Heat-pump compressor selection in five minutes
By DigiEntropy Engineering · 2026-02-07 · 6 min read
Air-to-water heat pumps need a compressor sized for HEATING capacity at the design ambient, not cooling at MT. Most selection tools default to cooling. DigiEntropy's Heat Pump page flips the entire workflow to heating mode so the selection actually matches the duty.
About this post — Authored by an AI assistant using DigiEntropy's polynomial corpus, the universal compressor predictor, and the project's chart-generation tooling. Charts are produced by Python scripts that read the same database the live site queries; tables and formulas are pulled from the same engineering modules. Findings reflect the project's current dataset and methodology — send corrections or deeper questions to admin@digientropy.com.
A heat-pump compressor is sized differently from a refrigeration compressor. The duty is heating capacity at the design ambient (often −7 °C outdoor in northern Europe), not cooling at a benign MT condition. The COP that matters is the heating COP across a whole season, not the nameplate at a single point.
Most vendor selection tools default to cooling. You have to know they have a heating switch, find it, flip it, and re-pick a model. The DigiEntropy Heat Pump page starts in heating mode and never leaves it.
−7 °C - typical design ambient - northern Europe
The duty is heating capacity at the design ambient (often −7 °C outdoor in northern Europe), not cooling at a benign MT condition.
75 °C - TC limit, sanitary water heat pumps - heating-specific envelope
The heating-specific operating envelope from the vendor: TE typically -15 to +15 °C on the source side, TC up to 75 °C for sanitary water heat pumps.
20 kW - R290 wins below this - R744 wins for sanitary DHW
R290 wins below 20 kW (charge-limited but cheap, A3-rated compressors widely available). R744 wins for sanitary DHW (high TC, GWP 1).
247 - R290-qualified compressors - across 8 brands
R290 hub page: 247 R290-qualified compressors across 8 brands.
30 % - oversizing that cycles short - 10 % undersized is usually the right trade
A heat pump oversized by 30 % cycles short, runs at poor part-load COP, and wears the contactor. A heat pump undersized by 10 % runs full-load most of the heating season and hits the heating element a few times a year, usually the right trade.
4 % - annual kWh margin, worked example - Bitzer, German Köln climate
Annual-cost simulation (German Köln climate): Bitzer wins by 4 % on annual kWh because the design-point dominates the energy bin distribution.
5 min - to a defensible spec - from the Heat Pump page entry
Total time: 5 minutes from the Heat Pump page entry to a defensible spec.
What changes in heating mode
Three things flip when you toggle Heating, one per tab:
Capacity polynomial
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