How often should you change your furnace filter?
A new homeowner asked me how often should they change their furnace filter, and if they should use a high efficiency type filter. The answer to these questions is dependent on the needs of the occupant (allergies, etc.), the home and their lifestyle. Air filters were originally designed to protect the HVAC equipment from large particles and did very little to purify the air for the occupants. But today there are many types of filters of varying air cleaning efficiencies and corresponding cost. Today filters have a MERV (Minimum Efficiency Reporting Value) rating. It is a number from 1 to 12 produced from an air filtration test designed by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). The higher the number, the higher the purifying of the air that passes through that filter. 3M, who manufactures “Filtrete,” also developed their own test which reflects their filters ability, with respect to the size of the particles it filters from the air. The Micro-particle Performance Rating (MPR) reflects the filters ability to trap very small particles. The higher the number, the smaller the particle it can trap.
The Needs of the Occupant:
If you have allergies you will probably want to use filters with an MPR rating of 1000 or higher, but the higher the rating, the higher the cost of the filter, and it may also increase your HVAC expenses. Higher efficiency filters have more air resistance to air flow. This means you furnace fan may have to work harder, and the furnace may need to run longer, to heat or cool the living space. Also, studies have shown that most dust and particles will settle before they are pulled into the ductwork and through the filter. Therefore, the best allergen prevention is to vacuum often with a good vacuum with a HEPA filter. Have people remove their shoes and don’t allow them to smoke in your home.
The Home and Your Lifestyle:
The home and how you live will determine how often you need to change your air filter. The higher the efficiency of the filter, the more often you will need to change it. A dirty filter can increase the air flow resistance by 3 times or more, thus costing you money in operation costs. A system that works harder will probably need to be replaced sooner and need more maintenance. Also, during air conditioning season, an air conditioning coil without sufficient air flow can freeze up and in the worst case, allow liquid refrigerant to reach the compressor. This will cause the compressor to “slug” and destroy itself. If you have a new construction home, I would recommend changing the filter once a month for the first 6 months. After that you should check the filter once a month, but you may only have to change it every 2 to 3 months. If your home is older and the ductwork is dirty, you may need to change the filter every 1 to 2 months. If you have pets or children, have your windows open often, run your furnace fan continuously, burn candles, smoke or have other particulate-generating activities, you may need to change your filter more often.
My Recommendation:
To be cost effective and save on HVAC operating costs, I would recommend a filter with a MERV of 8 to 11 and change it every 2 months, but as you can see, every home is different.