A Lennox Furnace not igniting can be due to lack of power, a clogged filter, blocked vent, damaged motor or thermostat, or pilot light being out, among other things. Quick fixes to these issues include checking the power and fuel sources, cleaning the filter and vent, replacing damaged parts, and igniting the pilot light.
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The furnace is a crucial piece of equipment in your home to help maintain the ideal temperature and provide warmth for you and your family. Even though furnaces are built to last, they can malfunction or break down over time. Lennox furnaces are no exception and can sometimes fail to ignite.
As a rule of thumb, you shouldn’t ignore these problems, as they can lead to further complications when left unfixed. Keep reading for more insights into why your Lennox furnace isn’t igniting and what you can do to fix it.
Related: Why Is My Lennox Furnace Not Staying On?
The first things to check are your energy sources…
1. Lennox Furnace Not Igniting – No Power
Even though gas is it’s primary fuel source for heat, a Lennox furnace requires electricity in order to function. It won’t ignite without adequate power to the circuit board, blower motor, various sensors, and other components. So you must verify that you have power to the unit.
How To Fix
First, check to see if there is power at the furnace. Do you see any lights on? Since some furnaces don’t always have visible lights, this might not tell you anything, but it’s worth a shot. If you have lights, you have power.
If you don’t see lights, check the disconnect switch located either on the unit or close by. It looks like a regular light switch. Make sure it’s in the ON position. In fact, flip it off and on once, to be sure.
Next, check the breaker in the house circuit panel. Make sure the breaker is not off or tripped. Go ahead and flip it all the way off, and then back on.
If all of these things check out fine, then there may be an issue with a wiring connection somewhere on the circuit.
If you’re confident in your abilities, you can start checking the connections on each end of the circuit. Otherwise, it’s best to call an electrician who can inspect further and identify the problem quickly.
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Caution: Unless you’re experienced, never touch your Lennox furnace’s wiring or internal parts while it’s still connected to the power supply. You risk incurring personal injury and causing further damage to the furnace.
To be certain you have power to the furnace (if it has no lights to indicate such), you will need to use a multimeter or voltmeter.
With the furnace disconnect switch off, open up the furnace cover and locate the power junction box (j-box). You can usually see where the wire (or conduit) enters the furnace. The j-box should be just inside from there, attached to the unit wall.
Open the j-box cover. You will see a black, white, and green wire – each connected to a corresponding circuit wire, usually by way of wire nuts (a.k.a. wing nuts or Marretts).
Check the connections for snugness and full contact between the connected wires in each wire nut. Then leave the wires sticking out of the j-box in such a way that you can easily and safely test them.
Turn the disconnect switch on and set your multimeter to VOLTS.
Stick one probe into the black wire nut and the other into the white wire nut, making sure the probes are contacting the copper wire. You should get around 120 volts, give or take.
If you get no voltage reading, it is a circuit issue. If you do have voltage, turn the disconnect off, button up the j-box, and move on to Step #2.
Read: Lennox Furnace Problems
2. The Furnace Isn’t Getting Gas
If your Lennox furnace is still not igniting, the next thing to check is the gas supply. Needless to say, if you don’t have a proper fuel supply, your furnace will not ignite.
How To Fix
If you’re on city natural gas, there are a couple of ways to check:
- Take a look at the shut-off valve at the riser meter (where the gas enters the house). The valve knob is a rectangular bar that should be oriented parallel to the pipe. If not, your gas is not fully on.
- You can test other gas appliances (dryer, stove, water heater, etc.) in your home to see if they are working properly.
- Check with the gas company to see if there is an outage.
If, on the other hand, you have a separate storage tank for liquid propane, then find the gas pressure gauge on or near your tank. The gauge should show 10% or above. If it’s lower, you might have to invite the propane company for a fill-up.
3. The Furnace Filter Is Clogged or Dirty
One major cause of your Lennox furnace not igniting is a clogged or dirty filter. This prevents the proper amount of air from reaching the furnace, which causes an improper air/gas ratio. With an incorrect mixture, the furnace will not ignite.
Note: A dirty filter can lead to dirt and dust accumulation in your home. Over time, it may even damage the furnace by causing extra stress on the motor and overheating.
How To Fix
It’s essential to regularly clean and replace your Lennox furnace filter. Also, ensure the filter is not dusty, thick, or located in a spot with limited airflow.
If your furnace requires filters, be sure to change them every 3 or so months and keep an extra supply on hand. In this case, I recommend the Amazon Basics MERV 11 Furnace Air Filter from Amazon.com. Thanks to its electrostatically charged filter material, it’s highly effective in capturing dust and lint.
Note: We earn a small commission on some purchases, at no additional cost to you.
Read: What Does MERV Rating Mean For A Furnace Filter?
4. The Vent Is Blocked
Another way airflow can be obstructed is by dust build-up, insulation, or other materials in the duct system. Again, the Lennox furnace won’t be able to draw in the fresh air it requires to ignite.
Usually, when you turn on your furnace and set the thermostat to a particular temperature, the fan will start up. But over time, dust or other objects may accumulate in the ducts or vents, blocking or restricting airflow.
This airflow impedance can cause the furnace to employ a failsafe mode which prevents it from igniting.
How To Fix
You can fix this problem by removing the obstruction and unclogging the vents. It’s crucial to keep your vents and ducts clean, lest you risk further issues with your furnace. These issues can end up being quite expensive if left unattended.
To avoid this, it may require taking apart some ductwork parts and cleaning them thoroughly before reassembling everything. If you feel confident in your own abilities, this is something that you may be able to do yourself.
Otherwise, consider calling a professional HVAC contractor who can perform this task for you.
5. The Draft Motor Is Damaged
The primary function of a draft inducer motor in a furnace is to pull in air and expel it through a vent pipe. If this motor is damaged or broken, air won’t be drawn in properly.
If the blower isn’t functioning, then the furnace will not ignite. You should hear humming noises coming from the system right before the burners ignite, if you don’t hear the sound, then the draft motor is dead.
If you hear the humming sound, but your Lennox furnace still doesn’t ignite, then there might be a blockage in the vent. Make sure that it is clear of snow, ice, debris, bird nests, and so on.
How To Fix
If your draft motor is broken, it must be replaced. You can do this on your own or hire a professional HVAC contractor to do the job for you.
If you decide to replace the motor on your own, here are the steps to follow:
- Turn off the furnace power using either the disconnect switch or the breaker.
- Using a screwdriver or nut-driver, remove the furnace’s front cover. Depending on your Lennox furnace model, you may need to access several different screw locations to remove the cover. And it may come apart in two pieces.
- If your model has pressure switches attached to the motor assembly, remove them (two screws each).
- Remove the four screws holding the inducer motor in place.
- Disconnect the wiring harness and remove the wiring bundle from its clips as needed.
- Loosen the hose clamp and disconnect the motor exhaust from the intake pipe.
- Disconnect the ground wire from its termination point and remove the motor from the furnace.
- Remove the limit switch from the motor (to be re-used on new motor).
- Install the limit switch on the new motor.
- Reverse the process to install the new motor in the furnace. Be sure the gasket is clean and not damaged where the motor housing mates to the header box.
Pro Tip: Keep a check on your draft motor regularly. It shouldn’t be dusty or damaged in any way or else it can affect your furnace’s performance significantly.
Read: Lennox Furnace Error Codes – Troubleshooting Guide
6. The Thermostat Has Malfunctioned
If the thermostat is functioning correctly, then you are going to hear a few sounds when you set the thermostat to a higher temperature.
If standing near the furnace, you will hear the fan, the swoosh sound of the ignition process, and, a few seconds after, the sound of the blower motor moving hot air through the ducts.
However, if you don’t hear anything happen, it’s possible that your thermostat is on the fritz.
A faulty thermostat can cause numerous problems with your Lennox furnace, including a failure to ignite.
Here are the common signs of a damaged thermostat:
- The display is erratic or unreadable.
- The temperature reading in your home fluctuates excessively.
- The furnace turns on, but the house doesn’t get warm–even when set to a high temperature.
- The thermostat is displaying an unusually high or low temperature.
How To Fix
If the thermostat is broken, it needs to be replaced. You can do this independently or hire a professional HVAC contractor to do the job for you.
If you are simply replacing the bad thermostat with a nearly identical new one, the job is very straight-forward. However, if you are getting a different style thermostat, sometimes the wiring terminations can be non-intuitive and not so easy to figure out.
Pro Tip: Check if your thermostat requires calibration. This ensures that it accurately displays temperature changes and facilitates proper furnace ignition. Look in the user manual (if you don’t have one, you can usually look up your manual online).
Related: Why Does My Thermostat Say HEAT ON, But There Is No Heat?
7. The Pilot Light Is Out
Most of the newer models use spark ignition. However, if you have an older Lennox furnace, then it might have a pilot light instead.
It is extremely simple to figure out whether your unit has a pilot light or not.
Such furnaces have a small round knob on the gas valve that says ‘off’, ‘on’, and ‘pilot’.
Lennox Pilot Light Location
Open the cover to the burner assembly and look toward the bottom. You will see a knob that says off/on/pilot.
There, you’ll find three tubes that are attached to the base of the gas valve housing. One of them travels to the gas burners, while the other two (they are typically smaller) will lead you to the pilot light.
Of course, it is relatively easy to spot a small blue flame, if the pilot light is burning. But if it’s not – use a flashlight to locate the end of the pilot tube (that’s where the pilot orifice is).
When you turn on your Lennox furnace, the pilot light ignites first; then, the entire heating cycle is initiated. Therefore, if there’s no pilot light (i.e. it has gone out), then nothing will happen — even when you turn the system on.
How To Fix
This problem can be frustrating to deal with at times, but it’s not hard to fix. Clearing the pilot light and restarting your furnace should only take a few minutes of your time.
Always read the instructions that you’ll find on the blower access panel.
- Set the thermostat to the lowest possible setting.
- Turn off the furnace and remove the burner access panel.
- Locate the gas valve knob, push it in, and turn clockwise to ‘off’.
- Wait for 5 minutes, so that the system has enough time to get rid of any leftover gas.
Warning! If you smell gas after this step, call your gas supplier or HVAC tech.
- Behind or under the gas valve, you’ll see the pilot access panel – remove it.
- Follow the tube from the gas valve to locate the pilot.
- Turn the gas valve knob counterclockwise to ‘pilot’.
- Push in the gas valve knob and hold it while you try lighting the pilot with a long match.
- Continue holding the knob for around 30-60 seconds, after you have lit the pilot.
Tip: Did the light go back out? Repeat the process.
If the knob didn’t pop up once you released it or the pilot won’t stay lit even after a few tries, then read section #2 (Furnace Isn’t Getting Gas) above or call your gas supplier.
- Put the pilot access panel back in place, turn the knob counterclockwise to ‘ON’, replace the burner access panel, and turn the unit on. Don’t forget to change the thermostat settings.
Read: Furnace Ignitor Lights, But There Is No Flame
Here’s an excellent video that demonstrates how to light a furnace pilot light:
8. Spark Ignitor Is Bad
If your Lennox furnace doesn’t have a pilot light, then it uses spark ignition. This operates much like the ignitor on a gas cooktop or barbecue. It will make a clicking noise when it’s trying to ignite the burners.
Here are a few signs of a faulty spark ignitor.
Symptoms Of A Bad Spark Ignitor
- The furnace stops operating: If the unit won’t turn on at all, then the chances are high that there is a problem with the ignition.
- The furnace blows cool air: If your thermostat is set to heat, but the unit is blowing cold air, then it looks like the ignitor is not lighting the burner.
- The furnace is short-cycling: Short-cycling is when your unit is turning on and off repeatedly. It is a safety feature that indicates that something is wrong.
- The ignitor is not glowing: An ignitor is located in the same place where you would usually look for a pilot light. This element should get extremely hot, in order to ignite the gas/air mixture. However, if you have turned the furnace on, and after your hear the gas valve open, the ignitor is not glowing brightly, then there is something wrong with the component.
Related: Why Furnace Is Running But Not Blowing Air?
How To Fix
There is really no repairing an ignitor; it must be replaced. This is quite simple to do in most cases.
- Turn off the disconnect switch and remove the front cover of the furnace.
- Identify the ignitor and remove the screws (usually two) holding it in place.
- Unplug the wiring harness connector.
- Check the ignitor for cracks or other damage. If you see no visible damage, check the connector pins for continuity with a multimeter to verify that the ignitor is indeed bad.
- Reverse the steps to install the new ignitor.
Lennox Furnace Ignitor Cleaning
In most cases, the ignitor will continue working even if it is dirty. But the element won’t get as hot if it’s covered with carbon deposits.
You might try cleaning the ignitor with compressed air. You can find such cans in stores and online; they usually come with long ‘straws’ which make it easy to apply the air directly to the ignitor.
You can also try manually cleaning your Lennox furnace ignitor.
Simply pull off the wires, loosen the screw, and gently pull the ignitor out. Carefully rub the ignitor probe with an emery cloth or fine steel wool.
Tip: Don’t touch the ignitor with bare hands as the oil from your fingers might damage the element.
9. The High-Limit Switch Is Open Or Damaged
Limit switches are used to limit the temperature of your furnace. For example, if the temperature surpasses a certain point, you can program the system to shut off automatically. If this part is broken or working incorrectly, it may be causing issues with your furnace’s ignition.
How To Fix
You need to replace your unit’s limit switch to fix this problem. You can do this on your own if you’re fairly handy.
For the best results, consult your furnace’s user manual or call customer support to determine the appropriate switch to use. Since different limit switches have different settings and tolerances, it is important to get the correct one for your furnace.
Related: How To Reset A Lennox Furnace
Conclusion
Whether you’re troubleshooting ignition problems or simply need to maintain your Lennox furnace, there are a few easy-to-recognize warning signs to tell you when something is wrong.
And remember: Keeping a clean furnace will go a long way to prolonging the life and efficiency of your system, thereby saving you money in the long run.
Moreover, heating systems should be checked overall at least once every year by a professional HVAC contractor. This ensures everything is working correctly and no safety issues are lurking around, which could affect you or your family members adversely.