Using the proper filter frame and securing it correctly can greatly improve your furnace filter's effectiveness at trapping contaminant particles. Build or adjust a filter frame following these steps:
Step 1: Find the Source of the Problem
If your furnace filter does not have a frame, it may be loose, and unable to trap airflow effectively. If the filter is in a frame, the filter itself may be damaged or the frame may be bent or out of position, causing furnace gaps.
Step 2: Make a Frame for Your Filter
Measure the size of the filter and add 1/4 inch all around. Make the frame top out of a wide solid piece of cast aluminum and leave the bottom open. The sides should be 2 open rectangular frames that allow airflow through the filter. The frame must fit snugly into the furnace and support the filter at top and bottom.
Step 3: Fix Your Existing Frame
Look for bends or warps in the edges of the frame. Straighten a metal frame with handheld vice grips. Replace bent sections of a wood frame and fasten them with heat resistant wood putty and wood screws.