Flow temperature: the one number that decides your running cost

What flow temperature actually means

Flow temperature is the temperature of the water leaving the heat pump and heading for your radiators or underfloor circuits. Return temperature is what comes back. A gas boiler system typically runs 70 to 80C flow. A heat pump runs most efficiently at 35 to 45C, and MCS design practice commonly sets a design flow temperature of 45 to 55C on a radiator retrofit.

Design flow temperature is the figure the whole system is engineered around. It is chosen once, at survey, and every other decision follows from it: the size of the unit, which radiators change, how much the job costs and what you pay to run it for the next two decades.

Why higher costs you money

A heat pump works by moving heat across a temperature gap. The wider the gap between the outside air and the water you demand, the harder the compressor has to work, and the more electricity it takes to deliver the same heat. This is not a manufacturer quirk. It is the physics the whole technology sits on.

Samsung publishes figures that make the cost visible in a single comparison. Its EHS Mono R290 5 kW model is quoted at SCOP up to 4.84 at 35C flow. Its EHS Mono HT Quiet range at 65C flow is quoted at SCOP 2.82 for the 8 kW, 3.01 for the 12 kW and 3.03 for the 14 kW. Same manufacturer, same generation of technology, roughly 40 percent more electricity for the same heat at the higher temperature.

Flow temperatures shown are design values for space heating. Hot water cycles run hotter regardless, which is a separate and much shorter part of the year's energy.

The trade you are actually making

Every room has one requirement: deliver enough heat to hold the design temperature on the coldest day. You can satisfy it with hotter water or with a bigger emitter. Those are the only two options, and the choice between them is a capital cost against a running cost.

Radiator output falls fast as water cools. At 45C flow and 40C return, a radiator delivers roughly 42 percent of its Delta T 50 rated output, so a 1,500 W radiator gives about 630 W. Holding the same room temperature at heat pump temperatures needs roughly 2.4 to 2.5 times the Delta T 50 rated output. In practice that means deeper panels rather than radiators the length of the wall, and it usually applies to a minority of rooms rather than all of them. The full arithmetic is in do you need new radiators for a heat pump.

Modern units can go high, which is a safety net not a plan

The move to R290 refrigerant across flagship lines changed what is possible. Vaillant quotes the aroTHERM plus at flow temperatures up to 75C, and states that this allows cylinder hot water without an immersion heater top up for legionella protection. The Mitsubishi Electric Ecodan R290 datasheet quotes flow up to 75C. Daikin's Altherma 3 H HT high temperature range quotes up to 70C, and Samsung's HT Quiet range quotes hot water up to 70C.

That capability is genuinely valuable. It means an older, hard to treat house is never technically stranded, and it means hot water does not need electric help. What it must not become is an excuse to design the whole space heating system at 65C because it saves changing four radiators.

What to ask for, in three questions

1. What design flow temperature is this quote based on, as a single number.
2. Which rooms fail at that temperature with their existing radiators, and what does each replacement cost.
3. What would the design flow temperature drop to if I upgraded those radiators, and what does that do to the projected annual electricity use.

Those three answers turn a quote from a price into a decision. They come out of the room by room heat loss calculation that the MCS design standard requires before any certified installation, described in what a proper heat loss survey covers.

Flow temperature and your tariff

Efficiency and price per unit are separate levers, and both are worth pulling. A lower flow temperature reduces how many kWh you buy. A heat pump tariff reduces what each kWh costs. Suppliers including Octopus, British Gas and E.ON Next run heat pump specific structures, and the interaction with your system design is covered in running costs versus a gas boiler and our tariffs guide.

Every county page on this site carries a local calculator, so check your county for the local picture, and the full heat pumps guide covers the rest of the system. If solar panels are also on your list, they change the arithmetic on daytime running further still.

Clearline gives independent guidance and never installs. Work is carried out by an MCS-certified installation team for your area. Product specifications are manufacturer sourced and were checked on the date shown.