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MCS, SCOP, COP, flow temperature: the five terms that decide your bill
Five pieces of jargon do all the work in a heat pump quote. Understand these and you can read any quote in the country; miss them and you are comparing prices with no idea what you are buying.
1. Flow temperature
The temperature of the water leaving the heat pump and going to your radiators. It is the most important number in your quote, because everything else follows from it. Every 5 °C lower is worth roughly 0.2 on the seasonal efficiency. It is set by your emitters, not by the heat pump. If a quote does not state a design flow temperature, the design has not been done.
2. COP
Coefficient of performance: instantaneous heat out divided by electricity in, at one specific pair of temperatures. A COP of 4 at 7 °C outside and 35 °C flow is a real measurement and a useless planning number, because your coldest days are not 7 °C. Treat a single COP figure in marketing material as a headline, not a spec.
3. SCOP
Seasonal coefficient of performance: the same ratio averaged across a defined heating season under EN 14825, at a stated flow temperature. This is the number to use — but only alongside the flow temperature it was measured at. “SCOP 4.2” without “at 35 °C” is half a sentence. See why installer SCOP figures run optimistic.
4. MCS
The Microgeneration Certification Scheme. In England and Wales an MCS-certified installer is a hard requirement for a Boiler Upgrade Scheme grant, and the installer — not you — applies for the voucher. MCS certifies the company and its processes; it is not a guarantee that a particular engineer designed your system well, so ask for the heat-loss calculation regardless. We are not MCS certified, not accredited and not an installer; check any company's certification on the MCS register itself.
5. Heat loss
The kilowatts your house needs at the design outdoor temperature, calculated room by room to EN 12831. This sets the size of the heat pump and, via each room's radiator, the flow temperature. Every number in the quote depends on it, which is why “we always fit a 12 kW” is a red flag rather than reassurance. Our sizing calculator gives a rough whole-house figure to sanity-check a quote against.
How they fit together
Heat loss decides the output. Output plus radiator size decides the flow temperature. Flow temperature decides the SCOP. SCOP plus your electricity tariff decides your bill. MCS decides whether you get the grant. That chain is the entire subject, and four of its five links are settled before anyone arrives with a heat pump.
Take the installer question checklist to your quotes and write the answers on it.
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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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Questions people ask
What is the difference between COP and SCOP?
COP is an instantaneous efficiency at one pair of temperatures. SCOP is a seasonal average measured under EN 14825 at a stated flow temperature. SCOP is the planning number, but only when quoted with its flow temperature.
Do I need an MCS-certified installer?
For a Boiler Upgrade Scheme grant in England and Wales, yes — and the installer applies for the voucher on your behalf.