I too am looking to do exactly what you have described in this post.
I have an older horstmann H37XL controller for 3 zone hearing only.
I have 1 room stat for the upstairs circuit and 1 room star for the downstairs circuit.
I would like to use the existing controller and replace the 2 room stats with something like the CT100 thermostats.
I plan on having ST temperature sensors (motion,open close etc) in every other room that I would like to feedback to the room stat.
Example, I only want to stop calling for heat when all rooms are above a certain temperature for each zone. I’m not sure if there is customer code or a neater solution already in place by other community members.
This way I can use the existing controller and let the stats do the work with temp sensors.
Does anyone else have a similar set up or even know if it’s possible?.
The nest thermostat works with ST. Takes a little bit of set up (NEST) and you need to keep polling it with pollster (https://community.smartthings.https://community.smartthings.com/t/nest/817/9/t/pollster-a-smartthings-polling-daemon/3447). But mine is up and running.
I also changed the code a bit to make the interface on the smartthings app a bit more user friendly for the uk version ( I.e. No cooling, etc.). But I am happy to share if you go down this route.