[Review] Eurotronic StellaZ – Heating Thermostat

The Stella Z

The Stella Z Heating Thermostat by Eurotronic is a nice looking Thermostat. It also is the only thermostat for Z-Wave which allows direct valve control. Running on two AA batteries this devices seems the perfect addition to every Z-Wave network – or is it?

Front view of Stella Z

Exterior

The device itself looks quite appealing. It has a blue button on the top which is used to include/exclude the device and besides the button there is a small green led. The led lights up, when the device is woken from sleep mode or to acknowledge inclusion/exclusion.

Top view of Stella Z

First inclusion

Inclusion of the device was not as simple as with other Z-Wave devices. The blue button had to be pressed quite often, but inclusion was not complete. Only after a couple of tries there was a confirmation of the controller. The behavior was reproducible with both the Aeotec Z-Stick Gen4 and Gen5 Controller.

Configuration

Unfortunately the device has no configuration parameters. The only changeable value is the wake up interval. This can be set from 240s increasing in 240s steps.

The shortest possible time for any value set on the device therefore is 240s or 4min. This eliminates nice features like opening the valve fully for one minute and then close it again to flush the radiator. Or quickly shutting off the heating when the window is open.

Another major downside of this is that there is no possibility to automatically report values. The Z-Wave controller has to query the values and wait for the device to wake up. Massive battery drain is the result.
With other battery powered devices there is a possibility to report values without wake up. The time sending a message is significantly shorter than a wake up cycle. The device has to wake up, signal all other devices that it is awake and then wait a certain amount of time to see if there are any messages for it. It then sees that the master requests e.g. the temperature value which it then sends.
Other devices automatically send the value in a preconfigured interval without wake up. That’s it

Wake up issues

While a minimum wake up interval of four minutes might be something which is annoying but still usable there seems to be more issues with the wake up interval. After erratic behavior of the device the average update time of the target and current temperature were measured.
To achieve this the wake up period of the device was set to 3600 sec (1h). The polling interval for the values on the Z-Wave controller was also set to 1h. The time between updates of the values was measured and plotted in a diagram.

Diagram

Chart showing the average update time Red: Current Temperature Green: Target Temperature

It is quite obvious that wake up is erratic and therefore setting/getting temperatures dynamically is not possible.

Used openhab configuration

This openhab configuration which was used to measure the report interval is also the currently used configuration.

Number	Heating_Battery				"Battery Heating [%d %%]"							{ zwave="6:command=BATTERY" }
Number	Heating_Mode				"Mode [%X]"											{ zwave="6:command=THERMOSTAT_MODE" }
Number	Heating_Temperature_Is		"Heating Temperature is [%.1f]"						{ zwave="6:command=SENSOR_MULTILEVEL,sensor_type=1,refresh_interval=3600" }
Number	Heating_Temperature_Set		"Heating Temperature set [%.1f]"					{ zwave="6:command=THERMOSTAT_SETPOINT,setpoint_scale=0,refresh_interval=3600" }

Openhab configuration for all values

Be aware that the device wake up interval has to be set to 240s.
This configuration did not work in the desired interval because of the described wake up issues!

Number	StellaZ_Battery				"Battery [%d %%]"						{ zwave="20:0:command=BATTERY" }
Number	StellaZ_Mode				"Mode [%d]"								{ zwave="20:0:command=THERMOSTAT_MODE,refresh_interval=240" }

Number	StellaZ_Temperature_Is		"Heating Is Temperature [%.1f]"			{ zwave="20:0:command=SENSOR_MULTILEVEL,sensor_type=1,refresh_interval=240" }
Number	StellaZ_Temperature_Target	"Heating Target Temperature [%.1f]"		{ zwave="20:0:command=THERMOSTAT_SETPOINT,setpoint_scale=0,refresh_interval=240" }

Number	StellaZ_Valve_Pos			"Valve Pos [%d]"						{ zwave="20:0:command=SWITCH_MULTILEVEL,sensor_type=1,refresh_interval=240" }

Current usage

Currently the device is only used with a fix temperature set (20°C). No dynamic temperature changes are made.

Summary

The device is absolutely not recommended as a Z-Wave device. The wake up issues and the lacking possibility to automatically report values are almost certainly reason for trouble!
If the device were mains powered and always awake both issues wouldn’t matter, but unfortunately it is not!

Pro

  • Nice exterior

Con

  • Wake up erratic
  • No automatic reporting of values
  • Long minimum wake up interval (240s)
  • No possibility to configure constant wakeup
  • No configuration whatsoever
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