Weather compensation controls

Weather compensation lowers a heating system’s target flow temperature as outdoor temperature rises, then raises it as conditions become colder. Its purpose is to provide only the water temperature the building needs at that time.

For a heat pump, that usually improves efficiency and supports long, steady operation. It also avoids repeatedly heating water to the coldest-day design temperature during mild weather.

The heating curve

The controller maps outdoor temperature to a target flow temperature. This relationship is called the heating curve or weather curve.

The cold end should be grounded in the building heat-loss and emitter design. At the design outdoor condition, the selected flow temperature must let each room’s emitter meet its calculated load. The warm end should provide little or no heat as the building approaches the point at which it no longer needs space heating.

Controls express the curve in different ways:

• two outdoor and flow-temperature coordinates
• a slope plus an offset
• a numbered curve with separate room-temperature influence
• a design flow temperature and a warm-weather cut-off

The labels are product-specific. A curve number copied from another brand may describe a different relationship.

The aim is the lowest temperature that maintains comfort

Heat-pump efficiency generally improves when the temperature lift between source and heating water is smaller. An unnecessarily high curve can increase electricity use, cause thermostats to close, shorten run times and bring supplementary heat into operation.

A curve that is too low leaves rooms below target in some weather. The best setting is therefore not simply “as low as possible”. It is the lowest practical curve that maintains the design indoor temperatures across the building.

No universal slope, minimum flow or maximum flow figure fits every property. Emitter output, insulation, air leakage, system volume, pump flow and occupancy all affect the result.

Current MCS commissioning position

As at 22 July 2026, MIS 3005-I:2025 says weather compensation should be enabled where it optimises heat-pump efficiency, using the maximum practical gradient. In plain terms, the curve should exploit the greatest sensible fall in flow temperature as weather becomes milder while still delivering comfort.

That requirement does not justify selecting arbitrary endpoints. The commissioning value needs to follow the approved system design and the manufacturer’s control method.

Outdoor sensor position matters

The controller can only respond to the temperature it measures. A wired sensor should be located according to the product instructions, away from false heat or cold caused by direct sun, boiler flues, extract vents, open windows and sheltered recesses.

Some controllers use an internet weather service or a sensor built into the outdoor unit. Each method has limitations. Compare the displayed outdoor value with local conditions if the curve behaves unexpectedly, and check time zone and location settings where online data is used.

Do not apply a casual software offset to hide poor sensor placement. Record any justified calibration.

Tuning the curve

Start from the commissioned design settings. Observe room temperatures during reasonably stable weather, allowing for the building’s thermal response before judging a change.

Useful diagnosis is:

• Too cold mainly in colder weather: the cold end or slope may be too low, or the system may lack emitter output or flow.
• Too warm mainly in colder weather: the cold end or slope may be too high.
• Too cold or warm in most weather: the whole curve may need an offset, subject to the product’s control logic.
• One room differs: investigate its emitter, flow, balancing, air leakage and controls before moving the whole curve.

Change one relevant value in a small step, note the previous setting and observe the result. Large daily changes make the evidence difficult to interpret.

Room controls and thermostatic valves

Weather compensation sets the supply temperature for the system. Room thermostats, thermostatic radiator valves and underfloor actuators still affect where water can flow.

If most zones close whenever they reach temperature, the heat pump may lose the minimum flow or active volume it needs. It can then cycle even though the weather curve is sensible. Hydraulic design must provide the manufacturer’s required flow in every permitted control state.

The main room control may act as:

• a high-temperature safety or limit
• an on/off demand control
• a room-temperature offset to the weather curve
• a modulating room-influence sensor

These modes are not interchangeable. The commissioning record identifies which one is active.

Combining weather and room feedback

Weather compensation anticipates the building’s changing load from outside temperature. Room influence corrects for gains and losses that the outdoor sensor cannot see, such as sunshine, cooking or occupancy.

Energy Saving Trust describes the combined approach as weather compensation with room influence. It can work well when the indoor sensor is representative and its influence is not so strong that it overrides steady weather-led control.

A sensor near a stove, in direct sun, behind a curtain or in an unusually cold hallway can distort the entire system. Multi-zone buildings may need a more considered control strategy than one influential sensor.

Domestic hot water is separate

The space-heating weather curve should not set the domestic hot-water cylinder temperature. A cylinder call normally uses its own target and may temporarily raise the heat-pump leaving-water temperature under hot-water priority.

If the controller appears to ignore the curve, check whether it is serving hot water, running a hygiene cycle, defrosting or using backup heat before altering settings.

Maximum limits are safeguards, not tuning tools

Installer menus often include maximum flow temperature, compressor limits and supplementary-heater thresholds. Raising them can mask an emitter or design problem while reducing efficiency or exceeding component limits.

Compare actual flow with target flow. If the target follows the curve but actual flow cannot reach it, investigate capacity, flow, defrost and controls. If actual flow repeatedly overshoots, investigate minimum output, system volume and cycling.

Commissioned settings

The handover and any later adjustment should preserve:

• control mode and heating-curve parameters
• design outdoor and flow temperatures
• warm-weather cut-off, if used
• room-influence value and sensor location
• minimum and maximum limits
• zone and valve behaviour
• date, reason and result of each change

This turns tuning into a reversible commissioning process. It also prevents an app reset or service visit from silently replacing a working curve with a generic default.

Related entries

• Design flow temperature
• Radiator emitter upgrades
• Load compensation controls
• Heat pump scheduling and setback
• Handover settings and user controls