Heat Pump Flow Temperature Explained (And What to Set)

If you have an air source heat pump, you have probably seen a setting called “flow temperature” and wondered what to do with it. Getting this setting roughly right is one of the biggest factors in how comfortable your home feels and how much you pay to run the system.

What flow temperature means on a heat pump

Flow temperature is simply the temperature of the water leaving your heat pump and travelling around your radiators or underfloor heating. The cooler water that comes back to the unit is called the return temperature.

On a boiler, it is common to set a high flow temperature and let the system blast heat in short bursts. A heat pump works differently. It prefers to run steadily at a lower flow temperature, topping up the heat gently over longer periods.

The lower you can run the flow temperature while still keeping your house warm, the more efficient the heat pump will be. In many homes this sweet spot is somewhere between about 30 °C and 45 °C for heating, but it depends on your radiators, insulation and local climate.

How heat pump flow temperature differs from a boiler

If you are used to a gas or oil boiler, it is natural to think “it is cold, I will just turn the flow temperature up.” With a heat pump, that approach can easily backfire. A higher flow temperature makes the compressor work harder, which reduces efficiency and can shorten component life.

Instead of short, hot bursts, a correctly set heat pump will:

• Run for long, steady periods at a modest flow temperature

• Keep rooms at an even, comfortable background temperature

• Use less electricity over the course of the day

It is normal for radiators on a heat pump to feel only warm or even lukewarm when the room is up to temperature. That does not mean the system is not working. It means it is maintaining the set temperature efficiently.

Signs your flow temperature may be too high or too low

Finding the right setting is partly about watching how your system behaves. Typical symptoms can point you in the right direction, although any major changes should be discussed with your installer or engineer.

Common signs the flow temperature is too high

A flow temperature that is set unnecessarily high can cause a few familiar issues. You may notice the heat pump starting and stopping frequently, especially on milder days, which is known as “cycling”.

Other clues include the unit running noisily at high power more often than expected, and radiators that feel very hot for short periods then go cold. You might also see higher than expected electricity bills, even though the house does not feel particularly warm.

Common signs the flow temperature is too low

On the other hand, if the flow temperature is too low for your particular property and radiators, the system will struggle to keep up. Rooms may never quite reach the set temperature, especially on colder or windy days.

You might find radiators are only lukewarm even when the thermostat is calling for heat, and the heat pump runs for very long periods without the house feeling truly comfortable. In extreme cases, some distant rooms or upstairs spaces may stay noticeably cooler than the rest of the home.

Heating curves and weather compensation made simple

Most good air source heat pump controls use weather compensation, also called a heating curve. Instead of one fixed flow temperature, the controller looks at the outdoor temperature and adjusts the flow temperature up and down automatically.

When it is mild outside, the system uses a lower flow temperature, saving energy. As it gets colder, the controller increases the flow temperature so the house stays comfortable. The “curve” or “slope” setting tells the heat pump how aggressively to raise the flow temperature as the weather changes.

If the curve is set too steep, the flow temperature and running costs will shoot up on cold days. If it is too flat, the system may not keep up in a cold snap. Getting this balance right is one of the main jobs during a professional commissioning or heat loss survey.

Why simply turning the flow temperature up can backfire

It is very tempting to head straight to the heat pump settings and crank the flow temperature up when you feel chilly. That may give a small short term boost, but it often creates new problems. Higher temperatures reduce the efficiency or COP of the heat pump, so the unit uses more electricity for the same amount of heat.

A higher flow temperature can also lead to more frequent cycling, shorter run times and louder operation. The system may hit the room set point too quickly, shut down, then have to restart again and again. Over time this can put extra strain on components and, in extreme cases, cause nuisance lockouts or faults.

Safe checks a homeowner can do without tools

There are a few sensible checks you can carry out yourself before calling an engineer. These do not involve opening the unit, using tools or changing advanced settings.

• Check room thermostats are in “heating” mode, not “off” or “holiday” mode

• Review the heating schedule to confirm the system is actually programmed to be on when you expect warmth

• Look at the main controller to see if weather compensation or “auto” flow temperature control is enabled

• Check for obvious error messages, warning lights or lockout symbols on the indoor or outdoor unit

• Make sure radiators are not blocked by large furniture and that basic radiator valves are open

It is also worth checking any backup immersion or electric heaters have not been left on unnecessarily, as that can mask underlying flow temperature or system issues while pushing up bills.

When to call an engineer

Some issues are not suitable for DIY investigation and need a trained heating engineer with heat pump experience. Repeated lockouts, unexplained error codes or a system that keeps tripping the electrical supply are all clear signs that professional help is required.

Freezing and defrost issues, such as a thick build up of ice on the outdoor unit or loud defrost cycles that do not seem to clear the ice, should also be assessed properly. Likewise, if your hot water cylinder is not reaching the set temperature, or you are running out of hot water more quickly than before, it is time to book a visit.

If you have already checked thermostats, schedules and obvious settings but rooms still never reach temperature, an engineer can measure real flow and return temperatures, check pump speeds, and confirm the heating curve and system design are appropriate for your home.

Flow temperature, heat loss and Kent / South East homes

Across Kent and the South East there is a real mix of properties, from Victorian terraces and 1930s semis to modern estates and new builds. The heat loss of the building and the size of the radiators or underfloor loops have a huge effect on what flow temperatures are realistic.

Older solid wall or 1930s homes with smaller original radiators may need higher flow temperatures on the coldest days, unless insulation and radiator upgrades have been carried out. Newer, well insulated properties, especially those with underfloor heating, can usually run at much lower flow temperatures and still feel warm.

A professional heat loss survey will calculate how much heat each room needs and check whether your existing emitters can deliver that at sensible heat pump flow temperatures. This is the foundation for setting an appropriate heating curve and achieving both comfort and efficiency.

Next steps and getting expert help

If you are unsure whether your heat pump flow temperature and heating curve are set correctly, it is worth having the system checked rather than constantly tweaking controls. A proper service and setup can pay for itself in lower running costs and fewer callouts.

The Heat Pump Guy offers specialist heat pump servicing and system optimisation, as well as detailed heat loss surveys to confirm what flow temperatures your home really needs. You can book a Heat Pump Servicing visit or arrange a Heat Loss Surveys assessment to get tailored advice for your property.

To talk through your situation or arrange an appointment, contact The Heat Pump Guy on 07577097132.