Gilles' Outlet

October 19, 2008

Replacing a Furnace Filter – Adding an Air Filter Return Grille

Filed under: HVAC — Gilles @ 6:14 am

A furnace filter is replaced. During this process, it becomes obvious that the filter is poorly installed. An air return filter grille is added to correct the problem.


Skill Level: 2 (Basic)

Time Taken: About 25 minutes

Forced Air Furnaces have a blower which takes air from the living space, warms it and pushes it through ducts to all rooms. It is important to make sure air blown in the system is free of dust, particles, allergens because:


  • You are breathing this air – dust, particles, allergens … can cause lung irritations or other respiratory diseases,
  • When dust in the air comes in contact of impellers in the blower, they tend to stick to it. This builds up a coat of dust on internal elements of the blower and causes the motor of the blower to have to work harder to pump air. This can lead to premature stress of the motor.

In this article, the filter is poorly held in the furnace and cannot effectively perform its function. We clean the dust accumulated on the blower’s motor and provide a better way to hold the filter.

When I tried to change the filter on the furnace, I could not find a "filter box" (a drawer like device where furnace filters are held). I had a hunch that the filter would be inside the furnace.


Just to be sure, I shut down the power at the furnace.

Left: I unscrewed the cover of the furnace and sat it aside.


Right: I inspected the furnace and noticed that the blower compartment was located at the bottom, hidden behind a protective panel. The panel was held by a few sheet metal screws. I removed them using a simple screwdriver.

Left: There was the filter – held by duct tape. Actually, the glue on the duct tape dried and the filter was pretty much floating freely at the bottom of the blower compartment. This is bad because when the blower starts, the flow of air causes the filter to be sucked tight at the bottom of the cylinder (the blower fan). This leaves large gaps around the filter where air can get through and bypass filtration.


Right: There were signs of dust and particles collecting on the blower. They are clearly visible on the back of the blower’s electric motor.

Left: I pulled the filter out. It is very dirty.


Right: Using a 5HP shop vacuum cleaner, I cleaned the blower as well as I could. I could not access the impellers in the fan assembly so I did not clean these. I was able to touch a few impellers with my finger and observed that some fine dust already started to collect on them.

I could have removed the complete blower assembly, pulled the motor’s arbor and cleaned the fan perfectly. This would have taken at least a few hours and I did not have this kind of time. 

However, it was now obvious I had to find a better way to hold the filter in place. I put the furnace back together and turned the power back on.

Left: All forced air furnaces have an air return grille: this is essentially where air is sucked in the system. This simple grille hides the duct work and prevents items to be sucked in by mistake.

In this installation, the air return grille is easily accessible, making it the perfect candidate to hold an air filter.


Right: This is a new Air Return Filter Grille purchased online. It is an air return grille which can hold a standard 1” thick filter. It offers a removable face which allows for easy filter replacement.

When ordering air return grilles (with filter or not), the width of the duct opening comes first and then the height. In my case, the duct opening is 14” wide by 20” tall so I ordered a 14 x 20. It is easy to forget this and order the wrong part.

Left: Using my impact driver, I removed the four screws holding the old grille in position.


Right: I slid the new grille in position and secured it to the framing at four corners using 2” coarse thread drywall screws.

Left: In order to prevent unfiltered air to enter the system, I applied metal foil tape to seal any gaps between the edges of the air return grille and the duct. I made sure all the duct work to the furnace was properly sealed as well.

While it is impractical to make ductwork perfectly air tight, sealing with metal foil tape is, in practice, pretty effective.


Right: The standard 14×20 1” furnace filter slides in the opening. The removable grille cover gets installed above and secured to the frame.

The brown mark at the top left of the filter is a paper tear I caused when I took the filter out of its packaging. It does not affect its effectiveness.

Tools Used:

  • Impact Driver
  • Screw Driver
  • Shop Vacuum Cleaner 
  • Utility Knife

Materials Used:

  • Air Return Filter Grille 14 x 20 
  • 2” Coarse Drywall Screws (4)
  • 14×20 1” furnace filter
  • Foil tape


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