Key information
- Starting point: Design heat load
- Unit check: Capacity at the required A/W condition
- Final check: Minimum modulation, hydraulics and power supply
Heat pump sizing: from building load to a suitable model.
Good sizing starts with heat loss at the design outdoor temperature and ends with a check of actual unit capacity at the required water temperature. Floor area and nominal output are not enough.
Collect the right inputs
You need location, design outdoor temperature, heat loss, emitters, water temperature, DHW, power supply and sound constraints. An estimate can guide early work but cannot replace technical verification.
- Separate space-heating and DHW duties
- Use the design leaving-water temperature
- Confirm space, sound and electrical limits
Compare capacity at real conditions
Use manufacturer tables at the proposed outdoor and water temperatures. Also check minimum modulation in mild weather and any planned electric backup.
- Available output below zero
- COP at the design point
- Cycling and active water volume at part load
Selection sequence
| Step | Result |
|---|---|
| 1. Load | kW required at the design outdoor temperature. |
| 2. Water temperature | Flow temperature required by the emitters. |
| 3. Model | Available capacity at the same A/W condition. |
| 4. System | Flow, water volume, DHW, electrical and sound checks. |
Mistakes to avoid
- Applying one W/m² figure to every building.
- Selecting from A7/W35 for a different design point.
- Ignoring minimum modulation and shoulder-season load.
Frequently asked questions
Can historical energy use size the heat pump?
It can help when adjusted for weather, generator efficiency and actual occupancy, but should be checked against design heat loss.
Must the compressor cover 100% of design load?
That depends on the energy strategy and design temperature; state the bivalent point and any backup explicitly.