If you're in the market for a 20x20x4 air filter for home use, you should opt for one with a MERV rating of 8 to 13. This rating will effectively remove particles that cause allergies and provide you with a comfortable breathing environment. For households with pets and allergy sufferers, the MERV 13 filter is the best choice. The reason behind this is that it can capture a large amount of waste using relatively porous filtering material, due to its large surface area through which dirty air passes - four times more filter material than a 1-inch filter. A MERV 13 is an excellent option in this size, as it offers great air quality at an affordable price.
On the MERV 13, the highest rating you'll find for most popular residential HVAC filters, you can count on the filter to stop bacteria, smoke, and other microscopic particles. However, these filters have some drawbacks. Firstly, they usually cost more than basic fiberglass filters. In terms of the most common filter levels sold in retail stores, MERVs from 1 to 16, experts say that this range “ranges from filters that can catch a golf ball (I'm exaggerating a little) to filters that catch just about anything”.
You may need to change the filter more often if you have some very furry pets, or if you live in an area with a lot of air pollution, such as from wildfires. We also control the extent to which each filter restricts air flow, using a differential pressure transducer installed in the air filter. All of them (and experts) told us that the MERV filters in this range restrict air flow more than filters with a low MERV content, as measured by the decrease in atmospheric pressure generated by the filters when they are installed. However, recent innovations in air filters allow for high MERV ratings (11 to 1) with a low pressure drop.
Compared to the cheapest basic filters available, medium-efficiency MERV filters such as the Nordic Pure MERV 12 can significantly reduce airborne dust, mold spores, pollen, and even smoke - helping to alleviate respiratory conditions according to an NIH review. When HVAC filters become dirty and clogged with debris, air can't flow freely through the ducts, making your home less comfortable and eventually causing damage to your HVAC equipment. You should expect to replace the filter every three to 12 months of use, depending on the size of the filter. The air is charged with particulate matter and sent through the filter only once before results are measured.
An easy way to choose a replacement filter is to simply buy the same one that you are already using in your system. However, some air conditioning systems have filters inside the return air ducts scattered throughout several rooms in the house.