Ever notice how your car seems to run better right after an oil change, especially if you wash and wax it? Well, it's the same for your furnace... don't laugh, I'm serious!

The main component of heating units, both forced air and hot water, is the heat exchanger. This component takes the heat produced by burning fuel and transfers it to the water or air for distribution throughout the house. In a hot water system this component is usually concealed from view, and in a forced air unit only 10 to 25% (sometimes it's completely hidden) of this component is typically visible without disassembly.

Cut-away view of a modern forced-air gas furnaceModern forced-air gas furnace:

1. Solid-state furnace control (Fan assembly visible at lower rear)

2. Draft inducer (fan-forced exhaust)

3. Igniter and flame sensor

4. Gas valve and manifold

5. Gas burners

6. Heat exchanger(s)

7. Air filters

(Configuration will vary between models)

What usually makes heat exchangers inoperative is developing a hole or a crack that allows the hot water to escape, or exhaust from the combustion fuel to escape into the interior air of the home. Constant heating and cooling from years of use will eventually cause a heat exchanger to crack, however some last longer than others. Under ideal conditions, many survive well beyond their predicted life spans.

It seems regular cleaning and maintenance play a factor in life expectancy, as does the environment surrounding the unit. Damp environments tend to assist the build-up of rust on the heat exchanger, shortening its life, while dry, clean environments tend to increase the life span of most furnaces.

Reduced airflow...

Dirty air filters and fan blades, dirty ductwork and obstructed air vents can all contribute to wear on fan motors, reduced efficiency and even premature failure of heat exchangers. Fuel-fired forced-air furnaces are prone to overheating due to obstructions to airflow. Modern furnaces are designed to shut down if temperatures become dangerously high... however, moderately elevated internal temperatures caused by dirt, dust and debris may not be high enough to switch off a furnace, while remaining high enough to cause metal fatigue over extended periods of time.