Use the lowest heating-curve (flow-temperature) setting that keeps every occupied room comfortable, and let weather compensation adjust that temperature as outdoor conditions change. Make changes in small steps, allow the house time to respond, and avoid boiler-style cycles of switching the heat pump off and demanding a rapid recovery. Keep setbacks modest—Natural Resources Canada recommends no more than 2°C unless a control starts recovery early—and follow the installed unit’s manual for hot-water, defrost and auxiliary-heat settings.
There is no universal “best” number. The right setting depends on the heat pump, radiators or underfloor heating, insulation, design heat loss, weather and the people in the home. Lower flow temperatures generally improve efficiency, but only when the rooms still reach a healthy, comfortable temperature.
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The settings that usually make the biggest difference
Lower the flow temperature, but only until comfort starts to suffer
Flow temperature is the temperature of the water leaving the heat pump for the heating system. Producing cooler water normally improves efficiency because the heat pump has a smaller temperature lift between its outdoor source and the heat delivered indoors. Energy Saving Trust’s domestic heat-pump guide explains this relationship and stresses that the appropriate value is determined by the system design, not by a universal target. Read the guide at Domestic Heat Pumps: Technical Considerations.
A flow temperature that is too high can make the compressor work harder, increase running costs and encourage short cycling. One that is too low can leave rooms cold, lengthen recovery times or trigger supplementary electric heat. Adjust the heating curve or maximum flow-temperature limit in small increments rather than making a large change.
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Use the heating curve to match output to outdoor temperature
The heating curve tells the controller how warm the heating water should be at different outdoor temperatures. A steep or high curve supplies hotter water in cold weather; a lower curve supplies cooler water. The aim is to match heat output to the building’s actual heat loss so the heat pump runs steadily instead of repeatedly stopping and starting.
Energy Saving Trust advises changing the curve by about one degree at a time, then observing the home through representative weather before changing it again. Do not copy a curve number from another house: manufacturers use different scales, and the same number can represent different temperatures. Radiator size, underfloor-loop spacing, insulation, emitter design and the local design temperature all affect the result. Practical guidance is available in The most efficient way to run a heat pump (19 May 2026).
Enable weather compensation when the system supports it
Weather compensation uses an outdoor sensor to vary flow temperature as the weather changes. In milder conditions, when the house needs less heat, the controller lowers the flow temperature; in colder conditions it raises it. This can keep room temperature steadier than a simple on/off thermostat and helps the heat pump spend more time at efficient operating conditions.
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Keep room targets realistic and stable
Set each occupied area to the lowest temperature that remains comfortable for the people using it. A small reduction in the room target can reduce heat demand, but a target that leaves occupants cold is not a successful efficiency measure. Energy Saving Trust states plainly that “Comfort and health should take priority over energy savings.” Its controls guidance is at Heating controls.
Heat pumps often deliver heat gradually, especially through underfloor heating or large, low-temperature radiators. A room thermostat that makes frequent, large corrections can fight the heat pump’s weather-compensation logic. If your system has room sensors, use them as the manufacturer intended and avoid closing so many valves that the system loses the water flow or emitter capacity it was designed around.
Use modest setbacks rather than a deep overnight shutdown
Turning a heat pump off for a long period and then demanding a fast warm-up is not automatically cheaper. The house, slab and furnishings may cool substantially, and the system may need a high flow temperature or auxiliary resistance heat to recover. Energy Saving Trust recommends longer operating periods because the heat pump can run more efficiently even though it runs for more hours.
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Treat hot water as a separate control problem
Many systems have separate schedules and temperature controls for space heating and domestic hot water. Follow the cylinder and heat-pump manufacturer’s instructions for hot-water temperature and the periodic sterilisation cycle. Do not disable a required hygiene cycle as an energy-saving shortcut. Hot-water heating may temporarily use a higher temperature or supplementary element, so schedule it when the household needs the water and check what the controller’s “boost,” “comfort” or “legionella” functions actually do.
Exact menus and allowable temperatures vary by model. Energy Saving Trust’s expert answers at Your heat pump questions answered by our experts explain why the manual and installer remain the authority for equipment-specific settings.
Leave auxiliary or boost heat as backup
Auxiliary electric heat is useful for protection, exceptional demand or a controlled hot-water cycle, but frequent activation is a warning sign. It may indicate a heating curve that is too low for the weather, undersized or poorly balanced emitters, an incorrect sensor, a blocked filter or a system that was not commissioned correctly. Record when boost operates and ask an installer to review the curve, emitter temperatures, flow rates and design assumptions rather than simply disabling the backup.
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Let the unit defrost normally and protect airflow
Air-source heat pumps periodically defrost their outdoor coil in cold, damp weather. Defrost is normal; stopping it can damage performance. Natural Resources Canada notes that an installer can configure an indoor fan to stop during defrost, reducing the amount of supplementary and heat-pump energy used, although the defrost period may take longer. The correct setting depends on the equipment.
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Keep outdoor coils, grilles and drainage paths clear of leaves, snow and stored items. For wall-mounted indoor units, adjust airflow direction for comfort according to the product manual; do not block the intake or outlet.
How to tune settings without losing comfort
- Record the starting point. Note the current heating-curve level, flow-temperature limit, room targets, schedules, hot-water settings and any auxiliary-heat indicator. Record indoor temperatures in the main rooms and the outdoor conditions.
- Check the basics first. Confirm that filters are clean, radiators or underfloor circuits are not obstructed, doors and windows close properly, and the outdoor unit has free airflow and drainage. A control change cannot compensate for a maintenance fault.
- Confirm the control mode. In the controller menu, identify whether the system is using weather compensation, a fixed flow temperature, room-temperature feedback or a mixture. Use the manual for the exact menu names; do not assume a generic thermostat is compatible.
- Lower the curve or flow limit by one small step. Change only one variable. A one-degree curve adjustment is the practical increment recommended by Energy Saving Trust. If the controller uses a different scale, make the smallest clearly defined change available.
- Allow a representative observation period. Keep schedules and room targets unchanged while the system experiences normal daytime and overnight conditions. Low-temperature systems may take many hours to show the effect, and a single mild afternoon is not enough evidence.
- Check every occupied room. Look for rooms that fail to reach the target, long recovery times, cold floors or radiators, unusual cycling, or frequent auxiliary heat. If comfort is lost, raise the curve slightly or have the system balanced; do not keep lowering it to chase efficiency.
- Set a modest setback only if it suits the household. Try no more than 2°C, or select a recovery schedule that starts early. People who need continuous warmth should use a stable target instead.
- Review the result across changing weather. A curve that works in mild weather may be too low during a cold spell. Adjust gradually and keep a simple log of outdoor temperature, room temperature, flow temperature and boost events.
Should you leave a heat pump on all the time?
Usually, a heat pump is better suited to long, steady operation than to the large temperature swings commonly used with a boiler. Continuous operation does not mean maximum output: weather compensation can reduce the flow temperature as demand falls, and the compressor modulates or cycles at a lower rate when the building is warm.
That does not make an all-day schedule mandatory. A well-insulated, low-mass home, a high electricity tariff during certain hours, or a household that is empty for a long period may justify a carefully designed setback. The practical test is whether the recovery period causes high flow temperatures, auxiliary heat or uncomfortable rooms. Compare those outcomes rather than assuming that “on” or “off” is always cheaper.
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Control approaches compared
| Control approach | How it operates | Potential benefit | Main risk or limitation |
|---|---|---|---|
| Fixed flow temperature | Supplies roughly the same water temperature until manually changed. | Simple and predictable when outdoor conditions are stable. | Can waste energy in mild weather or leave the home cold in a cold spell unless someone adjusts it. |
| Weather compensation | Uses an outdoor sensor and heating curve to vary flow temperature. | Matches output to heat loss and can maintain steadier rooms at lower temperatures. | Needs a correctly positioned sensor, a commissioned curve and compatible equipment. |
| Room thermostat with large setbacks | Allows the room to cool, then calls for rapid recovery. | May reduce demand during a long vacancy in some buildings. | Recovery can require high flow temperature or auxiliary heat; slow systems may feel cold. |
| Room-by-room zoning | Uses valves or sensors to vary temperatures by space. | Useful when rooms have genuinely different occupancy or comfort needs. | Excessive restriction or incompatible controls can reduce flow and interfere with heat-pump operation. |
| Smart or adaptive recovery control | Starts heating early so a scheduled target is reached gradually. | Can provide a modest setback without a sharp rebound. | Compatibility and commissioning are equipment-specific; verify both before purchase. |
When comparing controllers, check comfort stability, whether weather compensation is supported, how setbacks and recovery are handled, compatibility with the heat pump and existing sensors, and whether the schedule suits people who are home for much of the day. A controller marketed as “smart” is not automatically better if it overrides the heat pump’s intended logic.
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Symptoms that the setting is wrong
- Rooms never reach the target: the curve may be too low, emitters may be undersized or unbalanced, airflow may be restricted, or the heat-loss calculation may need review.
- Rooms overheat and the unit repeatedly stops: the curve may be too high, room feedback may be conflicting with weather compensation, or zoning may be closing too many circuits.
- Frequent auxiliary-heat or boost indications: check the curve, outdoor sensor, emitter capacity, defrost configuration and system sizing with an installer.
- Long recovery after a setback: reduce the setback, start recovery earlier or use a steadier schedule.
- Uneven room temperatures: check balancing, radiator or underfloor-loop settings, doors and insulation before raising the whole-house curve.
- Unexpected defrost behavior: keep the outdoor unit clear and consult the manual; do not disable defrost.
When to ask an installer to change the setup
Get professional help when you cannot identify a control’s function, the manual and controller disagree, comfort remains poor after small adjustments, or auxiliary heat operates often. Ask for a review of the heat-loss design, emitter sizing, hydraulic balance, weather-sensor location, minimum flow requirements, defrost settings and hot-water programming. The installer should also confirm that any replacement thermostat or controller is explicitly compatible with the heat pump and its communication protocol.
Do not alter electrical protection, refrigerant settings, safety limits or required hot-water sterilisation without qualified advice. Consumer controls are appropriate for small curve, schedule and room-target changes; commissioning parameters belong with the responsible installer or service engineer.
How to interpret the available guidance
Energy Saving Trust’s practical advice is dated 19 May 2026 and is aimed at household operation. Natural Resources Canada’s recommendations reflect its own climate and equipment context, so apply the 2°C setback guidance alongside the installed unit’s manual. The UK government review of compensation and time-proportional-integral controls, published on 12 October 2017, provides useful background on control mechanisms but predates the newer consumer guidance; it is available at Heating controls: compensation and time proportional and integral (TPI) controls.
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No reliable universal percentage or bill saving can be assigned to a particular setting from these sources. Savings depend on the building, weather, tariff, heat-pump performance, emitter temperatures and occupant behavior. The dependable rule is operational: reduce flow temperature and curve settings only as far as comfort allows, use weather compensation correctly, keep setbacks moderate, and investigate repeated backup-heat use instead of masking it.
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