Why Are Some HVAC Filters Bigger Than Others?

Choosing the right HVAC filter sizes for your air conditioner helps ensure optimal functionality of the unit. If you're not sure what size air conditioner filters you need, keep reading to find out how air filter sizes are determined.

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How Are HVAC Filter Sizes Determined?

The size of an HVAC filter is determined by the size of the opening in an HVAC unit's indoor air handler unit. This is the part of the unit that pulls in air from the inside of the home to be recycled through the HVAC system.

Common HVAC filter sizes include:

• 10x20x1

• 14x20x1

• 16x24x1

• 18x30x1

These measurements give the length, width and thickness of the air filter in inches. So, for example, the first measurement above indicates a filter that's 10 inches long by 20 inches wide by 1 inch thick.

Note that HVAC filters are labeled with what's called a nominal size. This is the approximate size of the filter. If you measured the filter carefully with a tape measure, it might be a quarter of an inch or so off of the nominal size, and you can convert to the nominal size generally by rounding to the nearest inch.

Why Are Some HVAC Filters Bigger Than Others?

HVAC air filters come in different sizes to fit various air intake slots. It's important that your filter fits well in the space.

Furnace filters that are too small leave gaps where air is pulled into the HVAC unit without passing through the filter. This can lead to debris building up in the unit, potentially damaging your furnace or causing it to work inefficiently. One outcome of this can be a higher-than-necessary utility bill.

When furnace filters are too small, they can also move around in the slot, which can cause an annoying rattle in the unit as the air moves through the filter.

Alternatively, if the filter is too big for the intake slot, you might have to cram it in. This can lead to bent or warped filters that don't function optimally or creases around the edge that let in unfiltered air.

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How Do I Know What Size Air Filter I Need?

Determining the correct HVAC filter size is easy. Here are some methods you can use:

• Look at your current filter label.

  Many filters are printed with their nominal sizes, which makes it easy to buy a replacement. If the current filter fits well, take it out and look at the front, back and sides to see if you can find a size.

• Measure the filter.

  Measure the length, width and thickness of the filter with a tape measure in inches. Remember to round to the nearest inch for each measurement to get a viable nominal filter size.

• Check the manual for your unit.

  If you have a manual or other documentation for your unit, scan through it to see if it mentions the correct filter size.

• Measure the slot.

  If you don't have a filter to start with, you can measure the slot where the filter should go. Measure from inside edge to inside edge to get the length and width. Measure the depth of the location where the filter rests to get the depth. You'll need to subtract 1/8 to 5/8 from those figures to get close to an appropriate nominal filter size, as the filter needs to be slightly smaller than the slot opening.

If you really aren't sure about what filter to use for your home HVAC system or believe you may not be getting optimal performance with the filter you're using, consider asking for a filter recommendation next time you have your unit professionally maintained.