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Thermostat

This feature is not included in precompiled binaries.

To use it, you must compile your build. Add the following to user_config_override.h:

#ifndef USE_THERMOSTAT
#define USE_THERMOSTAT
#endif

Control over heating and cooling as a true HVAC unit

Thermostat driver allows a Tasmota device, provided it receives the temperature input via MQTT or a locally connected sensor, to follow control heating/cooling strategies to reach the desired setpoint. The thermostat offers similar functions as feature rich commercial ones as the models found below:

Pinout

Typical setup: Heating floor system~

A typical setup for heating room systems can be found in the picture below. A conventional room thermostat is connected to a heating floor valve actuator, both running at AC voltage (f.i. 220V). The thermostat is connected to neutral as well as to the phase, the actuator to the same neutral connection of the thermostat and to its actuation signal. The actuation signal will switch between the neutral voltage (actuation Off) and the phase voltage (actuation On).

The conventional room thermostats offer nowadays either 2 point control with hysteresis or a more advanced PI (Proportional-Integral) control. The result of the PI control is typically transformed into a PWM signal with a pre-defined period and a variable duty cycle.

Pinout

Use of tasmota switch to bypass an existing wall thermostat~

A tasmota switch can be installed in a way that it bypasses the existing wall thermostat. The advantage of this setup is that the thermostat driver offers the possibility to follow the output of the existing wall thermostat or acting autonomously. This setup allows a seamless integration with existing wall thermostats and gives the user the freedom to still use them.

Below you can find an example of a Shelly switch bypassing a wall thermostat:

Pinout

Configuration for standalone application or bypass of existing wall thermostat~

The driver by default does not consider the input switch states even if available and its therefore suitable for standalone use. If the application requires to follow the command of the input once active (see bypass setup explained in previous section). This bypass function is specially useful to allow the user to use the device bypassed, in case of a wall thermostat, to allow this thermostat to be used if desired. To enable this bypass function, the following command is to be sent to the tasmota device:

cmnd/Tasmota_Name/INPUTSWITCHUSE 1

Note

Some devices (such as the Sonoff 4CH Pro R2) even if having input buttons to manually switch the state of the output, report always its inputs in active state no matter if the button is pressed or not. For these devices the parameter above needs to be set to 0, otherwise the thermostat driver will activate the output continuously and stay permanently in manual mode.

Once active, the thermostat, in case of its input being active, will switch to manual mode and set as output the same state of its input. The thermostat will switch back from manual to automatic mode after a defined time where the input is inactive. The following parameter can be set to modify the time window in minutes to switch back to automatic in case the input is inactive:

cmnd/Tasmota_Name/TIMEMANUALTOAUTOSET 60

The default value for the time window to switch from manual to automatic is 60 minutes.

Temperature input / setpoint~

Local temperature sensor~

The tasmota driver can receive the temperature either via the related MQTT command or via a local temperature sensor (see example of a DS18B20 temperature sensor and a shelly temperature sensor addon below).

Pinout

The default temperature input is MQTT. The following command can be used to select the local sensor as default input:

cmnd/Tasmota_Name/SENSORINPUTSET 1

Note

The default local temperature sensor is a DS18B20. In case a different Tasmota supported sensor is used, the following define in my_user_config.h is to be changed (or redefined in user_config_override.h) and a user specific tasmota software needs to be compiled:

#define THERMOSTAT_SENSOR_NAME                "DS18B20" // Name of the local sensor to be used

MQTT temperature value and setpoint~

The following commands can be used to provide the driver with the temperature value of the room and the desired setpoint:

cmnd/Tasmota_Name/TEMPTARGETSET 22.5
cmnd/Tasmota_Name/TEMPMEASUREDSET 21.8

Examples for room temperature of 21.8°C and temperature setpoint of 22.5°C

There are several ways to send the MQTT room temperature. For the development and testing of this driver, a Raspberry Pi with Domoticz and a Z-Wave stick has been used to gather all room temperatures from Z-Wave sensors and send them to the respective Tasmota room thermostats.

Customization of the controller for best results~

The thermostat controller includes a default parameter set that targets a typical floor heating application for mid-sized rooms (< 20m2) with one heating circuit. The controller is however highly configurable via MQTT. The following sections will guide the user to adapt the main parameters to improve the performance of the thermostat controller via customization.

Enable the thermostat driver~

The thermostat driver is by default disabled. To enable it the following command can be used:

cmnd/Tasmota_Name/THERMOSTATMODESET 1

Set the controller in degrees Celsius or Fahrenheit~

The default temperature format is degrees Celsius. The format can be easily switched to degrees Fahrenheit via MQTT command, see below:

cmnd/Tasmota_Name/TEMPFORMATSET 1

Set the control strategy~

The control strategy by default is a Hybrid one. The hybrid control strategy mixes a so called "Ramp-Up" strategy (invention during the development of this driver, initially as a LUA script and ported to Tasmota later) and a PI one. The "Ramp-Up" strategy is typically used to reach as fast as possible the setpoint for big deltas between desired temperature and setpoint, the PI on the other hand for most part of the normal thermostat operation.

The control strategy can be however modified, if the Hybrid one is not desired, the PI or "Ramp-Up" mode, can be forced. For that purpose the following command can be used:

cmnd/Tasmota_Name/CONTROLLERMODESET 1

The value 1 forces the PI operation and the value 2 the "Ramp-Up" one.

PI controller main parameters~

Cycle time

Depending on the heating system, the cycle time (PMW period) can be adapted. Very slow systems (high time constants) such as heating floor systems might need higher values (default value is 30 minutes), faster systems might need smaller cycle times. Below the command to adapt the cycle time can be found:

cmnd/Tasmota_Name/TIMEPICYCLESET 30

Proportional Band

Depending on the dimensioning of your heating system, the proportional band of the controller might be increased (if it takes too long to reach setpoint) or reduced (very high overshoot). The default proportional gain is 4, which means that the duty cycle due to the proportional part of the PI controller will be 100% for temperature deltas between setpoint and room temperature equal or bigger than 4°C. Below the command to adapt the proportional band can be found:

cmnd/Tasmota_Name/PROPBANDSET 1

Note

With the command above, the PI controller will output a proportional time equivalent to 100% of the duty cycle for delta temperatures between setpoint and room temp. above 1°C (f.i. for big rooms with weak dimensioned heating circuit).

Reset Time

The reset time is the time the PI controller takes to overcome steady-state errors. The default value for the reset time is 1800 seconds. This value can be for instance increased in case a stronger integral reaction of the controller is desired. Below the command to adapt the proportional band can be found:

cmnd/Tasmota_Name/TIMERESETSET 1800

Temperature for the anti-windup reset

To avoid the accummulated error and therefore integral component of the PI controller to grow too much and produce a high overshoot, a temperature delta can be defined within the integrator will work. Outside this range the accummulated error and integral part will be set to 0. The default value for the integrator to work is 0.8°C. Below the command to adapt the anti-windup temperature can be found:

cmnd/Tasmota_Name/TEMPANTIWINDUPRESETSET 0.8

Temperature hysteresis

A temperature hysteresis can be set to avoid any PI controller actions within a certain value arround the setpoint. The default value for the hysteresis is 0.1°C. In well configured controller this value should be as low as possible to avoid unwanted temperature oscillations which reduce efficiency and therefore increase costs. Below the command to adapt the anti-windup temperature can be found:

cmnd/Tasmota_Name/TEMPHYSTSET 0.1

Maximum action of the controller

The maximum On time (Duty Cycle) in minutes within a cycle can be set by this parameter. The default value is 20 minutes. This represents for the default cycle time of 30 minutes 2 thirds of the complete cycle. In case the controller takes too long to reach the setpoint, this value can be increased to values closer to the cycle time. Below the command to adapt the maximum action time can be found:

cmnd/Tasmota_Name/TIMEMAXACTIONSET 20

Minimum action of the controller

The minimum On time (Duty Cycle) in minutes within a cycle can be set by this parameter. The default value is 4 minutes. Below the command to adapt the minimum action time can be found:

cmnd/Tasmota_Name/TIMEMINACTIONSET 4

It is very important to adapt this value to your heating system to obtain accurate temperature control

If the value is very low, in case of floor heating systems for instance, the heating actuators might not have enough time to open the valves and the temperature will drop (depending on the actuator open/close time could take from 1 to 3 minutes) if it is too high, there will be unwanted oscillations arround the setpoint. One way to configure this value in heating mode is to manually tune it in worst case conditions (highest typically desired room temperature and lower winter temperature outside) checking that the proportional action generated by the controller is sufficient to raise slightly the temperature. If the temperature still goes down after the pulse plus delay time of the system and rises just once the accumulated error triggers integral actions then the value set is too low.

Ramp-Up controller main parameters~

Temperature delta to get into "Ramp-Up" mode

When the controller is configured in Hybrid mode (default), the control strategy will be a mix-up between "Ramp-Up" (for big deltas between room temperature and setpoint) and PI (arround the setpoint). The following parameter can be set to define at above which delta temperature between measured and setpoint the "Ramp-Up" controller shall be active:

cmnd/Tasmota_Name/TEMPRUPDELTINSET 30

The default value is 0.4°C.

Time passed after latest setpoint change to get into "Ramp-Up" mode

When the controller is configured in Hybrid mode (default), the activation of the "Ramp-Up" mode will not just depend on the defined temperature delta between measured and setpoint, but as well on the time in minutes passed since the last setpoint change occurred. This strategy matches the purpose of the "Ramp-Up" controller, which was developed to reach the desired temperature as fast as possible in very specific scenarios, f.i. after a night keeping the room temperature low. In hybrid mode, the controller active most part of the time should be the PI one. The following parameter can be used to define the time to allow switching to "Ramp-Up" in minutes.

cmnd/Tasmota_Name/TIMEALLOWRAMPUPSET 300

The default value is 300 minutes.

Cycle time

Depending on the heating system, the cycle time (PMW period) can be adapted. Very slow systems (high time constants) such as heating floor systems might need higher values (default value is 30 minutes), faster systems might need smaller cycle times. Below the command to adapt the cycle time can be found:

cmnd/Tasmota_Name/TIMERAMPUPCYCLESET 30

Maximum Ramp-Up time

The maximum time the ramp-up phase of the controller shall be active can be configured. The default value is 960 minutes. Below the command to adapt this time can be found:

cmnd/Tasmota_Name/TIMERAMPUPMAXSET 960

Advanced features~

Multi-controller~

The tasmota driver can be compiled to be used in devices with more than one output, allowing independant controllers for each one of the outputs. This feature has been successfully tested with a Sonoff 4CH PRO R2.

Pinout

To increase the number of controller outputs, modify the value of the thermostat controller outputs in my_user_config.h or redefine it in user_config_override.h and compile a customized tasmota software.

#define THERMOSTAT_CONTROLLER_OUTPUTS         1         // Number of outputs to be controlled independently

Alternative outputs: PWM duty cycle~

The driver provides the possibility to read the duty cycle in % (0-100) of the actuated relay. Below the command to read the duty cycle can be found:

cmnd/Tasmota_Name/CTRDUTYCYCLEREAD

The physical switch of the output can as well be disabled via command. Below the command to disable it can be found:

cmnd/Tasmota_Name/ENABLEOUTPUTSET 0

Future improvements~

Cooling~

The controller offers the possibility to switch from heating to cooling. Due to lack of cooling setup at the time of the development of the driver, this feature has however not been propertly tested. Testers for cooling are therefore welcomed.

The following MQTT command can be used to switch from heating (default) to cooling:

cmnd/Tasmota_Name/CLIMATEMODESET 1

Self learning process of the "Ramp-Up" controller to reduce overshoot~

The "Ramp-Up" controller evaluates the time constant of the system and predicts when to switch off the actuator to reach the desired temperature as fast as possible. This controller offers the best speed to reach the Setpoint. This controller will be improved by a learning process to evaluate how accurate the target value has been reached and therefore minimize gradually the overshoot. This feature will improve the behavior of the current controller which depending on the application and thermal capacity of the system might produce some overshoot. By default the controller set is the Hybrid one, enabling "Ramp-Up" for big temperature deltas between Setpoint and measured temperature and PI for smaller ones. If you are not satisfied with the performance of this controller in your system, you can disable it by MQTT and force the use of the PI controller exclusively (see Controller configuration section above).

PI Autotune~

A PI autotune feature following the Zigler-Nichols closed loop algorithm has been implemented. This feature is untested and will be further developed soon. To enable it for testing purposes add the following define in user_config_override.h and compile a customized tasmota software.

#define USE_PI_AUTOTUNING // (Ziegler-Nichols closed loop method)