Water law regulation

The "weather-compensated" control system adjusts the heating circuit's supply temperature based on the outside temperature. The colder it is outside, the hotter the water starts; the milder it is, the lower the temperature is set. The result: emitters (radiators, underfloor heating, fan coil units for heating) operating at optimal levels, consistent comfort, and improved efficiency (heat pumps/gas-fired boilers, condensing boilers). The weather-compensated control system is defined by a slope (sensitivity to cold) and an offset, with limits (min/max) and corrections (reference rooms, time of day, wind/sunlight).

Water law: principle and key settings

The outdoor sensor sends the outside temperature to the regulator, which calculates the desired starting temperature according to a curve (slope + offset).

Slope : steep for high temperature emitters (old radiators), gentler for low temperature (floors, large-sized radiators).

Offset (parallel) : shifts the whole thing if you feel too hot/cold in general.

Limits : Minimum starting temperature (avoid cold floors/baseboards) and maximum (material safety).

Corrections : ambient probe (trim of a few K), night setback, summer stop, anti-cycling, hysteresis on burner/compressor, VFD pump with ΔP setpoint.

Modern optimizations : dynamic “reset” based on returns/openings, predictive weather (heatwave/cold anticipation), consideration of solar radiation (south facades) and occupancy (BMS).

Good practice : calibrate the curve using field measurements (ambient temperature, departure/return temperature, ΔT) over several days and seasons, rather than "by guesswork".

Advantages, limitations and points to consider

Interests

• Stable comfort: less oscillation, homogeneous rooms.
• Increased efficiency: colder returns → condensation promoted; improved COP of the heat pump thanks to lower supply temperatures.
• OPEX ↓: less overheating and manual purging of setpoints.
• Longevity: reduced machine cycles, gentle regulation.

Boundaries

• Incorrectly adjusted curve = complaints (too hot/soft).
• Poorly exposed outdoor probe (sun, hot roof) = errors.
• Buildings with poor airtightness/inertia: susceptible to wind gusts and solar gains if not compensated.

Points to consider

• Emitters: aim for low temperature when possible (oversized floor/radiators).
• Balancing + descaling + thermal insulation: otherwise the water law masks hydraulic defects.
• GTB: realistic nighttime setbacks, summer shutdown, optimized restarts (sequencing).
• Probes: outside to the North/Northwest, sheltered from the sun; representative environments, far from intakes/openings.
• Validation: monitor start/return temperature, ΔT and complaints; adjust slope by steps of 0.1–0.2 and offset by 1–2 K max.
• PAC: lock max temperature compatible emitters to preserve COP; on boiler, favour returns < 55 °C for condensation.

Anecdote — “A curve that calmed Confluence (Lyon)”

In Lyon – Confluence, a tertiary plateau experienced alternating periods of intense heat in the morning and cooler temperatures in the afternoon. The heating curve was steep, without offset, and the outdoor sensor was receiving sunlight at 10 a.m. By repositioning the sensor to the north, softening the slope (-0.2), adding an offset of -1 K, and a slight nighttime setback, the return temperatures decreased, the heat pump's COP improved, and the complaints disappeared. The site manager concluded: "We haven't changed the weather, just the curve we use to interpret it."