Flame Rectification: What It Is, How It Works, And Why It Fails

Flame rectification is a complex-sounding term for a relatively simple process. Gas furnaces ignite a mixture of natural gas and oxygen, create a flame, and extract heat energy from the resulting exhaust stream. Both the resulting combustion products in the exhaust stream and the natural gas are potentially dangerous, so it's crucial to keep them out of your home.

While the exhaust system ensures that toxic fumes don't enter your home, a clean burn is the furnace's primary defense against gas leaks. As long as the gas burns completely, there's no risk of unburnt gas entering your home and creating an explosion hazard. Flame rectification is the process by which the furnace proves a complete burn process.

How Does Flame Rectification Work?

A typical gas furnace includes multiple burners. Although the burners are the stars of the show, they're fairly simple. Each furnace burner is effectively a nozzle that allows a measured amount of gas into the combustion chamber. During the ignition process, the gas moves across a device known as an igniter, and combustion starts.

Furnaces generally only include a single igniter near the first burner, relying on each previous burner in the line to ignite the next. The final burner will have a flame rod nearby, which is critical for flame rectification. Since each burner can only ignite if the previous one also ignites, a flame rod on the final burner can prove that all the burners ignited successfully and your furnace isn't leaking any gas.

What Happens When Flame Rectification Fails?

Flame rectification is among the final steps in your furnace's start-up process. Once the furnace's control board instructs the burners to ignite, it will then wait a preset amount of time for the flame rod to confirm successful ignition. If the flame rod doesn't detect a flame, the control board will shut the furnace down to prevent unburnt gas from leaking into your home.

If you aren't near your furnace, the only symptom you'll notice is that your heat won't run. You may also hear your furnace turn on briefly and then quickly shut down again. Since most furnaces won't have time to start their blower if flame rectification fails, you usually won't get any air from your vents. In some cases, you may get a brief burst of cold air before the furnace shuts down.

Why Does Flame Rectification Fail?

Faulty or dirty flame rods are among the most common causes of flame rectification issues and the cheapest to repair. However, there may also be a genuine problem with your burners. A faulty gas valve, dirty burners, or damaged burners can all cause incomplete combustion, allowing one or more burners to fail and ultimately causing the flame rectification process to trigger a shutdown.

Since flame rectification is an important safety feature, you should never assume that the flame rod is a  problem. If you're experiencing the symptoms of a flame rod failure, contact an HVAC technician to diagnose the issue, confirm your furnace is operating safely, and ultimately perform a complete repair. 

For more info about furnace repair, contact a local company.