How to Use a Filter Frame to Fill in Furnace Gaps

What You'll Need
Measuring tape
Handheld vice grips
Cast aluminum strips
Wood strips
Metal or wood screws
Duct tape
Heat resistant putty

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.