Furnace Systems Keep You Warm

A furnace runs for months at a stretch in a Chicago winter. Most of the failures we see in January were visible in October to anyone who looked. Here is what the system does, what wears out, and how to keep it working.

What happens when the thermostat calls for heat

The control board runs the inducer fan to clear the exhaust, the igniter glows or a pilot lights, the gas valve opens, the burners light, and the flame sensor confirms it. Once the heat exchanger is warm, the blower pushes air across it and through the ducts. Any link in that chain can fail, and each one has its own symptoms.

Filters

The cheapest part of the system and the one that causes the most trouble. A dirty filter restricts airflow, overheats the heat exchanger, and trips the high-limit switch, which is why a furnace with a clogged filter runs a few minutes, shuts off, and starts again. Change it every one to three months in heating season.

Flame sensors and igniters

A flame sensor coated with residue cannot see the flame, so the board shuts the gas off a few seconds after the burners light. The furnace lights, dies, lights, dies. Cleaning the sensor is part of every tune-up. Igniters wear out with age and crack; a furnace that clicks or hums but never lights often needs one.

Heat exchangers and carbon monoxide

The heat exchanger keeps combustion gases separate from the air you breathe. When it cracks, carbon monoxide can enter the house. You cannot smell it. Every home with gas heat needs working carbon monoxide detectors, and every furnace more than a few years old should have the exchanger inspected annually.

Warning signs

Short-cycling, cold air from the vents, a yellow or flickering flame instead of a steady blue one, a burning smell that does not clear, a rise in the gas bill, or a furnace that is simply louder than it used to be. Any of these is a reason to call before the cold snap, not during it.