The thought of the furnace failing to ignite during the coldest winter months is enough to give one the chills… just not as chilling as when the furnace actually does fail to heat up. One way to prevent this occurrence is to perform regular furnace maintenance including replacing the air filter at least twice a year, using a vacuum with a long attachment to clean out the interior of the furnace, and always making sure to keep the area around the furnace well-ventilated (for example: if the furnace is located in a closet, the closet doors should be louvered, or space left at the bottom of the doors, to ensure good airflow).
Still, even the best furnaces can go kaput after years of dependable use. What’s the number one reason for a furnace not igniting? That’s right, it’s a faulty igniter.
How the furnace igniter works
While the igniter may be the main culprit for a furnace failing to heat, it is just one part that has to function properly in the furnace’s timed ignition sequence. This sequence begins when the home’s thermostat calls for heat, prompting the circuit board to send 120 volts of alternating current to the furnace’s draft inducer fan motor. The inducer fan motor draws air into the burner assembly and, when the fan motor reaches its maximum speed, a pressure switch is closed to ensure that gas will not enter the furnace unless the toxic fumes can be exhausted safely. Once the pressure switch closes, 24 volts will travel through the switch, and one or more limit switches, allowing the circuit board to continue the combustion sequence.
The next step in the sequence has the circuit board sending 120 volts of alternating current to a glow-bar style igniter or a greater amount of voltage to a spark igniter. The glow-bar style igniter will begin to heat up and will literally begin to glow orange, whereas a spark igniter will begin to pulse to create a spark. At this point, the circuit board will send 24 volts to the gas valve, opening it and allowing gas to flow over the igniter where the gas is ignited into a flame.
Once the combustion sequence is successfully initiated, the circuit board will send 120 volts of alternating current to the circulation blower fan motor. The blower fan will draw air through the return vent, blow it past the heat exchanger, and force the now-heated air through the home’s venting to keep you comfortable regardless of how chilly the weather is outside.
Why won’t my furnace ignite?
As noted above, a faulty igniter is the number one reason the furnace won’t work when you expect it to. The igniter may be too weak to ignite the gas or it may lack the necessary electrical continuity to function properly. How can you tell if the igniter is faulty? You should first inspect the component for cracks in the element or base as this is a strong indicator that the part is defective. You can also use a multimeter to test the igniter for “continuity” – a continuous electrical path present in the part. If the igniter has good electrical continuity, the multimeter reading should show a result between 20 and 400 Ohms of resistance. A higher result could mean the igniter has weakened and a completely negative continuity result will mean the part has failed completely.
If the igniter appears to be in good shape and shows good electrical continuity, then it’s likely one of the other ignition sequence components is defective. Here are six additional parts you should consider inspecting:
- Flame Sensor – The flame sensor monitors the burner to detect whether or not a flame is present. A malfunctioning flame sensor may fail to detect the ignited flame, causing the control board to shut off voltage to the gas valve and prevent the furnace from heating. Since the flame sensor will collect carbon build-up after years of use, you can attempt to clean the sensor’s sensing rod with an Emery cloth or a fine abrasive pad to restore its ability to detect a flame. If the component is still malfunctioning, you can easily replace it with a new one.
- Draft Inducer Fan Motor – A defective draft inducer fan motor may be unable to maintain a consistent air flow, stalling the ignition process. Before you consider replacing this component, you should make sure that the area around the furnace is clear of boxes or other items that could impede ventilation. You should also confirm the exhaust vent is free of obstruction.
- Control Board – If the control board fails, it might not send voltage to the ignition system components. As with all electronic components, a control board can simply burn out after years of use. This is especially true of the transistors on the board. The heat generated by the furnace can also cause the solder joints on the board to expand which can create breaks in the connection. Additionally, since a furnace is normally only in use during the colder months, the relay switches on the board may become stuck in the open or closed position due to the furnace’s inactivity during warmer weather.
- Flame Roll-Out Switch – The flame roll-out switch monitors the heat surrounding the burner. If there are air flow problems, the switch will open and halt the ignition process. However, if the switch is defective, it may open even if the air flow is good. Most roll-out switches will have a reset button you should press to see if you can reset the switch. As with the igniter, you can also use a multimeter to test the switch for electrical continuity to determine if the part has failed.
- Gas Valve Assembly – If the gas valve assembly malfunctions, it may be unable to allow gas to enter the burner. As with the igniter and flame roll-out switch, you can use a multimeter to determine if the valve has electrical continuity or not.
- Pressure Switch – Proper airflow through the furnace causes the pressure switch to close so the ignition process can continue. A defective pressure switch may be unable to close. Again, you can use a multimeter to test the switch for electrical continuity, provided the switch is connected to the furnace and the draft inducer fan motor is running.
Find the right igniter for your furnace and more with Repair Clinic
Looking for a replacement igniter for your furnace? Repair Clinic makes it as easy as finding the right flame sensor, pressure switch, draft inducer fan motor, gas valve, or control board. Enter the full model number of your furnace in the Repair Clinic website search bar, then use the part category and part title filters to locate the specific part you need. Regardless of which replacement part you’re looking for, you’ll want to make sure it’s one that works with your particular unit, whether you own a Bryant, Lennox, Goodman, Rheem, York, Coleman, Ruud, Heil, or Payne heating and cooling product.