Flow temperature & efficiency

Flow temperature — how hot the water is that a heat pump sends to your radiators or underfloor heating — is the single biggest factor you control that affects efficiency. This guide explains why lower is better, the trade-off with your heat emitters, and how to read the flow-temperature data in this database.

What "flow temperature" means

The flow temperature (or leaving water temperature) is how hot the heat pump heats the water it sends out to the heating system. It is the "W" number in a rating such as A7/W35, where 35 is a 35°C flow. The water gives up its heat to the rooms and returns cooler — the return temperature.

Why lower flow temperature means higher efficiency

A heat pump's efficiency is governed by the "lift": the gap between the source temperature and the flow temperature. The smaller that gap, the less work the compressor does, and the higher the COP and SCOP. This is fundamental thermodynamics, not a quirk of any one model.

As a rule of thumb, every 1°C you can shave off the flow temperature improves efficiency by roughly 2–2.5%. Dropping from a boiler-style 55°C to 35°C can lift seasonal efficiency substantially — often taking a SCOP from around 3 to comfortably above 4. The numbers below are indicative, to show the shape of the relationship.

The trade-off: your heat emitters

The catch is that a lower flow temperature means each radiator gives out less heat, so you need more emitting surface to warm the room. Underfloor heating is ideal — a huge surface area running happily at 30–40°C. Existing radiators sized for a 70–80°C gas boiler are often undersized for 35–45°C operation and may need upsizing. Matching emitters to a low flow temperature is the central design task of a heat pump installation, and the main reason a proper heat-loss survey matters.

Weather compensation

Modern heat pumps use weather compensation: they raise the flow temperature only as the weather gets colder, and keep it as low as possible the rest of the time. Leaving this enabled — rather than fixing a high flow temperature year-round — is one of the easiest ways to protect efficiency, and it is how the low seasonal figures are actually achieved in practice.

Hot water is the exception

Domestic hot water has to be stored hotter — typically around 50–60°C, partly to manage legionella — so the heat pump briefly runs at a high flow temperature to reheat the cylinder, at lower efficiency. That is normal and unavoidable; what matters is that space heating stays at a low flow temperature the rest of the time. High-temperature R290 units can reach the hot-water temperatures directly without an electric backup heater.

Maximum flow temperature & high-temperature heat pumps

Every model has a maximum flow temperature (shown here as the upper end of its flow range). High-temperature heat pumps — usually running R290 (propane) — reach 70–75°C, which lets them replace a boiler on existing radiators with little or no upgrading. That is genuinely useful for difficult retrofits, but running permanently hot sacrifices efficiency. Treat a high ceiling as headroom for the coldest days, not an everyday target.

Reading the data here

• The flow temperature range is listed for each model; the upper figure is the hottest it can deliver.
• The COP/SCOP conditions tell you the flow temperature each efficiency figure was measured at — an A7/W35 COP and an A7/W55 COP describe very different operation.
• Design for the lowest flow temperature your emitters allow, then pick units that are efficient at that temperature.

Reviewed June 2026. This guide is general information, not professional or design advice; a heat-loss survey by a qualified installer is the basis for any real system design.