Weather Compensation and Load Compensation Heating Controls

Quick Answer: Weather compensation and load compensation both lower a boiler's flow temperature so it spends more time condensing, typically saving 6–15% on heating fuel. Both are recognised energy-saving measures under the Boiler Plus 2018 standard (England), meaning fitting either one satisfies the requirement that every new combi boiler include an additional efficiency control. Weather compensation uses an external temperature sensor to set flow temperature against outdoor conditions; load compensation uses the room thermostat's demand signal to modulate flow temperature instead.

Summary

A condensing boiler only condenses — and therefore only reaches its advertised efficiency — when its return water temperature falls below the flue-gas dew point of around 55 °C. In practice that means running the system at a low flow temperature, ideally 55 °C flow or lower, so that radiators extract enough heat for the return to drop into the condensing band. A boiler left at the factory default of 70–80 °C flow rarely condenses and can run 10–15% less efficiently than its data badge claims. Compensation controls exist to hold the flow temperature as low as conditions allow while still heating the home.

Weather compensation (also called outdoor reset) reads an external sensor and follows a heating curve: the colder it is outside, the higher the flow temperature; the milder it is, the lower the flow runs. On a 10 °C autumn day the boiler might run 45 °C flow and condense hard; on a −3 °C design day it ramps to 65–70 °C. Load compensation works differently — it has no outdoor sensor. Instead it uses the difference between the set room temperature and the actual room temperature (the "load") to decide flow temperature, easing the flow down as the room approaches setpoint. This avoids the overshoot and cycling of a simple on/off thermostat.

Both controls are listed measures under Boiler Plus, so installing either satisfies the regulation for new combi installations in England. They matter most on systems with generously sized radiators or underfloor heating, where low flow temperatures still deliver enough output. On systems with marginal radiators sized for 70 °C operation, aggressive compensation can leave rooms cold on the coldest days, so the heating curve or maximum flow temperature must be set to suit the emitters.

Key Facts

Quick Reference Table

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Feature Weather Compensation Load Compensation
Input signal Outdoor temperature sensor Room thermostat demand (load)
Needs external sensor Yes No
Needs OpenTherm/modulating stat Optional Usually yes
Anticipates cold weather Yes (proactive) No (reactive)
Best for Larger/zoned systems, heat pumps Single-zone combi systems
Typical fuel saving 6–15% 5–12%
Boiler Plus compliant Yes Yes
Key setting to tune Heating curve / slope Room setpoint accuracy
Common protocols Proprietary + OpenTherm OpenTherm
Sensor location North wall, shaded N/A

Detailed Guidance

How weather compensation works

A sensor on a cold, north-facing external wall feeds outdoor temperature to the boiler, which references a heating curve to set the flow temperature. The curve is defined by its slope and an offset. A typical radiator system might run a slope giving 45 °C flow at +10 °C outdoor and 70 °C flow at −3 °C outdoor. Because the boiler reads the weather directly, it pre-empts cold spells and rarely overshoots. The trade-off is that it ignores internal gains (cooking, sunlight, occupancy) unless paired with a room thermostat acting as an upper limit. Best practice is weather compensation with a room thermostat enabling a parallel shift of the curve.

How load compensation works

Load compensation needs no outdoor sensor. A modulating room thermostat — usually over OpenTherm — tells the boiler how far the room is from setpoint. When the room is far below target, the boiler runs a high flow temperature for fast warm-up; as the room approaches setpoint, the flow temperature ramps down so the last degree is delivered gently with the boiler condensing and the room not overshooting. It reacts to actual room conditions including internal gains, but it cannot anticipate the weather. It is the simpler retrofit because it needs only a compatible thermostat, not an external sensor and cable run.

Setting the heating curve

The single most common mistake is leaving the factory curve, which is usually too steep and keeps flow temperatures unnecessarily high. Tune it across the heating season: if rooms reach temperature easily in mild weather, lower the slope; if they fall short on the coldest days, raise it. The aim is the lowest flow temperature that still satisfies the design-day load — that is where condensing efficiency is maximised. Record the final settings on the commissioning sheet so the next engineer does not reset them.

Compatibility and radiator sizing

Compensation only saves fuel if the emitters can deliver the room's heat loss at the reduced flow temperature. Radiators sized for a 70 °C flow / 50 °C return (ΔT50) lose roughly half their output at a 45 °C flow. If you intend to run low flow temperatures, the radiators may need upsizing, or the maximum flow temperature on the curve must be set high enough to cover the design day. Underfloor heating, which inherently runs at 35–45 °C, is the ideal partner for weather compensation. Always check emitter output at the intended flow temperature before promising savings.

Retrofit vs new installation

On a new combi install in England, fitting either compensation type satisfies Boiler Plus, so it is effectively free design effort rather than an add-on. For retrofit on an existing condensing boiler, load compensation via an OpenTherm thermostat is the lowest-cost upgrade and needs no external sensor. Confirm the boiler supports the chosen method — not all boilers accept a third-party weather sensor, and OpenTherm compatibility varies by model. Lowering the boiler's maximum flow temperature manually is the cheapest "poor man's" compensation and worth doing even without smart controls.

Frequently Asked Questions

Which saves more fuel, weather or load compensation?

In a well-set-up system the difference is modest. Weather compensation has a slight edge because it anticipates cold weather and avoids overshoot proactively, and it pairs naturally with low-temperature and heat-pump systems. Load compensation reacts to real room conditions including internal gains. The bigger lever in both cases is getting the flow temperature genuinely low — a badly tuned weather curve saves less than a well-set load-compensation thermostat.

Do I need both?

No. Boiler Plus requires only one energy-saving measure, and the two compensate in overlapping ways. The strongest setup is weather compensation with a room thermostat that can apply a parallel shift, which captures both outdoor conditions and internal gains. Fitting two separate compensation systems offers little extra and adds complexity.

Will compensation make my home slow to heat up?

It can if the maximum flow temperature is set too low for the radiators. On the coldest mornings the system may take longer to reach temperature because it is running cooler water. Setting the curve's cold-end flow temperature high enough for the design day, and using optimum-start scheduling, avoids this. Comfort and efficiency are balanced by correct curve tuning, not by abandoning compensation.

Does it work with old radiators?

Only if they are large enough to output the room's heat loss at the lower flow temperature. Older systems with radiators sized for 70–80 °C operation may not deliver enough heat at 45–55 °C flow. Either upsize the radiators or set the maximum flow temperature on the curve high enough to cover the cold days while still benefiting in milder weather.

Regulations & Standards