Return plenum transition to furnace

[david8370]

I have a question about how my return is set up to the furnace. The return plenum is rectangle and drops vertically down to the floor. Directly connected to the plenum is the furnace, with no transition area. The contractor cut out a rectangle opening in the plenum and built in a sliding sheet metal door. I put the filter in the plenum on the outside of the furnace.

Based on how this is set up, I'm getting a .10 static before the filter and .40 after the filter (.30 pressure drop). This is using a cheap poly panel filter and about the same pressure drop with a blue fiberglass filter. Luckily, my supply static before coil is only about .30, so I'm not too far off where I want to be. My coil when wet is a pressure drop of .25.

I'm assuming my high pressure drop over the filter is due to the way the contractor positioned the furnace to the plenum. I'm also concerned that some air is bypassing the filter, particularly on the top part since the air comes straight down the plenum to the filter. Any suggestions on how I can seal off the filter so that no air can bypass?

I plan on having my return altered eventually, but now is not a good time financially.

Thanks.