Heat Pump Weather Compensation: How It Works | UK Guide

How does weather compensation work?

A home loses heat faster as the temperature outside falls. Weather compensation responds by changing the target flow temperature.

Energy Saving Trust explains that outdoor temperature may be measured by a sensor in the heat pump or by a separate external sensor. The controller then uses the selected heating curve to calculate how warm the heating water should be.

The refrigeration cycle itself does not change. Weather compensation simply changes the target water temperature leaving the heat pump.

For a fuller explanation of outside temperature, flow temperature and winter operation, see the Get Free Quote guide to air source heat pump temperatures.

You can also see Energy Saving Trust’s installer guidance on weather compensation for a technical explanation of how the control works.

What is a heat pump heating curve?

The heating curve links outdoor temperature to target flow temperature.

A simple controller may use two design points and draw a line between them. Other systems use numbered curves, slope settings or an offset. The principle is always similar: colder outside means warmer heating water; milder outside means cooler heating water.

Energy Saving Trust notes that many controllers allow an offset to be applied to the curve. This moves the target flow temperature up or down without completely changing its shape.

There is no universal “best” curve. The correct setting depends on the building’s heat loss and the output of its radiators or underfloor heating.

That is why copying another homeowner’s heating curve is unreliable. Two properties using the same heat-pump model may need different settings.

The aim is not simply to achieve the lowest number possible. It is to use the lowest flow temperature that can still maintain the required indoor temperature.

Why does weather compensation improve heat-pump efficiency?

Heat pumps generally work more efficiently when they do not have to produce unnecessarily hot water.

If a system is fixed at 50°C all winter, it may be working harder than necessary on a mild day. Weather compensation can lower the flow temperature while still providing enough heat.

That can improve coefficient of performance, or COP, because the compressor has a smaller temperature lift to achieve. It can also support steadier operation.

The saving is not a guaranteed percentage. It depends on the home, emitters, heat-pump model, controls and starting settings. However, Energy Saving Trust warns that an incorrectly set heating curve can increase running costs.

Its current advice recommends checking the heating curve and, where appropriate, reducing an unnecessarily high setting gradually rather than assuming hotter water is better. Read Energy Saving Trust’s heat-pump efficiency guidance.

For more detail on COP and seasonal performance, read Get Free Quote’s air source heat pump efficiency guide.

Weather compensation with radiators

Weather compensation can work well with radiators, including many existing ones, if they can provide enough heat at the planned flow temperature.

If one room struggles while the rest of the house is comfortable, the radiator could be too small, the system may need balancing, or water flow may be restricted.

Raising the whole heating curve to fix one weak room can make every other room run hotter than necessary. That can reduce efficiency and cause thermostatic valves or zone controls to close more frequently.

For more on emitter sizing, see Get Free Quote’s guide to using a heat pump with existing radiators.

What about underfloor heating?

Underfloor heating often works at lower water temperatures because it uses a large surface area to release heat.

However, MCS technical guidance notes that some emitters, including underfloor heating and fan convectors, do not respond linearly as flow temperature changes. Their compensation settings may therefore need specialist consideration.

Underfloor heating also has high thermal inertia. A change can take hours to show its full effect, particularly with a thick screed floor, so frequent adjustments can make tuning harder.

How can you tell if the curve is set too high?

Possible signs include the whole home regularly becoming warmer than the target, radiators feeling hotter than necessary in mild weather, room thermostats repeatedly closing zones, or frequent heat-pump cycling.

Those symptoms can also have other causes, including poor hydraulic design or control conflicts. If the home stays comfortable at a lower flow temperature, there may be scope to improve efficiency.

This is one reason weather compensation can affect running costs. See Get Free Quote’s air source heat pump running-cost guide for the other major factors.

How can you tell if the curve is too low?

The clearest sign is that the home cannot maintain the chosen indoor temperature during sustained heating demand.

If the house is comfortable in mild weather but too cool only during cold spells, the cold-weather end of the curve may need attention. If it is too cool across many outdoor temperatures, the whole curve may be too low.

Before raising it, check for closed valves, restricted flow, poor balancing or undersized emitters. A slow warm-up alone is not proof of a fault.

How should weather compensation be adjusted?

Your installer should set the initial curve from the system design. If fine-tuning is needed later, make small changes and give the house time to respond.

Record the outdoor temperature, indoor temperature and target flow temperature over several days. Change one relevant setting at a time, then observe comfort before changing anything else.

Energy Saving Trust recommends reducing an over-high curve in small steps while checking that the home remains comfortable. Manufacturer controls differ, so use the instructions for your exact model and avoid changing installer-level settings you do not understand.

If performance is poor, the installer should check the system against the room-by-room heat-loss and emitter design rather than simply raising the maximum flow temperature.

Weather compensation versus a room thermostat

A room thermostat usually decides whether more heat is needed from the indoor temperature. Weather compensation decides how hot the heating water should be from outdoor conditions.

The two can work together, but poor setup can make them fight. A high weather curve may overheat rooms, for example, causing thermostats to close circuits repeatedly.

Some systems add “room influence” or load compensation. Energy Saving Trust describes load compensation as additional control using indoor-temperature feedback, while combined strategies can use both indoor and outdoor conditions.

Does weather compensation control hot water?

Usually, the heating curve applies to space heating rather than the domestic hot-water cylinder.

Hot water normally has a separate target temperature and hygiene settings. Grant’s Aerona³ documentation, for example, distinguishes its weather-compensated space heating from hot-water operation.

See the manufacturer’s Grant Aerona³ weather-compensation guidance for one example of how those controls are separated.

Do not change cylinder hygiene settings simply because you are tuning the heating curve.

Is weather compensation required on an MCS installation?

The MCS Heat Pump Installation Standard MIS 3005-I:2025 says weather compensation should be enabled where it can optimise system efficiency.

MCS also recognises exceptions. The standard says there may be cases where weather compensation would reduce efficiency or provide no practical value, with examples including particular heat emitters, occupant usage patterns and how the building responds to heating.

You can review the relevant requirement in the MCS Heat Pump Installation Standard MIS 3005-I:2025.

For most conventional radiator and underfloor-heating systems, however, weather compensation is a central part of efficient heat-pump control.

What should you ask your installer?

Ask which weather compensation mode is active, what outdoor and flow-temperature design points are set, whether room influence is enabled and how to make a small comfort adjustment without disturbing installer parameters.

A good handover should also explain what to expect in mild weather and at the winter design temperature. Radiators may feel only lukewarm for much of the season, and that can be normal.

Ready to compare heat-pump systems?

Before comparing quotations, ask each installer what design flow temperature they propose and how the weather compensation curve will be commissioned. Those answers can tell you considerably more than the headline heat-pump model alone.