Quick answer
A furnace that keeps tripping the breaker is either drawing too much current or has a short to ground. A blower motor with seized bearings or a failed run capacitor is the usual overload; a shorted inducer motor, cracked igniter or chafed wire trips it instantly. Reset the breaker once. If it trips a second time, leave it off and call a technician.
When a furnace keeps tripping the breaker, the breaker is doing its job: something on that circuit is pulling more current than the wire can safely carry, or 120 volts is finding a path to ground. Most of what a gas furnace draws goes to the blower motor, and the total sits comfortably under the 15 amps a typical furnace circuit is rated for. A trip therefore points to a failing part, a crowded circuit or a tired breaker, and the timing of the trip tells you which.
When the breaker trips tells you what is wrong
- The instant you reset it, before the furnace does anything: a dead short in the wiring, the transformer or the control board. Leave the breaker off.
- A few seconds into a heat call, as the inducer fan starts or the igniter begins to glow: one of those two parts is shorting to the cabinet.
- About a minute in, right when the main blower should start: the blower motor is seized or its run capacitor has failed, and the motor is sitting stalled at several times its normal current.
- After 10 or 20 minutes of normal heating: a motor running hot on worn bearings, a loose connection heating up at the breaker, or other loads sharing the circuit.
- Only when something else kicks on, such as a freezer: the circuit is overloaded, and the furnace is one of several things on it.
Blower motor faults are the usual reason a furnace overloads its breaker
The blower is the largest motor in the furnace, so it is the first suspect. On a PSC (permanent split capacitor) motor, a failed run capacitor leaves the motor humming without turning. A stalled motor draws locked-rotor current, several times its running amps, until its internal overload opens or the breaker trips. Seized or dry bearings do the same thing more slowly: the shaft drags, the amps climb and the windings cook.
A clogged filter rarely trips a breaker by itself, but it matters. A PSC blower motor is cooled by the air it moves, and a plugged filter cuts that air, so the windings run hot and a motor already dragging on worn bearings is pushed over the edge. A variable-speed ECM motor reacts differently: it speeds up to hold airflow against the restriction and draws more power doing it. Check the filter before anything else, because it is the one cause you can fix yourself.
Shorts to ground inside the furnace, and circuits that are shared
A short trips the breaker fast. Three places account for most of them inside a furnace:
- The inducer motor. On a condensing furnace it sits beside the condensate collector, and a leaking gasket or a blocked drain lets water into the windings.
- The hot-surface igniter. On many furnaces it runs on 120 volts, and a cracked element or a broken ceramic base can touch the burner bracket.
- Chafed wiring. Years of vibration rub a harness through its insulation where it crosses a sheet-metal edge or the blower housing.
The other possibility is that the furnace is fine and the circuit is crowded. A furnace is meant to have a circuit to itself, but in older houses where the basement has been rewired in stages, the same 15 A breaker can end up feeding a chest freezer, a dehumidifier or a row of workshop receptacles. A condensate pump or a humidifier adds little on its own. A freezer compressor starting at the same moment as the blower is enough to tip it.
Reset the breaker once, then decide who to call
You get one reset, so make it a test. Turn the furnace service switch off, then push the breaker fully to Off and back to On. If the breaker trips with the furnace switched off, the fault is in the house wiring or in something else on that circuit. If it holds until you turn the furnace switch back on, the fault is inside the furnace.
A second trip means leave it off. Every reset sends fault current through the same damaged part and the same wire, and that is how a motor problem becomes a burnt control board or a scorched connection.
- Never swap a 15 A breaker for a 20 A one to stop the tripping. The wire in the wall is sized for 15 A, and the breaker is what protects it.
- Call an ESA-licensed electrician if the breaker is hot to the touch, the panel smells of burning, the breaker feels loose or will not latch, or it trips with the furnace switched off.
- Call a TSSA-licensed gas technician if the trip follows the furnace sequence: at the inducer, at the igniter or at the blower.
- Keep the breaker off and call right away if you smell burning at the furnace. A burning-plastic or electrical smell from a furnace has its own checklist.
A tripping breaker is also a different fault from a furnace that blows the small 3 A or 5 A fuse on its control board. That fuse protects the 24-volt thermostat circuit, and repeated control-board fuse failures have separate causes.
What a furnace breaker-trip repair costs
- Diagnostic visit: $90–$150, credited toward the repair if you go ahead.
- Run capacitor: $250–$500.
- Hot-surface igniter: $200–$450.
- Draft inducer motor: $400–$900.
- Blower motor: $500–$1,500, with PSC motors at the low end and ECM motors at the high end.
Work at the panel is priced by the electrician. On the furnace side, a technician measures the amp draw of each motor against its nameplate, tests the capacitor and checks each motor and the igniter for a path to ground, so the part that gets replaced is the one at fault. See furnace repair for how a ZK Mechanical visit works and our pricing page for the wider price list. If the house is already cold, call (647) 801-1252 or send us the details; related no-heat faults are grouped in the heating answers hub.