a huge proportion of the filter to air-gap ratio in the photo.

That's one resource. The Home Depot carries a filter labeled as a 16 x 25 x 5 but the specifications read as follows:

15.88 (15 7/8) x 24.75 (24 3/4) x 4.38 (4 3/8)

It's still an inch thicker than your current filter.

Amazon has a filter listing, but it matches Home Depot in dimension.

Lowes gets a bit closer but not in the important dimension.

15.75-in x 24.4375-in x 4.88-in

your filter shows 15 1/2" x 24 1/2" x 3 5/8 a quarter inch larger in the short direction, almost a perfect match on the long side, but once again, far too thick to fit.

One could conclude that NO is the answer to your first question and YES to the second.

If your measurements show otherwise, mock up a cardboard version with hot glue and see how well it fits.