Six Furnace Fixes to Try Before You Call for Service

Make sure it's actually telling the furnace to come on. Thermostats, especially programmable ones, can be complicated, and the more options one has, the more that can go wrong. Check that the program is displaying the right day and time, and replace the battery if needed.

It sounds unbelievable, but furnace technicians often find that the only "repair" a furnace needs is to be turned on. Look for a standard wall switch on or near the furnace (every furnace has one somewhere), then check the circuit breaker or fuse as well.

Dirty filters are the most common cause of furnace problems. A clogged filter restricts airflow, and if it gets too clogged, the heat exchanger will overheat and shut off too quickly, causing the furnace to shut down.

Furnace not blowing hot air? Someone may have turned off a gas valve and forgotten to turn it back on. Trace the gas line back from the furnace to the meter, and if you see a handle perpendicular to the gas pipe, turn it so it's parallel instead.

High-efficiency furnaces can drain off several gallons of water a day in heating season, and if the lines become restricted by sediment or mold growth, the furnace will shut down. If the drain hose looks dirty, remove it, fill it with a mixture of 25 percent bleach and water, then flush it after several minutes.

If you have a furnace that vents out the side of the house, make sure nothing is blocking the intake or exhaust. If you have a heat pump, clear away grass and leaves from the fins of the outdoor compressor unit, and before heating season starts, hose it down gently from the top to rinse out dirt and debris.