Homeowner guide · Cooling and heating
HVAC Short Cycling: Why Your System Keeps Turning Off
Short cycling means your air conditioner or furnace starts, runs for a short time, shuts off and starts again, over and over, without completing a normal cycle. It's hard on the equipment and often leaves the house less comfortable. The causes overlap between cooling and heating, but not completely.
Recognizing it
Is it really short cycling?
Illustration only. Normal run times vary with the weather, the home and the type of system.
There's no single number of minutes that applies to every system. Manufacturers give different rules of thumb (runs of only a few minutes, or a furnace shutting off in under about five minutes, are common warning signs), and cycle length changes with the outdoor temperature and the type of system. Some two-stage or variable-speed systems are designed to run long and slow. What matters most is the pattern compared with how your system normally behaves:
- it shuts off after only a few minutes, then starts again soon after;
- the house doesn't reach the set temperature, or swings around it;
- you hear it starting up far more often than it used to;
- on heating, the burners light and go out again, sometimes repeatedly.
If you can, note roughly how long it runs and how long it rests. That's useful information for whoever diagnoses it.
Cooling and heating
Causes that apply to both
- The thermostatA thermostat in direct sun, beside a supply vent or near a heat source can read the room wrong and switch the system off early. Settings and wiring problems can do the same. See thermostat problems.
- Restricted airflowA clogged filter or closed and blocked vents can make the equipment overheat on heating or freeze on cooling, and its protective controls stop it.
- A system that's too largeAn oversized system can satisfy the thermostat in a few minutes and shut off before running a full cycle. Sizing is decided when a system is chosen and installed (see AC installation). But if a system that used to run normally has started short cycling, its size hasn't changed, so look for another cause first.
- Electrical and control problemsA failing control board, relay, sensor or connection can interrupt the system partway through a cycle.
Where they differ
Cooling-specific and heating-specific causes
Air conditioning
When the AC cycles on and off
- A freezing coil. Low airflow or a refrigerant problem can ice the coil and trip protective controls. See frozen coil or leaking water.
- Refrigerant charge. A low charge, usually from a leak, can make pressure safety switches stop the compressor.
- An overheating outdoor unit. A dirty or blocked outdoor coil, or a failing fan, can make the compressor's overload protection shut it down.
- A backed-up condensate drain. On many systems, a float switch stops cooling until the water drains.
Furnace
When the furnace cycles on and off
- Overheating. A high-limit switch shuts the burners off if the furnace gets too hot, for example because of a clogged filter or blocked airflow.
- Flame sensing. If the burners light and then go out within seconds, the furnace may not be sensing the flame, for example because of a dirty flame sensor.
- Venting and draft. Safety switches stop the furnace if combustion gases aren't venting properly. That's a safety issue, so it needs a technician.
If you ever smell gas or a carbon monoxide alarm sounds, follow the safety steps on the furnace repair page first.
Before you call
What you can check
These won't fix a failing part, but they rule out the simple causes.
- Replace the air filter if it's dirty.
- Open supply vents and clear return grilles.
- Look at where the thermostat is: sun, lamps, a nearby vent or a drafty spot can all fool it.
- If the thermostat uses batteries, replace them.
- For cooling, make sure the outdoor unit is clear of leaves, plants and stored items.
- Don't keep switching the system off and on to "reset" it. If it keeps cycling, let it rest and call.
Why it's worth a diagnosis
Every start puts extra wear on motors, the compressor and ignition parts. On cooling, short runs remove less humidity, so the house can feel clammy. On heating, the thing stopping the furnace may be a safety control doing its job, which is a reason to find out why rather than keep restarting it.
System still cycling on and off?
Tell us what it's doing and for how long. No obligation to hire.