How to Reset a Furnace Safely (and When Not To)

To reset a furnace, turn the thermostat to off, switch off the furnace power, wait about 10 minutes, then turn power back on. If your furnace has a red reset button, press it firmly once, only when the unit is cool. If it shuts off again, stop and call an HVAC technician.

A furnace that will not start on a cold morning is stressful. Sometimes a simple reset gets it going. But a reset is not a repair, and pressing a button over and over can be dangerous. This guide explains what to check first, how to reset safely, and when to stop.

First: check the simple things

Before you reset anything:

1. Thermostat. Set it to Heat and at least 5 degrees above room temperature. Replace the batteries if the screen is blank.
2. Power. Check the furnace switch (it looks like a light switch near the unit) and the breaker.
3. Filter. A clogged filter is a common cause of shutdowns. See how often to change a furnace filter.
4. Vents and registers. Open them and move furniture away.
5. Gas supply. On a gas furnace the valve handle should be parallel to the pipe.
6. Smell or noise. If you smell gas, leave the house and call the gas company from outside. Do not reset anything.

Method 1: Power-cycle reset (works on most furnaces)

1. Set the thermostat to Off.
2. Turn off the furnace power switch, or flip its breaker off.
3. Wait at least 10 minutes. This lets the control board clear and the unit cool.
4. Turn the power back on.
5. Set the thermostat to Heat and raise the temperature.
6. Wait a few minutes. A furnace goes through a startup sequence: inducer fan, igniter, burners, then the blower.

If it starts and keeps running, replace the filter and watch it for a day.

Method 2: The red reset button

Some furnaces have a red button. Sources are clear about one thing: it is often not a "blower motor reset" at all. It may belong to a different safety device, such as a rollout switch or a limit switch. Many newer motors recover on their own and have no button.

1. Turn off the thermostat and the furnace power.
2. Let the furnace cool for at least 10 minutes.
3. Remove the access panel. It usually lifts or pops off. Do not touch wires.
4. Find the red button and read any label next to it.
5. Press it firmly once.
6. Replace the panel securely. The furnace will not run with the door open on most models.
7. Turn the power back on and set the thermostat to Heat.

Do not reset a rollout switch if you have not found why it tripped. A tripped rollout switch can mean a blocked vent, a blocked heat exchanger or a combustion problem. That needs a technician.

Oil furnaces: press once only

On an oil furnace, the red button is on the primary control. Press it a single time. Each extra press can push more oil into the combustion chamber, which risks a dangerous puff-back. If it does not start after one try, call for service.

What if the furnace trips again?

If it runs briefly and shuts off again, or the reset button pops again, stop. A repeat trip means something is wrong. Common causes include:

• A clogged filter or blocked airflow
• A dirty flame sensor
• A blocked flue or vent
• A failing blower motor or limit switch
• A cracked heat exchanger

See furnace short cycling causes and furnace clicking but not starting for how to narrow it down.

Error codes and blinking lights

Many modern furnaces have a small window showing a blinking LED. The number of flashes is a code. The label on the inside of the furnace door tells you what it means. Write down the pattern before you reset, because a reset can clear it. A technician can use that code to find the fault faster.

What not to do

• Do not press the reset button repeatedly.
• Do not bypass a safety switch or tape a button down.
• Do not reset a hot furnace. Let it cool first.
• Do not open the burner area or touch wiring unless you are qualified.
• Do not keep resetting every day. That hides a real fault.
• Do not use the oven or a gas stove to heat the house.

When to call a professional

• It trips again after one reset
• You smell gas or see soot or yellow flames
• A carbon monoxide alarm sounds
• Strange noises, such as banging or squealing
• The igniter glows but no flame appears
• The furnace is more than 15 years old and failing often

If you are not sure who to call, see who handles a broken furnace. When the weather is very cold, call early, because HVAC companies get busy.

Bottom line

Check the thermostat, power and filter, power-cycle the furnace, and press a red button only once on a cool unit. If it trips again, or you smell gas, stop and call a professional.