Take a few minutes to troubleshoot your furnace before calling a technician.

How to Troubleshoot a Furnace Before You Call the Repairman

by Matt Smolsky

Nothing equals the awful feeling when you realize your furnace isn't working. It always seems to happen on a weekend or, even worse, late at night, which sends everyone running for the extra blankets. Before you call the furnace repair technician, take a few minutes to assess the situation yourself. The movement will keep you warm and the troubleshooting will keep your mind from worrying about big repair bills.

1. Check the Thermostat

Check the settings on your thermostat. Ensure it's set to "Heat" and that the thermostat's temperature is set higher than the room temperature. This is where an old-fashioned room thermometer comes in handy. Raise the temperature on the thermostat until you hear the furnace click on. If your thermostat operates on battery power, there should a panel that slides on and off, revealing the batteries. Go ahead and change them even if you can still read the temperature, then change the temperature. There's also a chance that the thermostat itself may have gone bad. If it has failed, you might see a blank display or a flashing code. Older, nondigital thermostats can go bad as well. Installing a new thermostat is a job you can do yourself.

2. Turn On the Fan

Furnaces come with fans that you can turn on without blowing heat through the house. Turn the fan to the "On" position. It's usually right next to the "Auto" position. If the fan won't go on, then the furnace isn't getting power or the fan is malfunctioning. The furnace may be producing heat, but can't distribute the heat because the fan won't work.

3. Check the Circuits, Switch & Fuse

A power surge may have tripped a circuit or blown a fuse. Find your home's electrical panel and determine if any of the switches have been switched off. It will be easy to see because the rest of the switches will be in the "On" position. Locate the switch that controls your furnace. Try turning it off for a minute or two, then back on again. Sometimes this will reset the connection. Alternatively, your furnace may have a fuse. If it does, find it and check it to see if it's burned out. The fuse should be on the outside of the furnace, near a power switch or electrical wiring. Turn the power to the furnace off at the breaker box before removing the fuse. Check to ensure that the power switch is in the "On" position.

4. Check the Pilot Light & Gas Valve

On gas furnaces, open the combustion chamber door and look to see if the pilot light is lit. If it is out, it will need to be relit. Determine if the gas valve is open and allowing gas to flow to the furnace. If it's closed, check to see if your furnace comes with instructions on how to relight a pilot light. If yours does, the instructions will be on the inside of the combustion chamber door or very close to it. These instructions will explain in specific detail how to relight the pilot light and will likely include instructions for turning the gas valve on. If you do not see instructions for relighting the pilot light or you are not comfortable doing so, call a professional to do it for you. Some furnaces come with warnings not to attempt to light the pilot light yourself.

5. Other Troubleshooting Tips

If your furnace cycles on and off too often, it may be because of a dirty air filter, faulty thermostat or the fact that your home is simply too drafty and needs to be insulated better. If your furnace is noisy, consider the noise. A high-pitched squeal may indicate slipping belts, fans or poor lubrication. A grinding noise may mean the blower's bearings are wearing out. Yearly maintenance by a qualified technician can help avoid these problems. If your pilot light won't stay lit, it could mean that a strong draft is blowing it out, or that the gas supply or flame sensors are malfunctioning.

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