Switching on the furnace initiates several processes before hot air is sent through your building. The ignitor starts to glow followed by a spark that helps to create flames. When the furnace ignitor stays on at all times, it may be a sign of an underlying problem in your HVAC system.
If the furnace ignitor won’t turn off, you need to look at the control unit, consider how the ignitor is wired to the main system, and see if the relay is stuck. Fixing faulty wiring, replacing the control unit, and cleaning the relay should solve the problem.
In this post, I’ll explain possible reasons why a furnace ignitor won’t turn off. I’ll also walk you through how to fix each issue.
1. The Control Unit Is Faulty
When the furnace ignitor won’t turn off, start by looking at the most common reason behind the issue. This means you should begin by inspecting the ignitor control unit.
This control unit consists of a circuit board. The board sends a signal to the ignitor when the furnace is turned on. This signal causes the ignitor to turn on. When sufficient heat is available in your home, the control unit continues to regulate the function of the ignitor, turning it off when the additional heat is no longer needed.
If the control unit is faulty, there may be a problem with the communication between this unit and the ignitor. Perhaps the control unit is sending a signal to turn the ignitor on. However, once it’s on, it doesn’t continue to regulate its current power.
When this happens, the ignitor remains on, even when you reduce the temperature or switch the blower off. The ignitor may remain glowing when the thermostat is off.
How To Fix
There are two main ways to detect problems with the ignitor control unit.
The first is to look if you can see any physical problems with the unit.
If this process doesn’t yield any results, you may have to use a voltmeter to determine if there is a problem with the control unit’s power supply.
Here’s what you need to do:
- Open the furnace panel. Before removing the panel or door, switch the furnace off.
- Find the control unit that attaches to the ignitor. The unit is generally small yet plays a significant function in the operation of the ignitor and furnace.
- Perform a physical inspection of the unit and the board attached to it. See if all wires attached to this board are intact. Take note of any damage to the board itself. This may include small cracks or parts that look burnt.
- To determine if there’s a problem with the power supply, use a voltage meter or a multimeter. See if you can get a reading at the power cables that run to the board. Also, check the power cables that run to the ignitor.
If you switch power to the ignitor off, yet you still get a voltage reading when testing the wires that attach the switch to the furnace, that’s a problem.
Replacing the control unit is a simple and effective solution for this. Note that this board also controls other functions of the furnace. Be sure that all connections are replaced after installing the new board.
This video can be helpful if you plan to replace the control board in your furnace:
2. Ignitor Wiring Is Bad
Wiring can be another factor to blame when the ignitor isn’t turning off. The number of wires that run to the ignitor depends on your furnace model.
Some furnaces have more wires than others. Many modern ones use more of a “plug-and-play” solution — this makes it easier to remove or attach cables to the ignitor and other components of the furnace.
Various wiring issues can cause a furnace ignitor not to turn off. This may include wires not connected to the right switches or connectors on the ignitor and control board. The issue may also lie with loose or damaged wires.
How To Fix
Although a relatively uncommon problem, it’s still important to rule out the possibility that the wiring in the furnace isn’t right.
- Turn off all power to the furnace before opening the front panel that gives you access to the interior compartment.
- You need some kind of reference to use during this step. If you don’t call a professional to help you out, you’ll need to have some way to identify how the wiring should be done. If you have the owner’s manual, refer to it. Alternatively, see if you can find information on the internet about your furnace.
Make sure that all the wires are in the correct slots. Also, check if you can see any damage on or around the wiring. This includes wires with broken insulation on the exterior.
If there are problems with the wiring, address them. Once completed, see if the ignitor turns off after running for a while now.
3. The Relay Is Stuck
Another reason why your ignitor isn’t turning off can be related to the relay. There is a relay in the control switch that’s used to switch the ignitor on and off.
When the relay switch is stuck or faulty, it does not regulate the ignitor’s current operation properly. This situation can lead to the ignitor not turning off when it should — in which case you see a glow continuously, even when the temperature is set to a lower setting.
A stuck relay could mean problems with the board. It could also be a simple issue that a simple reset would fix.
How To Fix
You’ll first remove the relay switch to determine if this is where the problem lies. The specific procedure to follow depends on the furnace model you have. Look for physical damage to the relay switch and see if it’s stuck.
Micro-welding can happen in the contacts found in the relay switch. This is caused by a consistent opening and closing of the contacts. In such a case, a current is constantly delivered to the ignitor. This means the ignitor won’t turn off as there is a constant power supply sent to the hardware.
To clean the relay contacts, you can try using an appropriate tool, such as a burnishing tool. Cleaning the contacts can sometimes get them unstuck, allowing the contacts to close when the ignitor’s power supply needs to be cut.
The video below gives you more details on how to diagnose a broken relay and what you can do to fix this issue:
Summary
A faulty control unit is a common reason why a furnace ignitor won’t turn off, but the problem can lie with other parts of the furnace too. Be sure to check all possibilities, including the furnace’s wiring.
A professional can quickly get to the root of the problem and ensure your furnace won’t run into future problems due to an ignitor not turning off. Hiring a professional is easy with our system. Start by filling out the form below to tell us about your location and needs.
Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he’s not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.