Although gas furnaces rely on natural gas to fuel combustion, they still require both line and low voltage to operate correctly. Modern furnaces rely on electrical power to run secondary components such as the inducer motor and igniter. Additionally, low voltage signals from your home's thermostats command the control board to begin heating when necessary. 

While it's natural to think that a gas interruption is the most likely cause of a furnace that won't start, a power interruption is just as likely, if not more so. Several places may cause a power interruption for your furnace, but this article will discuss three of the most common.

1. External Issues

Your furnace can't receive power if an interruption occurs outside the unit. The two most common external issues that may be preventing your furnace from receiving power are the breaker and the emergency shutoff switch. Always confirm the furnace switch is in the "on" position and that your breaker doesn't require a reset.

However, don't forget that your home's breakers protect your wiring from faults that may cause fires. If your furnace trips its breaker, there's a problem somewhere that you'll need to address, although it may not be critical if these are infrequent. On the other hand, avoid using your furnace if it repeatedly trips its breaker, and call in a professional to locate the source of the wiring fault.

2. Safety Switch Problems

Another power interruption can occur in a surprising place: on your furnace's door. All modern furnaces include a door switch that determines whether the furnace door panel is on or off. The switch opens if you remove the panel, cutting power to the furnace. A faulty switch may remain open, preventing the furnace control board from receiving the power it needs.

Note that the door switch must supply the furnace with 120v power from your home's supply, so testing or replacing this switch can be dangerous. If you suspect your door switch may be faulty, it's a good idea to call an HVAC technician to diagnose and replace it.

3. Transformer Failures

Your furnace receives 120v of power from your home, but many components require substantially lower voltage to operate correctly. Likewise, your furnace control power needs to supply low voltage to the thermostats around your home. The transformer steps down line voltage as needed, ensuring these parts of your HVAC system can operate correctly.

A faulty transformer can interrupt power to critical parts of your HVAC system, causing your furnace to stop working. A bad transformer may also cause your furnace's breaker to trip. As with the door safety switch, it's crucial to remember that the transformer receives full line voltage, so it's best to rely on a professional to diagnose and repair this component when needed.

Reach out to a heating services company for more information.