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SCOP reality simulator: catalogue 4.2 vs field 2.8
The SCOP on the datasheet is measured at 35 °C flow temperature in a laboratory. Your system will not run at 35 °C unless someone designed it to. This shows what that difference does to your bill and your payback.
Where each SCOP actually comes from
A 4.2 on the box and a 2.8 in your house are the same machine. What changed is the water temperature it has to produce, and that is set by the size of your radiators relative to your heat loss. This is why the only quote question that matters is “what design flow temperature is this system based on?”
| Design flow temperature | SCOP | Typical system |
|---|---|---|
| 35 °C | 3.9 | Underfloor heating throughout, or a well-insulated new build |
| 45 °C | 3.4 | Radiators deliberately oversized during the design |
| 50 °C | 3.0 | Existing radiators, a couple of the worst ones replaced |
| 55 °C | 2.8 | Existing radiators untouched, insulation as found |
| 60 °C | 2.5 | Undersized radiators, or a system set high to stop complaints |
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Get quotes for your property
Compare at least three installers. Ask each one for the heat-loss calculation and the design flow temperature in writing — that number decides your running cost more than the brand on the box.
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