A heating curve tells a weather-compensated boiler or heat pump what space-heating flow temperature to aim for as the outdoor temperature changes. A lower setting can improve efficiency, provided the home still reaches a comfortable temperature. The right curve depends on the building, radiators or underfloor heating, and the appliance. Keep the original settings and change one user-adjustable value at a time.
Keep hot-water controls separate. A conventional boiler that heats a hot-water cylinder must not have its boiler flow turned down to 50°C as a DIY experiment. The Energy Saving Trust advises a boiler flow of at least 65°C for that arrangement so its cylinder can reach a safe temperature. Do not change the cylinder’s heating programme or any legionella protection setting while tuning space heating.
Hi there, fellow DIYer,
When the first cold evenings arrive, it is tempting to turn every dial up. If your controls already adjust the heating water to match the weather, careful observation can tell you more than a guess at a number. Here is how to recognise a heating curve, record what you have and decide whether an adjustment belongs in the user menu or with an installer.
What the curve does
As outdoor air gets colder, a house loses heat faster. Weather compensation responds by requesting warmer water for the radiators or underfloor heating. When it is milder, it requests cooler water. The Energy Saving Trust’s installer guide explains that some controllers use two temperature points, while others offer a numbered curve or a small offset.
A curve is not a room thermostat schedule. The thermostat asks for a room temperature or starts a heating period; the weather control decides the water temperature needed to provide heat. Some systems also use room feedback. A smart thermostat or a modulating boiler does not, by itself, prove that weather compensation is active. Look up the exact boiler, heat pump and controller models.
The accompanying graph is an example of the principle. Its temperatures are illustrative, not settings to copy into your system. A manufacturer may calculate the curve differently, and a mixed radiator and underfloor system may have separate circuits and mixing controls.
Why lower can be more efficient
A condensing gas boiler can recover more heat from its flue gases when the return water is cool enough. With a heat pump, a lower required flow temperature generally lets the machine work more efficiently. Neither result justifies lowering a setting until the house is cold. Energy Saving Trust guidance on heating controls says the flow must still be high enough to warm the rooms in cold weather.
A poorly performing room is a clue to investigate. Air in a radiator, a stuck valve, a control setting, poor balancing, insufficient radiator output or high heat loss can each cause a cold room. Changing the whole curve may hide the cause or leave other rooms too warm.
First, find out what controls the heating
| What you have | Sensible next step |
|---|---|
| Heat pump with a weather-compensation menu | Find the user guide and commissioning settings. Ask the installer which user adjustment is intended for you. |
| Boiler and controller with an outdoor sensor or online weather input | Check the manuals to confirm that weather compensation is enabled and which device owns the setting. |
| Modulating controller without confirmed weather compensation | Identify what it changes. A flow-temperature limit is not necessarily a heating curve. |
| Conventional boiler and hot-water cylinder | Do not copy a 50°C combi-boiler trial. Keep the cylinder and boiler safety settings as specified by the manufacturer and installer. |
| Communal or district heating | Follow the provider’s instructions; the boiler procedure here may not apply. |
Do not move an outdoor sensor, change an installer-only parameter, remove a mixing valve or override a high-temperature limit. Those parts of the system need the manufacturer’s instructions and, where appropriate, a competent heating professional.
A careful way to test a user-adjustable curve
- Record the starting point. Photograph the controller screens, room thermostat schedule and any displayed heating-curve setting. Note the date and outdoor temperature.
- Check that rooms are behaving normally. If a radiator is cold at the top, a valve is stuck or the heat pump is showing a fault, deal with that first. A heating curve cannot repair a flow problem.
- Read the exact control manual. Find out whether its user setting changes the curve’s slope, its position, a room-temperature offset or a maximum flow temperature. These are different adjustments.
- Change just one allowed value by one small step. For a heat pump, the Energy Saving Trust’s consumer guide suggests one-degree changes through winter while watching comfort, then moving back if the house becomes too cold. Follow the appliance instructions if they specify a different method.
- Give the system time. Log the outdoor conditions, room temperatures and morning warm-up in the same rooms. Underfloor heating can respond slowly. Compare similar weather and schedules rather than the next single afternoon.
- Restore comfort promptly. If someone is cold, put the previous setting back. A useful log records the failure as well as a successful setting.
The optimum setting may differ between a mild autumn and a cold snap. A curve that appears fine in September has not been proved at the design outdoor temperature for your area. Record cold-weather behaviour before drawing conclusions.
Which adjustment might help?
If every room is too cool in both mild and cold weather, the whole curve may sit too low, or the schedule and room target may be wrong. If the rooms are comfortable in mild weather but struggle only when it is very cold, a steeper curve or a design issue may need attention. If one room alone struggles, investigate that room and its radiator before raising the temperature for the whole home.
Those are diagnostic clues, not universal menu instructions. The Energy Saving Trust’s heat-pump design toolkit bases sizing on room heat losses and system output. An installer should assess persistent cold rooms, unusual cycling, flow faults or high electricity use.
Do not mix this up with the 50-degree trial
A 50°C heating-flow trial is a separate observation for a suitable combi boiler without a hot-water cylinder and with a distinct space-heating control. It is not a default heating-curve setting. It cannot, by itself, prove that a fully electric heat pump will work or that a hybrid is necessary. A heat-pump proposal still needs a room-by-room heat-loss calculation, emitter and pipework checks, and a hot-water design.
If you have a cylinder, heat pump or an uncertain control layout, ask the installer how to assess low-temperature performance safely. Never turn down domestic hot water as part of this test.
Keep a simple autumn log
| Date and outdoor conditions | Curve or offset | Room temperatures and comfort | Heating behaviour | Action |
|---|---|---|---|---|
| Example: cool evening, 8°C outside | Original setting, photographed | Living room 20°C, bedroom 18°C | Normal | Keep as baseline |
| Your entry |
A room that stays comfortable at a lower flow temperature is a useful result. So is discovering that one room cools too quickly. Give the log and appliance details to a heating professional. You can then discuss whether the next step is a control adjustment, system balancing, a larger radiator or heat-loss work.
The practical rule: start with the existing settings, protect the hot-water arrangement, change only a permitted user control in small steps and judge it by comfort in real weather. Do not borrow a temperature from somebody else’s house.
Best wishes from Fred Schouten