Thermostat

The thermostat is the device that measures a temperature (often ambient) and controls HVAC equipment (boiler, heat pump, chiller, valve, fan) to maintain a set temperature. Depending on the technology, it operates on/off, modulates (0–10 V, PWM, OpenTherm), or uses integrated PID control. When properly chosen, positioned, and adjusted (setpoint, hysteresis, time periods), it stabilizes comfort and reduces energy consumption by preventing overheating and short cycling.

Thermostat: types, functions and integration

1. Room thermostat: Measures the indoor temperature of a zone and controls a generator (dry contact), a valve (0–10 V / PWM) or underfloor heating via a motorized manifold. Programmable (timer) or connected (apps, geolocation, scenarios) versions available.
   
   
2. Modulating thermostat: Digital (e.g., OpenTherm) or analog (0–10 V) communication to adapt the power: local water law, flow setpoints, gentle ramp → fewer cycles, better COP/efficiency.
   
   
3. Thermostatic radiator valves: Mechanical or electronic; regulate room by room. Wireless/wired versions integrate with a zoned control system (gateway).

Sensors & algorithms

Hysteresis/anticipation (adaptive learning), open window detection, sun compensation, min/max limits, locks (hotel/tertiary).

Implementation

Height ~1.2–1.5 m, representative area, away from radiation (windows, radiators), drafts and point sources of heat (copiers, kitchens).

GTB/EMS coupling

Reporting of states, remote reading, occupancy periods, scenarios (night/weekend), instructions by usage profile; consistency with water law and hydraulic balancing.

Advantages, limitations, and points to consider when using a thermostat effectively

Interests

• Stable comfort: less yo-yo effect thanks to anticipation and modulation.
• Energy: nighttime setbacks, fine zoning, reduced cycles → kWh and OPEX down.
• Control: BMS scenarios, temperature history and alerts (under/overheated premises).

Boundaries

• Poor location = biased measurements (glass, airflow, sunlight).
• Regulation conflicts: room thermostat vs. water law too rigid, heads closed everywhere → zero flow, cycles.
• Usage habits: pushing the settings to "24°C" instead of an earlier start.

Points to consider

• Placement: representative area, not on cold walls or under blowing air.
• Settings: realistic setpoint (tertiary often 21 °C), short hysteresis, softened water law slope if modulating.
• Zoning: link heads/servomotors to a central logic (opening ↔ VFD pump ↔ ΔP), avoid "chasing".
• Inertia: with underfloor heating, prioritize modulation + anticipation, limit large temperature drops.
• Consistency: thermostats = local control; BMS = global strategy (schedules, holidays, free/night-cooling).
• Maintenance: check probes, recalibrate if drifting, update firmware of connected models.

Anecdote — “One degree cooler in Angers”

In Angers, offices heated by an air-to-water heat pump and underfloor heating experienced morning temperature spikes followed by complaints in the afternoon. The room thermostats were poorly positioned (near windows) and operated on an on/off basis. They were replaced with modulating thermostats (0-10V) featuring learning and movement to representative zones, a softened water temperature curve, and a controlled nighttime temperature setback. The result: the setpoint was maintained without overheating, the average setpoint was reduced by 1°C while maintaining the same level of comfort, pump/compressor cycles were reduced by 40%, and occupants "stopped the constant temperature fluctuations.".