Dispose of it and replace it with a new filter of the same size when it gets dirty.
Check furnace filter.
As a rough guide disposable furnace filters start at around 7 or 8 each for 20 x 20 x 1.
Here are some general guidelines for how often to change furnace filters although as always it is a good idea to check your owner s manual for the manufacturer s maintenance recommendations.
Check the filter monthly to see when it needs to be replaced.
This will tell you how long your filter will last.
Just make sure to check the filter monthly and change it when it s dirty not just every three months as recommended.
A good furnace filter isn t expensive.
Changing a home furnace filter regularly is the fastest and easiest way to make sure your furnace is running smoothly and efficiently as well as to help purify the air in your home.
Thick mechanical air filter to an electronic filter plate system involve electrical or ductwork changes by heating cooling contractors.
However it is difficult to give accurate prices because of the enormous variety of materials and sizes and the competitive nature of the furnace filter market.
Electromagnetic filters however do not need replacing unless they are severely damaged.
All other options from a 4 in.
This is how to buy furnace filters at the home depot.