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Short Cycling and Runtime Calculator

Turn a simple observation of how often your system turns on and how long it runs into cycles per hour, average run time and duty cycle — the numbers that reveal short cycling.

Method reviewed October 25, 2026. Version 1.0.0. How it works

Your inputs

Minutes you watched (or that the thermostat history covers).

Minutes, if the equipment or thermostat documentation gives one.

Result

Cycles per hour
6
Average run time
4min

6 cycles per hour, averaging 4 minutes on and 6 minutes off (40 % running).

  • Average run time (4 min) is shorter than the 5-minute minimum you entered.
  • Frequent short cycles in steady weather often point to oversized equipment (DOE), but also to thermostat placement or settings, a dirty filter, low airflow, or a safety control tripping. A technician can tell which.
Average off time
6min
Duty cycle (share of time running)
40%
Show the working
  1. Cycles per hour = cycles × 60 ÷ period
    6 × 60 ÷ 60 = 6
  2. Average on time = total running ÷ cycles
    24 ÷ 6 = 4 min
  3. Duty cycle = running ÷ period
    24 ÷ 60 = 40 %

How to observe your system

Pick a period of steady weather — an hour or two in the afternoon for cooling, or early morning for heating. Count how many times the system starts, and add up the minutes it runs. A smart thermostat’s runtime history makes this easier.

The calculator gives cycles per hour, the average on and off time and the duty cycle (the share of time it runs). If you enter the manufacturer’s minimum run time, it flags cycles that are shorter.

What short cycling means

Short cycling is a system turning on and off many times with very short run times. The U.S. Department of Energy notes that oversized air conditioners short-cycle, which wastes energy and removes less moisture, because a cooling coil needs several minutes of running before it starts to dehumidify.

Oversizing is not the only cause. A thermostat near a supply register, wrong thermostat settings, a dirty filter, low airflow, low refrigerant or a safety control tripping can all cause it. A technician can tell which.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Observation period
60
Number of on cycles in that period
6
Total minutes running in that period
24
Manufacturer minimum run time (optional)
5

Result

Cycles per hour
6
Average run time
4 min
Average off time
6 min
Duty cycle (share of time running)
40 %

6 cycles per hour, averaging 4 minutes on and 6 minutes off (40 % running).

Assumptions

  • Averages over the observation period; a longer period in steady weather gives a more representative picture.

Limitations

  • Runtime alone cannot diagnose the cause. Multi-stage and variable-speed equipment cycles differently from single-stage units.

Frequently asked questions

How many cycles per hour is normal?

There is no single number; it depends on the weather, the equipment and the thermostat. In mild weather a correctly sized single-stage unit may cycle a few times an hour; in very hot weather it should run long, steady cycles. Very short run times of only a few minutes in steady weather are the warning sign.

Does a high duty cycle mean my AC is too small?

Not by itself. Running almost continuously on the hottest days is what a well-sized system does. If it runs continuously and still cannot hold the set point in ordinary weather, have it checked.

Do variable-speed systems short-cycle?

They are designed to run long cycles at low speed instead. Frequent on-off cycling from a variable-speed system is unusual and worth having checked, often for settings or communication problems.

Sources

  1. Proper Sizing of HVAC Systems — U.S. DOE Building Science Education Solution Center. Rules of thumb lead to oversized systems; loads should be calculated with Manual J.

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