Just when you thought you could make it through the end of winter in comfort, your furnace has begun behaving erratically. You can hear the furnace starting up, but the unit seems to shut down just as the heated air starts to warm the rooms or even before the air has a chance to make its way through the home’s venting. What causes a gas furnace to start then stop? Repair Clinic has five likely causes you should troubleshoot, but let’s review how a properly-functioning gas furnace is supposed to operate first.
How a gas furnace is supposed to function
Gas furnaces have a lot of components that need to work together to successfully heat your home. Let’s start with the home’s thermostat.
The home’s thermostat gives the orders
The thermostat on your living room or hallway wall is wired directly to the furnace’s control board. Increasing the temperature setting on the thermostat is what gives the order for the furnace to start the process of heating your home. When you increase the thermostat’s temperature setting, or the temperature of the room air drops below what the thermostat is set at, the furnace goes through a timed ignition sequence. This sequence begins with the control board sending 120 volts of alternating current to the furnace’s draft inducer fan motor. The job of the draft inducer fan motor is to draw outside air into the heat exchanger.
A pressure switch ensures that toxic fumes won’t spread inside the home
Once the draft inducer fan motor reaches its maximum speed, a pressure switch will close. The pressure switch ensures that the gas valve will not allow gas to enter the burner assembly unless the toxic fumes can be safely exhausted to the outside of the home. If the pressure switch malfunctions, it’s possible that carbon monoxide can mix with the air being forced through the home’s venting. This is why it’s always important that you keep carbon monoxide detectors in each bedroom in your home and change the batteries in the detectors regularly. Once that pressure switch closes, 24 volts will travel through the switch, and one or more limit switches, allowing the control board to continue the ignition sequence.
A glow-bar igniter or spark igniter ignites the flame
The next step in the sequence has the control board sending voltage to an igniter. Depending on the model, your furnace may use a glow-bar style igniter or a spark igniter. If it’s a glow-bar igniter, the control board will send 120 volts of alternating current to the component; a spark igniter will receive even more voltage. As its name suggests, the glow-bar igniter will become hot enough to glow once energized by those 120 volts. When the voltage reaches a spark igniter, it will begin to pulse and, that’s right, create a spark. The control board will then 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 by the glow-bar igniter’s heat or the spark igniter’s spark.
A circulation blower fan motor sends the heated air through the venting
With the ignition sequence successfully initiated, it’s time to heat the air throughout the home. This starts with the control board sending 120 volts of alternating current to a circulation blower fan motor. A blower wheel fan attached to the motor shaft will draw air through the return vent, blow it past the heat exchanger, and force the now-heated air through the venting to spread the heat throughout the house. This process will continue until the room temperature rises to the designated setting on the thermostat. At that point, the furnace will shut off until the thermostat detects the room temperature has begun to drop.
The 5 reasons why your gas furnace starts then stops
A gas furnace that starts then stops unexpectedly is likely the result of one of five possible causes you can troubleshoot and, potentially, fix yourself:
- Malfunctioning flame sensor – The furnace’s flame sensor monitors the burner to detect whether or not a flame is present. A malfunctioning flame sensor may fail to detect the flame, causing the control board to shut off the voltage to the gas valve which will stop the furnace from heating. If the flame sensor is dirty, you can try cleaning it with a fine abrasive pad. If suspect the sensor is still not functioning after cleaning, you should replace it with a new one.
- Faulty pressure switch – Adequate air flow through the draft inducer fan motor housing will cause the pressure switch to close, allowing the ignition process to continue. A faulty pressure switch may be unable to close, stalling the ignition process and shutting off the furnace. You can use a multimeter to determine if there is continuity, a continuous electrical path, present in the switch, provided the switch is connected to the furnace and the draft inducer fan motor is running. If there is no electrical continuity, you’ll know the switch has failed and will need to be replaced.
- Defective control board – The control board regulates the power supply to all of the furnace components. A defective control board may not send voltage to the ignition system, causing the furnace to shut off after a few minutes. While a control board can be difficult to test, you can inspect the board for any burn marks or evidence of a shorted component.
- Faulty draft inducer fan motor – The draft inducer fan motor draws air into the heat exchanger and then exhausts that air out of the flue. Since proper air flow is required for the pressure switch to close and the ignition process to begin, a faulty draft inducer fan motor unable to maintain proper air flow can stall the ignition process and shut the furnace off after a few minutes. You should listen to the motor just as the furnace starts up. Are you hearing a buzzing, grinding, or screeching sound? Then it’s likely the motor bearings are failing and you will probably need to install a new draft inducer fan motor.
- Air flow problem – In order to continue running, the furnace requires sufficient air flow through the burner assembly. Poor air flow is often the result of insufficient ventilation or a restricted inlet or exhaust vent. Make sure the furnace is in a properly ventilated location free of boxes or other items placed near the furnace cabinet and check that the vents are clear of obstructions.
You can fix the furnace yourself with help from Repair Clinic
As your repair partner, Repair Clinic has the resources you need to fix the furnace yourself. Looking for a procedural video showing the correct way to replace a broken flame sensor on a Model TG8S080B12MP11A York Furnace? Or do you need some guidance showing how to install a new pressure switch on a Model GMUH75E3A Rheem furnace? Visit Repair Clinic’s vast content library of videos, step-by-step guides, articles, and schematics to unlock the knowledge you need, whether you’re a do-it-yourselfer or a professional technician.
A major part of successfully fixing a furnace is using only genuine manufacturer (OEM) parts for the repair. OEM parts are specifically designed by the manufacturer to work with the furnaces that manufacturer makes. Whether the furnace you’re attempting to repair is made by Bryant, Lennox, Goodman, Rheem, York, Coleman, Ruud, Heil, or Payne, you’ll find the exact part you need at Repair Clinic. Enter the full model number of the unit in the Repair Clinic website’s search bar for a complete list of parts compatible with the furnace you’re fixing.