Furnace Keeps Tripping the Breaker? Causes and What to Check

A furnace breaker usually trips for one of two reasons: a short in the igniter, inducer motor, blower or heat strip, which trips it as soon as heat starts, or restricted airflow that overworks the blower and trips it after the furnace has run a while. Replace a dirty filter, open blocked vents and, if it keeps tripping, call a licensed technician.

Timing is your best clue. Note when the trip happens before you start checking.

What the timing tells you

Common causes

• Dirty filter and blocked vents. The most frequent airflow problem. The blower works harder and draws more current.
• Failing blower motor. A seized or shorted motor draws a lot of current. One source notes a motor can spin freely by hand and still trip the breaker if its windings are damaged.
• Bad run capacitor. It stops the motor starting properly and causes high inrush current.
• Cracked igniter. Hot surface igniters can crack and short.
• Wet inducer motor. Water getting into it can short the motor.
• Electric heat strip. A short in the strip trips the breaker when heat is called.

How to fix it

1. Turn the thermostat off and switch the furnace off at its switch.
2. Replace the air filter if it is dirty.
3. Open all supply registers and clear the return vents of furniture and rugs.
4. Reset the breaker once and turn the furnace on.
5. Note when it trips. If it trips right away, stop and call a technician.

If you notice an odd smell when heat starts, see what causes a burning smell from the heat.

Gas furnace or electric furnace?

A gas furnace uses electricity only for the blower, the inducer fan, the igniter and the control board, so it normally draws little power. An electric furnace uses heating elements that draw far more, and it is on larger dedicated breakers. The nameplate on the furnace lists a minimum circuit ampacity and a maximum overcurrent protection value. Your breaker should match those numbers, and a technician can confirm it.

Other causes worth knowing

• A shared circuit. In some homes the furnace shares a circuit with basement or garage outlets. A vacuum, a space heater or a freezer on the same circuit can push the total load past the breaker rating just as the blower starts.
• Condensate problems. High-efficiency furnaces make water as they run. A clogged drain or a failed condensate pump can leak onto wiring or the control board and cause a short.
• Pests. Mice and other rodents sometimes chew wiring inside a furnace cabinet, which can cause a short.
• A worn breaker. An old breaker can trip below its rating, especially on a hot day in the basement.

What a technician will check

Knowing what the visit involves makes the call easier to price and plan:

1. The nameplate ratings against the breaker size and wire gauge.
2. The blower motor and its run capacitor, including how much current the motor draws compared with its rating.
3. The igniter and the inducer motor.
4. The wiring and control board, looking for burn marks, moisture or chewed insulation.
5. The air filter, ductwork and return airflow, since poor airflow overworks the blower.

Ask for the findings in writing, and ask whether the part that failed is covered by a warranty.

When to call a professional

• It trips as soon as heat starts or the blower runs.
• A new filter and clear vents make no difference.
• You see scorch marks or smell burning.
• The breaker will not reset.

If the house is cold in the meantime, see a plan for heating during an outage for staying warm safely, and why hot water fails in winter if the water heater is acting up too.

What not to do

• Do not install a bigger breaker.
• Do not keep resetting the breaker.
• Do not open electrical panels on the furnace with the power on.
• Do not run the furnace with a blocked filter.

Bottom line

Check the filter and vents first. If the furnace still trips, or trips at once, a motor, igniter or heat strip has a fault that needs a technician.