Resources / Heat pumps
Understanding Flow Temperature and Heat Pump Efficiency
Flow temperature is one of the biggest levers on running cost and comfort. Lower — when the house can still get warm — is almost always better for efficiency.
Quick Answer
Flow temperature is the temperature of the water leaving the heat pump into the heating system. The lower it can be while still meeting room comfort, the more efficiently the heat pump runs (higher SCOP, lower bills).
Well-designed systems often run space heating around 35–45°C. Systems pushed to 50–55°C+ because radiators are too small or heat loss is high give away efficiency. Emitters and fabric decide how low you can go — not the outdoor unit brand alone.
View printable checklist Planning aid — not a quote
What flow temperature is
It is the target (or actual) temperature of water sent to radiators or underfloor heating loops. Return temperature is what comes back; the difference relates to how much heat was dropped into the rooms.
Old gas and oil systems often ran much hotter water. Heat pumps prefer cooler water because the refrigeration cycle works harder the larger the temperature lift from outdoor air to the water you want.
Why it matters so much
| Lower flow temperature | Higher flow temperature |
|---|---|
| Better seasonal efficiency (SCOP) | Worse efficiency for the same outdoor conditions |
| Lower running costs when comfort is still met | Higher electricity use per unit of heat |
| Needs enough emitter surface (or UFH) | Often a patch for small radiators or high heat loss |
| Matches underfloor heating well | Can still “work” but costs more to run |
Turning the flow up can mask weak emitters for a while. It does not fix design. Running costs guide: heat pump running costs.
Typical temperatures in practice
- •~30–40°C — common design territory for good UFH and low-loss homes
- •~40–45°C — frequent real-world space-heating band when radiators are upgraded sensibly
- •~50°C+ — often a sign the system is compensating for emitters or fabric; efficiency suffers
- •Hot water — cylinder prioritisation is separate from steady space-heating flow; design both honestly
Exact numbers depend on heat loss and emitter schedules. Ask what flow temperature the design assumed — and whether radiators/UFH were sized at that figure.
Weather compensation
Weather compensation adjusts flow temperature with outdoor temperature: cooler water in mild weather, warmer only when needed. Done properly, it keeps average flow temperatures down over the season.
A fixed high flow all winter is simple and inefficient. A curve that is set too aggressive leaves rooms cold; too weak wastes electricity.
Commissioning the curve to your emitters and house matters as much as the hardware.
Weather compensation vs deep setbacks
Deep overnight setbacks on a heat pump (especially with underfloor heating or solid walls) can force a hard recovery: higher flow, longer run times, more energy — and rooms that feel “always behind.”
Steady setpoints with weather compensation often outperform aggressive on/off schedules that people bring from old gas habits. Response times: UFH not warming fast enough.
Emitters decide how low you can go
The heat pump can only run low flow temperature if the radiators or floor can still put enough heat into each room.
- •Small original radiators → need higher water temperature or upgrades
- •Larger radiators / fan-assisted emitters → lower flow possible
- •Well-designed UFH → naturally low-temperature friendly
Older South West homes
Solid-wall cottages and mixed-age stock often start with high heat loss and small emitters. The efficiency story only works if you either reduce loss, increase emitter capacity, or accept higher flow — and higher bills.
Hoping the outdoor unit will “just run hotter” forever is how people end up saying heat pumps are expensive. What works in older homes.
What to ask on a quote
- What design flow temperature were emitters sized at?
- What outdoor design temperature and heat loss (kW) are you using?
- What capacity does the unit have at those conditions?
- Will weather compensation be set up and explained at handover?
- If we keep existing radiators, what flow temperature is implied?
Honest bottom line
Flow temperature is not a minor setting. It is the bridge between the heat pump’s efficiency and the house’s ability to emit heat.
Design for the lowest flow that still delivers comfort. Size emitters and fabric for that goal. Use weather compensation. Treat permanent high flow as a warning light, not a strategy.
Frequently asked questions
Is 55°C always bad?
It can be necessary for hot water or short recovery periods. As a permanent space-heating flow on a heat pump, it usually means efficiency is being left on the table.
Will underfloor heating always run at 35°C?
Often in that region when designed well — not a law of nature. Room heat loss, pipe spacing and floor finish still decide output.
Can software fix high flow temperatures?
Controls help optimise within physical limits. They cannot make small radiators emit more heat than physics allows at a given water temperature.
Does a higher SCOP always mean lower bills?
Higher SCOP helps, but tariff, heat loss and how you operate the system matter too. Flow temperature is a major driver of the SCOP you actually achieve.
Next step
Get a realistic planning band for your home, then use the checklist when you speak to installers.