Some central air conditioning installations use a return air vent opening that is as large as 24×24 inches or larger. Once the replaceable corrugated paper filter gets a little clogged, the power of the return air suction can suck the filter through the opening. This is bad because now all of the air returning to the air conditioning unit is not being filtered any more. That is bad because the dirt in the air will collect inside the unit on the evaporator coils. If that continues for very long, the evaporator coils will get clogged and the efficiency of the unit will decrease. The more clogged the evaporator coils get, the more they will hold more dirt, and eventually the unit will fail.
The solution to this problem is to install a wire mesh, also called hardware cloth, in the return air opening behind the replaceable corrugated paper air filter. Hardware cloth is made of a weave of thin wires to form a mesh. Most lumber stores carry hardware cloth in a variety of sizes. I used ½” hardware cloth, which means that the wires are spaced ½” vertically and horizontally.
This method assumes that the return air opening is framed with a removable metal vented access door and frame. This method mounts the hardware cloth to the backside of the access door frame. You can also just install the hardware cloth from the front using metal screws, but I think mine is a neater installation because it doesn’t require an exact fit.
You will need:
A piece(s) of hardware cloth as big as the frame of the access door.
Metal cutting shears to cut the hardware cloth.
A drill and 1/8″ (or so) drill bit.
Eight number 10 (or so) ½” bolts with nuts and washers.
Gather all the parts then turn off the air conditioner at a time when it’s not too hot. Open the access door and remove the corrugated paper filter. Look for the screws that mount the access door frame to the surrounding wood framing on the inside edges of the access door frame. Remove those screws and remove the access door and frame from the opening.
Lay the access door/frame on the floor with the back side of the frame facing up. Lay the hardware cloth on top of the frame and mark the size to the nearest wire without going too wide in either direction. Cut the hardware cloth to size. For a 24×24 inch frame, I drilled two holes at the third points on each side. Place the hardware cloth on the back side of the access door frame and insert the bolts through the front and through the hardware cloth. Use a washer and nut to secure the hardware cloth to the back side of the access door frame. Repeat all around.
Reinstall the access door and frame into the opening using the screws that you removed earlier. Install the corrugated air filter but don’t close the access door yet. Turn the air conditioner back on and check to make sure that the filter stays in place. Close the access door. All done!