Bosch CT200 thermostat connected to Bosch 4300i Boiler — heating not starting

[MarcoITA]

Hi everyone,


I recently installed a Bosch CT200 thermostat on my Bosch Condens 4300i boiler. I connected the thermostat to the two BUS pins on the boiler’s board (red and yellow/green cables comes from CT200 straight in the BUS connector). The thermostat powers on and seems to work fine.

Here’s the issue:

• When I set a heating temperature, the radiators don’t start.
• However, if I open the hot water tap, the CT200 correctly detects hot water usage.

So the BUS communication works, but it looks like the thermostat can’t actually start the heating.

I also asked the technician who originally installed the boiler, but he didn’t know how to fix this. Since I bought the CT200 second-hand from a private seller, I can’t return it, so I really need some help. Could this be just a connection issue? Maybe I need to add a jumper somewhere else?


Any ideas on what I might be missing? Thanks!