This is a picture of the opening to the air filter on a furnace.
If you know what a furnace filter looks like---and hopefully all of you with forced air heating systems do—the thing you will notice is that it is flat against the side of the plenum (duct). This should tell you that it cannot possibly be functional. This type of filter is designed to be installed across the inside of the plenum either diagonally or flat.
In this instance, as you can see in the next picture, because very long screws had been used to attach and exhaust vent pipe to the same plenum, the filter could not be installed properly and had likely been like this for the 6 year life of the heating system.
This type of filter, in general, is not very effective, but installed in this manner might as well not be there at all.
It is time to get the HVAC contractor to fix the issue so that the filter can be properly installed.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board