Weather compensation is a control strategy that continuously matches the heating flow temperature to the outdoor temperature along a preset heating curve — the colder it gets outside, the warmer the flow. By keeping the flow temperature as low as each moment allows, it exploits the 2–3% efficiency gain per avoided degree, and EU energy labelling credits such controls with up to 5% on the package label.
How does weather compensation work?
An outdoor sensor — mounted on a north or shaded wall — feeds the outside temperature to the heating controller, which computes the flow temperature setpoint from a heating curve: a line relating outdoor cold to water warmth. At +10 °C outside an underfloor system may run 27 °C; at −10 °C the same curve commands 35 °C.
The generator then modulates to hold exactly that setpoint. The building always receives just enough warmth to balance its current losses, instead of full-temperature bursts chopped by a thermostat.
How do you read and set a heating curve?
Two parameters define the curve. The slope says how strongly flow temperature rises as it gets colder; the offset (parallel shift) moves the whole curve up or down. Underfloor systems use shallow slopes around 0.3–0.5, radiator systems 1.0–1.5.
| Outdoor temperature | Underfloor, slope ≈ 0.4 | Radiators, slope ≈ 1.2 |
|---|---|---|
| +10 °C | ≈ 27 °C | ≈ 38 °C |
| 0 °C | ≈ 31 °C | ≈ 48 °C |
| −10 °C | ≈ 35 °C | ≈ 58 °C |
Values are indicative — tuning is empirical. The classic method: if rooms are too cold only in cold weather, raise the slope; if they are too cold at all outdoor temperatures, raise the offset. Then keep trimming downward over the first winter until comfort just holds — every degree trimmed is efficiency banked.
How much energy does weather compensation save?
Each avoided degree of flow temperature is worth roughly 2–3% of heat pump electricity, and over a season a compensated system runs far below its design maximum most of the time. EU energy labelling quantifies the control benefit on the package label per Regulation (EU) 811/2013:
| Control class | Description | Efficiency credit |
|---|---|---|
| II | Weather compensation, modulating generator | +2% |
| III | Weather compensation, on/off generator | +1.5% |
| VI | Weather compensation plus room sensor, modulating | +4% |
| VII | Weather compensation plus room sensor, on/off | +3.5% |
| VIII | Multi-room-sensor control, modulating | +5% |
For a heat pump the label credit understates the real prize: the seasonal efficiency difference between a tuned curve and a fixed high setpoint routinely shows up as several tenths of a point of JAZ.
What about room thermostats and TRVs?
Pure weather compensation is open-loop — it never sees the rooms. Best practice adds gentle room influence: a sensor in a reference room trims the curve for solar and internal gains. The trap to avoid is the opposite extreme, an on/off room thermostat chopping a modulating generator into cycles.
Thermostatic radiator valves stay useful as upper limiters in secondary rooms, but the reference room's emitters should stay open so the controller retains an honest signal — the same open-circuit logic that governs buffer tank decisions in heat pump systems.
Why is weather compensation standard on heat pumps?
Because heat pumps punish high flow temperatures where boilers merely shrugged. Modern inverter heat pumps ship with an outdoor sensor and treat the heating curve as the primary operating instruction, modulating compressor speed to follow it continuously. A condensing boiler also benefits — lower return temperatures mean more condensation — but for a heat pump the curve is not an optimisation, it is the difference between an efficient system and an expensive one.
Frequently asked questions
Should my heating curve be steep or shallow?
As shallow as comfort allows. Underfloor systems typically need slopes of 0.3–0.5, radiator systems 1.0–1.5. Start at the installer's estimate, then lower the settings stepwise across the first winter — the correct curve is the lowest one that still keeps rooms warm on the coldest day.
Does weather compensation work together with a room thermostat?
Yes, and the combination earns one of the highest control-class credits on the EU label (class VI, +4%; only multi-sensor class VIII scores higher): weather compensation sets the baseline flow temperature and a room sensor applies fine correction for solar or internal gains. What defeats it is a simple on/off thermostat forcing a modulating generator to cycle.
Do I still need TRVs with weather compensation?
Keep them as maximum-limiters in bedrooms and secondary rooms, but leave the reference room open. If every valve throttles, the controller loses its feedback and the reduced water flow can disturb generator operation — particularly for heat pumps needing constant flow.
Does weather compensation help a gas boiler too?
Yes. Lower flow setpoints mean lower return temperatures, and a condensing boiler condenses more flue-gas moisture the cooler its return — the same control earns label credits of 2–4% for boilers as well. The relative gain is simply larger for heat pumps.