Cycles per Hour to Hertz for Thermostat Cycle Rates

Reading a Thermostat's Cycles-per-Hour Setting as a Frequency

Buried in the installer menu of almost every modern thermostat is a cycle-rate parameter, written CPH. It decides how many times an hour the equipment is allowed to start and stop, and technicians change it far more often than homeowners realise — usually after a complaint about a compressor that will not stay running, or a room that swings warm and cold between calls.

Expressed in hertz, that same setting stops being an abstract menu number and becomes a period you can hold a stopwatch against. Nine cycles per hour is a complete on-and-off sequence every six minutes and forty seconds, which immediately raises the question of how much of that window the compressor is allowed to occupy once its own protective delays have taken their share.

Conversion factor: divide by 3 600 — 1 cph = 0.000 277 78 Hz. A cooling stage set to 3 CPH works out at 3 ÷ 3 600 = 0.000 833 33 Hz, one full on-off cycle every 1 200 seconds, or twenty minutes end to end.

What the number in the installer menu really controls

A ceiling, not a timetable

The setting caps the cycle rate rather than enforcing it, and the cap is only reached around half load, where run and off time are roughly equal. On a mild day the equipment satisfies the space in one long call and cycles far less; in a heat wave it may barely stop at all. The control logic behind it is time-proportioning rather than a plain temperature differential, so the thermostat is aiming for a cycle length, not a deadband.

Thermal mass picks the value

Steam and gravity systems keep pushing heat for a long while after the call ends, so they are set to one cycle an hour and any faster rate simply overshoots. Electric resistance heat stops the instant it is switched off, which is why strip heat and baseboards are given the fastest settings. Forced-air gas and a compressor circuit sit between those extremes.

Capacity that never turns up

An air conditioner does not deliver its rated output the moment the contactor pulls in. Refrigerant pressures need to settle and the evaporator has to get cold and genuinely wet before latent capacity appears, which takes minutes. Chop the run short enough and the house is cooled but never dried, and the occupants report clamminess at a thermostat reading that looks perfectly correct.

Checking a Cycle Rate Before You Change It

Enter the CPH value from the installer setup

Type the number shown against the cycle-rate parameter for the stage you are adjusting into the left field. Heating, cooling and auxiliary stages usually carry separate values, so convert them one at a time rather than assuming the equipment shares one rate.

Turn the frequency into a cycle length

One divided by the hertz value is the seconds in a complete cycle. Five cycles an hour gives 0.001 388 89 Hz and a 720-second window — twelve minutes covering one burn and one rest, which is the figure worth comparing against the burner's own timings.

Fit the protective delay inside that window

Halve the cycle length for the balanced-load case and check the off portion still clears the anti-short-cycle timer, which is commonly five minutes on a compressor circuit. At twelve cycles an hour the off period is only two and a half minutes, so the delay, not the thermostat, ends up setting the real rate.

Reverse it after you have timed the equipment

Stand in front of the unit, time one complete start-to-start interval and you have a period rather than a setting. Press the swap arrows, or type into the right-hand field, and 0.002 5 Hz measured on site reads back as the nine cycles an hour the thermostat has been configured for.

Cycle Rate Settings Across Heating and Cooling Equipment

Values that appear in installer menus, the frequency and period each one corresponds to, and the run and off split when the load happens to sit at half of the equipment's capacity.

CPH setting	Equipment it usually belongs to	Frequency (Hz)	Cycle period	Run / off at half load
1	Steam and gravity heating	0.00027778	60 min	30 / 30 min
2	High-mass hot-water radiators	0.00055556	30 min	15 / 15 min
3	Cooling stage and heat pump compressor	0.00083333	20 min	10 / 10 min
4	Low-mass hydronic fan coil	0.00111111	15 min	7.5 / 7.5 min
5	Gas or oil forced-air furnace	0.00138889	12 min	6 / 6 min
6	Forced air in a leaky or lightweight house	0.00166667	10 min	5 / 5 min
9	Electric furnace and strip heat	0.00250000	6 min 40 s	3.33 / 3.33 min
12	Electric baseboard, fastest response	0.00333333	5 min	2.5 / 2.5 min

Menu values typed as they appear

Whole settings, half steps such as 4.5 on controllers that allow them, and comma-written values from a European display all convert without being reformatted first.

Small numbers keep their meaning

Rates this slow land far below a thousandth of a hertz, and the eight decimal places kept in the output are what separates a one-hour cycle from a twenty-minute one.

From a stopwatch back to a setting

Swap the direction and an interval timed at the condenser becomes the cycles-per-hour figure you can compare against what the installer menu currently holds.

Short-Cycling Questions From the Service Call

What does raising or lowering the CPH setting actually change?

It changes how long each call for heat or cool is permitted to last, not how much capacity the equipment has. A lower number means longer runs separated by longer rests, so the room temperature wanders further either side of setpoint but the equipment starts less often. A higher number chops the same total run time into more, shorter pieces, holding the room tighter at the cost of extra starts. Typical factory values reflect the equipment type rather than the house: three for a compressor, five for gas or oil forced air, nine for electric resistance heat, one for steam or gravity systems.

What counts as short-cycling, and what normally causes it?

Runs of only a few minutes, repeated far more often than the configured cycle rate would allow, are the usual working definition — and the giveaway is that the equipment is stopping on something other than a satisfied thermostat. Oversizing is the most common root cause: a system with far more capacity than the load hits setpoint before the coil or heat exchanger has settled, whatever the CPH value says. After that come a low refrigerant charge tripping the low-pressure switch, a clogged filter or blocked coil starving airflow until a limit or freeze protection opens, a furnace tripping its high-limit, and a thermostat sited in a draught from a supply register so it sees air rather than the room. Changing the menu setting hides none of these; it only reshapes the cycles that the equipment is actually able to complete.

Why must a compressor sit out a minimum off time?

Because restarting against an unequalised system is one of the hardest things you can ask of it. When the compressor stops, high-side and low-side pressures are still far apart and take a few minutes to bleed toward each other. Energise the motor before that has happened and it must break away against a large pressure difference, drawing locked-rotor current for far longer than normal; windings heat, the start components are stressed, and on a scroll the mechanism can briefly run backwards as gas blows back through it. That is why controls carry a delay of roughly five minutes after every stop, sometimes as a separate anti-short-cycle timer. It also explains why a very fast cycle-rate setting achieves nothing on a compressor circuit — the timer simply overrides it.

How does cycle rate trade off against the swing in the room?

They are two ends of the same lever. Between the moment the equipment stops and the moment it restarts, the room drifts toward whatever the outdoor conditions are pulling it to, and the size of that drift is roughly proportional to the off period. Halving the cycle length therefore roughly halves the swing. What it does not do is halve the discomfort, because faster cycling brings its own problems: more starts on the compressor and the blower, more time spent at the beginning of a run when a coil is producing little useful capacity, and on ducted systems a draught of not-quite-conditioned air each time the fan restarts. Comfort complaints that survive a sensible CPH setting are usually about airflow distribution or a poorly placed sensor rather than the cycle rate itself.

Does a two-stage or inverter system still have a cycle rate?

Much less of one, and that is the point of buying it. A two-stage unit spends most of a mild season on its low stage, which is a better match to the load, so calls are longer and starts fewer — the cycle-rate parameter then governs mainly how the control moves between stages and how long it waits before calling the second one. A variable-speed inverter goes further and modulates output continuously, ideally settling at whatever capacity balances the load and running for hours without stopping, in which case the cycle rate falls close to zero and the setting becomes largely irrelevant. The residual cycling that remains happens at the bottom of the modulation range, when even minimum output exceeds the load, and at that point a very small system serving a very small load is back to ordinary on-off behaviour.