TRVZB : Can't retrieve constant temperature and "warm" radiator that I had with my dumb TRV

[Antoine948398]

Hello,

Before installing Versatile Thermostat and the TRVZB I manage to had constant temperature (at least with few variation) with a dumb graduated TRV.

e.g. during a typical day :

The drops are just because I opened the window. The radiator were always warm, not very hot (except to compensate the windows opening).

Now I have installed a VTherm to control a climate of the TRVZB. They are installed on my radiators with no adapter, and the screw is well tighten. I am also using external temp sensors

With following parameters

• Default TPI parameter
• max_closing_degree = 10
• min_opening_degree = 15
• opening_threshold = 10

I am having hard time maintaining constant temperature since. I observe a lot of oscilliations : Next picture is showing temperature variation for two days + the valve opening degree. During the days the valve is closed (at 90%) so obviously the temperature is dropping, but then it is hard to maintain the 19 degrees like before. (there is a windows opening around 8 an, hence the temperature drop)

I am experimenting now with the `coeff_int`, (0.6-->0.5) to see if there is an improvement

The thing that scared me is the sensitivity of the valve_opening_degree.

I witness the followings

• 19% --> radiator stay cold
• 30% --> radiator is very hot
• after 30% --> no perceptible change.

I have the same behavior in the other rooms. It seems that the playground is only between 20 and 30%. So I am not sure the work I am doing with the coeff_int will have an influence at all. Basically I have the impression to have a on/off radiator.

For the record here is the full configuration

hvac_modes:
  - heat
  - sleep
  - "off"
min_temp: 7
max_temp: 35
target_temp_step: 0.1
friendly_name: Thermostat chambre parents
supported_features: 401
current_temperature: 18.7
temperature: 19
hvac_action: heating
hvac_mode: heat
preset_mode: comfort
ema_temp: 18.7
specific_states:
  is_on: true
  last_central_mode: null
  last_update_datetime: "2026-01-07T18:40:50.840324+01:00"
  ext_current_temperature: 1.5
  last_temperature_datetime: "2026-01-07T18:30:37.995744+01:00"
  last_ext_temperature_datetime: "2026-01-07T18:27:16.058685+01:00"
  is_device_active: true
  device_actives:
    - number.0x8c73dafffe3a5ac2_valve_opening_degree
  nb_device_actives: 1
  ema_temp: 18.7
  temperature_slope: 0
  hvac_off_reason: null
  total_energy: 1.94
  last_change_time_from_vtherm: "2026-01-07T18:40:30.838086+01:00"
  messages: []
  is_sleeping: false
  is_locked: false
  is_recalculate_scheduled: true
  auto_tpi_state: "off"
  auto_tpi_learning: {}
configuration:
  ac_mode: false
  type: over_climate_valve
  is_controlled_by_central_mode: false
  target_temperature_step: 0.1
  timezone: Europe/Paris
  temperature_unit: °C
  is_used_by_central_boiler: false
  max_on_percent: null
  have_valve_regulation: true
  cycle_min: 5
  minimal_activation_delay_sec: 10
  minimal_deactivation_delay_sec: 0
preset_temperatures:
  frost_temp: 7
  eco_temp: 17
  boost_temp: 21
  comfort_temp: 19
  frost_away_temp: 0
  eco_away_temp: 0
  boost_away_temp: 0
  comfort_away_temp: 0
current_state:
  hvac_mode: heat
  target_temperature: 19
  preset: comfort
requested_state:
  hvac_mode: heat
  target_temperature: 20.5
  preset: comfort
is_presence_configured: false
is_power_configured: false
power_manager:
  device_power: 1
  mean_cycle_power: 0.33
is_motion_configured: false
is_window_configured: true
is_window_auto_configured: true
window_manager:
  window_state: unknown
  window_auto_state: unknown
  window_action: window_frost_temp
  is_window_bypass: false
  window_sensor_entity_id: null
  window_delay_sec: 30
  window_off_delay_sec: 30
  window_auto_open_threshold: 3
  window_auto_close_threshold: 0
  window_auto_max_duration: 60
is_safety_configured: true
safety_manager:
  safety_state: "off"
  safety_delay_min: 60
  safety_min_on_percent: 0.5
  safety_default_on_percent: 0.1
is_lock_configured: true
lock_manager:
  is_locked: false
  lock_users: true
  lock_automations: true
  lock_code: false
timed_preset_manager:
  is_active: false
  preset: null
  end_time: null
  remaining_time_min: 0
is_auto_start_stop_configured: false
is_over_climate: true
regulation_accumulated_error: 0
regulated_target_temperature: null
vtherm_over_climate:
  start_hvac_action_date: null
  last_mean_power_cycle: 0
  underlying_entities:
    - climate.0x8c73dafffe3a5ac2
  is_regulated: true
  auto_fan_mode: auto_fan_none
  current_auto_fan_mode: auto_fan_none
  auto_activated_fan_mode: null
  auto_deactivated_fan_mode: null
  follow_underlying_temp_change: false
  auto_regulation_use_device_temp: false
  regulation:
    regulated_target_temperature: null
    auto_regulation_mode: auto_regulation_valve
    regulation_accumulated_error: 0
  temp_synchronisation:
    sync_entity_ids:
      - number.0x8c73dafffe3a5ac2_local_temperature_calibration
    sync_with_calibration: true
    sync_attributes:
      number.0x8c73dafffe3a5ac2_local_temperature_calibration:
        value: null
        min_sync_entity: -12.7
        max_sync_entity: 12.7
        step_sync_entity: 0.2
valve_open_percent: 33
power_percent: 33
on_percent: 0.33
vtherm_over_climate_valve:
  have_valve_regulation: true
  valve_regulation:
    underlyings_valve_regulation:
      - - number.0x8c73dafffe3a5ac2_valve_opening_degree
        - number.0x8c73dafffe3a5ac2_valve_closing_degree
    on_percent: 0.33
    power_percent: 33
    function: tpi
    tpi_coef_int: 0.5
    tpi_coef_ext: 0.01
    tpi_threshold_low: 0
    tpi_threshold_high: 0
    minimal_activation_delay: 10
    minimal_deactivation_delay: 0
    min_opening_degrees: "15"
    opening_threshold_degree: 10
    max_closing_degree: 90
    valve_open_percent: 33
    auto_regulation_dpercent: 5
    auto_regulation_period_min: 1
    last_calculation_timestamp: "2026-01-07T18:40:30.836014+01:00"
  underlying_valves:
    number.0x8c73dafffe3a5ac2_valve_opening_degree:
      hvac_action: heating
      percent_open: 33
      last_sent_opening_value: 37
      min_opening_degree: 15
      max_opening_degree: 100
preset_modes:
  - none
  - frost
  - eco
  - comfort
  - boost

Any help appreciated :)

Thanks in advance,

Best !

[caiusseverus]

I witness the followings
19% --> radiator stay cold
30% --> radiator is very hot
after 30% --> no perceptible change.

I have the same behavior in the other rooms. It seems that the playground is only between 20 and 30%. So I am not sure the work I am doing with the coeff_int will have an influence at all. Basically I have the impression to have a on/off radiator.

Yeah this is an issue with valves and radiators. They are not inherently linear devices, whereas the tpi algorithm is written with linear power output change in mind.

There are a few things you can try.

Firstly, if your radiator does not start to get hot until the valve is open at 19%, then set the minimum opening degree to 19%. That means that as soon as the VTherm calculates it needs power, it will move the valve to the 19% position.

Note that this is separate from the opening threshold, which acts like a cut-off for the minimum calculated power - eg below that threshold any calculated power is considered to be zero, so VTherm calculates it wants 4% power and the opening threshold is 5%, then nothing happens until the power required increases to 5% or above.

There is no mechanism to set the maximum valve opening at the moment, but there is a feature request open for it here: #1554

Assuming that gets added, this would scale the 0-100% power request to the functional range of the valve, but that still doesn't mean its heat output will be linear with that range because the heat output depends on the difference between the average temperature of the radiator and the room, it's not fixed like an electric heater is. As the room temperature gets closer to the water temperature the power output of the radiator drops. If you have heating with a low flow temperature (eg a heat pump at 20-30ºC) the this change could be considerable. If you have an older style boiler with a high flow temperature, it won't be as significant.

It's also worth looking at what the two TPI coefficients do - internal sets the responsiveness to internal temperature error, eg how far away is the room from the set temperature, and external sets the amount of output needed to offset the heat loss to the outside. That is directly proportional to the temperature difference between inside and outside.

The steady state heat loss requires a lot less power to replace than actually heating the room does, so you want the output from the internal coefficient to be much more significant than the external.

Start with your internal coefficient at something like 0.5, and the external at either 0 or 0.001. See how it behaves, and only then increase the external coefficient by small amounts - 0.001 or less. If you find the temperature constantly overshoots, you have gone too far and should reduce the external coefficient a bit.

Personally I think the TPI algorithm is not great for controlling radiator valves - it expects a linear response, but radiators do not give that so it's hard to get a satisfactory result. I have switched to using the internal logic of the TRVZB with the dead band set to 0.2º

There is a proportional/integral algorithm in development which should work better as it can inherently compensate for the non-linear response.

[Antoine948398]

to illustrate, here are the plots of the temperature and the valve opening degree

please ignore the spike at 21 degrees (I was manipulating the sensor to change the reports). However it is a good marker to see when the frequency of reports has changed.

Also, it is possible that the outside temperature that rose + 10 degreea on January 8 might had an influence…. Unfortunately can’t let this parameter constant :D

I will continue to monitor and give updates

[jmcollin78]

Hello, I'm a bit offtopic but looking at your curve, it seems your coef_int is too high. Those oscillations are not efficient at all and you will drawn you battery:

Reduce the coef_int until there is no more oscillation the increase or reduce coef_ext until the stability is at the setpoint.

You can also use now the experimental (but it works fine) Auto-TPI feature: https://github.com/jmcollin78/versatile_thermostat/blob/main/documentation/en/feature-autotpi.md

Wait for 8.5.2 release which comes this week-end to use the Auto-TPI if you want to give it a try.

[Antoine948398]

Thanks @jmcollin78 . My coeff_int is 0.5. I’ll try to decrease it.

indeed the valve is moving 12 times in 6 hours in the zoom in graph you sent (valve moving once every 1/2 hour)

last night was better with the same parameters (valve moving once every hour) But that night was colder.

the problem is that at 15% valve opening there is no flow in the radiator and at 25% the radiator seems to be at his maximum heating capacity.

[jmcollin78]

Those are very good curves now. Keep it like that. If you could have 15% instead of 10 or 20% for power_percent it will be better.

You can do a test (but just a test), changing parameter one by one to reduce this 10% step. Starts with:

minimal_activation_delay: 10
auto_regulation_dpercent: 5

Set it the 0, do a test and see if they have a influence on the 10% step you observe. Not sure they will have an impact. Don't keep it at 0 else it could drain your battery.

the problem is that at 15% valve opening there is no flow in the radiator and at 25% the radiator seems to be at his maximum heating capacity.

15%-25% is not a great interval 🫤 . I will implement a max_opening so that VTherm will distribute the power between min and max. If you set the max to 30%, then you will have 0-100% power distributed to 15% - 30%. So more granularity and more finesses in the tuning.

See: #1554 and vote for it.

[Antoine948398]

Thanks ! These curves are indeed better, even if no parameters were changed (only the outside temperature that was higher).

it’s not power percent that is displayed but the valve opening degree (even if they are moving with the same trend).

here is the same time frame with the power percent

You have my upvote on #1554 :)

Thank you very much for the help and the awesome work on this software.

[Antoine948398]

Update : with a coeff_int of 0.4, I have now a super stable temperature. With the radiator being warm (not hot as hell) like before the installation of the TRVZB, and a constant power percentage.

even if I am not reaching the target temperature by 0.2 degrees I prefer this behavior.

the tests are difficult to do because some night it is -2 degrees outside, and some night it is 12 degrees 🙃

[jmcollin78]

To reach the target just increase the coef_ext. See tuning here: https://github.com/jmcollin78/versatile_thermostat/blob/main/documentation/en/algorithms.md#principle

The max_opening_degree has been release in beta today. You can have a look if you want to test it. https://github.com/jmcollin78/versatile_thermostat/releases/tag/8.6.0.beta1. Feedback appreciated if you test it.
To install a pre-release you must force the version in HACS.

[Antoine948398]

I'll give it a try. I will increase the coeff_ext from 0.01 to 0.011.

EDIT : during the night, the temperature lighly oscillated. (Because maybe I was in the room being myself a efficient radiator :)

Best,

[jmcollin78]

0.1 or 0.2 each 2 hours is perfectly normal. This could be sensor precision. If you want more precision you will have to reduce:

auto_regulation_dpercent: 5
auto_regulation_period_min: 1

but you risk to drain your battery. For me your can keep this values now.

Now that you are sure your installation is working correctly, you can try auto-TPI algo to find the best coef_int, coef_ext for your installation. Follow the documentation, install the auto-TPI card and let the algorithm find the best coefs for you.