The short answer
Flow temperature is the temperature of the water a heat pump sends out to your radiators or underfloor heating. It is one of the most important numbers in a heat pump system, because efficiency is tied directly to it: the lower the flow temperature, the higher the efficiency (SCOP) and the cheaper the system is to run. Heat pumps are designed to run at a much lower flow temperature than gas boilers — commonly around 35 to 55 °C, versus a boiler's typical 60 to 80 °C. The reason is physics: the smaller the gap the pump must bridge between the outside heat source and the flow temperature, the less work the compressor does. To stay comfortable at a low flow temperature, a home needs larger radiators or underfloor heating and good insulation. Many systems also use weather compensation to nudge the flow temperature up and down with the outside conditions.
If you understand flow temperature, you understand why heat pump installations focus so heavily on radiator sizing and insulation. It is the lever that decides whether a heat pump is cheap or expensive to run.
Flow temperature basics
- What it isTemperature of water leaving the pump
- Typical heat pump rangeAround 35 to 55 °C
- Typical gas boiler rangeAround 60 to 80 °C
- Lower flow temp meansHigher efficiency, lower bills
- Enabled byLarger radiators / underfloor heating
Flow temperature in plain terms
Every wet central-heating system circulates hot water from the heat source out to the rooms and back again. The temperature of the water as it leaves the heat source on its way to the radiators is the flow temperature; the cooler water returning is the return temperature. With a gas boiler this is rarely discussed, because boilers simply run hot — often 70 °C or more — and the householder never thinks about it.
With a heat pump, flow temperature becomes the headline number, because the pump's efficiency rises and falls with it. A heat pump is not trying to make water as hot as possible; it is trying to deliver just enough warmth to keep the home comfortable, as gently as possible. That is a different design philosophy from a boiler, and flow temperature is where it shows up. Once you grasp that lower flow temperature equals higher efficiency, the rest of heat pump design — bigger radiators, better insulation, weather compensation, steady running — follows logically from the single goal of keeping that number down.
Why a lower flow temperature means higher efficiency
A heat pump's job is to lift heat from the outside source up to the temperature needed to heat your home. The bigger that lift, the harder the compressor works and the more electricity it uses. Flow temperature sets the top of that lift.
Run the system at 35 °C flow — typical of underfloor heating — and the lift is modest, so the COP and SCOP are high. Push the flow to 55 °C to suit smaller radiators, and the lift is larger, so efficiency falls and running costs rise. As a rough guide, every few degrees of flow temperature you can shave off improves efficiency. This is why a well-designed heat pump installation is, in large part, an exercise in getting the home comfortable at the lowest practical flow temperature.
| Flow temperature | Typical use | Relative efficiency |
|---|---|---|
| Around 35 °C | Underfloor heating | Highest |
| Around 45 °C | Larger radiators | Good |
| Around 55 °C | Standard / smaller radiators | Lower |
Illustrative relationship between flow temperature and efficiency. Sources: Energy Saving Trust; MCS.
The link to radiator sizing
A radiator's heat output depends on how much hotter it is than the room. A gas boiler running at 70 °C makes a radiator very hot, so even a small radiator throws out plenty of heat. A heat pump running at 45 °C makes the same radiator much cooler, so it emits far less heat.
To deliver the same warmth at a lower flow temperature, the radiator must be larger (taller, wider, or double-panel) or replaced by underfloor heating, which has a huge surface area and works beautifully at low temperatures. This is why a heat pump installation often involves upsizing some radiators. The MCS heat loss survey calculates the output each room needs and specifies emitters that can deliver it at the chosen low flow temperature.
It helps to see flow temperature and radiator size as two ends of the same trade-off. For a given room, you can deliver its heat with a smaller radiator at a higher flow temperature, or a larger radiator at a lower flow temperature. The first option is cheaper to fit but more expensive to run; the second costs a little more up front in radiators but saves on every heating bill because the pump runs more efficiently. A good design leans towards the larger emitters wherever it is practical, because the radiator is a one-off cost while the efficiency saving repeats for the life of the system. Not every radiator has to change, though — rooms that already have generously sized or double-panel radiators may need no work at all, and the survey identifies exactly which ones do.
Weather compensation: flow temperature that adapts
Most modern heat pumps use weather compensation, a control feature that varies the flow temperature automatically based on the outside temperature. On a mild day the system runs at a lower flow temperature (saving energy); as it gets colder, it raises the flow temperature just enough to keep the home warm.
This is more efficient than running a fixed high flow temperature all winter, because the system only uses a high flow temperature when it genuinely needs to. An outdoor sensor feeds the controller a continuous reading, and the controller follows a "heating curve" set up during commissioning. Properly configured weather compensation keeps the flow temperature as low as conditions allow, which is exactly what keeps a heat pump's running costs down.
This is also why a heat pump is happiest left running steadily rather than switched on and off like a boiler. Turning the system off for long periods lets the house cool, and recovering that lost warmth later forces the pump to run at a higher flow temperature for a while — exactly the opposite of what weather compensation is trying to achieve. A heat pump that holds a stable indoor temperature, gently topping up the heat the house loses, spends most of its time at the lowest practical flow temperature and therefore the highest efficiency. The mindset shift from "blast it when cold, off when out" to "leave it ticking over" is one of the simplest ways a household keeps its flow temperature, and its bills, down.
Frequently asked questions
Why do heat pumps use a lower flow temperature than boilers?
Because efficiency depends on it. The smaller the gap between the outside heat source and the flow temperature, the less work the pump's compressor does, so a lower flow temperature gives a higher SCOP and lower running costs. Boilers can run hot with little efficiency penalty, but heat pumps are designed around keeping the flow temperature as low as the home allows.
Do I have to replace all my radiators for a heat pump?
Not always all of them. The heat loss survey checks each radiator's output at the design flow temperature; some will already be large enough, while others may need upsizing. Better insulation reduces a room's demand and can avoid changes. Underfloor heating, where present, works especially well at low flow temperatures.
What is weather compensation on a heat pump?
Weather compensation automatically adjusts the flow temperature based on how cold it is outside. On mild days it runs a lower flow temperature to save energy, raising it only as the weather gets colder. An outdoor sensor and a configured heating curve keep the flow temperature as low as conditions allow, improving efficiency over a fixed setting.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific home. They are guidance, not a quotation or guaranteed saving.