In the most recent monthly meeting of our Midwest Association of Home Inspection (MAHI) the guest speaker was Peter Fiedler of Sabre Plumbing, Heating & Air Conditioning. He conveyed some very enlightening information about the standard forced air furnace systems and what causes some to fail prematurely. Here is your basic forced air furnace system:
At its most basic operation it pulls air from the return duct, across the filter element, and powered by the blower, sends the air across the heat exchanger and up the supply duct to the home.
There is a fair amount of engineering mathamatics that goes into calculation the size of the a furnace, but it comes down to the amount of (cfm) cubit feet per minute the blower can send across the heat exchanger. That number, say 400 cfm for an average sized furnace, is needed to not only extract heat from the heat exchanger, but to actually keep the heat exchanger cool. Absent that cooling the heat exchanger will fail prematurely.
Assuming the furnace is sized properly, the biggest killer is a dirty filter. Now you know why. A heat exchanger that is starved for cooling, overheats, and prematurely wears out. Another culprit is return air registers that are blocked or sealed off. The systems are calculated with "X" number of return registers bringing air back to the furnace via the return duct. When that air flow is blocked or diminshed the heat exchanger suffers.
There are two quick field test that can be done to access the relative health of a forced air furnace.
- With the unit running, lift off the filter cover. If you discover a substantial vacuum when pulling up the cover, then the furnace is/has been starved for return air. Consider crytically the health of the heat exchanger.
- With the unit shut down, remove the filter element. If this is changed regularly there will be a minimal about of debris on the element. If it is filthy, bowed or ill-fitting, it will have been busy reducing the lifespan of the heat exchanger.
A basic furnace filter that fits well in the filter bay, has a sheet metal cover, and is changed regularly is the cheapest insurance you can buy for your forced air furnace.