TPI tuning - Avoiding overshoot and frequent on/off

[koichirose]

I'm using a Sonoff TRVZB in two different rooms. In one of them, it takes very little time to reach the target temperature, but the room also cools itself down very quickly. This results in frequent opening/closing of the valve and overshooting the target temp as you can see in this graph. Setpoint is 20C from 1pm, 19C before as I was away and I'm using presence detection. I have another temp sensor directly on the radiator (Radiator Temperature):

Some overshooting (it reached 21.7C) is not an issue, but I'm wondering if/how it could do better and save some battery/wear on the TRVZB and maybe some gas. Maybe the valve could be open for longer but at a smaller percentage? Does it make sense?
I tried reducing coef_int and increase the cycle, but didn't see much difference. Thanks!

My config:

hvac_modes: heat, sleep, off
min_temp: 7
max_temp: 35
target_temp_step: 0.5
preset_modes: none, frost, eco, comfort, boost
current_temperature: 21.1
temperature: 20
hvac_action: off
preset_mode: comfort
hvac_mode: heat
ema_temp: 21.22
specific_states: 
is_on: true
last_central_mode: Auto
last_update_datetime: '2025-12-29T15:13:54.872660+01:00'
ext_current_temperature: 10.46
last_temperature_datetime: '2025-12-29T15:13:54.871247+01:00'
last_ext_temperature_datetime: '2025-12-29T15:00:19.046199+01:00'
is_device_active: false
device_actives: []
nb_device_actives: 0
ema_temp: 21.22
temperature_slope: -0.77
hvac_off_reason: null
total_energy: 0.02
last_change_time_from_vtherm: '2025-12-29T15:01:39.053722+01:00'
messages: []
is_sleeping: false
is_locked: false
is_recalculate_scheduled: false

configuration: 
ac_mode: false
type: over_climate_valve
is_controlled_by_central_mode: true
target_temperature_step: 0.5
minimal_activation_delay_sec: 10
minimal_deactivation_delay_sec: 0
timezone: Europe/Rome
temperature_unit: °C
is_used_by_central_boiler: false
max_on_percent: null
have_valve_regulation: true
cycle_min: 25

preset_temperatures: 
frost_temp: 15
eco_temp: 19
boost_temp: 22
comfort_temp: 20
frost_away_temp: 15
eco_away_temp: 18
boost_away_temp: 21
comfort_away_temp: 19

current_state: 
hvac_mode: heat
target_temperature: 20
preset: comfort

requested_state: 
hvac_mode: heat
target_temperature: null
preset: comfort

is_presence_configured: true
presence_manager: 
presence_sensor_entity_id: binary_sensor.anyone_home
presence_state: 'on'

is_power_configured: false
power_manager: 
device_power: 1
mean_cycle_power: 0

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_turn_off
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: 30

is_safety_configured: true
safety_manager: 
safety_state: 'off'
safety_delay_min: 15
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

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
underlying_entities:
  - climate.termosifone_camera
is_regulated: true
auto_fan_mode: auto_fan_high
current_auto_fan_mode: auto_fan_none
auto_activated_fan_mode: null
auto_deactivated_fan_mode: null
follow_underlying_temp_change: true
auto_regulation_use_device_temp: true
regulation:
  regulated_target_temperature: null
  auto_regulation_mode: auto_regulation_valve
  regulation_accumulated_error: 0

valve_open_percent: 0
power_percent: 0
on_percent: 0
vtherm_over_climate_valve: 
have_valve_regulation: true
valve_regulation:
  underlyings_valve_regulation:
    - - number.termosifone_camera_valve_opening_degree
      - number.termosifone_camera_valve_closing_degree
      - number.termosifone_camera_local_temperature_calibration
  on_percent: 0
  power_percent: 0
  function: tpi
  tpi_coef_int: 0.4
  tpi_coef_ext: 0.01
  tpi_threshold_low: 0
  tpi_threshold_high: 0
  minimal_activation_delay: 10
  minimal_deactivation_delay: 0
  min_opening_degrees: '7'
  opening_threshold_degree: 7
  max_closing_degree: 100
  valve_open_percent: 0
  auto_regulation_dpercent: 7
  auto_regulation_period_min: 25
  last_calculation_timestamp: '2025-12-29T14:37:05.248558+01:00'
underlying_valves:
  number.termosifone_camera_valve_opening_degree:
    hvac_action: 'off'
    percent_open: 0
    last_sent_opening_value: 0
    max_opening_degree: 100
    min_offset_calibration: -12.7
    max_offset_calibration: 12.7
    step_calibration: 0.2
    min_opening_degree: 7

friendly_name: Thermostat Camera
supported_features: 401

[jmcollin78]

Hello, reduce drastically the coef_int. Try 0.2 in your case.

[caiusseverus]

Sorry, long post incoming - it got longer than I anticipated.

From my experiments, overshoots with radiators are largely a result of too much external coefficient. If you look at how the tpi formula works, it becomes apparent why this is:

Power = kint x (set temperature - current temperature) + kext x (current temperature - outside temperature)

with the output limited to a maximum of 1 ( which is 100%).

The two parts of the formula are not dependent on each other one does not affect the other. Each part has a different function:

The first part is concerned with internal temperature, and the room temperature error. The larger the error, the more power is requested. As the room temperature rises, the error reduces and so does the power. When the room temperature reaches the set point temperature, the power level resulting from this part of the formula is zero, meaning no more power is required to get the room to the required temperature.

The second part of the equation is concerned with maintaining the room temperature at the set level. Heat loss varies with outside temperature - the larger the difference between inside and outside, the more heat will be lost. You want the external coefficient to be set so that when the internal temperature error is zero, eg, your room temperature is at 20ºC when your thermostat is set to 20ºC, the only power required from the radiator is the heat loss of the room.

Now lets look at your current settings, with kint at 0.5 and kext at 0.01 when the room temperature is at the set temperature, so the output is decided entirely by the external coefficient :

Outside temperature is on the x axis and power output on the y axis.

At 5 degrees outside temperature, the power requested will be 15% - that's almost certainly going to result in a significant temperature overshoot, unless the room is really poorly insulated.

Note that lowering the internal coefficient will not usually help with this because again, when the temperature is at the set point, the internal coefficient isn't doing anything.

So how to decide what coefficients to use?

The internal coefficient relates to how big the radiator is in the room compared to the heat loss. If the radiator is appropriately sized, then somewhere around 0.5 should be fine. If it's a very large radiator, then a smaller number here might be appropriate and this is when the internal coefficient might be contributing to overshoot - too much power close to the set point means that heat still gets dumped into the room even after the valve is fully closed because the radiator stores a lot of heat. If the radiator is a bit too small for the room, you want it to be giving a higher output to bring the temperature up faster, so a higher coefficient might be necessary.

Start with the external coefficient at 0. Then set the internal coefficient to 0.5 and let it run. Does the temperature shoot up over the set point? If so, lower the internal coefficient and try again. Does it take a long time to heat up? Then increase it a bit. If the temperature comes up smoothly towards the set point without a large overshoot, then it's at the right sort of level.

The external coefficient relates directly to the heat loss of the room, so you need to experiment. Once you have the internal coefficient set add a small amount of external coefficient - 0.001 or 0.002 and observe how it behaves. If it oscillates about the set point, it's good. If it's below the set point, add 0.001 at a time to see how it settles. You should end up with the room temperature staying within a small amount either side of the set point.

Bear in mind that the minimum valve position will come into play here - I have mine set to 5% to try and prevent the valves going out of calibration, so there is going to be some on/off cycling. If you are using VTHerm to control your boiler then a small amount of hysteresis is good, or the boiler would remain on continuously trying to output a small amount of heat, which it probably can't do.

I'm using internal coefficient of 0.5 and an external coefficient of 0.0045 at the moment, with a cycle time of 5 minutes. That means the radiators open fully when the set point changes from 18.5ºC overnight to 20.5ºC in the morning and then gradually close as the room temperature comes up. Here's what the response looks like:

The error is remaining within about 0.2ºC, which is not really noticeable in the room.

There are some other factors which can affect things - if the room has a lot of external heating, eg a kitchen where there is a lot of cooking or sun through a window, you might need to tweak the external coefficient down a bit.

[caiusseverus]

Do they actually go flat though? NiMh rechargeables are 1.2V, so will report lower than alkalines which are 1.5V.

I'm just using Costco's own brand alkalines. I've not tried them with rechargeables but Sonoff advise against it:

Can rechargeable batteries be used?
AA rechargeable batteries (e.g., nickel-metal hydride) have a voltage of only 1.2V. When three are connected in series, the voltage is slightly higher than the minimum operating voltage of the device, but even fully charged, they cannot maintain long-term use.

[koichirose]

Oh, did not notice that. Never had issues with rechargeables in other products though, IKEA ones are particularly good.
Will switch to alkaline. Thanks!

[koichirose]

I further lowered ext to 0.3, int is 0. I now have basically no overshoot, but the frequent on/off issue is still there. Maybe it's not even an issue, but I'll try increasing the cycle/min regulation period to see if it makes sense to keep the valve open at a lower opening degree for longer.

[Lipown]

Sorry, long post incoming - it got longer than I anticipated.

@caiusseverus .... I think your long post should be pinned somewhere or put as detailed explanation to the documentation. Honestly I reread it again carefully after still struggling with the TPI proper setup and now it does make 100% sense to me... I will once again reset my setup and start from scratch.

[jmcollin78]

I pin this discussion. Thank you @caiusseverus for this explanation !