When I bought my home, my furnace had a 16x20x1 nominal-size filter installed, and that's what I've been buying for replacements. This size doesn't fit the opening very well; there are large gaps around the sides. I measured the opening at 16"x20"x1-1/4".
I assume that the gaps are a problem, as they will allow unfiltered air into the furnace. What can I do to eliminate them?
Here's a picture of the filter's position when the furnace is on. I can still see the gaps around the edges:
This picture shows the entire slot — note that there is no "door" or "panel" to close
Best Answer
Those gaps are normal and don't usually cause a problem. When the furnace is running the air pressure forces the filter against the downstream face of the slot, effectively sealing the opening.
Also, it's not critical that you have a complete seal. The air in your home isn't (or shouldn't be) particularly dusty. It's important that most of the dust gets caught before it passes through the fan and heat exchanger(s), but it's not required to have clinically clean air.
They're pretty much all this way. If the filter was to fit tightly enough to create a seal against both faces it would be extremely difficult to install them. The alternative would be some sort of clamping mechanism like you'd see on a car engine, but it's just not that critical.