Why is My Furnace Not Working?

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You wake up at 4 a.m. curled up under layers of blankets and comforters, shivering even though you set the thermostat to a pleasant 68-degrees Fahrenheit just before you came to bed. As you speak the words “What the…?”, you can see your breath exiting your mouth. There’s no doubt about it; the furnace has stopped working.

If the furnace isn’t heating as expected, is there something you can do to fix the problem yourself? Quite possibly. But before we tackle the solutions, it’s a good idea to review how a furnace in good working condition is supposed to operate.

The short version of how a furnace operates

When the home’s thermostat calls for heat, the furnace goes through a timed ignition sequence that is initiated when the circuit board sends 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 which ensures 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.

Depending on the furnace model, 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 ensure you won’t wake up freezing in the middle of the night.

7 reasons why your furnace won’t heat

So, the furnace has a lot of working parts that need to work together. Here are the top seven parts that, if defective, will likely prevent the furnace from heating, and all of them can be replaced by the average homeowner:

  1. Faulty Igniter – If the igniter is faulty, it will be unable to ignite the gas into a flame to create heat. You should inspect the existing igniter for cracks in the element or base. You can also use a multimeter to test the igniter for “continuity” – a continuous electrical path present in the part. If the igniter tests negative for continuity, it has definitely failed and will need to be replaced.
  2. Malfunctioning 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.
  3. Defective 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.
  4. Failed 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.
  5. Defective 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.
  6. Malfunctioning Gas Valve Assembly – If the gas valve 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.
  7. Defective Pressure Switch – Proper air flow 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 furnace parts with Repair Clinic

Repair Clinic makes it easy to find the exact furnace parts like an igniter, flame sensor, pressure switch, draft inducer fan motor, gas valve, or control board that works with your furnace. Simply 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’re looking for. Regardless of the part’s function, you always need to identify the right component for your particular unit, whether you own a Bryant, Lennox, Goodman, Rheem, York, Coleman, Ruud, Heil, or Payne heating and cooling product.

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