How to identify the correct size / part number of the filter that fits your HVAC system.
Filter changes should be done when required to keep your HVAC system running efficiently. Even so, figuring out the correct air filter size can be a challenge if you don't know what the numbers imply.
When it comes to air filter replacement, size, timing and MERV rating (Minimum Efficiency Reporting Value) matters. This article will assist you in obtaining an accurate measurement and determining the right filter for your HVAC System.
How does filter size work?
Filters have three-part dimensions labelled on them. Generally, the Height (Width) is the first number, the Length is the second, and the Depth(Thickness) is the third.
For example, a filter labeled 16 x 25 x 1 is approximately 16 inches high, 25 inches long, and 1 inch thick, but this size is referred to as the Nominal Size in the HVAC ( Heating Ventilation Air Condition) industry, the Actual or Exact Size is different, commonly smaller than the nominal size but there are exceptions, so basically do not assume you know the correct size until you have verified by physically measuring the internal dimensions of your air cleaner, find the brand / model number of your air cleaner or go with information on the existing filter as long as its a snug fit.
Nominal Size vs Exact or Actual Size.
With a nominal size 16 x 25 x 1 Filter, the actual or exact size can be 15 1/2" x 24 1/2" x 3/4" this varies by 1/16" depending on manufacturer.
Common filter Depth or Thickness
1" Thick Standard - The most common thickness used in homes with an actual thickness of 3/4" across all brands/manufacturers. Examples of popular 1" filters 16x25x1, 20x25x1, 14x25x1, 20x20x1, 16x20x1.
2" Thick Standard - Used in Commercial and Industrial HVAC systems, almost non-existent in residential homes. Examples of 2" filters 16x25x2, 20x25x2, 20x20x2, 14x25x2, 16x20x2, 24x24x2.
3" Thick Standard - Only one known size exist that is labelled 3" Thick, 16x25x3.
4" Thick Standard - There are 2 different types, Standard and Non-Standard 4" filters. Just like 1", 2" and 3" filters a Standard 4" filter is approximately 1/4" to 1/2" less than the Nominal Size, with 1/16" variable between the different manufactures/brands and all standard 4" filters are interchangeable. For example, a standard 16x25x4 filter can have an actual/exact size of 15 3/8" x 24 3/8" x 3 5/8" or 15 1/2" x 24 1/2" x 3 3/4" or 15 5/8" x 24 5/8" x 3 5/8" or 15.4" x 24.4" x 3.68".
4" Thick Non-Standard - Examples of Non-Standard 16x25x4 labelled filters, Honeywell 15 15/16" x 24 7/8" x 4 3/8", Carrier 15 3/4" x 24 3/4" x 4 15/16", basically a non-standard 4" filter is more than 4"thick, can be 4 1/4", 4 3/8", 4 5/16" and 4 1/2". 
5" Thick - There is no standard when it comes to 5" thick filters, this is where homeowners need to know exactly which filter is required for their air cleaners, there are many manufacturers/ brands and each one has proprietary actual/exact dimensions although a few are interchangeable, but care must taken to measure all 3 dimensions of the filter and the internal dimensions of the air cleaner to be certain of compatibility. Examples of 16x25x5 filters, Lennox 15 3/4 x 24 3/4 x 4 3/8", GeneralAire 15 5/8"x 24 3/16" x 4 15/16"(With Foam Gasket), Carrier / York / Goodman / Electro-Air 15 3/8" x 25 1/2" x 5 1/4". Almost all 5" filters are labelled with a model number or part number, actual/exact size and MERV rating. Once you find the correct filter you will need to stay with that particular size / brand, the only other criteria you will need to know is which MERV rating you should get, if there are options available.
Signs that the filter in your air cleaner isn't the right size.
The improper size HVAC air filter might put your entire system at risk of failure during heating and cooling seasons. A filter that is either too small or too big will not provide the necessary seal to filter all the air. Dust and grime will be able to flow through the gaps causing dust to accumulate on the blower / fan, the secondary heat exchanger, the air conditioning evaporator coil and even recirculate back into your home's air.
Below are a couple indicators that you're using the incorrect filter size. If you're still not sure, measure the internal dimensions of your air cleaner and then look for the actual / exact size of a filter that closly matches,
You can also check your filter cabinet for any model number or manufacturer or brand to narrow down the search for the correct filter.
- It's possible that your filter is too small if there is a gap between the filter and the air cleaner frame at the top or the sides, basically not a snug fit.
- It's too huge if you must cram or push your filter into the slot.
If you still facing any issue on identify the correct size, please reach out to us on our communication channels on www.furnacefilters.ca or www.alberta.furnacefilters.ca
You can also check your filter cabinet for any model number or manufacturer or brand to narrow down the search for the correct filter. Finding the right size filter is an easier process than it may seem. Listen and look for cues your current filter may be under or oversized.
Happy Shopping.