By James Mitchell, Lead Writer, Renewable Energy · Energy efficiency analyst · Last reviewed
Heat Pump Flow Temperature: The Setting That Decides Your Bills
The One Number That Quietly Sets Your Running Costs
Ask most heat pump owners what their flow temperature is and you will get a blank look. Yet that single number does more to decide your winter electricity bill than the brand on the outdoor unit, the tariff you are on, or how often you fiddle with the thermostat. Get it wrong and a perfectly good heat pump can cost as much to run as the gas boiler it replaced. Get it right and the same kit can shave hundreds of pounds a year off your heating.
Flow temperature is the temperature of the water your heat pump sends out to your radiators or underfloor pipes. A gas boiler typically pushes water out at 70 to 80 degrees. A heat pump is at its best when it runs much cooler, often 35 to 45 degrees. The lower that number, the harder your heat pump works for you, because the gap between the cold air outside and the warm water inside is what the machine has to bridge. A smaller gap means less electricity for the same heat.
This guide explains exactly what flow temperature is, why it matters so much, what good figures look like for a UK home, and how weather compensation lets your system find the lowest safe setting on its own. It also covers the trade-offs, because chasing the lowest possible number without thinking can leave a cold room or an empty hot water cylinder.
Why Flow Temperature Drives Efficiency
A heat pump does not burn fuel. It moves heat from the outside air into your home using a refrigerant cycle, the same physics as a fridge running in reverse. The measure of how well it does this is the coefficient of performance, or COP. A COP of 4 means the unit delivers four units of heat for every one unit of electricity it draws. You can read the full breakdown in our guide to how heat pump COP is calculated.
The single biggest factor that moves COP up or down is the flow temperature. The wider the gap between the source temperature outside and the flow temperature your system has to produce, the harder the compressor works and the lower the COP falls. As a rough rule of thumb, every one degree you can drop the flow temperature improves efficiency by around 2 to 2.5 per cent.
Put numbers on that and the picture becomes stark. A system running a 35 degree flow temperature on a mild day might hit a COP of 4.5. The same system forced up to 55 degrees might drop to a COP of 2.8. That is the difference between paying for heat at roughly a quarter of the cost of the electricity and paying for it at over a third. Across a heating season the gap runs into the hundreds of pounds.
This is why the design of the whole system matters. A heat pump set up to run cool needs enough radiator surface area or underfloor pipe to deliver the required heat at that lower water temperature. That is why properly sized emitters are non-negotiable, a point we cover in detail in our guide to running a heat pump with radiators.
What a Good Flow Temperature Looks Like in a UK Home
There is no single correct figure, because the right flow temperature depends on your heat emitters, your insulation, and how cold it is outside. But there are sensible bands to aim for.
For a home with underfloor heating, flow temperatures of 30 to 40 degrees are normal and comfortable. Underfloor systems spread heat across a huge surface area, so they deliver plenty of warmth even from gently warm water. This is why underfloor and heat pumps make such a strong pairing, explored in our piece on underfloor heating with a heat pump.
For a home heated by radiators, the target depends on whether the radiators were sized for a heat pump. A well-designed system with correctly sized emitters can run at 35 to 45 degrees in all but the coldest weather. A system using existing radiators that were sized for a 70 degree gas boiler may need 45 to 55 degrees to keep up, especially on the coldest days of the year.
A useful way to think about it is the design flow temperature. This is the figure your installer chooses for the coldest expected day, often around minus 2 to minus 3 degrees outside in much of the UK. A good modern install aims for a design flow temperature of 45 degrees or below. Many older retrofits sit at 50 to 55 degrees because the radiators were never upgraded. If your design flow temperature is up at 60 degrees, your running costs will suffer and it is worth asking why.
Here is a simple reference for typical UK setups.
| Emitter type | Typical flow temperature | Expected COP range |
|---|---|---|
| Underfloor heating | 30 to 40 degrees | 4.0 to 5.0 |
| Heat pump sized radiators | 35 to 45 degrees | 3.5 to 4.5 |
| Existing radiators, lightly upgraded | 45 to 55 degrees | 2.8 to 3.8 |
| Existing radiators, no changes | 55 to 60 degrees | 2.3 to 3.0 |
The lesson is plain. The closer your home sits to the top of that table, the cheaper it is to run. Most of the work of getting there happens at the design stage, but plenty can still be tuned after installation.
What Weather Compensation Actually Does
Here is the problem a fixed flow temperature creates. Your home only needs its hottest water on the very coldest days. For most of the heating season, the weather is mild and the house loses heat slowly. If you set a single fixed flow temperature high enough to cope with the worst day of the year, you are running that high temperature all winter, including the many mild days when you simply do not need it. That wastes electricity.
Weather compensation solves this. It is a control feature, built into almost every modern heat pump, that links the flow temperature to the outdoor temperature using a sensor on the outside of the building. When it is bitterly cold outside, the system raises the flow temperature to keep the house warm. When it warms up, the flow temperature drops automatically, the COP rises, and your costs fall.
The relationship between outdoor temperature and flow temperature is set by something called the heating curve, sometimes called the compensation curve or the weather curve. It is usually drawn as a sloping line. At one end sits the cold weather design point, for example 45 degrees flow at minus 2 outside. At the other end sits the mild weather point, for example 30 degrees flow at 15 outside. The system slides smoothly between those two points as the weather changes.
A well-tuned curve means your heat pump spends most of the season running at the lowest flow temperature your home can tolerate, automatically, without you touching anything. This is the single most powerful efficiency tool you have after the system is installed. It is also the one most often left on a lazy factory default that is set too high to protect the installer from comfort complaints.
How to Lower Your Flow Temperature Safely
Lowering your flow temperature is one of the few genuinely free ways to cut your heating bill. But it has to be done in stages, because pushing it too low too fast leaves cold rooms. The right approach is patient and methodical.
Start by finding your current heating curve setting in the heat pump controller or app. The exact menu varies by brand, but you are looking for a weather compensation curve, heating curve, or a slope and offset pair. Note the existing values before you change anything so you can return to them.
Next, drop the curve a small amount, the equivalent of two or three degrees of flow temperature, and live with it for several days of similar weather. Comfort lags behind changes because a building takes time to settle, so give each adjustment time to show its true effect rather than reacting after a single evening. If every room still reaches a comfortable temperature, drop it again. Repeat until you reach the point where one or two rooms struggle to get warm on a cold day. That is your floor. Nudge the curve back up by the last step and you have found the lowest flow temperature your home can sustain.
A few practical points make this work better.
- Run the heating continuously, not in sharp bursts. A heat pump at a low flow temperature works best ticking over steadily through the day rather than blasting on and off. This pairs well with the steady, low and slow approach covered in our guide to heat pump thermostat settings.
- Open your radiator valves fully. Heat pumps want water flowing freely through every emitter. Throttled thermostatic valves fight against a low flow temperature and create cold spots.
- Balance your radiators. Make sure each radiator gets its fair share of flow so the far end of the house warms as well as the near end.
- Check the coldest day. The real test of your curve is a genuinely cold spell. If the house holds temperature through a hard frost, your setting is sound.
If you reach a point where rooms cannot get warm even with valves open and the curve nudged up, that is a signal your emitters are undersized rather than a flow temperature problem. The fix there is larger radiators, not a hotter, more expensive flow temperature.
The Hot Water Exception
There is one important caveat to the whole low flow temperature philosophy, and it catches people out. Your space heating and your hot water do not use the same flow temperature.
To heat your home efficiently you want low flow temperatures. But to heat a hot water cylinder you need higher ones, typically 48 to 50 degrees to deliver usefully hot water, and periodically a hotter cycle to manage legionella bacteria. The heat pump handles this by switching modes. It runs cool for space heating, then temporarily raises the flow temperature when it heats the cylinder, then drops back down again. You can see how the cylinder side works in our guide to how a heat pump heats hot water.
This matters because hot water heating runs at a lower COP than space heating, simply because of those higher temperatures. The way to limit the cost is to schedule the cylinder reheat for a cheap window. If you are on a time of use tariff, heating your water overnight at an off-peak rate keeps the cost of that less efficient cycle to a minimum. Our guide to heat pump smart tariffs explains how to line those windows up.
Common Flow Temperature Mistakes
Several recurring mistakes push flow temperatures higher than they need to be and quietly inflate bills.
Treating a heat pump like a boiler. The instinct to turn everything up when you feel cold, then off when you feel warm, fights against a heat pump. High setbacks force the system to drive the flow temperature up hard to recover lost heat, exactly the opposite of efficient running.
Leaving the factory curve untouched. Manufacturers often ship a conservative default curve set higher than most homes need, because a too-cool setting generates support calls. That default is a starting point, not a finished tune.
Undersized radiators left in place. A retrofit that reuses old radiators without checking their output at heat pump temperatures forces a high flow temperature to compensate. This is the single most common reason a heat pump runs expensively, and it traces back to the design stage rather than the controls.
Restricted flow. Closed-down thermostatic valves, sludge in the system, or a pump set too low all reduce water flow and push the controller to raise the flow temperature to deliver the same heat. Keeping the system clean and the flow free is part of basic upkeep, covered in our guide to heat pump maintenance costs.
Ignoring insulation. A leaky home needs more heat, which means a higher flow temperature to deliver it. Loft and cavity insulation lower the heat demand and let you run the system cooler, with the savings stacking on top of the efficiency gain. Insulation and winter performance go hand in hand, as set out in our guide to heat pump performance in winter.
How Much Can You Actually Save
The savings from getting flow temperature right are real and measurable. Field trials run by the Energy Saving Trust and Nesta have repeatedly found that the gap between a poorly tuned heat pump and a well-tuned one is large, often 20 to 30 per cent on running costs for the same comfort.
Take a typical UK home using around 4,000 kWh of heat-pump electricity a year. At a price cap rate of roughly 24.5p per kWh, a 20 per cent efficiency improvement from dropping the flow temperature by ten degrees is worth nearly £200 a year, every year, for the life of the system. Over a twenty year lifespan that is several thousand pounds, achieved by changing a setting rather than spending a penny.
That figure feeds directly into the bigger sums on whether the system pays for itself. If you are weighing up the economics, our breakdown of real heat pump running costs puts these savings in the context of a full year of bills.
When to Call Your Installer
Most flow temperature tuning is safe for an owner to do through the controller, because you are only adjusting comfort settings, not touching the sealed refrigerant circuit. But there are moments to bring in a professional.
Call your installer if you cannot find the weather compensation setting, if the system will not hold temperature even at a high flow setting, if you suspect your radiators are undersized, or if the heat pump short cycles, switching on and off rapidly, which points to a system design or flow problem rather than a curve that needs nudging. A good MCS-certified installer should be willing to revisit the commissioning and tune the curve properly, and many will do this as part of an annual service.
Under the MCS scheme, your installer is required to commission the system correctly and hand over the settings. If your design flow temperature was set high and you were never told why, it is a fair question to put to them.
FAQ
What is the ideal flow temperature for a heat pump?
There is no single ideal figure, because it depends on your emitters and insulation. As a guide, underfloor heating runs well at 30 to 40 degrees, heat-pump sized radiators at 35 to 45 degrees, and existing radiators often need 45 to 55 degrees. The lower you can go while keeping the house warm, the cheaper the system runs.
Does lowering flow temperature really save money?
Yes. Every degree you drop the flow temperature improves efficiency by roughly 2 to 2.5 per cent. Field trials show that moving from a poorly tuned high flow temperature to a well-tuned low one cuts running costs by 20 to 30 per cent for the same comfort, often worth around £200 a year for a typical home.
What is weather compensation and should I use it?
Weather compensation links your flow temperature to the outdoor temperature, automatically lowering it on mild days and raising it only when it is genuinely cold. It is the most effective efficiency tool you have after installation, and it should be switched on and tuned rather than left on a high factory default.
Why does my hot water need a higher temperature?
A hot water cylinder needs water at around 48 to 50 degrees to be usefully hot, plus a periodic hotter cycle to control legionella bacteria. Your heat pump runs cool for space heating and temporarily raises the flow temperature only for the cylinder. Scheduling that reheat for an off-peak window keeps the cost down.
Can I change the flow temperature myself?
Yes, in most cases. Adjusting the heating curve through the controller or app is a comfort setting, not a repair, so it is safe for an owner to do in small steps over several days. Call your installer only if you cannot find the setting, the house will not hold temperature, or the system short cycles.
The Setting Worth Learning
Of all the things you can do to a heat pump after it is installed, tuning the flow temperature is the one with the biggest payoff for the least effort. It costs nothing, it is reversible, and on a well-designed system it can take a fifth or more off your winter bills.
The plan is simple. Switch on weather compensation, lower the curve in small steps until comfort is the limit, keep the radiator valves open and the system running steadily, and schedule hot water for cheap hours. Do that and you turn a heat pump that merely works into one that works the way it was meant to, delivering far more heat than the electricity it draws.