gtbloi-eau

What is a heating curve (loi d'eau)? In hydronic heating, the colder it is outside, the hotter the water sent to the emitters must be to offset the building's heat losses. The heating curve (known in France as loi d'eau, or courbe de chauffe) formalises this relationship: flow temperature = f (outdoor temperature). The boiler room's weather-compensated controller reads the outdoor sensor and automatically applies the matching flow temperature.

The slope sets how aggressively the system responds to cold. With a steep slope (HT radiators, poorly insulated building), the flow temperature climbs quickly as the outdoor temperature drops. With a shallow slope (underfloor heating, well-insulated building), the curve is flatter.

The parallel shift moves the whole curve vertically: +2 °C of parallel shift = every flow temperature is raised by 2 °C, without changing the dynamics. It is the fine-tuning tool when perceived comfort is slightly too low or too high across the whole season.

Why pivot on room temperature? When the outdoor temperature reaches the desired room temperature, the building no longer needs heating. All curves therefore pass through this pivot point — typically (20 °C, 20 °C). Above it, the controller stops heat production.

Typical slopes by emitter (zone H1, pivot 20 °C):

• Underfloor heating — slope ≈ 0.4 to 0.9
• Low-temperature radiator — slope ≈ 1.0 to 1.6
• Hot-water fan coil unit — slope ≈ 1.2 to 1.8
• High-temperature radiator — slope ≈ 1.6 to 2.4
• Unit heater — slope ≈ 1.6 to 2.4

Tuning procedure in practice:

1. Pick a really cold day (close to the zone's design temperature).
2. Measure the perceived room temperature at the worst spot (the coldest room).
3. If too cold in cold weather → increase the slope. If too hot → lower it.
4. Wait for a mild day (5–10 °C outdoors). If it feels too warm in mild weather → lower the parallel shift.
5. Iterate over 2 to 3 seasons. Always leave 24 h between two adjustments (inertia).

Condensing boiler case: condensing only occurs if the return temperature stays below the flue-gas dew point, in practice < 55 °C. This means aiming for a low-temperature regime (LT radiators or underfloor heating) and capping max temperature. With a condensing boiler on non-resized HT radiators, savings are limited in cold weather.

Underfloor heating case: standard NF EN 1264 caps the floor surface temperature at 28 °C in occupied zones (29 °C in bathrooms), i.e. ~45 °C max flow. Beyond that, risk of screed damage and discomfort. Very shallow slope, moderate parallel shift, and always a hydraulic safety thermostat in series.

Related OpenGTB tools: unit converter · 3-way valve · heating degree days (DJU).