woman trying to get warm cupping hands with blanket because furnace wont turn on

A warm home is something we take for granted until the furnace won’t turn on. Furnaces pick the worst times to go out: the coldest month of the year, an hour before your party guests arrive, and the day before you leave on vacation. Sometimes, you can’t wait until a professional inspects your furnace. You have to solve the issue yourself.

An untrained homeowner can quickly resolve some furnace problems. Simple tasks such as flipping a switch, changing a battery, or swapping out a filter might be enough to restore heat. Try these ten troubleshooting tips when you need your furnace up and running quickly.

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How To Troubleshoot When Your Furnace Won’t Turn On

Try these troubleshooting measures if your furnace won’t turn on. Know your limits, and only attempt tasks that you can safely manage. Furnace components vary across the different types of furnaces. Bear in mind that some tips might not apply to your furnace.

person checking tripped furnace circuit breaker

1. Check your circuit breakers.

Your furnace might not have power because the circuit breaker has tripped. Circuit breaker panels are typically located in utility rooms, laundry rooms, or basements. Check the circuit breaker panel for a switch labeled “furnace,” “FAU” (shorthand for a forced-air unit), or “air handler.”

  • If the switch is in the “on” position, check if your other appliances have power. If they don’t, the power might be out.
  • If the switch is on the “off” position, flip the switch to the “on” position.
  • If the breaker has been tripped, the switch will be in the center position. If you flip it in that direction, it won’t stay in the “on” position. Flip the switch to “off” and then “on” to reset it. Call a professional to diagnose the problem if the breaker trips immediately or in the near future.
man checking thermostat furnace settings

2. Check your thermostat.

The thermostat tells the furnace when to turn it on. The furnace won’t turn on if something is amiss with the thermostat.

  • Bad Thermostat

Ensure the display is on and active if you have a programmable thermostat. Replace the thermostat’s batteries. Increase the temperature by 5 degrees to induce the furnace to turn on. Some thermostats are wireless. An interruption in communication between the thermostat and furnace occurs if the thermostat is faulty.

  • Damaged Thermostat Wiring

Loose or broken wires disrupt communication with the furnace. Unscrew the thermostat, and ensure the wires are intact and undamaged.

  • Incorrect Thermostat Settings

Ensure the thermostat is set to “heat,” the fan is set to “auto,” the temperature setting is high enough for the heat to turn on, and there is no programmed schedule that prevents the furnace from turning on.

person replacing old furnace air filter

3. Replace the air filter.

A clogged air filter restricts airflow. Inadequate airflow can cause the internal furnace temperature to increase and trigger the high-limit switch. The high-limit switch shuts down the furnace when the temperature exceeds the limit.

Locate the point where air flows into the furnace from the return ducts to find the filter’s position. Slide out the filter. Note the filter’s model number and size before you purchase a replacement.

Note the arrow on the replacement filter before inserting it. Position the filter so the arrow points to the furnace and slide the filter into the slot. How often you should change the air filter varies, but filters generally need to be replaced every 30 to 90 days.

furnace gas valve

4. Check your fuel supply.

Gas furnaces require a supply of natural gas to operate. Ensure the gas line valve is open; someone working on the furnace might have closed it.

Follow the gas line from the furnace to the valve. The valve will be located within six feet of the furnace. Furnace gas valve appearances vary. The valve might have a brightly colored lever or knob or a small box with a disk.

The gas valve is open if the lever or handle is parallel to or in line with the gas pipe. The valve is closed if the lever or handle is perpendicular or crosswise to the gas pipe. Turn the handle or lever parallel to the line if it's closed.

If the valve was already open, ensure your other gas appliances, such as your stove or dryer, are operational. If they are not, the gas company might have shut off the gas supply to work on the main line, or your service might have been interrupted because of a missed payment or system error. Call the utility company for more information. A gas leak or blockage in the line can also interrupt service. If you smell gas or a rotten egg odor, call the gas company or 911 immediately.

furnace power switch

5. Check the furnace’s power switch position.

Ensure the furnace’s power switch is on. Someone might have turned it off when changing the filter, performing maintenance, or by accident.

The switch resembles an ordinary light switch. It will be close to the furnace, but the actual position varies. Check the sides of the furnace, nearby walls or studs, and ceiling and floor joists. Typically the up position is “on.” Flip the switch to “on” if it’s off, and wait for the furnace to start.

person looking at furnace flame sensor

6. Examine the flame sensor.

The furnace flame sensor is a safety device that stops the gas supply to the burners if they don’t ignite. Flame sensors become coated with soot and carbon deposits because they extend into the burners’ flames. Dirty sensors are incapable of sensing flames.

Shut off the power to the furnace using the furnace switch or the circuit breaker. Shut off the gas valve if your furnace doesn’t use an electronic control. If the furnace has recently been running, let it cool for thirty minutes before working on it.

The furnace’s manual can help you locate the flame sensor. Generally, the access panel needs to be removed with a screwdriver. The flame sensor resembles a thin, possibly bent metal rod extending near the burners. The sensor is usually mounted with a quarter-inch hex screw and attached to wires.

Remove the screw, and carefully slide the sensor out of the housing. Disconnect the sensor from the wires. If the metal rod appears blackened or encrusted, clean the sensor. If it seems damaged or broken, or if the furnace won’t turn on after cleaning it and the sensor is more than five years old, replace the flame sensor.

furnace pilot light with blue flame

7. Check the furnace pilot light.

Older furnaces have pilot lights. The pilot light’s flame constantly burns, even when the furnace is idle. The pilot light ignites the burners to initiate the heating cycles. If it goes out, the furnace won’t heat up. The pilot light is usually visible through a small window near the furnace’s base. It looks like a small blue flame.

Remove the access panel to observe the pilot light if no window exists. Look for the gas valve’s knob, which is labeled “on,” “off,” and “pilot.” The large pipe running through the valve is the gas line. The smaller tube that leads to the burner is the pilot tube. Follow the pilot tube to the end. If the pilot is lit, you will see a blue flame from the pilot's nozzle in the burner.

If there is no flame, the pilot is out, and you should relight the furnace pilot light. If the gas valve’s knob is only marked “on” and “off,” the furnace uses an electronic ignition and does not maintain a standing pilot light.

closeup of furnace condensate line and drain pan

8. Inspect the drain pan.

The condensation from condensing furnaces accumulates in the drain pan and flows away via the condensate line. Check if there’s standing water in the drain pan. If so, there might be a clog in the drain or condensate line.

Some units have condensate pumps that pump the water away from the system. Accumulated water in the drip pan triggers the pump’s float switch. The float switch is a safety device that shuts down the furnace to prevent water from spilling over the drip pan when it doesn’t drain properly.

Check if the float switch is in the up or down position. The switch should be down when the pan is dry. If the float switch is up and the pan is dry, clean around the switch and ensure it moves correctly. If the switch is up and water is in the pan, push the float down and hold it. The pump should resume operation within a minute. However, the pump is faulty and should be replaced.

man using flash light to inspect furnace blower motor

9. Check the blower motor.

The blower motor moves air through the furnace and ducts. A dirty or faulty blower motor inhibits airflow. The furnace’s internal temperature rises when there’s a lack of airflow, and the high-limit switch shuts the furnace down.

Look through the furnace’s window or remove the access panel to locate blinking LED lights on the control board. These lights blink error codes when the system malfunctions. Write down the pattern of short and long blinks. Ascertain the code’s meaning using the chart on the access panel, the furnace cabinet, or the owner's manual.

If the control board yields no answers, there are two more things to inspect. Only undertake these tasks if you are comfortable working with a high-voltage appliance. First, turn off the power to the unit with the furnace switch and the circuit breaker. Verify the unit has no power with a voltage pen before touching the furnace’s internal components. Place your hand near the blower motor, typically located on the right side of the furnace. If you sense heat, the motor has likely overheated. Carefully spin the blower fan’s blades, typically located on the left side, with your hand. The edges can be sharp! If the fan is difficult to turn, the blower is probably bad.

closeup of furnace high limit switch

10. Consider the high-limit switch.

The high-limit switch measures the temperature of the air passing over the heat exchanger and shuts down the furnace if it overheats. Overheating typically results from poor airflow through the system due to a dirty filter. Frequent tripping of the limit switch can cause it to fail.

Indicators of a faulty limit switch are:

  • Short heating cycles.
  • The blower is running without the furnace igniting.
  • The furnace is going into a hard shutdown or lockout mode.

If a lockout occurs, the furnace won't turn on until it's serviced and reset.

Hire a professional to deal with a faulty high-limit switch. The limit switch can be difficult for a novice to find and replace.

What to Do When Furnace Won't Turn On

Some furnace problems can be resolved without professional help or HVAC knowledge. Checking the circuit breaker, thermostat, fuel supply, furnace power switch, and pilot light are noninvasive tasks that don’t require touching or opening the furnace. Replacing the filter requires some contact with the furnace, but it's a relatively simple task.

If you are competent working with appliances, you might examine the flame sensor, relight the pilot if it’s out, inspect the drain pan, and check the blower motor.

While there are many things homeowners can attempt to get their furnaces operational, some issues are too big to handle, such as a faulty high-limit switch. Call an HVAC technician to assess your furnace issues if you can’t resolve them on your own.

Frequently Asked Questions

Will a furnace work without power?

Furnaces, even gas furnaces, require electricity to operate. Your furnace will not work if the power is off.

Why is the furnace not responding to the thermostat?

Common issues that can cause an interruption in the communication between the thermostat and furnace are a loss of power, incorrect thermostat settings, a dead thermostat battery, faulty thermostat wiring, an incompatible thermostat, a tripped circuit breaker, or a dirty furnace air filter.

Why is my furnace running but not blowing hot air?

There could be several causes of a furnace not blowing hot air, including issues with the thermostat, air filter, high limit switch, condensate line, fuel source, ductwork, pilot light, gas valve, burners, and flame sensor.

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