HeatPumpSwitch › Articles › Why your installer's SCOP number is optimist
Why your installer's SCOP number is optimistic — and what to use instead
Datasheet SCOP is measured at 35 °C flow in a laboratory. Your house is not a laboratory. Here is how to convert a quoted SCOP into something you can put in a spreadsheet.
Every heat pump datasheet carries a SCOP, and it is not a lie. It is a measurement made under EN 14825 at a specified flow temperature — usually 35 °C, sometimes 55 °C — with a defined climate profile. The problem is not the number. The problem is that it is quoted for a system that will not run under those conditions.
Three separate optimism gaps
Flow temperature. The 35 °C figure applies to underfloor heating, or to radiators sized generously for low-temperature operation. If your quote does not state a design flow temperature, assume the installer has not calculated one, and assume 55 °C. That takes a 4.2 datasheet SCOP to something near 2.8 in practice.
Hot water. A cylinder has to be reheated to 48–50 °C, and periodically higher for legionella control. Domestic hot water typically runs at a COP between 2.0 and 2.5 regardless of how good your heating design is. If hot water is 15 % of your annual heat, it pulls the whole-system figure down by roughly 0.2.
Cycling and controls. An oversized heat pump at part load switches on and off instead of modulating. Field studies consistently find that badly commissioned systems lose 10–15 % against identical hardware correctly set up. This is the gap no datasheet can capture, because it is a property of the installation rather than the machine.
A defensible way to estimate your own number
- Start from the datasheet SCOP at the flow temperature closest to your design — the table in the technical manual, not the headline on the brochure.
- Subtract 0.2 for hot water, if it is included in your annual demand.
- Subtract a further 10 % if the quoted output is more than 30 % above your calculated heat loss.
- Use the result. For most British retrofits it lands between 2.7 and 3.3.
Our SCOP simulator does this in money terms: annual cost and payback at every flow temperature from 35 °C to 60 °C, so you can see what the difference between an optimistic and a realistic figure is worth over ten years.
The one question that settles it
“At the design outdoor temperature, what flow temperature does this system need, and which room sets that limit?” An installer who has done a room-by-room heat-loss calculation answers in one sentence. An installer who has not will change the subject to the brand of the unit.
Advertising
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.
Ad. If you request a quote through this link we may be paid a commission by the installer. It does not change your price, and we do not rank companies by how much they pay.
Questions people ask
Is datasheet SCOP accurate?
It is an accurate laboratory measurement at a stated flow temperature, usually 35 °C. It is not a prediction of what your system will achieve, because your radiators determine the flow temperature your system has to run at.
What SCOP should I assume for a UK retrofit?
For existing radiators with no emitter work, 2.8 to 3.0 is a defensible planning assumption. With deliberately oversized radiators at 45 °C, 3.4. With underfloor heating throughout, 3.9.