AC Short-Cycling — or Never Shutting Off? Both Are Telling You Something.
Two opposite complaints, one page, because they share causes. Short-cycling (on and off every few minutes) usually means a thermostat problem, an oversized system, a dirty coil overheating the compressor, or low refrigerant tripping safeties. Running nonstop without hitting setpoint usually means a dust-choked system, duct leakage into a 130° attic, or a system genuinely undersized for the July load. Nonstop running costs you on the bill; short-cycling costs you the compressor — the start cycle is the hardest thing a compressor does.
Ranked by Likelihood
Ordered by what we actually find on Valley service calls — start at the top.
What's most likely causing it
Thermostat placement or failure
Safe to check yourselfA thermostat over a supply draft or in afternoon sun reads the room wrong and cycles the system on its own schedule. Cheap to rule out first.
Dirty filter, coil, or condenser
Safe to check yourselfDust-blanketed coils overheat the system into safety shutdowns (short-cycling) or strangle capacity (nonstop running). Phoenix's default failure mode — start at the filter.
Duct leakage
Schedule service soonSupply leaks dump cooling into the attic, so the system runs forever chasing setpoint. If your bills outrun your neighbors' and rooms never balance, the ducts are suspects before the equipment is.
Low refrigerant / safety trips
Stop and callPressure safeties cut the compressor mid-cycle, it cools off, restarts, trips again — a destructive rhythm that reads as short-cycling. Needs gauges on it.
Oversized system
Schedule service soonAn oversized AC blasts the thermostat satisfied before the house (or the humidity) actually is, then stops — over and over. Common where a previous contractor 'went a size up to be safe.' The fix conversation belongs in the next replacement, sized by Manual J.
Safe checks before you spend a dollar
- Time it: under 10-minute cooling cycles repeatedly = short-cycling; hours without reaching setpoint = capacity/leakage
- Replace the filter and rinse obvious dust mats off the outdoor coil (system OFF first)
- Check the thermostat isn't in direct sun or over a register
- Note breaker trips — a tripping compressor circuit is a call-now item
When it's time to call
- Cycles under ~10 minutes that repeat after a fresh filter
- Any breaker involvement
- Nonstop running with rooms that never reach setpoint (leakage/capacity diagnosis)
- Bills that jumped without a rate change
Interactive
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