Should you cut HVAC air filters to fit the space?
The answer - NO!
The purpose of the HVAC air filter is to keep the system clean.
Yes, the TV commercials say that if you use this or that filter the air in the house will be cleaner and your children will be healthier.
While it is true that while the air filtered by the high-efficiency filters is cleaned of harmful particulates in the air, the HVAC system does not handle enough volume of air per hour to actually keep the air in the house clean.
I had a post on this years ago. You can read it by clicking here.
But cutting any filter that was not sized properly to begin with is never an appropriate thing to do.
FILTERS SHOULD BE PROPERLY SIZED TO FIT THE AREA THAT THE SYSTEM IS DESIGNED TO FILTER AIR.
As stated above, keeping the system clean is the primary thing the air filter is intended to do.
How long has the filter in the photo above, or other air filters, been used in the unit in this subject house? It's impossible to know. But just look at the amount of dust that is coming out from INSIDE the unit! Imagine how dirty that system must be!
My recommendation - it's worth a peek inside the air filter access port to view the filter itself. Is it caked with dust? Does it look to have been replaced recently? Is the area around it dusty and dirty? Is it properly sized? Looking at the filter in the photo above, and the area before and after it, it was easy to report that the system needed a complete service and cleaning. Any why not? Bad practice should always be followed by good practice. Good filter maintenance and habits are not only good, but BEST practice!