When I think of homeowner maintenance, the first thing I think of is changing the furnace filter. Most homeowners know about this regular chore, and at least one out of five homeowners is diligent about doing this. As for the rest of you...
Do you really know why you're supposed to change your furnace filter? Hopefully your dad, your real estate agent, or your home inspector told you about doing this regularly. There are a couple obvious reasons to change your furnace filter - your heating or cooling system will run more efficiently and the air in the ducts will hopefully be cleaner, but those aren't the most important reasons for changing your furnace filter.
The most important reason for changing your furnace filter is to preserve the life of your furnace.
To make it very simple, your furnace works by passing relatively cool household air over a big hunk of metal, which is called your heat exchanger. As the cool air passes over the heat exchanger, it gets warmed, which dissipates heat from the heat exchanger. This warm air gets distributed throughout the entire home, and this is what keeps the indoor air temperature at 70 degrees when it's -10 degrees outside. That heat exchanger is responsible for transferring a lot of heat to the rest of the house.
Now think about what would happen to that heat exchanger if it couldn't transfer it's heat to the rest of the house: it would get extremely hot, much hotter than it was ever supposed to get, and it would probably fail (i.e. -crack) prematurely.
When you operate your furnace with a dirty filter, this is what you're doing to your furnace. You're preventing the cool household air from passing over the heat exchanger, heat doesn't get dissipated the way that it should, and the life of your furnace gets reduced... besides costing you more money in heating bills.
In other words, don't forget to change your furnace filter. It's not just about indoor air quality.
Reuben Saltzman, Structure Tech Home Inspections