Furnace short cycling: why it turns on and off, and what to do

A healthy furnace in an Ontario winter runs longer cycles when it is cold outside and shorter, less frequent ones in mild fall weather. Short cycling is different: the furnace stops before the house is warm, or before it finishes a normal heat cycle, and does it again and again. Every start wears the igniter, the inducer and blower motors, and the control board, and the house heats unevenly. In a few cases the shutdown is the furnace protecting itself from something that matters, which is why it is worth finding the cause.

How do I know if my furnace is short cycling?

Time it. Note when the burners light and when they go off, over two or three cycles, on a cold day. Manufacturers describe short cycling as cycles that are often shorter than 5 to 10 minutes. Also watch what the furnace does, because the pattern points to the cause:

• Burners run a few minutes, go off while the blower keeps running, then come back: overheating and the high-limit switch.
• Burners light and go out within seconds, then the furnace retries: flame sensor or gas supply.
• Inducer fan starts, the furnace stops before lighting, and the status light blinks: pressure switch, vent pipes or condensate drain.
• The whole furnace, blower included, shuts off because the thermostat says the room is warm, then restarts soon after: thermostat location or settings, or an oversized furnace.

What causes a furnace to short cycle?

Trane's troubleshooting guide names an oversized furnace as a very common cause. A furnace that is too big heats the air around the thermostat quickly, stops, and then the rest of the house cools and it starts again. That cannot be fixed with a part. A technician can sometimes reduce it with blower settings, a two-stage furnace setting or the thermostat's cycle-rate setting, and it is worth raising when the furnace is replaced.

What can I check myself?

1. Replace the filter. It is the most common fix, and it costs the least. Use the size printed on the old filter's frame.
2. Open every supply register and clear the return grilles. Closing vents in unused rooms to save money makes a furnace overheat.
3. Look at where the thermostat is. If it sits above a supply register, near a lamp, a TV, a kitchen or a sunny window, it reads the room as warmer than it is.
4. Check the thermostat settings. Many smart and programmable thermostats have a cycle-rate or temperature-swing setting. A very tight swing makes the furnace start often. One RedFlagDeals user fixed their cycling by lowering the setting from 5 cycles an hour to 3.
5. Replace the thermostat batteries and make sure the thermostat is level and firmly on its base.
6. Outside, clear snow, ice, leaves and nests from the two plastic vent pipes of a high-efficiency furnace.
7. Read the status light through the window in the furnace door and write down the blink code from the chart inside the door.

Is a furnace short cycling dangerous?

Usually it is costly rather than dangerous: the parts wear faster, fuel is wasted on starts, and rooms far from the thermostat stay cold. It can be dangerous when the cause is a cracked heat exchanger or a venting problem, because both can let carbon monoxide into the house. Signs that point that way are soot around the burner door, a yellow or wavering flame, a smell when the heat comes on, water or rust stains on the furnace, and any CO alarm. Ontario requires working CO alarms outside sleeping areas and on every storey of homes with a fuel-burning appliance, so check yours when the heating season starts.

Why does my furnace short cycle only in the morning?

A furnace that short cycles at the first morning heat call and then runs fine is a pattern RedFlagDeals users in the GTA describe often. The coldest start of the day is when the furnace is working hardest. A partly blocked condensate drain, a pressure switch hose with water in it, or frost in the outdoor vent pipes can trip the furnace on the first cycles and then clear as things warm up. A thermostat setback that asks for a big temperature jump can also push an airflow-starved furnace to its limit. Note the time, the outdoor temperature and the blink code, and give that to the technician.

What will a technician do, and what does it cost on Doyo?

A technician checks the filter and airflow, measures the temperature rise across the furnace against the rating plate, cleans or tests the flame sensor, checks the pressure switch and its hose, clears the condensate trap, inspects the vents and the heat exchanger, and looks at the thermostat wiring. In Ontario this must be done by a Technical Standards and Safety Authority (TSSA) registered fuels contractor using a certified gas technician.

On Doyo, Furnace Repair is priced by written quote: a TSSA-certified gas technician, a call-out fee credited against the repair, and a diagnosis with a written price before any work, with parts quoted separately. If the furnace has not been serviced this year, Furnace Maintenance is $206.50 on Doyo, the full price with Doyo's fee included. It covers a full clean and safety check, with burners, flame sensor and filter checked, and a written report you keep.