There's nothing more frustrating during a cold snap than realizing that your thermostat is turned up but you're still not getting any heat. Furnaces can fail for a variety of reasons, and as a homeowner, it is helpful to be able to perform some basic troubleshooting when yours seems to be on the fritz.
How Does A Furnace Work?
To understand why your furnace might not working, it's helpful to understand how a furnace works in the first place. A home furnace isn't a particularly complicated device and it is not difficult to understand the basic principles of its operation. For a gas furnace, natural gas is ignited in a burner and used to warm a metal heat exchanger. The heat exchanger, as the name implies, allows the heat from combustion to be transferred to a plenum, where it can then be distributed to the rest of the house.
One common misconception is that the combustion happening in the furnace heats the air directly. Furnaces never work this way, because the combustion exhaust gases are extremely dangerous to breathe in an enclosed space. In fact, the combustion chamber is sealed and gases are directed to the outside of the house through an exhaust flue, making the heat exchanger a critical part of the furnace's operation.
Now that you have a foundational understanding of how your furnace operates, it's time to look a few reasons that it may not be turning at all.
Rule Out External Problems First
If your furnace won't turn on, the problem may not have anything to do with the furnace at all. The two most likely external culprits in this case are your thermostats and your breaker box. A thermostat is necessary to control the furnace, and if your thermostat is failing or has failed the thermostat might not turn on at all. A bad thermostat will sometimes cause your furnace to short cycle (turn on and off too rapidly) as well.
If the thermostat isn't the problem, don't neglect to check your breaker or fuse box as well. If your gas furnace uses electric ignition, then it won't be able to light if the breaker has tripped or the fuse has blown. This shouldn't happen regularly, however, so this may potentially indicate another problem with the furnace or an electrical issue.
A less likely but potentially more serious issue is an interruption in gas flow. If the gas has been turned off to your house by the gas company, then obviously your furnace won't function, but if you notice any smell of gas in your home at all, you should leave the building and call the gas company immediately. Never attempt to locate or fix a leak yourself.
Check the Pilot Light
If your furnace is equipped with a pilot light, make sure that it is lit. If the pilot light is out, your furnace won't be able to turn on at all. Relighting a pilot light is generally fairly simple, and your furnace should have instructions to do so printed somewhere on the casing. If your pilot light is out, getting it back on should solve the problem.
Determine If the Furnace Is Overheating
It may seem counterintuitive, but your furnace can overheat. Technically speaking, this happens when the heat exchanger becomes too hot. The result is the furnace's safety tripping and the furnace shutting off. The heat exchanger relies on fresh air moving through the hot-air plenum to keep it cool, so this is usually the result of an air restriction such as a clogged filter. Try cleaning your furnace and replacing the filter to see if the problem resolves.
Seek Professional Help
Most furnace issues end up being simple problems that can be fixed with some basic maintenance know-how, but if the above troubleshooting doesn't turn up the source of your trouble, then it is time to enlist the help of a professional. Most larger furnace repairs are fairly difficult and require expert knowledge, so bringing a pro into the mix will likely save you time, money, and frustration in the long run. Talk to a company such as American City Plumbing & Heating to learn more.