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Your high efficiency condensing furnace is a high technology machine that lets you heat your home comfortably and with low energy consumption. But like any machine, it can sometimes stop working, and when that happens it usually means a call to the heating and ventilating service repair shop.
But if your furnace is not firing up, there are actually a number of things you can check or repair yourself before you need to call a service pro. These repairs range from the simple to the more technical. If you're not comfortable with any of the following tips, then please call a service professional, but most are things the weekend-warrior can tackle with ease.
The following list identifies the most common reason a high-efficiency furnace does not run, listed in order of likelihood. Details of each item are described on the following pages:
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- Faulty Thermostat
- Furnace Power Switch is Off
- Clogged Furnace Filter
- Faulty High-Limit Switch
- Electronic Ignition: Faulty Hot Surface Igniter
- Electronic Ignition: Faulty Flame Sensor
- Faulty Pressure Switch(es)
- Clogged Condensate Line to Floor Drain
- Obstructed Furnace Flue Vent Pipe
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The thermostat controlling your high-efficiency furnace is the first place to check. Make sure it is properly operating and calling for heat. Chances are you have a digital thermostat if you have a high-efficiency furnace.
Possible problems could include:
- The circuit breaker has tripped or fuse is blown.
- Thermostat components are dirty.
- Loose wires or open connection.
- A bad battery on a digital electronic thermostat.
See the tutorial Troubleshooting a Home Furnace Thermostat for more information.Continue to 3 of 10 below.
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Furnace Power Switch is Off
One of the most common (and embarrassing) reasons for the furnace not running is that it has inadvertently been shut off. There is a switch usually found on the side of the furnace, make sure the switch is in the ON position.Continue to 4 of 10 below.
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Clogged Furnace Filter
Believe it or not running your furnace often with a severely clogged furnace filter can cause more damage than just circulating dirty air in your home. It can also cause the furnace to overheat and frequently cycle a safety switch—called a high-temperature limit switch—on and off, eventually causing the limit switch to fail.
Check your filter, which may either be a fiberglass or paper media furnace filter or a high efficiency pleated paper media furnace filter, and replace as necessary.
Once the clogged furnace filter has been replaced, turn the furnace switch off for ten minutes and then turn the furnace switch back on. This should reset the furnace electronics in the computerized ignition module.Continue to 5 of 10 below.
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Faulty High-Limit Switch
Note: If you have proceeded to this point or any further in this tutorial, it will require removing the front of the furnace cover. As a result, make sure to turn off the power to the furnace at the appropriate branch circuit in the main power panel if the front furnace cover panel is removed.
The high-temperature limit switch is a safety device that monitors the internal furnace temperature and shuts down the gas valve to the burner if temperature limits are exceeded. Once the temperature drops to the lower limit of the switch, the switch allows the gas valve to turn back on, allowing the burner to operate again.
As mentioned in the previous page, the high-temperature limit switch may become damaged if subjected to frequent tripping due to overheating and low air flow from a severely clogged air filter. A very dirty air filter will cause low air flow and heat buildup near the combustion chamber, which will cause the switch to cycle on and off frequently. This eventually causes the switch to fail completely from frequent overheating, or to get stuck in the off or "open circuit" position.
You can test the switch by using a multi-meter to check for continuity:
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- Set the multimeter to Ohms (resistance).
- Disconnect and note the wires that go to each switch terminal.
- Place a probe on each of the two switch terminals on the switch's faceplate located on the outside of the furnace.
- If the resistance is 0 Ohms the switch is good, if the resistance is infinite then the switch is bad.
- Replacement is easy, just unscrew the switch plate and replace with the appropriate replacement part.
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Electronic Ignition: Faulty Hot-Surface Igniter
Your high-efficiency condensing furnace most likely uses a special electronic ignition device called a hot surface igniter (HSI), which may have cracked or broken apart.
The HSI is usually made from a material such as silicon nitride or silicon carbide (more fragile), and as electricity passes through the igniter it will get hot and glow red when 120 volts are applied—similar to an incandescent light bulb. The hot-surface igniter is placed in the flow of gas entering the burners to ignite the fuel gas.
Under normal conditions, the HSI should last for three to five years. However, it will eventually crack and need to be replaced—quicker if the oils from your skin get on the element by improper handling. These hot surface igniters cost between $25 to $50, depending upon the model of furnace you have.
These, too, are easy to replace and are usually attached by a screw or clip. Remove the attachment and wire leads and replace with the appropriate replacement part. Make sure not to get oil from your hand on the filament.Continue to 7 of 10 below.
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Electronic Ignition: Faulty Flame Sensor
Complementing the HIS on an electronic ignition system is the flame sensor, which is used to determine proper burner operation. Once a proper flame is sensed by the flame sensor, the HSI is shut down.
You should note that on some furnace designs, the HSI also acts as a local flame sensor instead of having a remote sensing rod in the burner flame, as the above photo shows.
The flame sensor can sometimes fail due to corrosion; other times it just dies like any other piece of hard-uses equipment.
Replacement of a removable type flame sensor is a simple repair. See "Replacing an Electronic Flame Sensor" for more information.
You can also try to just remove and clean the flame sensor with emery cloth sandpaper or fine steel wool such as #0000.Continue to 8 of 10 below.
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Faulty Pressure Switch
The furnace pressure switch is another device to check. The pressure switch is safety device located near the draft inducer motor that will prevent operation of the furnace if correct venting air pressures are not detected.
Sometimes it gets stuck open or can fail because of improper exhaust gas flue venting, clogged condensate drains, clogged switch tubing, or other reasons.
To test the switch see the tutorial "Troubleshooting a Furnace Pressure Switch".
Also, check for a clogged hose port at the draft inducer, or cracked or broken hose(s) running from the pressure switch to the draft inducer or condensate collector box, which can cause a tripped switch (failure) condition.Continue to 9 of 10 below.
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Clogged Condensate Line to Floor Drain
A seemingly remote problem to check for is a clogged furnace condensate drain line (see photo). A clogged line can cause a pressure switch failure because the pressure switch senses the accumulation of condensate in the furnace drain pan. The furnace will not operate until the condensate drain has been cleared and the condensate flows freely.Continue to 10 of 10 below.
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Obstructed Flue Vent Pipe
Lastly, another seemingly remote problem than can indirectly stop the furnace from running is a clogged or obstructed flue vent pipe. An obstructed exhaust flue gas vent pipe can also cause the pressure switch to trip.
Ensure the pipe is properly sloped and free of obstructions.