If you know the actual size of your filter, it's easy to determine the nominal size. All you have to do is round each dimension to the nearest inch. The filters are labeled with three-part dimensions, with the first number being its length, the second its width, and the third its depth or thickness. Filters are usually listed by Nominal Size, which is a rounded number based on the actual dimensions of the filter.
However, they may add an additional surcharge to compensate for their services. If that doesn't work, you may be able to find the proper dimensions of the air cleaner in your HVAC manual. If your HVAC system uses a whole-house filter (close to your central oven or air conditioning unit), the model number will likely appear instead of the dimensions. Understanding the most common air filter size can be difficult if you're not sure what the numbers mean.
The most common AC filter sizes and oven filter sizes are 16x20x1 air filter, 20x20x1 air filter, 16x25x1 air filter, and 20x25x1 air filter. Changing the HVAC filter is just one part of routine maintenance you must perform on your system. It's best to have a little wiggle room in the slot of the HVAC unit, so you can easily slide the filter in and out without it bending or breaking. If you have to jam the air filter to install it, you should try a different brand of oven filter or a custom-sized air filter with a slightly smaller actual size.
Carrier and Lennox's superior systems offer some of the best residential filters available on MERV 15 and 16 respectively. Lennox has a MERV 16 filter option as an offer on their Lennox Pureair system, which is among the highest-efficiency residential filters available and Lennox also recommends it as an upgrade to specific Carrier, Trion Air Bear and Honeywell systems. Do not stack several smaller air filters on top of each other to place them in a larger air handler. You can choose not to use factory-installed filter grids and have an HVAC professional install an external filter grid to get a coarser filter. You can find the recommended filter size for your unit model in a table titled “Find your air conditioner and oven filter by make, model number and size”.