This is not intended to be a “DIY Tips” blog, and I don’t consider myself to be a “home handyman”. However, I recently found a solution to a maintenance problem that has frustrated us for the entire 11 years since we bought the house we’re living in. Since none of the so-called “professionals” that we’ve hired during that time seemed able to come up with this solution, it may help other homeowners if I share my tip.

If you don’t live in North America, this may be irrelevant to you, because most homes here rely on a ducted-air central heating and cooling (HVAC) system. This is so that the same ducting can be used for both heating and air-conditioning. By contrast, most homes in Europe rely on central hot-water-based heating systems, and have no central air-conditioning at all.

Ducted-air HVAC systems use a series of delivery vents, in ceilings, floors, or walls, and then one or more return vents to return the air to the furnace/air conditioner. Unfortunately, our house was built in 1999 with a non-standard size a/c return vent, so the filters have to be custom sized. The filters have to be replaced every 6 months or so. The image below shows the main return duct filter in the hallway ceiling. The aperture size is roughly 29″ x 22″.

The following photo shows the efforts of so-called professionals, who at great expense hacked up 2 smaller filters to fit. As you can see, part of the filter’s frame is now missing. The problem with the missing frame is that, when the filter is in position, air pressure tends to force it up into the duct, which means that it does not do an effective job, since there are now gaps around the edges. You can also see that the maker didn’t quite get the size correct, with the result that he had to crush the filter material.

Recently, I took matters into my own hands, and discovered that cutting the edges of a standard 24″x30″ filter and sliding the cardboard over itself can achieve an exact fit, without all that mess! Also, a pack of 3 filters costs only $40!

The steps are:

• Determine the actual aperture size
• Cut the cardboard sides of the oversized filter neatly along the edge that is too long. If the filter is too long and too wide, you’ll need to do this separately in both directions
• Use a knife to cut the filter material away from the shorter portion of the cut frame
• If necessary, shorten the filter material to fit the smaller aperture. I used tinsnips to do this, because the filter material contains a mesh of wire reinforcement
• Slide the edges of the shorter portion of the frame over the edges of the longer portion, until the frame is a snug fit for the aperture. I used 2 large slide clamps, set respectively to the length and width of the aperture, to check for a snug fit
• Tape the cut edges firmly back together

The result is as shown below. You can see where the edges were cut in both directions, then slid over each other and taped back together.

I’ve sometimes felt that the HVAC professionals we hire are “not the brightest sparks in the fire”, and I think I’ve just demonstrated that.

[Added 11/1/24]. I found a photograph of an earlier failed “professional” solution to our problem. In this case, the technicians used a piece of porous fiber material, but reinforced it with steel rods. As shown in the photo below, the result was that the filter did not fit properly into the aperture. When the circulation fan was running, the filter was pulled up into the duct, which meant that not only did it not function properly, but there was a danger that it would be sucked into the furnace intake.
The company that created this inept solution was https://www.hansonheatingandair.com/. A few months after they had installed their attempt at a filter, we noticed that it wasn’t working, and called to invite them to see the problem. Instead of showing any concern for the quality of their work, their receptionist started screaming at me, falsely claiming that I was “trying to get something for nothing”! Needless to say, we will never consider hiring them again, and we recommend that you also should avoid their questionable competence.