PID Thermostat over a Climate?

[MBRetrofit]

Is it possible? To use a Climate entity as a heater - to override the Climate settings and to apply PID only to modulation of the flame? The modulation algo in mostly gas boilers is quite the same as the PID algo, - difference between request and current temp over the exterior temp...-.

[ScratMan]

Hi, yes, it's possible, but you need an intermediate switch helper entity, for example input_boolean.chauffage_chambre_3 and an automation that will trigger on this entity's state changes and drive the climate entity to set either high or low setpoint to start and stop the boiler.

Example, I'm using this to control some Netatmo climates in bedrooms.
PID thermostats declared like this:

- platform: smart_thermostat
  name: Chambre parentale
  unique_id: chambre_parentale
  heater: input_boolean.chauffage_chambre_3
  target_sensor: sensor.popp_mold_chambre_3
  outdoor_sensor: sensor.fgbs222_smart_module_1_garage_temperature_2
  min_temp: 7
  max_temp: 30
  ac_mode: False
  target_temp: 19
  away_temp: 14
  sleep_temp: 18
  eco_temp: 19
  comfort_temp: 20
  boost_temp: 25
  keep_alive: 00:01:00
  min_cycle_duration: 00:05:00
  precision: 0.1
  kp: 100
  ki: 0.007
  kd: 125000
  ke: 0
  pwm : 900

And the automation to manage the link between virtual and physical thermostats:

alias: Chauffage chambres
description: ""
triggers:
  - entity_id:
      - input_boolean.chauffage_chambre_1
      - input_boolean.chauffage_chambre_2
      - input_boolean.chauffage_chambre_3
    trigger: state
  - minutes: /30
    trigger: time_pattern
actions:
  - if:
      - condition: state
        entity_id: input_boolean.chauffage_chambre_1
        state: "on"
    then:
      - data:
          temperature: 25
        target:
          entity_id: climate.netatmo_chambre_1
        action: climate.set_temperature
    else:
      - data:
          temperature: 14
        target:
          entity_id: climate.netatmo_chambre_1
        action: climate.set_temperature
  - if:
      - condition: state
        entity_id: input_boolean.chauffage_chambre_2
        state: "on"
    then:
      - data:
          temperature: 25
        target:
          entity_id: climate.netatmo_chambre_2
        action: climate.set_temperature
    else:
      - data:
          temperature: 14
        target:
          entity_id: climate.netatmo_chambre_2
        action: climate.set_temperature
  - if:
      - condition: state
        entity_id: input_boolean.chauffage_chambre_3
        state: "on"
    then:
      - data:
          temperature: 25
        target:
          entity_id: climate.netatmo_chambre_3
        action: climate.set_temperature
    else:
      - data:
          temperature: 14
        target:
          entity_id: climate.netatmo_chambre_3
        action: climate.set_temperature
mode: parallel
max: 10

[MBRetrofit]

Nio sense to use On/Off when the boiler can modulate the flame.

[ScratMan]

The boiler uses it's own regulation loop to modulate water output temperature and pump speed based on outdoor temperature and delta T° between water departure and return.

My original "intelligent" Chaffoteaux thermostat (from 2017) was very bad at regulating temperature in the living room (with underfloor heating), clearly there was no PID inside. I installed a Netatmo thermostat that has a PID, and it works much much better, while it only controls the boiler in on/off mode.
Unfortunately, their Netatmo valves are not so good, because there is no remote temperature sensor, they only rely on their integrated sensor and some algorithms to estimate the room temperature while heating. I can confirm it doesn't work well. That's why I'm using my PID thermostat to control the valves (that are seen as climates in HA).
Coupled with fine tuning of the water temperature rules of the boiler Vs outdoor temperature, it gives really good results, comfortable and stable.

[MBRetrofit]

If your boiler has Modbus or Opentherm protocol, you can install a very cheap ESP32 based board and you can get a lot more from your boiler.

I would forget about on/off