Using the right size air filter for your HVAC unit is essential to keep your system running smoothly and get the most out of your air filter. An improperly sized or installed filter frame can cause air leaks, reducing the filter's effectiveness. To make sure you get the right size filter for your system, you should measure the inside dimensions of the air cleaner inlet. The model number of your HVAC system will usually appear on the whole-house filter, which is close to your central oven or air conditioning unit.
If the return grille is in the ceiling, you may want to use a filter with a slightly larger actual size, so it doesn't fall on you when you open the vent. Filters are usually listed by Nominal Size, which is a rounded number based on the actual dimensions of the filter. Round to the nearest inch to create the nominal filter size, which is what you'll use when purchasing a filter. Cheaper fiberglass flat filters may need to be changed every 30 days and can even be labeled as a 30-day filter.
Lennox offers a MERV 16 filter option as an upgrade to specific Carrier, Trion Air Bear and Honeywell systems. How often you change your air filters depends on the type of air filter you have and environmental conditions like smoke, dust or construction.