Why Does Your Furnace Care If Its Burners Light?

As the temperatures drop, you'll likely rely on your home's furnace more and more to keep you and your family warm and comfortable. A typical gas furnace will likely cycle many times throughout the day and night. Each cycle involves a multi-step start-up process, including proving that the gas burners have ignited.

While this final step might seem purely diagnostic, it's a crucial safety element. If your furnace can't confirm successful ignition, it will usually shut down before trying again. Some furnaces may even go into a special lockout mode if ignition fails often enough. But what makes this step so important, and what problems can affect it?

How Flame Rectification Works

Flame proving (or flame rectification) is the process whereby your furnace checks for a successful ignition. A typical gas furnace will include numerous burners, but your furnace doesn't need to test each one. Instead, most furnaces include a flame rod only on the final burners. Since the burners light one after another, checking the last burner is sufficient to prove full ignition.

The actual process of flame rectification is relatively straightforward on most modern furnaces. Your furnace will likely have a flame rod placed before the final burner. This rod can "sense" heat by sending a small voltage signal when exposed to a flame. When the furnace control board receives this signal, it knows that it's safe to continue running the furnace.

Why Flame Rectification Matters

Combustion in a gas furnace is surprisingly simple. Your burners are simple nozzles that release a particular amount of gas into the combustion chamber. The igniter (usually a hot surface igniter in modern furnaces) provides heat to light this gas. Once the first burner ignites, the flame rapidly ignites the other burners.

Of course, you may already see an issue with this design. If the burners don't ignite, they will continue to spew unburnt natural gas into the combustion chamber. This gas can enter your home or even cause an incredibly dangerous explosion. The flame rectification process prevents these outcomes by stopping the gas flow if the burners fail to ignite.

What Causes Flame Rectification to Fail

The most obvious reason why flame rectification would fail is that one or more burners failed to light. Burners may not light due to clogs, issues with the gas supply, or even damage to the burner nozzles themselves. A faulty igniter can also prevent ignition. It's usually easy to tell if your burners aren't lighting by observing through the site glass or removing the outer panel.

However, the flame-proving process can also fail even if your burners light. This situation may occur if your flame rod is damaged or dirty or if a wiring problem prevents the control board from registering its signal. These problems can be tricky to diagnose, so it's usually best to call in an expert instead of replacing parts blindly.

For more information, contact a heating service near you.