If you've spent any time troubleshooting your gas furnace, you may have come across a strange bit of terminology. When discussing the various safety switches in your furnace, technicians may describe them as "proving" some function or state. While this phrasing might sound odd at first glance, it's not as unusual as it sounds.

How Your Furnace Keeps You Safe

Gas furnaces work by burning fuel to produce heat, but they need to contain a scorching flame that produces toxic exhaust gases. Furnace designers must spend a substantial amount of time and effort ensuring that your furnace operates safely and cannot fail in a way that endangers your family or property. Modern furnaces accomplish this by using a variety of safety switches and sensors.

When your thermostat calls for heat, your furnace's control board goes through a fairly lengthy start-up sequence. Before igniting the burners, it must first check that its safety equipment is operating as it should. When a technician talks about a sensor or switch "proving" something, they're talking about these checks.

For example, before igniting the burners, your furnace must prove that the draft inducer can pull air away from the combustion chamber. The negative pressure switch can check for the draft, but the control board must confirm that the switch is in its default (open) state before the inducer starts and closed after the inducer turns on.

Your furnace goes through several of these checks before producing heat, with each sensor proving that the furnace is in a safe state to begin combustion.

Why Safety Switches Are Common Repairs

Unfortunately, these safety devices can also leave you out in the cold. Ideally, each switch or sensor fails in a way that allows the furnace to recognize that something is wrong. This design prevents your furnace from creating potentially dangerous conditions. For example, a failed pressure switch should prevent the furnace from operating; otherwise, you wouldn't know if your draft inducer motor failed.

As a result, failed switches will usually cause your furnace to throw an error code and refuse to turn on. You should never try to override these switches or run your furnace with a failed safety switch. Without proving that it's in a safe condition, your furnace could potentially create a situation that damages your furnace's internal components or even allows harmful gases into your home.

If your furnace is shutting down to a safety switch, always contact a qualified HVAC professional to investigate the problem. You will only need to replace the relatively cheap switch in many cases, but an expert can check your furnace to ensure it's not suffering from a more severe and dangerous problem.

Call a heater repair contractor to look at your safety switches.