Unfortunately, it happens more often than you can imagine. Perhaps you’re just getting home from work on a cold winter evening. So, you head straight to the furnace and switch it ON – but nothing happens.
On checking inside the furnace, you find that the pilot light is ON, but the burners aren’t. What could be wrong? Is it a problem with the burners? Could they be blocked? Do you need to call the technician? These are just some of the questions that may run through your mind at that moment.
Below, we discuss the most common reasons your burners may be off even when the pilot light is ON and what you can do about the situation.
First, though, let’s try to understand how the furnace works from the moment you switch it ON until the burners ignite to provide the heat necessary to warm your home.
How the Furnace Heats
We’ll assume that your furnace is plugged ON and you have power. We’ll also assume that the breaker works normally and it’s ON, as is the main gas valve (the one on the wall next to the furnace). The gas valve is ON if it’s parallel to the gas line.
When you press the ON button on your furnace, the following chain of events ensues, leading to heating;
- The draft inducer motor engages. It’s the first part of the furnace to start when you turn on the appliance.
- The thermostat checks whether you need heating. You need heating if indoor temperatures are lower than the thermostat setting (typically 68°F to 72°F).
- If #2 is true, the thermostat calls for heating.
- The control board performs preliminary checks. This process sets the stage for heating by ensuring that the furnace isn’t locked out and that there aren’t any other safety concerns to prevent heating. If so, the actual heating process begins. Steps #3-#8 apply to electronic ignition systems only. Skip to #9 for traditional standing pilot systems.
- The control board engages the ignition system, allowing voltage to flow through the ignition system.
- The ignition system sparks or becomes red hot, depending on your ignition system.
- The control board opens the pilot gas valve, allowing gas to flow to the pilot.
- Instant combustion occurs, lighting the pilot system instantly.
- The control board verifies that the pilot light is stable and healthy.
- The board opens the gas valve to the burners.
- The burners ignite.
- A flame sensor continuously monitors the burner area to ensure that a flame is present and the flame is healthy. If so, heating continues uninterrupted until the ON cycle ends or the furnace encounters setbacks that force it to shut down mid-way. Common issues that may force the furnace to shut down abruptly include overheating and flame rollout.
Why’s the Pilot Light Lit, But Not the Burner?
The burner can be off even when the pilot light is lit for several reasons. The following are the most common causes and recommended solutions;
1. You’re Using a Standing Pilot Furnace
Before you begin to panic, you may want to check the type of furnace you’re using. If it’s a standing pilot unit, nothing is wrong, and your furnace is acting normally. That’s because the pilot light is lit 24/7 in standing pilot systems, even when the thermostat isn’t calling for heating.
It only goes off when you shut off the furnace at the breaker and turn off the gas at the main valve (the one on the wall next to the furnace). This is unlike electronic ignition systems, where the pilot light only comes on when the thermostat calls for heating.
Solution: You don’t need to do anything if the thermostat isn’t calling for heating. However, you have a problem if the thermostat is calling for heating and the pilot is ON, but the burners won’t come on, you have a problem.
2. The Thermostat is Drifting or Dead
This also only applies to traditional standing pilot furnaces. It’s a little challenging to know if the thermostat has malfunctioned when using a standing pilot furnace because the pilot light is ON throughout. So, you may blame other parts of the furnace when it’s indeed a thermostat problem.
For instance, imagine that the furnace is ON and set to AUTO, but the thermostat is drifting or dead. The burners won’t come definitely because the dead thermostat cannot call for heating.
However, it becomes difficult to point out the issue because the pilot light is ON. You may even mistakenly blame the burners.
Solution: Check whether the pilot is working and accurate. Can you see readings on the screen? Are the readings accurate and steady? If not, maybe you need to change the batteries, repair the thermostat, or replace the thermostat.
3. The Furnace is in “Lockout”
It’s also not easy to know if you have a furnace lockout when using a traditional standing pilot furnace. Again, the 24/7 pilot system can be misleading.
The light is ON throughout even if the furnace is in a lockout, unlike electronic ignition systems in which the pilot system cannot ignite if the furnace is locked out.
Lockout essentially means that the furnace is non-responsive to prevent gas poisoning, fires, or other hazardous events.
Solution: Fix the cause of the lockout, then reset the furnace. Lockouts typically result from overheating, flame rollouts, leaking gas, and blocked filters.
NB: The rest of the reasons here apply to all furnaces – both standing pilot and electronic ignition models.
4. The Burners aren’t Getting Gas
First, ask yourself whether the burners are even getting gas in the first place. And, no, the fact that the pilot is lit doesn’t mean the burners are getting gas.
What if the gas lines are blocked at the section between the pilot supply and burner supply? What if the valve controlling the burners is compromised?
Solution: We strongly recommend calling an HVAC professional if you suspect that the burner line or valve is blocked or leaky. Switch off the furnace at the breaker, turn off the gas at the main (on the wall), and call an HVAC pro.
5. The Burner Jet and Orifices are Blocked
Still, on gas supply, the burners cannot light if the orifices (small holes through which gas gushes out of the burner) or the burner jet (the metal piece that supplies the orifices) are blocked.
Both can become blocked due to dirt buildup or corrosion. Dust is another common culprit.
The two parts can become wholly or partially blocked.
Solution: You can attempt to DIY clean the burners, orifices, and even the burner jet. If so, use compressed air to blow out the dirt. However, an even better idea is to call an HVAC technician to look at the damage.
6. The Flame Sensor is Compromised
The flame sensor’s sole purpose is to make sure that there’s a flame in the burner area. Why’s this important? Because it would be dangerous to supply gas to the burners if there’s no flame. It creates a massive risk of poisoning and fire.
So, if there’s none or if the existing flame is too weak, the sensor informs the control board, which cuts off the burner’s gas supply within 10 seconds at most. Unfortunately, a defective flame sensor can erroneously report a missing flame even if the burners are lit, causing the burner to go off.
Solution: You should begin by cleaning the flame sensor to rid it of dust and carbon that usually settles on the surface. If that doesn’t work, replace the sensor.
7. Electrical Issues
Finally, your burners could also be off because of electrical malfunctions. You’re probably wondering how. Well, the furnace’s control board controls all activities within the furnace, including the burners. Indeed, the board has circuits dedicated to the burners.
If the circuits are interrupted or broken, the burners can remain off even if the pilot light is lit. Notably, this can happen in both standing pilot and electronic ignition furnaces. Common causes include overheating in the furnace, electric short-circuits, and physical damages to the circuitry board.
Solution: We strongly recommend professional input here, given the delicate nature of the furnace’s electric circuitry. The technician will essentially test whether the circuits are complete and recommend repairs or even replacements to fix the problem.
That’s all. Now you know common reasons why the furnace burners can be off even with the pilot light lit. The most important thing is to remain calm.
However, if you feel uneasy, shut down the furnace at the breaker, turn off the gas supply at the main valve, and call our HVAC technician.
We must also stress the need for scheduled professional maintenance and regular maintenance. Some of the above issues, such as blocked burner jets and orifices can be easily prevented by cleaning the furnace regularly and scheduling a tune-up at least once before the heating season begins.