[amateurplumber]

It was time to replace my furnace filter so I bought a new one that apparently was a different brand from the ones I regularly buy. It’s slightly smaller than the regular one despite having the same dimensions. Is that ok? Wouldn’t it technically not filter everything?

The “actual size” apparently differs by like 0.138”, which seems like nothing but it actually looks like a decent amount smaller than the size of the filter holder. I don’t know much about these things, but wouldn’t some air bypass the filter in that small empty area?

 

[CodeMatters]

My guess would be that that .318" will be covered once it's sucked in by the
negative fan pressure. Maybe try to install it so it so the space is equal on all
sides.

 

[lenaitch]

Assuming you are talking about a pleated filter in a cardboard frame, they are not built to absolutely exacting standards. Even with automotive cabin air filters, I've bought some that were 'a bit of a push'.

 

[MTN REMODEL LLC]

I'm really not sure of your question.... are you asking whether their is a difference among the efficiency of an air filter and is it correlated to it's thickness.

Yes, there is a big difference in the quality of air filters.You have to read....the degree of size particals it filters, washability etc.....

(When I remodeling a whole empty home, I buy the cheapies and put them in double, and toss them out every week or so depending on what I'm doing. Upon completion, I like the electrostaic and washable ones with greater filter capacity)...a lot .more expensive, but good for living in.

 

[MTN REMODEL LLC]

Sorry, I just reread your inquiry, and I guess you are worried about an undersize filter that let's air bypass the filter.

Yes....some air will bypass it, probably insignificant with a clean filter and increasing as your filter gets dirty.

In normal circumstances, I would not worry about it.

 

[surferdude2]

I've never measured my filter down to the mil level but it's sold as a 16X20X1 and it usually measures a full ¼" smaller in both dimensions. I appreciate that and expect it is intentional since I find it regardless of the brand or type.

Even so, I have trouble getting mine out and in the channel the tin knocker provided so I have to take the lower panel off the furnace to help it along.

So you see amateurplumber, although I sympathize with you, things could be worse! I'd prefer your situation over mine. :wink2:

If you measure the actual opening between the flanges provided for the filter to press against when they get sucked toward the blower, you may find that there is plenty of tolerance for a slightly undersized filter to work just fine without allowing any bypass of the flow. After all, the first 1" of the filter is solid cardboard frame anyway.