For 3 weeks now I own a Stiebel Eltron WWK-I 300 W Plus and I wanted to share some experiences (and questions 
).
The WWK-I is a domestic hot water heat pump with 291 liter nominal content and an extra heat exchanger which I current don’t use. The WWK-I has built-in WiFi interface which is used to send a lot of data to the MyStiebel cloud. The device can be controlled via the MyStiebel app. There is no Home Assistant integration available, therefore I use classic means to integrate the pump in Home Assistant:
I measure the water temperature in the upper quarter of the boiler through a hole using a PT1000 temperature sensor:
Power consumption of the pump is measured with a Shelly PM Mini Gen.3 (12,55 € at Hornbach/Germany). The pump has 2 binary inputs to select between 4 operating modes:
- Normal
- Standby
- SG Ready State 3
- SG Ready State 4
SG Ready State 3 and 4 can be configured individually in the smart phone app. For each state, you can set:
- Heating mode: Efficient (pump only), Balanced (optionally uses the built-in heating rod), Quick (pump + heating rod)
- Target temperature, up to 65°C
- Charge level (in German: “Aufladung”), which is a percentage value between 30% and 100%.
I use a cheap 2 channel relais module from Aliexpress which I reflashed with ESPHome to get rid of any Chinese software to control the 2 binary inputs.
Both modules fit easily under the removable front cover. The Shelly is the device on the right, the 2 channel relais module is the one on the left side.
This is an example of a pumping cycle. Ambient temperature is around 18°C. The pump takes 3:30 hours to charge the temperature from 42°C to 53°C (measured with my own temperature sensor). The configured target temperature is 55°C.
Together that is 1,65 kWh, using less than 500W.
So far I’m quite happy with the device and also the integration into Home Assistant. But ideally, I want to have all the values that are available in the MyStiebel app also in HA. And I’m annoyed that I have to use their app and that they don’t even have a browser based access to the MyStiebel cloud. Therefore I’m looking for other ways to get the data. Does anyone know if there is a local API available? If not, is there a way to grab the data from the MyStiebel cloud? Please let me know if you have insights.