Schedule soon
Short cycling means the system starts and stops every few minutes. It's most often caused by a system that is oversized for the house, a thermostat in the wrong spot or failing, restricted airflow, or a safety switch stopping the system because of a refrigerant, drain or electrical problem. It wastes energy, leaves the house humid and wears the compressor, so it's worth diagnosing soon.
Pick one to see how likely it is, who can deal with it, and what it means.
Common
Technician fixAn oversized system cools the air near the thermostat so quickly that it shuts off before it has run long enough to pull moisture out of the air. It's common when a replacement was chosen by matching the old tonnage instead of calculating the home's load.
Common
You can check thisA clogged filter or dirty coil can drive pressures and temperatures out of range, and the system's own safeties shut it off, wait, and try again.
Occasional
You can check thisA thermostat near a supply register, in afternoon sun or next to a kitchen reads the wrong temperature. Some thermostats also have cycle-rate settings that were changed by mistake.
Occasional
Technician fixToo little or too much refrigerant can trip low- or high-pressure switches, which stop the compressor until pressures recover. Low charge means a leak.
Occasional
You can check thisA partly clogged drain can lift a float switch, shut the system off, drain down and let it restart, over and over.
Occasional
Technician fixA failing capacitor or contactor, or a compressor shutting off on its internal overload because it's overheating, also produces short on-off cycles.
A cycle should end when the house reaches the setpoint. Each of these can end it sooner.
Turn the system off and call a technician if:
Timing a few cycles, the last step, is the most useful thing you can bring to a service call.
Done all of these and it’s still not right?
That points to a part that needs a technician’s meter, not another reset.
In a humid climate an air conditioner has two jobs: lowering the temperature and pulling water out of the air. The second one takes time: water only starts running off the coil once it has been cold for a while, so a system that keeps stopping early cools the air without drying it.
That is why an oversized system so often feels cool and clammy. It reaches the setpoint quickly, shuts off before it has wrung much moisture out, and some of the water on the coil evaporates back into the house before it reaches the drain. ENERGY STAR notes that oversized units "may cycle too frequently, resulting in less comfort and a shortened lifespan," and Florida's extension service advises calculating the cooling load rather than estimating it from square footage.
A system can't be made smaller, but some short cycling can be eased now through airflow, thermostat placement and settings, and the right size can be calculated when it's time to replace it. Our guide to Manual J sizing walks through how that calculation works, and high humidity in the house covers the clammy-house side of the same problem.
A technician times the cycles, checks whether a pressure switch, float switch or compressor overload is stopping the system, measures refrigerant charge and airflow, and compares the system's capacity with the home's cooling load. If the system is oversized, the technician explains what can be adjusted now and what to plan for at replacement. Free service call with repair.
There's no single number, but on a hot day a properly sized system typically runs long cycles, often well over ten minutes at a time. Cycles of just a few minutes, over and over, are a sign of oversizing or a safety switch stopping the system.
Sometimes it can be improved. Blower settings, thermostat placement and fixing duct or airflow problems can lengthen cycles and help with humidity. But a system that's far too large for the house can only really be corrected when it's replaced with one sized by a load calculation.
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