Can you cut down air filters to fit? (Yes, BUT…!)

HVAC systems are hardly a rarity in places that go through extremes of both hot and cold weather. Even without the need to control temperature, HVAC systems are also popular for the improved quality of the air they circulate back inside the homes. This is possible only because of air filters that form an integral part of the HVAC.

The task of an air filter is simple and exactly as it sounds. It filters the air. But the air is a gas and what possible filtering can it need? Well, you will be surprised to know just how much dust, pollen, allergens, and such are freely suspended in the air and get into our system with every breath we take.

This was not the case a couple of decades ago but with the rise of urbanization and subsequently, pollution and climate change, the air quality is just not pure anymore. That is why air filters are necessary for filtering out those tiny particles which might otherwise irritate our respiratory tract.

But here’s the thing, the air filters need to be attached to the ducts or vents and may not fit be of accurate size. This is an often-seen scenario because houses are pre-made and duct lines are already cut through. The newly bought air filters may not fit through the duct or vent and in such a case the remaining option is to slice or cut the air filter to be the required size so that it can fit.

Can you cut down air filters to fit?

Can you cut down air filters to fit
Dirty air filter being removed from a home furnace air intake vent / Serenethos /

The simple answer is, yes. You can cut air filters to fit the size. But there is a not-so-simple answer to this too. It will do you well to hear it out.

There are many sorts of air filters in the marker. Depending on the quality of materials used to make them and their flexibility, some air filters are unsuitable to be cut or modified. Their structural integrity rests on their initial shape and any alteration to it will make the entire thing useless and obsolete and may even lead to an inefficiently performing HVAC.

However, some manufacturers permit the cutting of air filters for easy modification to fit pre-made ducts and vents. This is the type of air filter that will still work absolutely fine even after some part of it has been cut off.

However, cutting it does not mean you will take a random machete and just brutally slice through it. It has to be done slowly and professionally and definitely not by a layman who has never had anything to do with air filters.

Long story short, yes you can cut air filters to fit size as long as you know what you are doing.

What if my air filter is too big?

What if my air filter is too big
Replacing filter in the central ventilation system / VidEst /

This is a fair possibility. Older constructions seem to have all the weirdly shaped vents. Although rectangular is the standard, there is also diamond shaped, circular, and even semi-circular. There is no way that any store-bought air filter will fit these sizes and will always be too big for these uncommon shapes of vents.

In such a case, there are only two options- you will either have to get customized air filters to fit that particular shape or just take regular cuts to fit air filters and modify them accordingly. But as already mentioned the method to cut and modify an air filter isn’t exactly simple. The first step is to measure the place where the air filter is going to be installed. It can be an air duct, a vent, a filter holder, etc.

How you measure it depends on what you are using the air filter for. For instance, a return vent usually has air filters that overlap on the sides whereas filters for a window AC have to be smaller than the vent so that the filter goes inside the vent.

Then it is time to work on the filter itself. You can use a pencil to mark on the material the places that need to be cut and use a very sharp knife to cut it. But the air filter has to be handled very carefully and that is why it is best to leave it up to professionals. 

Now for the installation part, most air vents have filter holders on the side but there is no need to fret if there is no hook or clamp to hold the air filter. In such cases, contractors use air filter installation tapes. The surface of these tapes is textured so that it sinks into the material of the air filter and grips it well.

This tape is used along the perimeter of the air filter and then attached to the vent. It is going to stay in place.

What happens if your air filter doesn’t fit?

The air filter, not fitting can actually be two different scenarios- the filter being too small or the filter being too big. In case it is too small, you have no choice but to find a replacement because if space is left around the vent, impure air can pass through and defeat the whole purpose of the filtering.

In case it is too big, then cutting it in shape is a sound option. But as has been well mentioned above, the air filter in question should be suitable for cutting. This you can find in the manufacturer’s manual and if it does not recommend cutting, then unfortunately you have to get the right-sized filter for your vent.

Do air filter sizes have to be exact?

No, air filter sizes do not necessarily have to exactly fit the vent or duct.

Not all air vents and ducts are the same or even serve the same purpose and thus the difference in how they take air filters depends entirely on the purpose. For example, return vents usually have air filters that exceed their size and overlap on the sides, as do ducts on the ceiling. But air filters for window AC have to be smaller than the actual vent so they can fit snugly inside.

On the other hand, air filters for split ACs have to be a particular size because split ACs do not usually have vents but a sliding panel system for the filters. And if the filter is even a centimeter more than required, it won’t fit and it cannot be cut either.

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Final Words

No matter what, the one thing that you need to keep in mind is that there must not be any empty spaces around the air filters from where air can escape. Escaped air will carry with it dust and such other irritants that can be detrimental to the respiratory system.

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