Connecting Combi Boiler Thermostat

[goldmansean]

Hi all. First time posting on the forum so I’m hoping someone can point me in the right direction with regard to wiring a new boiler thermostat?

I have a Viessmann Vitodens 100-w combination boiler currently connected to an intermittently working Honeywell CM921 wireless thermostat. At some point the thermostat has been dropped causing the display to disappear and the programme function to fail. We live in an old Victorian house with very thick walls and on occasion the thermostat finds it hard to communicate with the boiler unless we move it quite close for it to connect. A neighbour suggested we get a wired thermostat to solve this which we have. Purchased a Honeywell Home T3 1-Channel Wired Programmable Thermostat (battery operated) from screwfix and need some advice on the wiring? The boiler and current stat receiver is connected via a fused spur. Black wire is power to the boiler. White two core cable connects to the boiler next to the mains and then to the connector block. (see pics).

Can I connect my new wired stat to the current wiring configuration in the connector block and if so where do I connect too?

Or, do I connect my two wires directly to the boiler? In which case, which wire goes where? On my new stat there are a T1 and T2 marked connection. Where should these connect to on the boiler terminals? ( 1, L or N) Is there a correct way round? I just want to make sure. I hope that all makes sense?

Thanks in advance.

 

[ericmark]

I am not sure which model you have? But basic same connections with all three if using on/off control. The centre picture has option to work with Opentherm and it seems also the boiler has the option as well. I have never used OpenTherm, so no idea how good it is.

But it seems your also changing thermostat position, so step one is the talk about what you want the thermostat to do? May seem an odd question, however in the main our homes are split into rooms, so although we have a single boiler, in each room we have a TRV to control the temperature of that room.

However the simple TRV can't turn the boiler on or off, it will cause the boiler to modulate, and once fully modulated (min output) the boiler will start to cycle on/off and adjust the mark/space ratio for output required, but can't actuality turn it off.

So some way to turn off boiler in the summer is required. So the instructions are put a thermostat in a room kept cool, that way it turns boiler off before we get stinking hot weather, down stairs as heat raises, in a room with no outside doors, and no alternative heat, and that includes heat through windows with the sun, and at least in my house, there is no such room.

So we have to consider carefully where to place the thermostat, easy with wireless we can try it out, wired however we need to get it right first time, and I didn't, I put mine in the hall, and the hall cools too slowly, you can adjust heat up time with the lock shield valve, but not cool down time.

The fool proof way is a hub which connects to programmable linked TRV heads, but at around £60 per head, it gets expensive. I have ended up with multi-thermostats due to the first one cooling too slow.

So are you using OT or on/off, and which diagram matches yours?

 

[goldmansean]

Hi there. Thanks for your reply. Very informative. We are not using Open Therm but the on/off system. The diagram that best looks like what we have I would say is diagram 4 (fig.13) but with no wire connected to number 1 terminal on the right hand side of the diagram (marked ‘D’) I am assuming that I will need to make connections to 1 and L on the boiler but does it matter which connects to L1 and L2 on the thermostat?