- Ensure thermostat is set to “HEAT”
- Change the furnace air filter if it’s dirty
- Check the furnace circuit breaker
- Check the furnace power switch
- Check the pilot light (old furnaces only)
- Check the gas shutoff valve
- Keep outside vents clear (high-efficiency furnaces only)
Now, let’s explain those steps in more detail.
Step 1: Ensure the thermostat is set to “HEAT”
If your furnace does not come on at all, mosey on over to your thermostat. And make sure it’s set to “HEAT” and not “COOL.” We know, we know. This might be too obvious, but we don’t want to make any assumptions.
If your thermostat is set to heat, then…
Step 2: Change the furnace air filter if it’s dirty
Start by turning off the furnace. Remove the existing furnace filter, which is located inside the furnace or the return air vent.
Why does a dirty filter keep the furnace from turning on? Well, once the filter gets too dirty, it blocks airflow to the heat exchanger (the part that heats the air), causing it to overheat. And once it overheats, it shuts the furnace down.
If changing the filter didn’t solve the problem, then…
Step 3: Check the furnace circuit breaker
Your furnace may not be turning on because it’s got no electric juice! The circuit breaker may have tripped, preventing the furnace from getting electricity. So head to your
home’s electrical panel and find the breaker labeled “furnace” or “HVAC.”
If your breaker has tripped (the switch is in the middle position), you’ve found the problem. Turn the switch to the OFF position and then back to the ON position. If the switch trips again, you may have an issue elsewhere or the breaker itself is bad.
WARNING: Do not keep switching the breaker back to ON if it trips continuously. The breaker’s job is to protect the furnace’s wires from overheating. Eventually the breaker will fail, which could lead to the wires overheating and causing a fire.
If the breaker wasn’t the issue, then….
Step 4: Check the furnace power switch
Another reason the furnace may not be getting electricity is because its power switch is flipped to “OFF”.
Yes, your furnace has a power switch. And because it looks like a normal light switch, it may have been flipped by accident. You should be able to find this switch near the furnace itself.
If the switch was already set to ON, then…
Step 5: Check the pilot light (old furnaces only)
If the furnace pilot light goes out, it won’t run. The pilot light’s job is to ignite the furnace’s larger burner. But if the pilot light goes out, the furnace’s sensors detect this and shut off the gas valve to prevent your home from filling up with gas. (Because gas explosions are bad news.)
To relight a furnace pilot light, follow this video tutorial or follow these instructions:
- Turn off the furnace at the circuit breaker (or the power switch).
- Take the furnace cover off.
- Look for a pilot flame.
- If no flame, turn the furnace control valve (it’ll be a knob near pilot light) to the “Off” position.
- Wait five minutes.
- Get a match or barbecue lighter.
- After five minutes, turn the control valve to “Pilot.”
- Press the pilot button and hold it while you use a match or barbecue lighter to light the pilot. You can locate the pilot by following the tubing from the control valve.
- Once the pilot flame is lit, continue to hold the pilot button for one minute.
- Release the pilot button. The pilot flame should stay lit. If it goes out, start the steps over.
- Standing back at arm’s length, turn the control valve slowly from “Pilot” to “On.”
- Turn the furnace power switch on.
- Look for the furnace burners to light.
- Carefully put the cover back on the furnace.
If the pilot does not stay lit, then you may have a few different issues like:
- A bad thermocouple
- Pilot light issues, such as the opening is too small or dirt has gotten into the opening
If the pilot light is fine, then…
Step 6: Check the gas shutoff valve lever
Your furnace may not ignite because it’s not getting any gas. So check the furnace’s gas level and make sure that it’s in line with the gas pipe.
No one should have touched this recently, but you never know. So it does not hurt to check.
If the lever is in line, then…
Step 7: Keep outside vents clear (high-efficiency furnaces only)
In many houses, high-efficiency furnaces vent out the side of the building. And if those vents get blocked, the furnace will shut off. In New Jersey, these vents usually get blocked by snow during heavy snowstorms and extended periods of extreme cold.
But not all furnaces shut down in this situation, which can create a potentially lethal situation. If the exhaust vent is blocked and the furnace doesn’t shut off, carbon monoxide (an invisible toxic gas) produced by the combustion process in the furnace can fill your house.
Regardless, if your furnace has shut down, try checking the outdoor vent to see if it’s blocked by snow.