Is your furnace not kicking on when the temp drops inside your home? Your thermostat is supposed to signal the furnace to turn on when the indoor temperature drops below the thermostat’s setting. If your house is cold and the furnace doesn’t kick on, some issue prevents the furnace from responding.
A lack of heat doesn’t necessarily mean that the furnace is broken. Sometimes, a minor issue prevents the furnace from kicking on. Homeowners can often resolve problems related to the thermostat, gas, power supply, or airflow without professional help. If your furnace suddenly stops working, investigate some common causes and implement the solutions.
Why is my furnace not kicking on when temp drops?
Multiple issues can prevent the furnace from turning on. You don’t need any HVAC experience to address the problems listed below, but if you are uneasy with any of the following tasks, call a professional to assess the situation.
1. There is an issue with your thermostat.
Your thermostat tells the furnace when to turn on and off. Something amiss with the thermostat can prevent the furnace from turning on.
Change the thermostat’s batteries. Make a habit of replacing the batteries at the beginning of each heating and cooling season to avoid interruptions.
Check the thermostat’s settings. The heat should be switched on, and the temperature setting should be lower than the indoor temperature. Increase the temperature by five degrees to induce the furnace to turn on.
Verify that the correct day, time, and a.m. and p.m. settings are displayed. Check if the thermostat is running a programmed schedule. Bypass the program by punching the desired temperature and pressing “hold.”
Remove the thermostat from the wall. Look for dust accumulations behind it that might be affecting its sensors. Gently clean away the dust with a paintbrush or other soft brush. Look for damaged wires and carefully repair them with electrical tape.
2. There is an issue with the gas supply.
If your furnace is gas and not electric, gas supply issues can occur at several levels. The home’s main gas supply might be interrupted because of an unpaid bill or gas line work in the area. The furnace supply line or the furnace gas valve might be off. If the furnace runs on propane, the supply tank might be low.
Ensure the gas supply is on. Ensure your other gas appliances, such as the stove, work. Call the utility company to verify the bills have been paid and the supply hasn't been interrupted.
Make sure the furnace supply valve is open. Follow the gas pipe out from the furnace to find the valve within six feet of the furnace. The valve often has a red or yellow lever or knob. 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.
Ensure the gas valve inside the furnace cabinet is on. Follow the black metal gas pipe into the furnace to a small box with a switch or dial labeled “on,” “off,” and “pilot” if the furnace has a pilot light. Ensure the button or dial is in the “on” position.
Check the fuel in the propane tank if you have a propane furnace.
3. The circuit breaker was tripped.
Circuit breakers can be tripped when something short-circuits or by an electrical surge, a fault, or a defective fuse. Another possibility is someone turned the breaker off and forgot to turn it back on.
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 on the “off” position, flip the switch to the “on” position. If the breaker tripped, the switch would 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 again.
4. You have a defective circuit control board.
The furnace's circuit control board sends power to the ignitor and blower when it receives the thermostat's signal. The heating cycle won't initiate if the control board is defective.
In some cases, an overheated circuit board or fan motor might cause the furnace to smell like it's burning. Shut off the system and call an HVAC service.
You can check the voltage in the control board if you own a multimeter and know how to use it safely. The control board is typically located in the furnace’s lower compartment in front of the blower housing. Be extremely careful, and don’t touch any terminals to avoid an electric shock.
Verify the furnace is receiving power. Some furnaces have a diagnostic indicator light that blinks when it has power. Check the voltage across the “W” and “C” terminals on the thermostat terminal strip with the multimeter’s probes. “W” controls the heating, and “C,” the common, supplies power to the thermostat. The meter will read around 24 volts if the control board is working.
5. Your flame sensor is dirty or your pilot light went out.
The furnace flame sensor is a safety device that ensures flames are burning when the gas valve is open. If the burners don’t ignite, the flame sensor closes the gas valve to prevent a gas leak.
The flame sensor extends into the burners’ flames and becomes coated with residue. A dirty flame sensor can’t detect the burners’ flames and shuts the gas valve each time it opens at the beginning of a heating cycle.
Pilot light issues also prevent furnaces from heating up. Today, most furnaces use electronic ignitions to light the burners, but some outdated furnaces still use pilot lights. The pilot light is a constantly burning flame that ignites the burners.
Clean the flame sensor. Shut off the power to the furnace using the furnace switch or the circuit breaker. Remove the furnace access panel. 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. Gently rub the deposits on the metal rod with a lightly abrasive material such as steel wool or fine sandpaper and clean the rod. Do not rub the white ceramic housing.
Ensure the pilot light is burning. The pilot light might be 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. If the pilot is out, you must relight the furnace pilot light.
6. The ignitor is defective.
Modern furnaces use electronic ignitors, rather than pilot lights, to light the burners. A hot surface ignitor is a small metal piece that becomes red-hot. Intermittent pilots and direct spark ignitions produce sparks to light the burners.
Ensure the ignitor is functioning. If you have a hot surface ignitor, you can observe it. Shut off the furnace switch on the side of the unit, and remove the furnace’s access panel. Visually follow the thick, black gas pipe to the burners. The ignitor is behind the burners and looks like a thin, flat piece of rectangular metal.
Lower the thermostat’s temperature to initiate a heating cycle. Switch the furnace on, and watch the ignitor. After the inducer motor starts, you will hear the gas valve click open and gas hiss out. At this point, the ignitor should begin to glow red. If it doesn’t, there is an issue with it.
If you have an intermittent pilot or direct spark ignition, listen near the furnace for a clicking noise when the furnace turns on. These ignition systems produce sparks to light the burners and make audible sounds. If your furnace repeatedly clicks, the ignitor is likely defective.
7. The condensate pan is full.
High-efficiency condensing furnaces produce condensation that collects in the condensate pan and drains away via the condensate line. A clog in the pan’s drain or pipe will cause the condensate pan to fill with water and prevent the furnace from turning on.
Drain the water from the pan with a wet/dry vacuum. Look for a clog near the drain and remove it. Then, clear the line. Turn off the power to the furnace by turning off the switch on the furnace or flipping off the circuit breaker. Locate the white PVC pipe that exits the furnace and leads to the floor drain. Inspect the end of the pipe for debris, and remove it with a brush or other tool. Remove the cap near the top of the line. Flush the tube with equal parts white vinegar and hot water. Repeat the process several times if needed.
8. The air filter is dirty.
The air filter traps contaminants from the air that enters the furnace to keep the HVAC system operational and improve your home’s air quality. Dust, hair, and other debris accumulate and eventually clog the filter if it’s not often changed. Airflow to the furnace is restricted, and the furnace can overheat.
Replace the filter with a new one. 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. Filters come in various types, sizes, and MERV ratings.
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. Check the filter every 30 days, and replace it when it appears dirty.
9. There is something restricting the airflow.
If the furnace turns on, but some rooms are much colder than others, something could be restricting or interrupting the airflow through the ducts and vents.
Make sure all of the vents and dampers are open. Examine the ducts for large dents and gaps between the sections. Seal the holes with metal duct tape.
10. Your furnace is overheating or there is a malfunctioning part.
The high-limit switch measures the temperature of the air passing over the furnace heat exchanger and shuts down the system if it overheats. Overheating typically results from poor airflow due to a dirty filter, but it can also be due to cracked heat exchanger or another broken component. Furnaces have many parts that work together to produce heat. If one part malfunctions or breaks, the furnace might cease operation.
Change your air filter. If the furnace still won’t turn on, and you’ve inspected it for other possible problems, call a professional to diagnose your furnace issue.
Furnaces stop heating for various reasons, but professional help is only sometimes required. Thermostat issues, gas and power supply problems, clogged air filters, full condensate pans, and dirty flame sensors can often be dealt with by homeowners. Hire an HVAC technician if you’re uneasy about troubleshooting furnace issues, fail to identify or resolve the problem, or encounter complicated matters such as a faulty control board, defective ignitor, or overheated furnace.
Frequently Asked Questions
Can a furnace not have a filter?
No, you need a filter on your central heating system. Every furnace requires a filter. A filter’s primary function is to remove contaminants from the air entering the furnace to protect the HVAC system’s components. Additionally, air filters improve your home’s air quality.
Why won't the furnace not turn on with the thermostat?
Multiple issues can prevent a furnace from turning on. If the thermostat says “heat on” but no heat issues from it, examine the air filter, gas supply valves, thermostat settings and functions, power source, and flame sensor.
Why is my furnace not blowing hot air?
The cause of your furnace not blowing hot air could be the thermostat configuration, a dirty air filter, the high limit switch, a clogged condensate line, insufficient fuel, leaky ducts, ignition problems, a blocked burner, flame sensor issues, gas valve malfunctions, a cracked heat exchanger, or a clogged coil.