Replacing a Cosy installation
We had a Cosy Smart Thermostat installation which was reasonably good. The idea was to move the device with you around the house and your room temperature would reach the level you had set.
Problem was the whole house heated up to produce the required temperature in the occupied room - often overheating the rest of the house.
So we switched to Tado thermostatic radiator valves so we could control the room in each room of our 12 room, 3500 sq ft house. I installed the tado valves myself
The advice from Cosy was to unplug the Cosy Hub and Display and leave wired into the system, the Hot Water Control and Heating Switch (https://cosy.support.geotogether.com/en/support/solutions/articles/7000011269-moving-home).
After a few weeks we decided to extend the system to include control of the boiler and hot water using a Tado extension kit replacing our Honeywell programme controller.
The complete Tado system of wireless Thermostat, Thermostatic valves and Extension Kit worked for a week or so and then there was no heating and intermittent hot water. Investigating the cause of the problems, the symptoms replicated those of a defective motorised diverter valve and I tried the trick of switching these manually.
However, nothing I tried worked and eventually I resorted to calling out a plumber before I took the step of purchasing replacement gear. I wasn't 100% convinced my diagnosis was correct. It seemed my Tado Extension unit was 'demanding action' from the boiler, according to the Tado app.... but I couldn't verify this. I tried setting the Extension unit manually to demand action - but again there was no demonstrable way to know if it was working. There were audible clicks - but no confirmation of what these meant.
The plumber came and investigated the diverter valves and determined they were ok. Eventually he found there was power into the Cosy Hot Water Controller and Heating Switch - but no power out! He bypassed these devices and hey presto the system is working.
The advice from Cosy is erroneous and their devices should be disconnected or bypassed to make the Tado system work.
Hope this story helps and prevents someone else going through our experience and expense.😉