Flow temperature is one of the most important settings on your heat pump, yet it is rarely explained clearly to homeowners. Simply put, flow temperature is the temperature of the water your heat pump sends to your radiators or underfloor heating. Getting this setting right can make a big difference to your home's comfort, energy efficiency, and heating bills. Understanding flow temperature helps you avoid problems like noisy operation, short cycling, or rooms that never feel warm enough. With basic knowledge, you can adjust your system for better performance and know exactly when it’s time to call in a heating expert for a system check or optimisation.
What flow temperature actually means
In simple terms, flow temperature is the temperature of the water leaving your heat pump and heading into your radiators or underfloor heating pipes. It is different from the room temperature shown on your thermostat.
The heat pump warms the water to a set flow temperature, sends it around your system, then cooler water returns to be reheated. The difference between the flow and return temperatures tells the heat pump how hard it needs to work.
Unlike a boiler, which can often push out very hot water, a heat pump works best with lower flow temperatures running for longer periods.
How flow temperature affects efficiency and comfort
Heat pumps are most efficient when they lift the temperature of the water by a smaller amount. The higher the flow temperature, the harder the unit has to work and the more electricity it uses.
Keeping the flow temperature as low as practical usually means:
Lower running costs and better efficiency (higher COP)
Steadier room temperatures with fewer hot and cold swings
Less stress on components and often quieter operation
However, the flow temperature still has to be high enough to keep the house warm on the coldest days. The right setting is a balance between comfort and efficiency.
Heat pump thinking vs traditional boiler thinking
With gas and oil boilers, many people set the flow temperature very high and let the boiler blast heat into the radiators in short bursts. This on/off approach does not suit a heat pump.
Heat pumps like to run for long periods at a low flow temperature, gently topping up the heat in your home. Instead of quick blasts, you are aiming for continuous, even warmth.
This is why you may be advised to leave your heating "on" more consistently, especially in colder weather, and adjust the flow temperature rather than constantly changing room thermostats.
Radiators, underfloor heating and flow temperature
The type of emitters you have makes a big difference to the flow temperature you need. Traditional radiators have a smaller surface area so usually need a higher flow temperature than underfloor heating.
Underfloor heating covers a large area, so it can comfortably run at much lower temperatures. This is ideal for heat pumps, which are happiest with cooler water.
If your home has older, small radiators, your heat pump may have been designed to run at a higher flow temperature. In some cases, upgrading radiators to larger or modern low-temperature models allows the flow temperature to be reduced and efficiency to improve.
Weather compensation and outdoor temperatures
Many heat pump systems use a control strategy called weather compensation. Rather than using one fixed flow temperature, the controller automatically adjusts it based on the outdoor temperature.
On a mild day, the system can run a lower flow temperature and save energy. As the weather gets colder, the controller raises the flow temperature to keep the house warm.
This relationship between outdoor temperature and flow temperature is shown as a weather compensation curve in the settings. Small tweaks to this curve, done correctly, can make a noticeable difference to comfort and bills.
When flow temperature is set too high
If the flow temperature is higher than it needs to be, the heat pump may heat rooms too quickly and then shut off. It will then start again shortly afterwards, a pattern known as short cycling.
Short cycling can lead to:
Higher electricity use and increased running costs
More wear on components due to frequent starts and stops
Potentially noisier operation, especially outdoors
You might also notice rooms overheating slightly then cooling, rather than holding a steady temperature. If this sounds familiar, it is worth checking whether your flow temperature or weather compensation settings are unnecessarily high.
When flow temperature is set too low
If the flow temperature is too low for the size of your radiators or the heat loss of your home, the system may struggle to reach the target room temperature. The heat pump can run for long periods without the house ever feeling properly warm.
Common signs include rooms stuck a few degrees below the thermostat setting, radiators that feel only lukewarm even after hours of operation, and cold spots in certain areas of the house.
Sometimes this is simply a settings issue. In other cases, it points to design problems such as undersized radiators, poor insulation or an incorrectly sized heat pump.
Homeowner checks before calling an engineer
There are a few safe checks you can carry out yourself to rule out simple issues before booking a service visit. Always refer to your user manual and avoid opening the unit or changing advanced menus you do not understand.
1. Look at the controller settings
Check the heating flow temperature or weather compensation settings on your main controller. If a fixed flow temperature is set very high (for example, above 50–55°C for radiators, or above 40°C for underfloor), there may be room to reduce it gradually.
Lower the setting a few degrees at a time over several days and see if comfort is maintained. Do not make large jumps, especially in cold weather, as this can leave the house chilly.
2. Confirm thermostats and zone controls
Make sure all room thermostats and zone valves are correctly set and actually calling for heat. If many zones are turned down or off, it can cause short cycling because the heat pump has nowhere to send the hot water.
Check that time schedules are sensible and that no thermostat is placed next to a heat source, such as a TV or direct sunlight, which can mislead the system.
3. Check radiators are bled and roughly balanced
If you have radiators, feel them carefully when the heating is on. Cold spots at the top often mean trapped air, which can be released with a radiator key and a cloth.
Radiators that are hot at the top and cold at the bottom may be sludged and need professional cleaning. If some rooms are much warmer than others, the system may need balancing so that each radiator gets its fair share of flow.
When to ask an engineer to review the system
If you have checked the basics and still have issues with comfort, noise or high bills, it is sensible to ask a qualified engineer to review the design and settings.
A professional can check emitter sizing (radiators or underfloor loops), the hydraulic balance of the system, pump speeds, weather compensation curves and the overall heat pump sizing. They can also confirm that any buffer or mixing arrangements are set correctly.
This type of optimisation visit often pays for itself through lower running costs and improved comfort, particularly in properties around Portsmouth, Hampshire, Chichester and Fareham where winter conditions can vary quite a bit.
A quick note on hot water temperatures
Hot water cylinder temperatures are controlled separately from heating flow temperature, which can be confusing. Your cylinder usually needs to reach higher temperatures periodically to control bacteria such as Legionella.
It is common for a heat pump system to run a special hot water cycle at a higher temperature once or twice a week. This is normal and should not be reduced without professional advice, even if you are lowering heating flow temperatures for efficiency.
Get help optimising your heat pump
If you are unsure whether your heat pump flow temperature is set correctly, or if you are experiencing high bills, short cycling or rooms that never quite warm up, a professional check is well worth considering.
For homeowners in Portsmouth, Hampshire, Chichester and Fareham, GEO M&E Ltd can review your system settings, assess your radiators or underfloor heating and fine-tune your controls for better comfort and efficiency. To arrange a system optimisation visit or book a service, call 02392388799 today.
