Opentherm + multiple floor heating zones: boiler runs with no demand / no heat request without OT thermostat

[XtraEngineering]

Hello,

I’m testing Smart Autotune Thermostat v4.2.1 and I’m a bit confused about the correct configuration when using Opentherm together with multiple floor-heating zones.

System setup

• Boiler: Baxi Luna Confort HT 1.280
• Original thermostat: AVS77
• Opentherm gateway from nodo-shop.nl with OTGW firmware 0.10.2+50c3ed2
• Home Assistant 2026.1.0
• Heating type: floor heating only
• 7 heating zones, each controlled by a valve
  → valves are controlled via Shelly relays, exposed in HA as Generic Thermostats
• SAT integration via Serial (MQTT fail during calibration)

Boiler does not expose return water temperature via Openther, but I do have external temperature sensors on:

• boiler supply line
• boiler return line

Calibration issue

Calibration always failed via MQTT, but I managed to complete it successfully via Serial.

Question:

• Is MQTT calibration expected to work in this setup? It gets stuck in this view:

• Is there anything specific I should check for MQTT calibration with Opentherm?

Configuration questions:

I’m unsure how SAT is supposed to be configured in my case:

1. should I select an Opentherm Thermostat and 7 Rooms?
   Or
2. should I only configure the 7 Rooms and no Opentherm Thermostat?

Observed behavior / problems

Case 1: Opentherm Thermostat + 7 Rooms

• Room temperatures reach target
• All zone valves close
• Boiler keeps running anyway, even though there is no heat demand

Case 2: Only 7 Rooms (no Opentherm Thermostat)

• Boiler does not turn on, even when there is an active heat request from a room

Main questions

What is the correct SAT configuration for:

• Opentherm boiler

• multiple floor-heating zones

• zone valves controlled separately in Home Assistant?

• How should boiler demand be calculated when all rooms are satisfied but Opentherm is still active?

• Is external return-temperature sensing supported / recommended in this case?

• Is there a way to log and see, what SAT is doing in background ?

Few more things:

In General tab, I don't see Heating Curve selection:

Even in Home Assistant SAT is configured, it still shows:

Any guidance or examples for a similar setup would be greatly appreciated.

Thanks!

[XtraEngineering]

I changed configurations yesterday and SAT was working (at least how I understand +- correctly) until today.

Moved Bedroom to the Radiator:

I still can't figure out, why SAT is still on, while all the Rooms are on target:

`hvac_modes:

• "off"
• heat
  min_temp: 7
  max_temp: 35
  target_temp_step: 0.1
  preset_modes:
• none
• activity
• away
• home
• sleep
• comfort
  current_temperature: 19.5
  temperature: 19
  current_humidity: 35.6
  hvac_action: heating
  preset_mode: away
  error: -0.2
  error_pid: -0.2
  error_source: climate.living_room_climate_control
  integral: 0
  derivative: 0
  proportional: -5.787
  integral_enabled: false
  boiler_flame_timing: 252.01108704979157
  boiler_temperature_cold: 33.5
  boiler_temperature_tracking: false
  boiler_temperature_derivative: 0.02
  pre_custom_temperature: 19.299999999999997
  pre_activity_temperature: null
  derivative_enabled: true
  derivative_raw: 0
  current_kp: 28.933333
  current_ki: 0.003444
  current_kd: 17012.799804
  rooms:
  climate.1st_floor_hall_climate_control: 19.5
  climate.living_room_climate_control: 19.5
  climate.kitchen_climate_control: 19.5
  climate.home_office_heating_control: 18
  climate.gabrieles_climate_control: 20
  climate.bernardo_climate_control: 20
  climate.bedroom_climate_control: 19.4
  setpoint: 45
  summer_simmer_index: 20.9
  summer_simmer_perception: Cool
  valves_open: false
  heating_curve: 43.1
  requested_setpoint: 37.3
  minimum_setpoint: 45
  outside_temperature: -14.8
  optimal_coefficient: 1.8
  coefficient_derivative: 0
  relative_modulation_value: 0
  relative_modulation_enabled: false
  relative_modulation_state: "OFF"
  pulse_width_modulation_enabled: true
  pulse_width_modulation_state: "ON"
  pulse_width_modulation_duty_cycle:
• 777
• 422
  friendly_name: SAT OpenTherm Gateway
  supported_features: 401
  `

when there is bigger demand to increase temperature and more zones are open, then Boiler flow temperature are following target nice, but when one or 2 smaller zones are left open only, then Boiler water flow temp overshoots a lot:

with quite big innertia of heated floor it also overshoots targets in the rooms, but this is next thing to think later.

[sergeantd83]

In the radiators section you should add the climate entities that are linked to the inside temperature sensor and Thermostat in the thermostat section. Maybe your OTGW_Thermostat requests for heat, that’s why SAT is still ON.

So far home since Sunday feels warmer overall, even temperatures outside are -14 C.
Before switching to SAT, home was controlled by Baxi AVS77, demand temperatures were much lower, than now, ~34C at 4C outside temp.

Can you explain a bit further?

[XtraEngineering]

I already understood the radiator section and added same climate entity of that room that temperature sensors were selected, even I don't have any radiators at home :) Is it ok, that this Room has climate entity that is controlled via Shelly relays to open/close zone valves?

No the OTGW_Thermostat is not requesting for heat, I set it to low temp, so it couldn't do any requests.

Can you explain a bit further?

I think this feeling is because floor it is heating with higher temperature, it heats faster overshoots by ~0.4-0.5C and this makes the biggest difference for feeling, that floor stays warmer. Before SAT we couldn't feel any warm on the floor, even room temperatures vere on the same target.

[sergeantd83]

If it overshoots then you may adjust the heating curve coefficient to a lower value to prevent overshoot.

Regarding the zones balancing you need to have lower flow in zones that are heating faster and higher flow in the zones that are heating slower.

The pilot zone ( is the zone that needs to have the highest flow ) should be the zone that is heating in a slower rate.

[XtraEngineering]

Why SAT delivers such a high water flow temperature?

If it overshoots then you may adjust the heating curve coefficient to a lower value to prevent overshoot.

By overshoots you mean boiler water flow temperature ? Currently it is set to 2.

In General tab, I don't see Heating Curve selection, it is normal ?

Underfloor:

• Classic Curve
• Quantum Curve
• Precision Curve ( Recommented )

My bigger issue with overshooting is innertia of heated floor, look how short time zone must be open with 45C water flow delivery temperature and with closed zone overshoots room temperature by 0.7C

Regarding zone balancing:

I have pump in boiler and external addition pump Grundfos Alpha 2 connected in line to the feed to all heating system spread acros my house with 4 Manifolds for Floor Heating. So should I close a bit vavles for Manifold input that are closer to the boiler (or the zones that heats faster?)

[sergeantd83]

In General tab, I don't see Heating Curve selection, it is normal ?

It is the Heating curve Coefficient slider in the general settings. You can change it to a lower value ( 1.7 is a good option ).

I have pump in boiler and external addition pump Grundfos Alpha 2 connected in line to the feed to all heating system spread acros my house with 4 Manifolds for Floor Heating. So should I close a bit vavles for Manifold input that are closer to the boiler (or the zones that heats faster?)

Yes, close the these ball valves. And close a bit the zone ball valves that are heating faster.

[XtraEngineering]

I still don't understand if it is "normal" not to have an option to select Heating Curve version?
Using: 4.2.1

Heating Curve Version: Represents the 3 formulas of calculation. The available options are:

[sergeantd83]

It is the Heating Curve Coefficient slider!

[XtraEngineering]

I see the slider, but it is written, that in general tab there are 3 versions for Curve from dropdown meniu: Classic, Quantum, Precision ?

[sergeantd83]

Oh correct, we need to update the README in order to reflect the ONLY heating curve we use in the latest version ( Precision Curve ).

From the version 4.2.1 you can tune only the Heating Curve Coefficient.

Sorry for the confusion.

[XtraEngineering]

thanks. After heating curve change from 2 to 1.7 made difference:

[sergeantd83]

Can you please lower the Overshoot Procection value to 40 °C. Probably your boiler will be able to handle this value.

[XtraEngineering]

Can you please lower the Overshoot Procection value to 40 °C. Probably your boiler will be able to handle this value.

Changed, will see how night goes. Is it normal that after changing settings need to restart HA? otherwise some of the sensors is shown missing.

Also closing valves step-by-step and looking at each zones return temperatures to see effectiveness of closing valves.

[sergeantd83]

Is it normal that after changing settings need to restart HA? otherwise some of the sensors is shown missing.

Yes, for some reason it needs to restart HA.

Also closing valves step-by-step and looking at each zones return temperatures to see effectiveness of closing valves.

The main goal of the balancing is to maintain as many zones possible open.

[XtraEngineering]

What could mean these sensors:

Control Setpoint Synchro: Problem
Flame healt: Problem
Flame status: STUCK_OFF

OTGW don't show any errors