It's Murphy's law: If your furnace conks out, it's likely to do so during a bout of particularly cold weather or on a Sunday when fixing it means paying emergency service call rates. Depending on the issue, your best option is to have a professional assess and fix the situation. There are, however, a few simple things you can check. Troubleshooting will help you determine if you have an issue you can fix yourself or if you need a technician.
Furnace Won't Turn On
If your furnace isn't kicking on at all, a power problem may be to blame. First, check the circuit breaker in your home and make sure it hasn't tripped and turned off. Check the furnace itself as well. Some have an external on/off switch that resembles a light switch. This switch is located on the outside of the furnace and gets bumped easily if you're working around the furnace for some reason.
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While checking the power switch, verify that your furnace doors and access panels are firmly closed. If not, a safety switch could prevent the furnace from running. If all looks good at the furnace, take a look at your thermostat, making sure it's set at least five degrees above the current temperature. If it is, try changing the batteries if applicable. If your thermostat is hardwired, remove the cover and use canned air or a soft brush to remove any dirt and dust bunnies from the mechanism.
It's also a good idea to make sure a nearby heat source isn't giving your thermostat a false temperature reading. For example, did your kids just get a new reptilian pet that requires a heat lamp, and is that lamp near your thermostat? Warming the air next to your thermostat can fool it into thinking your house is toasty warm when it really isn't.
Furnace Is Too Cold
Sometimes, a furnace will kick on — perhaps even running for an extra-long time — yet fail to produce heat. The first step to solving this problem is to check your air filter and replace it if necessary. If you can't see light through the filter, your furnace can't get enough air through it either. Replace the filter or wash it and see if that helps.
When checking for airflow, remember to examine the area around your furnace. Too much clutter around the furnace can also reduce airflow and make it very difficult for your furnace to run effectively or at all. An occasional vacuuming or dusting around your furnace never hurts either.
If you have a gas furnace, check the flame color on the burners. A good, hot gas flame appears blue in color. Yellow flames don't burn as hot and may keep your furnace from heating properly. In this case, it's a good idea to turn off the furnace and clean the burners. If that fails, call a tech.
Furnace Constantly Cycles On and Off
Sometimes, furnaces turn themselves on and off too frequently, running for only short periods of time. A clogged or dirty filter could be to blame, so start there. If a new filter doesn't solve the problem, it's time to call for service. A continually cycling furnace usually indicates an issue with the blower motor or a belt, neither of which are DIY fixes.
Furnace Blows Air Continually (Or Not at All)
If your furnace blows air constantly, check your thermostat first. Make sure you don't have the fan set to run continuously. Try turning down your thermostat temperature by a few degrees as well to try to get the blower motor to turn off. No luck? You probably need a new limit control switch and a professional to install it.
At times, homeowners experience the opposite problem and can't get their furnace blower to turn on. In this case, find the inspection window on your furnace cover and peek through it. If you see a green light, things are good. If it's red, it's time for service. The problem could lie with the thermostat, blower motor, run capacitor, control board, or a transformer.
Furnace Is Noisy
It's perfectly normal to hear your furnace kick on, and you can often hear it running as well. Clangs, bangs, pops, squeals, and grinding noises, however, indicate an issue. Pings and pops usually indicate that the air in your ducts is expanding and applying pressure to the ductwork. The simplest solution is to insulate your ducts to minimize the noise.
Rattling noises, on the other hand, indicate loose ductwork that needs tightening. As is true in your car, squeals and squeaks denote a problem with a belt somewhere in the system. The belt may need to be tightened or replaced altogether. If you hear a grinding noise, have a technician check the motor bearings in your furnace.