A furnace generates heat by burning a fuel source, usually natural gas, oil or propane, within a combustion chamber. This releases heat, which is circulated through the property via ducts and vents. A heat pump transfers heat between the indoors and outdoors. In winter, it extracts it from the outside air or ground and pumps it into the building. In summer, it removes heat from indoors and releases it outside, which cools the indoor space.

Typically, you should replace or clean a dirty filter every 90 days. That said, during heavy use periods when your furnace system is running often, it can be a good idea to replace your filter once a month. How often to change a furnace filter also depends on what type of air filter you’re using.