It’s Super-Important to make sure your Air Filter Fits Right!
This might be a boring news flash to some, but if you’re concerned about the air you breathe, we advise you to check your HVAC filter!
Making sure your HVAC filter fits right, is sealed and covered is just as important as changing that filter on a regular basis. I know, it’s just another thing to add to a to-do list, but we suggest you bump it up the list. This is because you want to avoid drawing unfiltered air into your air handler, where it can cause mold and allergens to have access to your entire home. Here’s what we’re talking about:
Nominal Size vs. Actual Size
Older air handlers often have 1 or more “slots” just before the air handler where the filter is supposed to slide in. Here’s the thing: filters from different manufacturers may have the same “nominal size”, for example 18x24”, but different “actual size”: some may be up to ¾” less than either of those dimensions! If the filter does not fit snugly, it can allow unfiltered air to bypass the filter, so that your filter is having minimal effect. We’ve seen filters that fit too loosely in the slot, and also ones that stick out: both of these are not good! Since we sell high-quality filters on our website, we get questions all the time about what filter would fit best in our customers’ systems. Measure your filter slot(s) or grille(s) as accurately as possible and shoot us a message about what allergens you are concerned about, and we’ll do our best to recommend a good fit on a new set of filters. It also helps to know the model of your air handler, so that we recommend the right MERV (filtration level) for your machine.
No cover (or loose cover) on the filter slot
Here’s the next problem: if you can see part of your filter without even opening a door or cover, there’s a problem. This is because air can again bypass the filter, being drawn from outside the duct to the downstream side. We would recommend one of two things every time you change the filter:
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Use a wide painter’s tape to cover over the filter and the edges of the slot, pressing firmly to seal the slot from outside air.
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Purchase 1-2 AllergyZone Filter Slot Seals, to cover over the slot (they are magnetic and will stick firmly to metal ductwork). You can overlap them if necessary to get a wider seal.
Ill-fitting filter in a return grille
We’ve all had those filters that just don’t want to fit into the return grille, so mashing the edge to make it fit seems like the best solution to get it in there and over with. However, the mashed corner or edge is also a prime source of filter bypass– air would rather go around the filter than through it, because of the resistance of the filter. Here’s where adding a piece of wide painter’s tape on that edge, or really, all around the filter, is a great way to seal it up (and make a note to look for a filter with a slightly smaller “actual size”.)
A worthwhile upgrade
The next time you have an HVAC tech do maintenance or service on your system, ask them about adding a filter cabinet. Filter cabinets typically accommodate 2-6” thick filters. The advantages of adding a thicker filter to your system are:
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Thicker filters last longer (up to a year between filter changes in some cases)
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Thicker filters often have “gaskets” on them to seal the filter to the cabinet, eliminating air bypass
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Thicker filters typically have a lower pressure drop, which is better for filtration and better for your HVAC system
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Because of the lower pressure drop, thicker filters may allow you to increase filtration of smaller particles by increasing the MERV rating of the filter.
Although installing a filter cabinet has an upfront cost, and thicker filters cost more, the cleaner air and reduced frequency of changing filters might be really attractive to you! We discuss some filter cabinets options in this article.
Since your HVAC system is an expensive piece of equipment that provides a critical function in your home, installing the right filter on it in a timely manner is just as important as installing the right oil filter on your car. Prioritizing your filter = prioritizing your lungs and health!
