Advice
My furnace keeps starting and stopping
The possible causes, from the simplest — which you can check yourself in two minutes — to the one that needs a technician.
What you can check yourself
Start here. These three causes account for a good share of the calls we get.
1. The filter
A blocked filter chokes airflow. Heat then builds up inside the unit, the high-limit safety trips, and the furnace shuts down — restarting once it cools, and doing it again.
Check the filter. If you can barely see light through it, that is your answer.
2. Registers and returns
A piece of furniture in front of a return air grille, closed registers in unused rooms, a rug over a floor vent: anything that restricts circulation does the same thing a blocked filter does.
The system needs a clear return path. Closing registers in unused rooms does not save money — it unbalances the system.
3. The thermostat
A thermostat mounted above a baseboard, next to a lamp, in direct sun or on an exterior wall is measuring something other than room temperature. It thinks it is satisfied and cuts out early.
What needs a technician
Oversized equipment. It reaches the setpoint too fast, stops, restarts. This is the most common cause once everything else checks out, and unfortunately the one no adjustment fixes. On an existing system you can soften the symptom by adjusting blower speed or modifying ductwork; you cannot make the unit smaller.
A flame or ignition fault. On a gas furnace, a dirty flame sensor makes the unit cut out seconds after ignition. It is a simple cleaning, but it means opening the equipment.
Low refrigerant charge. On a heat pump or air conditioner, a low charge triggers safety cut-outs. A low charge means a leak: it has to be found, not just topped up.
An electrical fault. A relay, contactor or capacitor near the end of its life produces irregular cut-outs.
Why not to leave it
Short cycling is not just irritating: every start-up is the hardest moment for the compressor and the ignition system. Equipment that short cycles wears out fast.
And on a combustion furnace, repeated overheating stresses the heat exchanger. A cracked exchanger lets combustion gases into the household air — which is why our annual maintenance includes a heat exchanger inspection and a carbon monoxide check.
If you are still unsure
Call us at 514 626-3913. Equipment that short cycles is equipment sending you a signal — better read in October than in February.
Read next
Frequently asked questions
Is it dangerous?
Short cycling itself is not dangerous, but it wears equipment out early. On a combustion furnace, repeated overheating can weaken the heat exchanger — and a cracked exchanger is a safety problem requiring immediate attention.
How long should a normal cycle last?
In heating, expect 10 to 15 minutes per cycle in moderately cold weather, and longer cycles in deep cold. Repeated three-to-five-minute cycles indicate a problem.
Let’s talk about your project
One of our advisors comes to your home, measures what needs measuring and walks you through your options — free, and with no pressure.
Monday to Friday · 8:30 a.m.–12 p.m. and 1 p.m.–5 p.m. Outside business hours, our 24-hour emergency service is reachable at the same number.