Furnace Will Not Turn On? Easy Troubleshooting in 7 Steps

You come home from work, and your house is freezing: the furnace has stopped working. Most of the time when a furnace will not turn on, it is because the thermostat or furnace itself has lost power. However, there are other issues that can cause a furnace to not start up, things which, if you know what to look for, you can fix yourself. Try the following troubleshooting steps before calling an HVAC technician. You might save yourself the expense of a service call.

Step 1: check the thermostat

The first thing you will want to do when figuring out why you furnace does not work is to check the thermostat. Make sure the thermostat is set to “heat,” and look at the temperature. It is possible someone (accidently) changed the settings.

If you own a digital or programmable thermostat, you might have another problem — the power. If your thermostat does not turn on, you will need to check on its power supply. Replace the batteries if the thermostat is battery operated. Batteries in these units typically need replaced every couple of years. If changing the batteries does not help, go to the circuit breaker or fuse box and see if the circuit for the thermostat has been tripped.

Step 2: check to see if your furnace has power

If everything with your thermostat is fine, your furnace might lack power. You can use thermostat to test it. On your thermostat, turn the setting to “on” instead of “auto.” If the furnace’s fan comes on, the furnace has power. If not, it lacks power.

At this point, check the fuse box/circuit breaker. If the fuse is blown, replace it. If the circuit breaker is tripped, turn it all the way off then back on (should hear click). The furnace should turn back on within five minutes (many thermostats have time delay in them), but if it does not, visit the furnace itself.

“Furnace Switch,” courtesy of Jen Durfey. CC license.


At the furnace, find the furnace’s power switch. The switch will look like a light switch. People sometimes turn off these switches accidentally. If the switch is on and the furnace still lacks power, check the furnace’s main and secondary electrical panels.

If the circuitry has been tripped in these panels, turn back on the circuit. The furnace should start within five minutes. If the furnace starts, great. If not, go to the next step.

As a note, you will want to investigate the source of the tripped circuit. If the furnace is not on a dedicated circuit, another appliance might have caused the power surge that tripped the furnace’s breaker. Ask yourself: Were you running too many appliances/devices on the circuit? Did the power go off through the entire building? Or has an appliance been acting funny or not running well? Failing appliances can send surges through your electrical system. These appliances need repaired or replaced.

It is important to identify what tripped the circuit tripped circuits breakers and blown fuses mean your electrical system encountered a power surge. Power surges damage electronics and appliances and, at worst, can cause thousands of dollars in damage. If you find the source of the power surges, address it.

However, if your circuit breaker trips or fuse box blows a fuse again and you cannot find the source of the power surge, call a qualified electrician to address the problem.

Step 3: check the furnace filter

Many people neglect to replace their furnace filters. If it has been a while since you have replaced it, dust and other particles might be clogging up the filter and not letting air through. If this is your problem, replace the furnace filter.

Most disposable, pleated furnace filters need changed every 90 days, per manufacturer instructions. You should check filters every 30 days, however, to make sure they have not accumulated too must dust. Replacing furnace filters is especially important during seasonal changes when furnace or AC system kick on for the first time.

Another way to determine whether your furnace will not turn on due to your air filter is to listen for whistling sounds. If a furnace can’t get air through its filter, it will suck it in anywhere it can, including small crevices.

Step 4: look at the condensate pan

High-efficiency furnaces pull enough heat out of the air that water condenses. This water drips down to a pan. If the furnace is working properly, the water will drain out of the pan, or a pump will pull it into a reservoir.

If the pan has a drain, check to see if a clog is blocking the drain. Consider adding condensate pan tablets to your tray when this happens. These tablets will keep the drain from clogging.

If a pump empties the condensate pan, see whether the pump is stuck in the “up” position. Clean the pump if it is, making sure to wash your hands afterward (water condensed from furnaces has acid in it). If cleaning the pump does not work, you might have to replace it.

When you can fix the water issue, the furnace should kick back on within a minute. If you cannot fix it, go to the next step.

Step 5: check the vents

It might sound obvious, but make sure your indoor vents are open. Closed vents will not allow air to circulate in your house. Also, check to see whether something has blocked your outdoor heat vents. If a bird or mouse blocked up the PVC pipe or flue and the furnace’s fumes cannot escape, the furnace might stop running.

Step 6: clean the pilot

If you have an older furnace with a pilot light, it is possible the flame might have gone out. To turn the furnace back on, relight the pilot light. If it will not stay lit, however, turn off the furnace’s power and gas supply and try the following.

1) Take a fine wire and poke it into the holes where the pilot flames burn to knock out debris.

2) Turn back on the power switch and gas.

3) Adjust the flame using your furnace’s pilot light screw (if needed). When the pilot light is on, slowly turn the screw until the flame is a consistent 1½ to 2 in.

If debris (buildup) have accumulated on the pilot, the furnace likely needs additional maintenance. Call a licensed HVAC company to tune up the unit, and consider enrolling in an annual maintenance plan to keep your unit as efficient as possible and to save money on additional repairs.

“Gas,” courtesy of Daniel R. Blume. CC licensing.

Step 7: check the gas

If your pilot light will not come on after cleaning the furnace, check your gas or propane supply as a final step to find out why your furnace will not start. Make sure the gas line is turned on. If it is and you’re not receiving gas, you will need to call a plumber to address the issue. On the other hand, if you own your own gas or propane supply, make sure the storage tank still has fuel.

If you cannot figure out why your furnace will not turn on after going through these steps, it is time to contact Patriot Heating and Air Conditioning, located in Franklin, Indiana, or another HVAC professional near you. Patriot has more than 25 years of HVAC experience and is available 24/7 for emergency repairs.

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