Set to "Heat"
“This sounds obvious, but it’s true: A lot of people don’t have their thermostat set right,” says Bobby DiFulgentiz, director of product management for Lennox International. Double-check that the thermostat is set correctly. The switch can easily get moved—say, during dusting. DiFulgentiz also advises to make sure the set point is at a temperature that will actually turn on the furnace.
Filter Out Trouble
Check your filter for obvious dirt. Don’t try to skimp by cleaning and reusing cheap hardware-store filters, says Mike Bonner, at Gray Furnace Man. They have been sprayed with an oil that catches dirt, and once saturated they are no longer effective. “I recommend that homeowners replace their filters once a month,” says Bonner. “A monthly routine will be much easier to remember than every two months—and it’s that important.”
Batteries Not Included
Some thermostats are wired to the home’s electrical system, while others use batteries. How is yours powered? Sometimes those that use batteries will flash a low-battery symbol when they need a replacement, but the signal often goes unnoticed, says Bonner.
Do You Have Juice?
You need to know if the furnace is getting electricity, so check. Most thermostats have a switch for the fan that says either “On” or “Auto” (which means that the fan turns on when the equipment comes on). Throw the switch to “On.” “If the fan comes on, then you know you’ve got power to the furnace. If it doesn’t, you know you’ve got other problems,” Bonner says.
Look at the Circuit Breaker
Still haven’t found the problem? Go to your breaker panel and look for the circuit that controls the furnace. “You’re looking for the one switch that seems in a different position from all the others,” says Bonner. “To fix it, throw it all the way off, then back on.”
Throw ANOTHER Switch
Furnaces have another switch that often looks like a regular light switch. It can be located either on the unit or on a wall nearby. Often this switch is unlabeled. If installed correctly, the switch in the up position is “On.” Unfortunately, this switch can sometimes be mistaken for a light switch and be accidentally turned off. Throw this switch and give it a few minutes, as some furnaces have a few minutes’ delay.
Break the Code
Furnaces built after 1990 have a tiny window where a light shows through. That light can flash a code to help you know what’s going on. If you’ve flipped the furnace switch off, then back on, note the sequence of the flashing light. Then open the furnace’s access panels. Inside one will be a key that tells you what the code means. That meaning will be useful information to tell a technician if the furnace still won’t start after you replace the panels.
Follow the Light
“If your furnace has a pilot light—anything less than 20 years old won’t—there are instructions in your owner’s manual for how to relight the pilot,” says Bonner. A modestly capable homeowner should be able to do it. You’re dealing with fire, however, so don’t do anything you’re not comfortable with.
Check the Gas Valve
If all else fails, check the furnace’s gas valve to make sure that it hasn’t somehow been turned to the “Off ” position. Any gas furnace has a gas cock, or valve, that has to be located within six feet of the furnace, Bonner says. This is usually never touched, but you could check it. Another way to double-check: If you have more than one gas appliance, find out if it’s working. If it is, you know that the gas line into the home is OK.
Call the Pros
So when should you give up troubleshooting your furnace yourself and call in the cavalry? That point varies for every homeowner. “When you get uncomfortable, call somebody,” Bonner says.
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