A central thermostat alongside TRVs

Better Thermostat has no concept of a boiler. It controls TRVs, one instance per room, and it never decides whether the heat source runs. If your system has a central thermostat as well, that thermostat stays outside Better Thermostat and you drive it yourself.

This page exists because the question keeps coming back and the answer was never written down.

The shape that works

Give each room its own Better Thermostat instance with that room’s TRVs and its temperature sensor, and leave the central thermostat out of every one of them. Keep the central thermostat as its own climate entity, and drive it from an automation that watches the rooms.

The signal to drive it with

Use hvac_action on the Better Thermostat entities. It reports heating while a room wants heat and idle once the room is satisfied, which is exactly the aggregate a boiler needs.

triggers:
  - trigger: state
    entity_id:
      - climate.living_room
      - climate.bedroom
    attribute: hvac_action
  - trigger: homeassistant
    event: start
actions:
  - action: climate.set_temperature
    target:
      entity_id: climate.central_thermostat
    data:
      temperature: >
        {{ 21 if expand('climate.living_room', 'climate.bedroom')
             | selectattr('attributes.hvac_action', 'eq', 'heating')
             | list | count > 0
           else 15 }}

Swap the two temperatures for whatever your central thermostat treats as “run” and “stand down”.

This only works while the central thermostat is in a heating mode. climate.set_temperature leaves the current mode alone, so a thermostat sitting at off takes the new setpoint and still does not fire the boiler. Either leave it in heat permanently and let the setpoint do the switching, as above, or drive the mode as well with a separate climate.set_hvac_mode action ahead of the setpoint — not through hvac_mode inside set_temperature, which many integrations ignore.

The start trigger covers the restart. A state trigger only fires on a change it is listening for, and there is no guarantee the automation is attached by the time the room entities come back, so without it the central thermostat can sit on its pre-restart target until the next room transition.

One attribute to stay away from

call_for_heat looks like the right signal and is not. It is the weather-based shutdown flag: it says the outdoor temperature is below the cut-off, not that this room wants heat. It is identical across every room served by the same weather entity.

Why the central thermostat cannot be left alone

Leaving the boiler thermostat on its own schedule produces a failure that is easy to misread as a Better Thermostat problem.

While the central thermostat is satisfied, no flow reaches the radiators. The rooms keep missing their targets, so Better Thermostat opens their valves further, which is the correct response to a room that will not warm up. When the central thermostat calls for heat again, those valves are open wider than the rooms now need, and the ones that opened furthest overshoot. The symptom shows up as overshoot; the cause is the ceiling above the valves.

Wiring the central thermostat to the rooms removes that ceiling, because the boiler runs when a room asks and stands down when none does.

A note on flow temperature

Better Thermostat does not modulate flow temperature either, and a flow-temperature loop is a second controller with its own time constant. Keep it much slower than the room loop: every change in flow temperature is a disturbance to every room at once, and a fast outer loop will fight the inner ones into an oscillation. Weather compensation from outdoor temperature, corrected slowly, is the usual well-behaved version.

For what Better Thermostat does contribute to distribution across rooms, see Hydraulic balance.