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. What changes in heating mode Three things flip when you toggle Heating: - Capacity polynomial evaluates to Qh = Qe + P (heating output = evaporator capacity + compressor power). The Heat Pump page uses Qh directly so the column you sort on is the one that matters for the duty. - Envelope is 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). The cooling envelope's HP corner doesn't apply. - Catalog filter drops models without a published heating envelope or heating polynomial. Many cooling compressors are not qualified for the high TC heat pumps run at — the page hides them rather than letting you spec one that fails at commissioning. The 5-minute workflow 1. Set design ambient + supply temperature. Northern-Europe air-to-water: -7 / 55 °C is the EN 14511 standard. Southern-Europe: 2 / 45 °C. Sanitary DHW: 7 / 65 °C. 2. Pick refrigerant. R290 wins below 20 kW (charge-limited but cheap, A3-rated compressors widely available). R744 wins for sanitary DHW (high TC, GWP 1). R32 still dominates in residential splits but is being phased toward A2L low-GWP successors. 3. Read the ranked list. The page returns compressors sorted by heating COP at the design point. The capacity column is Qh, not Qe. 4. Pivot to Benchmark for top 2-3 candidates. The Heating switch carries through, so you can overlay heating curves directly. Where the COP curve goes wrong The seasonal COP is not the nameplate COP. It's a load-weighted average across the year's ambient distribution. A compressor with a strong nameplate at +2 °C ambient but a…