Heat pump flow temperature and SCOP
The single biggest design decision for a heat pump's running cost is the flow temperature. The cooler the water it has to make, the less work the compressor does, and the more heat you get for each unit of electricity.
Here is how flow temperature, SCOP and radiator size fit together.
What SCOP means
COP (coefficient of performance) is the heat out divided by the electricity in at one moment. SCOP (seasonal coefficient of performance) is the average over a whole heating season. A SCOP of 3.5 means 3.5 kWh of heat for every 1 kWh of electricity across the year.
Running cost ≈ heat needed (kWh) ÷ SCOP × electricity price
How flow temperature changes SCOP
Every degree lower makes the heat pump more efficient. As a rough rule, efficiency improves by a couple of percent for each degree the flow temperature drops. A system designed for 35 °C underfloor heating will usually have a much higher SCOP than one designed for 55 °C radiators.
| Design flow temperature | Typical emitters | Efficiency |
|---|---|---|
| 35 °C | Underfloor heating | Highest |
| 40 to 45 °C | Larger or upgraded radiators | High |
| 50 to 55 °C | Mostly original radiators | Lower |
The exact figures depend on the heat pump model. Use the manufacturer's SCOP data at your design flow temperature.
The trade-off with radiators
Lower flow temperatures need bigger radiators, because each radiator gives out less heat. The design sweet spot is the lowest flow temperature you can reach without unreasonable radiator changes, often between 40 °C and 50 °C in a retrofit.
Weather compensation
The design flow temperature is only needed on the coldest day. Weather compensation lowers the flow temperature automatically as the outside temperature rises, so for most of the season the heat pump runs well below its design flow temperature. Setting the compensation curve properly is one of the cheapest ways to improve a system's real-world SCOP.
Do it with the free tool
Choose a design flow temperature in the free tool and it shows the radiator sizes it needs room by room, so you can find the sweet spot.
Frequently asked questions
What is a good flow temperature for a heat pump?
As low as the emitters allow. Underfloor heating often runs at 30 °C to 40 °C, and radiator systems are commonly designed for 40 °C to 50 °C. Lower is more efficient.
What is a good SCOP for a heat pump?
It depends on the flow temperature and the model, but a well-designed radiator system often achieves a SCOP of 3 or more, and low-temperature underfloor systems higher still.
What is the difference between COP and SCOP?
COP is the efficiency at one set of conditions. SCOP is the average efficiency over a whole heating season, so it's the better guide to running costs.
What is weather compensation?
A control that lowers the heat pump's flow temperature as the weather gets milder. It keeps the heat pump running as efficiently as possible for most of the year.
Does a lower flow temperature mean a colder house?
No, as long as the radiators are sized for it. The rooms reach the same temperature; the radiators are just larger and run cooler.
Related guides
Sources: manufacturer SCOP data (EN 14825); MCS MIS 3005-D; Energy Saving Trust heat pump guidance. Free Heat Pump Design follows the MCS method but is not an MCS product and is not endorsed by MCS. Check designs against MCS MIS 3005-D and the official MCS Heat Pump Calculator.