Heat Pump Weather Compensation Curve Explained
Last Updated: 12 November 2026
Quick Summary
A heat pump weather compensation curve is a control setting that adjusts the flow temperature according to the outdoor temperature. Colder weather means hotter water is sent to the radiators or underfloor pipes, while milder weather means cooler water, which keeps running costs down.
The curve has two settings: slope and parallel shift.
Lower flow temperatures make a heat pump more efficient.
Change the parallel shift first, one step at a time, and wait 24 hours between changes.
Radiator size matters more than the curve when one room stays cold.
Check the flow and return temperatures to confirm the system is running efficiently.
Is your heat pump curve set right?
Heatgeek Wales checks heat pump curves and emitters so your home runs efficiently.
Table of Contents
What Is Weather Compensation on a Heat Pump?
Weather compensation is a control method that changes the heat pump's flow temperature according to the outdoor temperature. When it is cold outside, the system sends hotter water to the radiators or underfloor pipes (Weather compensation and other controls).
A heat pump works best at lower flow temperatures. The lower the water temperature, the less work the compressor does, and the more efficient the system becomes. Weather compensation keeps the flow temperature only as high as the house needs at that moment.
Weather compensation relies on an outdoor temperature sensor. The controller reads the outside air temperature and uses a set rule to decide the flow temperature. That rule is the heating curve, and it is the part most homeowners need to understand.
How a Heat Pump Heating Curve Works
The heating curve is a graph. It links the outdoor temperature on one axis to the flow temperature on the other. Each point on the line tells the controller what water temperature to aim for at a given outside temperature.
The curve has two main settings:
Slope: how steeply the flow temperature rises as the weather gets colder
Parallel shift: an offset that raises or lowers the whole line
A steeper slope means the system sends more heat as the temperature falls. A higher parallel shift means the house runs warmer at every outdoor temperature.
Watch out: raising the slope or shift to fix a cold room can push flow temperatures up and cut efficiency. Make small changes and wait before judging the result.
Heat Pump Flow Temperature and Outdoor Temperature
Flow temperature and outdoor temperature are linked through the curve, but they do not move in step. A small change in outdoor temperature can call for a larger change in flow temperature, depending on how well the home holds heat.
A well-insulated home needs less heat, so it can run on a lower flow temperature. A draughty or poorly insulated home needs more heat and may need a higher flow temperature to keep up. This is why the same curve setting can feel right in one house and wrong in another.
Matching the Curve to Your Heat Emitters
The type of heat emitter in the home decides how low the flow temperature can go, and therefore how flat or steep the curve needs to be. The same curve setting that suits a well-insulated new build with underfloor heating will usually leave a 1970s semi with standard radiators cold on the coldest days.
A useful way to think about it is the design flow temperature, the water temperature the system needs on the coldest expected day to keep the house at target. The curve is simply a line drawn from that point back to a milder outdoor temperature. Emitter type sets that design flow temperature, and everything else follows from it.
Typical curve shapes by emitter type
Underfloor heating: designed to run at low flow temperatures, often in the 35°C to 45°C range on the coldest day. The curve can be relatively flat because the large floor area gives plenty of output even at modest water temperatures. Slopes are typically gentle, and the parallel shift does most of the fine-tuning.
Oversized radiators (often called low-temperature radiators): sized to deliver the room's heat loss at around 45°C to 55°C flow. The curve sits between underfloor and standard radiator settings, steeper than underfloor, flatter than a like-for-like radiator swap.
Standard radiators sized for a gas boiler: these were usually specified for a 70°C to 80°C flow temperature. On a heat pump they will only work at lower flow temperatures if they are large enough for the room, which often means upsizing. The curve has to be steeper to compensate, and on the coldest days the system may still struggle.
Mixed systems: a house with underfloor heating downstairs and radiators upstairs is common.
The exact numbers depend on the manufacturer's controller, but the shape of the relationship holds.
Why radiator sizing matters more than the curve
No curve setting can make an undersized radiator deliver more heat than its surface area allows at a given water temperature. If a room is cold at the correct flow temperature, the answer is usually a larger radiator, not a steeper curve. Raising the slope to compensate pushes the whole system to a higher flow temperature, which reduces efficiency across the entire heating season, not just in the one cold room.
The better fix is to check the radiator in that room against the room's heat loss and replace it if it is short.
Watch out: raising the curve to fix one cold room can increase running costs across the whole heating season.
How to Adjust a Heat Pump Weather Compensation Curve
Changing the curve is a careful process. Small steps are safer than large ones, and it helps to change one setting at a time. The aim is a curve that keeps the house at a steady temperature on the coldest days using the lowest flow temperature that still works, because every degree of flow temperature you avoid is energy the compressor does not have to produce.
Reading the curve settings step by step
Note the current slope and parallel shift settings, and write them down. Photograph the controller screen if it helps.
Wait for a day when the outdoor temperature is near its typical winter level, not a mild spell, and not the coldest snap of the year.
Check whether the rooms are warm enough. Most homeowners aim for a steady temperature rather than a fast one.
If the house is cold, increase the parallel shift by one step.
If the house is too warm, reduce the shift by one step.
Wait at least 24 hours before making another change.
Adjust the slope only after the shift is right, and only if the cold-weather performance is still poor.
Always keep the flow temperature within the range set by your installer. Your installer can show you the safe limits for your system.
A seasonal tuning protocol
The first cold snap of the year is the best time to fine-tune the curve, because it exposes problems that mild weather hides. A workable protocol:
Before the cold snap: confirm the current slope and shift, and note the room temperatures you are aiming for.
During the first cold spell: check room temperatures each morning and evening for three or four days. Look for rooms that are consistently cold rather than briefly cold.
Adjust the shift first: if the whole house is slightly cold, a one-step shift increase is usually enough. If only one room is cold, the problem is the emitter, not the curve.
Adjust the slope second: if the house is fine in mild weather but cold on the coldest days, the slope is too shallow. If it is too warm on mild days but fine when cold, the slope is too steep.
Record what you changed: a simple note of the date, the outdoor temperature, the setting and the result makes next winter's tuning much faster.
What to measure, not just feel
Feeling a room is unreliable. Two readings do most of the work:
Room temperature in the coldest and warmest rooms, taken at the same time of day.
Flow and return temperatures at the heat pump, which show whether the system is delivering the water temperature the curve is asking for. A large gap between flow and return suggests the emitters are giving up heat quickly; a small gap suggests the system is running efficiently at low output.
Short cycling at high flow temperatures is the pattern to avoid.
The efficiency trade-off
Weather compensation saves energy because heat pumps produce more heat per unit of electricity at lower flow temperatures. The exact gain depends on the heat pump, the emitters and the weather. That is why the goal is not "the warmest setting that works" but "the lowest flow temperature that still keeps the house comfortable".
Watch out: lowering the curve too far in pursuit of efficiency can leave the house cold on the coldest days, and the fix is usually a small slope increase rather than a large shift change.
Allowing time for the home to respond
A house does not warm up or cool down quickly. Heat stored in the walls, floors and furniture can take many hours to shift. This means the effect of a change may not show for a day or two.
Heat pumps are designed to run for long periods at a steady, lower output. Frequent large changes can make the system work harder and can make it hard to judge what is happening. Give each adjustment time before you decide whether it has worked.
The steps below summarise the adjustment process, with the timing for each.
Step | What to do | Timing |
Note current settings | Record the slope and parallel shift, and photograph the screen | Before any change |
Choose the day | Pick a day near typical winter outdoor temperature | Not a mild spell or the coldest snap |
Check room temperatures | Aim for a steady temperature, not a fast one | Before changing anything |
Adjust the shift | Raise by one step if cold, lower by one if too warm | One step at a time |
Allow time to settle | Leave the system before judging the result | At least 24 hours |
Adjust the slope | Change only after the shift is right, and only if cold-weather performance is still poor | After the shift is correct |
Heat Pump Room Thermostat Settings and the Curve
Room thermostats and the weather compensation curve work together, but they do different jobs. The curve sets the water temperature, while the room thermostat tells the system when a room has reached its target.
If a room thermostat is set too high, the system may keep running at a higher flow temperature to chase that target. Setting the room thermostat to a realistic level helps the curve work as intended.
Underfloor heating with room thermostats needs particular care. Underfloor pipes respond slowly, so a sudden thermostat change can leave a room too warm or too cold for hours. Keep the room settings steady and let the curve do most of the work.
Booking a Heat Pump Set-Up Check with Heatgeek Wales
Setting the weather compensation curve is much easier with a specialist's help. Contact Heatgeek Wales to discuss your heat pump's set-up.
Contact Heatgeek Wales to arrange a set-up check and get your curve working for your home.
Frequently Asked Questions
What is a weather compensation curve on a heat pump?
A weather compensation curve is the relationship the controller uses to set the water flow temperature sent to your heating circuit based on the outdoor temperature. When it is cold outside, the system raises the flow temperature; when it is milder, it lowers it. This keeps the home at a steady temperature without running the heat pump hotter than needed. The curve is usually shown as a graph with outdoor temperature along one axis and flow temperature along the other, and it is adjusted through the controller's slope and parallel shift settings.
Should I adjust the curve or the room thermostat?
Adjust the heating curve first if the house is consistently too cold or too warm across the whole day and night. The room thermostat is best used for fine control in individual rooms or for short-term changes. Changing the curve by large amounts can make the system work harder, so make small changes and allow time for the home to respond before judging the result. Heatgeek Wales can review your settings during a set-up check.
How long should I wait after changing a heat pump curve?
Wait at least 24 to 48 hours after each change before judging the result, because the building fabric and radiators respond slowly. Homes with thick walls or underfloor heating can take longer to settle. Make one adjustment at a time and record the indoor temperature at the same times each day, so you can see whether the change has helped rather than reacting to a single cold evening.
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