How To Test Furnace Ignitor

It’s important to know how to troubleshoot and, hopefully, fix the furnace ignitor in case your ignitor refuses to glow in the middle of the night on a freezing winter weekend.

Fortunately, it’s easy to test the furnace ignitor. All you need is a multimeter to measure the voltage or resistance (or both) through the ignitor and basic DIY skills.

We’ll show you how to proceed. First, though, let’s learn how the ignitor works and why it may fail.

How Does the Furnace Ignitor Work?

The furnace ignitor is the sole ignition mechanism on modern hot surface furnaces. A hot surface ignition furnace is different from the traditional standing pilot and intermittent spark ignition systems. A silicon-carbide or silicon nitride igniter piece replaces the standing pilot or spark plug.

High-current, low-voltage electricity is forced through the silicon ignitor, causing it to glow red hot. In many cases, the ignitor reaches temperatures up to 2500°F.

At this temperature, the ignitor produces instant combustion when it comes in contact with natural gas or propane – both of which ignite at much lower temperatures.

For example, natural gas instantly combusts (bursts into a flame) when heated to 1200°F. Meanwhile, propane combusts at an even lower temperature of about 1000°F.

One of the most notable advantages of hot surface ignition systems is that they have a safety mechanism to confirm whether the pilot is working normally.

It waits five seconds to check the state of the pilot system and gas circulation and will instantly shut down the system if any safety issues emerge. However, if everything is fine, the pilot light requests the control board to open the main gas valve, and your burners are lit.  

Where is the Furnace Ignitor Located?

The hot surface furnace ignitor is located in the burner assembly. You can easily find it when you lift off the front panel. It’s a small M or fork-shaped device with a white plug base located next to the burners on one end and protrudes into the path of the flame on the other. Two wires attach to the base and run to the main control board.

How to Tell If Your Furnace Ignitor is Bad

A defective furnace ignitor will display many signs. However, you want to keep an eye out for the following particularly;

  • The pilot light won’t light: Of the many things that can cause a pilot light to refuse to ignite, a bad ignitor is top of the list. If the igniter doesn’t work anymore, it will not produce the flame necessary to light the pilot system. Thus, the furnace will remain OFF no matter how many times you try to light it.
  • The furnace short-cycles: A compromised furnace ignitor can also result in short-cycling – the tendency of furnaces to start and stop frequently. What usually happens here is that the igniter attempts to light the furnace but fails because of low charge, so the furnace goes off. Then it attempts to re-ignite and fails again. This can go on for a long time until you fix the underlying issue.
  • The ignitor doesn’t glow: As we’ve mentioned, high-voltage electricity is passed through the ignitor when you switch on the furnace, causing the ignitor to glow red hot. If your ignitor doesn’t glow red after about five seconds, it’s likely compromised.

What Happens When the Furnace Ignitor Goes Bad?

Several things can happen when you have a bad furnace ignitor. However, the standard outcome is usually cold air from your ducts. Why? Because heating cannot go on without furnace ignition. You need the ignitor to light the burners, which, in turn, produce the heat to warm your rooms.

Take the ignitor out of the equation, and the entire chain of events falls apart. However, you will still likely have air coming out of the ducts. But it will be cold – not warm – air.

That’s because the blower system comes on even before heating begins properly to generate the air movement inside necessary to pull cold air into the furnace as warm air re-enters the home.

Troubleshooting a Furnace Ignitor that Isn’t Glowing

The troubleshooting process will depend on the current issue. Is the ignitor completely cold, i.e., not glowing in the first place, or is it glowing but not lighting the burners?

If the hot surface ignitor doesn’t glow, you likely have no power, have a faulty transformer, a malfunctioning thermostat, or a faulty limit switch. Alternatively, the igniter itself could be faulty, or its control system is compromised.  

However, if it glows but the burners don’t light, the issue could be different. Perhaps the gas valve is faulty, or the ignition control is compromised. Alternatively, the furnace’s polarity may be reversed, or the ignitor alignment is not proper.

Whichever the case, a quick physical check is often enough to reveal the underlying issue. Here’s what to look for;

  1. Check the unit for cracks and signs of damage.
  2. Check the sleeving over the wire for chaffing
  3. Check the wire for cuts or burnt portions
  4. Check that the connects are properly seated
  5. Make sure the connectors are free from oxidation and corrosion
  6. Ensure the wires aren’t disconnected inside the ceramic holder
  7. Ensure there’s no white silica dust around the igniter legs

How to Test a Furnace Ignitor

You should also strongly consider testing the ignitor with a multimeter to ensure that it has the correct voltage and resistance performance. Here’s how to test the furnace ignitor;

  1. Switch off the furnace at the breaker switch
  2. Turn off the gas supply at the valve
  3. Switch off the furnace at the onboard switch, i.e., the switch on the furnace itself
  4. Remove the service panel. You may need to unscrew a few screws, though some panels come off without much effort.
  5. Detach the ignitor from the socket. Only hold the ceramic base when pulling it out as the metallic tip is highly delicate.
  6. Test the voltage: Set the multimeter to voltage (V) readings. Then attach the multimeter probes to the tips of the ignitor plug and take the reading. You should get 24V. If it’s less, you likely have a defective ignitor.
  7. Check for ignitor resistance: Detach the multimeter and set it to resistance readings (ohms). Then reconnect it to the ignitor probes and check the reading. A normally functioning ignitor will give a resistance value in the region of 40 ohms to 200 ohms. If your ignitor has lower resistance, it’s probably dying. If it’s zero, it’s dead.

How to Clean the Furnace Ignitor

If the above tests reveal that the ignitor conducts electricity, i.e., the resistance is between 40 ohms and 200 ohms, it’s salvageable. All you need to do is thoroughly clean it to remove the layer of grime and dust on its surface. That harmless-looking layer of grime can make the ignitor slow to respond as it compromises the device’s sensitivity.

The igniter may even refuse to ignite. Gentle cleaning can solve the problem. There are two main ways to clean a hot surface ignitor. First, you can use compressed air.

Alternatively, you can clean it physically. You must begin by switching off the appliance at the breaker and turning off the gas supply in both cases. Then detach the ignitor and proceed as follows;

Clean it with compressed air

In this case, you need to purchase a can of compressed air. You can easily find one at your local hardware store. The cans feature long applicators that resemble straws. Hold the ignitor in one hand and the compressed air can in another, pointing the applicator at the ignitor.

Then release a blast of air across the ignitor to blow away the layer of dirt. You may need to blow it two or three times to get rid of the dust completely. 

Clean it physically 

Find a ball of fine steel wool or emery cloth and use it to wipe the probe gently. You must be extremely careful as the furnace ignitor is delicate.

When you’re satisfied that you’ve removed as much dust and carbon deposits as necessary, put the ignitor back in place, hook the connectors, and turn on gas and power. Then test the furnace to see if it works. If it still doesn’t work, you need to replace it.

What’s the Cost to Replace a Furnace Ignitor?

It costs about $120 to replace a furnace ignitor. However, the cost varies depending on many factors, such as the quality of the replacement part, your geographic location, and whether you opt for DIY or professional installation. DIY replacement costs $120, on average, as you only need to purchase the ignitor.

However, a professional replacement can cost up to $250, including labor charges, as most HVAC professionals charge $60 to $150 for general services. In both cases, make sure to purchase the right replacement. Otherwise, your furnace may still refuse to work.

Summary

Now you know how to test the furnace ignitor. Remember to always start with a physical inspection before an electrical test. More importantly, if you must replace the ignitor, beware that professional installation can save you a lot of money in the long run.

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