A clean air filter is very important to how well your furnace and air conditioner run. If your filter is dirty and clogged, it can block air flow and cause your furnace to stop working, or the evaporator coil (“A” coil) to freeze up causing your air conditioner to go out.
Depending on the size of your air filter, it varies on how often they should be changed. Generally a 1” filter should be changed each month, and 4” filter every 3-6 months. If you use a wood burning fireplace or burn a lot of candles your filter could dirty much faster. A good rule of thumb is if you use a 4” filter, check for cleanliness with each season change.