Furnace ignitors: how do they work?

HVAC Logo IconBy Tom MoorAugust 1, 2023
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When you turn up the thermostat to initiate your furnace, you might notice a clicking sound before the warm air arrives. That noise comes from the furnace ignitor.

As its name implies, the furnace ignitor creates a spark, igniting the gas to heat the air circulating through your home. The ignitor is a critical part of your furnace. Our guide covers the causes of faulty ignitors, when to replace them, and if a furnace ignitor replacement is a DIY project or you should call in the pros.


How to tell if the furnace ignitor is bad?

After you start your furnace, check the vents. If the air blowing matches your current indoor temperature, it could indicate the furnace ignitor isn’t heating the gas. Before diagnosing the gas furnace ignitor as the source, check your thermostat to ensure you have the setting switched to heat. If so, then the ignitor isn’t creating the combustion needed to warm the air.

There are several symptoms of a faulty gas furnace ignitor, including:

Furnace short cycles

If you notice the furnace turning on and off again repeatedly, the unit is short cycling. When this happens, the furnace detects something is wrong, and as a safety measure, shuts it off.

A failed furnace ignitor could be the cause of the short cycles. If the ignitor fails to heat the gas, then the furnace will try to turn on repeatedly (usually three more times) until a safety feature locks the unit from further attempts. A furnace ignitor replacement is likely the best solution. Call in an HVAC professional to complete the job.

Furnace ignitor not glowing

Unlike older furnaces that require an actual flame to operate, modern furnaces use an electronic ignition that has a hot surface ignitor. The ignitor sits beside the gas burners, and when the thermostat communicates that it’s time to distribute warm air, the hot surface ignitor can reach 2500 degrees Fahrenheit.

The gas valve then opens, and gas is ignited by the burning ignitor. The ignitor will burn out over time. When the thermostat communicates to the furnace that it’s time to turn on, look through the louvers of the front cover. The ignitor glows brightly when working properly. If you don’t see a glow coming from the area, it’s time to call a furnace specialist.

It may be time to change the furnace ignitor, but the problem could also lie with the inducer motor. The inducer motor is responsible for providing airflow that allows the hot surface ignitor to turn on. An HVAC pro will inspect the system to determine which component is causing the issue.

The furnace shuts off

If you hear the furnace click on but it never fully fires up to begin the heating cycle, a faulty furnace ignitor is likely to blame. In this situation, the furnace detects there’s a problem (often with the ignitor’s inability to create combustion) and shuts down to keep you safe. Call your local furnace technician to perform a furnace ignitor replacement.

What causes a bad furnace ignitor?

There are many reasons a furnace ignitor may malfunction, with some not having to do with the component. Here are some tips to help you find the reason your furnace does not function properly:

Inspect the circuit breakers

When you notice a problem with the furnace, inspect your home’s circuit breakers. The furnace pulls a large amount of power to operate at full capacity. If too much power is demanded at once, your breaker may flip as a safety precaution.

Test the gas lines

Check other gas-powered appliances in your home to ensure the gas lines are functional. If the challenge occurs with other appliances, call your utility company for support. Never attempt to fix a gas line yourself. If the problem appears to be isolated to the furnace, call in an HVAC technician to inspect the unit.

Check the furnace drip pan

High-efficiency furnaces create condensation when they operate, so the water may acquire in the drain pan (positioned under the furnace). The water is designed to exit the system through the drain line. If that line becomes clogged and the water cannot drain properly, the unit will be prompted to shut off by the float switch.

Dirty air filter

The furnace needs sufficient airflow to operate properly. If your air filter becomes clogged with lint, pet dander, and dust, airflow is restricted and the furnace will not function optimally (or at all, in some cases). Be sure to change your filter at least every 90 days, more frequently if directed by the manufacturer, but check its cleanliness monthly.

Age of furnace ignitor

If you need to know how to tell if the furnace ignitor is bad, consider how long you’ve had the furnace. Most ignitors are designed to last for around five years. If you’ve lived in your home for nearly five years or longer or you know the ignitor has not been repealed in the unit, a replacement may be your needed fix.

Too much power

If a problem exists with the ICM ignition control module, more simply known as the control board, the ignitor may receive improper voltage. The ignitor will not cause your breaker to trip, but issues with the control board will. Find a local electrician to inspect and repair the problem.

Dirty furnace ignitor

A dirty ignitor can also prevent the furnace from operating well. Have regular inspections done to ensure it is in peak condition. It is possible for you to clean the ignitor yourself before calling in an expert.

How to clean furnace ignitor

Routine maintenance of your furnace should keep the ignitor in good working condition, but if you need to clean it in between inspections, begin by turning off all power to the unit.

Disconnect the furnace ignitor from the unit by detaching connected wires and loosening the screw that holds the component in place. The furnace ignitor sensor is typically the part that needs to be cleared of debris, but be very careful. The part is easily broken.

Emery cloth is the best tool to use to clean a furnace ignitor. Touching the surface of the ignitor with your bare hands will permanently disable the component. Gently rub away dirt and carbon residue and reconnect the piece to test the furnace. Adjust your thermostat to force the furnace on. If nothing happens, it’s likely time to replace the ignitor.

How to test the furnace ignitor

Before taking on a furnace ignitor replacement project, test the component to see if it has any remaining life. You’ll need a multimeter to complete this test.

Safety is paramount, so start by turning off all power to the furnace and closing the gas. If the furnace has been attempting to work, allow the unit to cool before testing. Once the system has reached a safe temperature, remove the panel door.

Locate the ignitor, which is usually positioned near the gas port on a V-shaped bracket. If the ignitor has visible damage, there’s no need to test the part. Just replace it.

If there’s no visible damage, detach the wires and loosen the screw to remove the piece.

Connect the multimeter probes onto the tips of the plug for the ignitor. Use this how-to video as a guide. If the multimeter displays infinity or stays at zero, the ignitor needs to be replaced.

Where to buy a furnace ignitor?

If you or a qualified technician determine the furnace ignitor is the culprit, then you can pick one up from a local hardware store. Your furnace manual includes information on the correct model to buy. You can also check with your retailer, who can look up that information.

How much does a furnace ignitor cost? Costs vary between $30 to $50. That price, however, doesn’t include a service or labor fee if you have a local technician make the repair. Home Advisor reports a furnace ignitor replacement cost can range from $150 to $250.

Furnace ignitor replacement

If you’ve walked through the different steps to troubleshoot your furnace with no luck, call an HVAC technician to inspect the unit and make any necessary repairs. Keep in mind, a furnace typically lasts 15 to 30 years. Routine maintenance will support a longer lifespan. If your unit is reaching the end of its life, consider replacing the system with one of these best furnace brands.

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