Heating curve

The heating curve (water law curve) sets the heating system's starting temperature based on the outside temperature. The colder it is, the hotter the water leaves the system; the milder it is, the lower the temperature. When carefully adjusted (slope, offset, and limits), it stabilizes comfort, improves efficiency (cooler return water ⇒ condensation/better COP), and prevents overheating. Conversely, a curve that is too steep or poorly offset leads to complaints, unnecessary cycling, and wasted kWh.

Heating curve: principle, parameters and fine-tuning

The outdoor sensor measures the outside temperature; the controller calculates the flow temperature via a curve defined by:

1. Slope : sensitivity to cold. High for high temperature emitters (older radiators), gentler for low temperature (floor, wide radiators).
2. Offset : shifts the curve if the entire building is too hot/cold.
3. Limits : Minimum/maximum starting temperature (protect materials, avoid cold floors).

- Corrections : ambient probe (trim of a few K), night setback, summer stop, anti-cycling.

- Optimizations : dynamic reset according to return temperature/openings, weather prediction (anticipation of peaks), taking into account sunshine (South facades) and occupancy (BMS).

- Recommended setup : record ambient temperature, supply/return temperature and ΔT over several days (cold, inter-season), adjust the slope in steps of 0.1–0.2 and the offset by 1–2 K max, check the balancing and water quality (otherwise the curve masks hydraulic defects).

Concrete benefits, limitations and points to consider

Benefits

• Stable comfort: less oscillation, homogeneity of rooms.
• Increased efficiency: colder returns → easier condensation / better heat pump COP.
• OPEX ↓: end of overheating, reduced cycles, better GTB management.

Boundaries

• Poorly adjusted curve = complaints (hot/soft) and overconsumption.
• Poorly exposed outdoor probe (sun, hot roof) = biased calculation.
• Buildings with poor airtightness/low thermal inertia: sensitive to wind and solar heat gain if not compensated.

Points to consider

• Emitters: aim for low temperature when possible (floor/radiators sized).
• Hydraulics: balancing, descaling, insulation before fine-tuning the curve.
• GTB: realistic setbacks, summer shutdown, reset of setpoints according to valve opening.
• Sensors: external probe North/Northwest, sheltered; representative environments.
• Validation: follow reference ΔT (e.g., 15–20 K in heating), complaints and kWh/DJU; document settings in the DOE.

Anecdote — “The curve that calmed Dijon”

In Dijon, an office building experienced alternating periods of intense heat in the morning and coolness in the afternoon. The heating curve was too steep, the offset was zero, and the outdoor sensor was exposed to direct sunlight. By repositioning the sensor to the north, softening the slope (-0.2), applying an offset of -1 K, and a slight nighttime setback, the return temperatures decreased, the condensing boiler condensed more efficiently, and the complaints disappeared. As the building manager summarized: "We didn't increase the power output, we adjusted the curve."