Furnace Keeps Turning On and Off? Causes and Fixes

A furnace that keeps turning on and off is short cycling. Carrier defines it as a furnace turning on and off repeatedly within a short timeframe, often less than 5 to 10 minutes. Carrier calls a clogged filter a leading cause. Other causes it lists are closed vents, a thermostat with faulty wiring, dead batteries or a poor location, and a furnace too large for the house.

Is my furnace short cycling?

Time a few cycles, from the moment the burners come on to the moment they shut off. Carrier's definition is a furnace that turns on and off repeatedly within a short timeframe, often less than 5 to 10 minutes. Carrier also notes the system may shut down as a safety precaution before the home reaches the target temperature.

Why does my furnace turn on and off?

Carrier and Trane list the causes below. Carrier's homeowner checklist covers the first three.

Quick checks you can do safely

1. Check the filter. ENERGY STAR says to inspect, clean or change air filters once a month. Carrier recommends checking filters monthly and generally replacing 1-inch filters every 1 to 3 months.
2. Open every supply register. Carrier's checklist says to confirm all supply registers are open and unobstructed. It notes that closed vents are a frequent but overlooked problem.
3. Check the thermostat. Carrier's checklist says to check its settings and batteries. Carrier recommends a thermostat on an interior wall away from direct sunlight, exterior doors, windows, air vents, drafts and heat-producing appliances.
4. Reset once at most. Trane says a soft lockout is the first one or two times the furnace controller stops the ignition sequence. If it happens a third time, the furnace enters a hard lockout, and a technician has to reset it. Trane says your owner's manual may suggest a power-cycling reset.
5. Stop there if it keeps happening. If airflow is fine and short cycling continues, Carrier says to turn the system off and contact a qualified HVAC professional for diagnosis.

Trane's lockout page says changing the air filter is the only thing you should do yourself. It says the rest of the troubleshooting is best left to an HVAC professional.

Can poor airflow cause furnace short cycling?

Yes. Carrier says a clogged filter is a leading cause of short cycling. It restricts airflow, which causes heat to build up inside the unit. That triggers the limit switch to shut the system down to prevent the heat exchanger from overheating.

Carrier names three airflow problems you can check: dirty filters, closed supply registers and blocked vents. If those are clear and the furnace still short cycles, Carrier's advice is to turn the system off and call a professional.

Can an oversized furnace cause short cycling?

Yes. Carrier says that if a furnace is too large for the calculated heating load of your home, it will heat the space too rapidly. Carrier points to a load calculation such as ACCA Manual J instead of rules of thumb.

ACCA's blog on Manual J puts it plainly: "You have to do an actual load calculation." If you are getting bids for a new furnace, ask each contractor whether they did one. Our guide to comparing HVAC quotes covers what else to ask.

Can the thermostat cause short cycling?

Yes. Carrier says faulty wiring, dead batteries or poor placement can cause incorrect temperature readings, so the furnace shuts off too early or cycles too often. Some thermostats also have an installer setting for cycle rate, counted in cycles per hour.

Honeywell's TH3110D installation guide lists 5 for gas or oil furnaces of less than 90% efficiency, 3 for furnaces of over 90% efficiency, and 9 for electric furnaces. That is one older model. Check the installer settings in the manual for your own thermostat.

Heat pump turning on and off in cold weather?

Some stops are the defrost cycle. Trane says its defrost cycles usually last 3 to 5 minutes and are more common when it is in the 20s or 30s outside. The outdoor fan may stop, and you may see a cloud of steam coming from the outdoor unit. Trane says the heat pump returns to heating shortly after.

The EPA requires technicians who maintain, service, repair or dispose of equipment that could release ozone-depleting refrigerants to be certified under Section 608 of the Clean Air Act.

What it costs to fix a furnace that keeps turning on and off

The two cost sources we found disagree, so treat the ranges as rough. Fixr reports that the average cost of furnace repair is between $349 and $650, with a national average of $485. Trane's cost guide says a service call can average $50 to $200. It gives repairs as $100 to $3,000 in one section and $150 to $3,000 in another.

Fixr labels its figures as the cost of a service call plus replacement parts. Trane gives its own estimates and names no outside source.

Is it safe to keep running a furnace that short cycles?

Carrier warns that over time, short cycling can lead to a cracked heat exchanger, which can leak carbon monoxide. Carrier calls that a significant safety risk. Our guide to cracked heat exchangers covers it in more detail.

The CPSC says to install battery-operated CO alarms or CO alarms with battery backup on every level of the home and outside sleeping areas. It lists headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion as symptoms of CO poisoning. More in our furnace carbon monoxide guide.

Call a pro now if

• You smell gas or a CO alarm sounds. Get out first, then call, as PURA and the CPSC advise.
• The furnace has gone into a hard lockout. Trane says a technician needs to reset it.
• Short cycling continues with a clean filter and open registers. Carrier says to turn the system off and call a professional.
• The house has no heat at all. See what to do when there is no heat.

ENERGY STAR recommends annual pre-season service by a contractor, with the heating system checked in the fall. Our furnace tune-up guide covers that visit.

Questions people ask