I managed to solve the problem. The problem was the TRV using an algorithm for heating depending on a profile for an average room with an average count of windows and draught. It’s possible to disable this algorithm by following the guide by brainzap:
This document describes on page 7 how to disable this algorithm. This guide is in german. For all non german speaking people, here is the translation:
3.1. The thermostat does not close on reaching the desired temperature
The thermostat uses by default an algorithm for temperature control, which is based on
average values for room size, ventilation, windows, …
Sometimes this setting doesn’t work correctly, so it has to be deactivated.
To deactivate this algorithm, you have to carry out the following instructions:
- Press the Home symbol () for about 5 seconds (until the symbol lights up).
- Release the Home symbol () and press it again for about 10 seconds
- Now you are in the settings and the ( ) symbol flashes
- Press the (+) symbol until the number 4 lights up and the (SET) symbol flashes.
- Confirm by simply pressing the Home Symbol ().
- Press the Home Symbol () about 10 times until you see the letter “A” to the right. Below should no blink a “0”.
- Press the Plus symbol (+)
- Now a “1” should flash
- Confirm by simply press the Home Symbol ().
The algorithm is disabled and the thermostat should work correctly.