Compressor modulation is an inverter heat pump's ability to vary output continuously between a minimum and maximum — typically 30–100% of rated capacity, a turndown ratio of about 3:1. Instead of stopping and starting, the compressor slows to match the heat load, which lengthens run times, prevents short cycling and lifts seasonal efficiency.
How does compressor modulation work?
An inverter compressor is driven by a variable-frequency drive that changes the motor speed, so refrigerant mass flow — and with it heating output — rises and falls smoothly with demand. A fixed-speed compressor knows only on and off; a modulating one behaves like a dimmer instead of a switch, holding a steady flow temperature at a fraction of full power.
Modulation is what allows a single machine to serve both a -10 °C design day and a mild October evening. The controller reads the load — via return temperature, weather compensation curve or room feedback — and settles the compressor at the lowest speed that holds the target, where the machine is also at its quietest.
What is a turndown ratio and a modulation floor?
The turndown ratio compares maximum to minimum continuous output: a unit that modulates between 30% and 100% has a 3.3:1 turndown, and the 30% point is its modulation floor — the smallest output it can sustain without stopping. The floor, in kW rather than percent, is the number that matters in design.
| Rated output | Floor at 30% | Typical mild-weather load, insulated home |
|---|---|---|
| 6 kW | 1.8 kW | 1–2 kW |
| 12 kW | 3.6 kW | 1–2 kW |
| 16 kW | 4.8 kW | 2–3 kW |
The table shows why oversizing hurts twice: a 12 kW unit on a house that needs 6 kW at design conditions spends most of the year below its 3.6 kW floor, forced into the stop-start operation described under short cycling. A correctly sized 6 kW unit rides the same load comfortably inside its modulation band — one reason Is my heat pump oversized? is the first question to ask about a cycling system.
Can good modulation replace a buffer tank?
Sometimes. The classic case for a buffer tank is absorbing the mismatch between a fixed compressor output and a variable load; a deep modulation range shrinks that mismatch at the source. A correctly sized inverter unit feeding an open emitter circuit — no zone valves choking flow, radiators or underfloor loops permanently available — can often run without one.
Modulation does not, however, add water volume. Systems with aggressive zoning, microbore pipework or very small emitter content may still need litres for stable operation and defrost energy — supplied by a buffer tank, which also separates zoned circuits hydraulically, or, where only volume is missing, by an in-line volumiser. The decision tree is laid out in Do I need a buffer tank with a heat pump?.
Does running at part load improve efficiency?
Yes — within the modulation band, part-load operation is where inverter heat pumps shine. At reduced compressor speed the heat exchangers are oversized relative to the refrigerant flow, temperature differences shrink, and the compression ratio eases, so COP at 50% speed is typically higher than at full speed. The European seasonal metrics are built around this behaviour: EN 14825 measures performance at several part-load points to produce the SCOP, and European field programmes consistently show that units allowed to modulate steadily outperform the same hardware forced to cycle. Solimpeks applies this logic in its Varm Up Series, whose EVI DC inverter compressors let the unit spend the heating season low, slow and continuous.
Frequently asked questions
Is a bigger turndown ratio always better?
A deeper turndown gives more flexibility, but the absolute floor in kW matters more than the ratio. A 6 kW unit with a 3:1 turndown reaches a lower minimum output than a 16 kW unit with a 5:1 turndown. Size the unit to the heat load first, then compare modulation ranges.
What happens when the heat load falls below the modulation floor?
The compressor cannot run slower, so it stops, waits for temperatures to drift, and restarts — short cycling. Controls soften this with hysteresis and run-on logic, but sustained sub-floor loads need either more connected volume or a smaller unit.
Do all inverter heat pumps modulate equally well?
No. Modulation floors commonly range from roughly 25% to 50% of rated output depending on compressor design and how the manufacturer maps the drive. Datasheets state minimum capacity at defined conditions — compare that kW figure, not just the marketing term inverter.
Does modulation make a heat pump quieter?
Yes. Sound power rises with compressor and fan speed, so a unit idling at 30–40% output is markedly quieter than at full power. Correct sizing therefore helps with noise compliance as well as efficiency.