Replacing wired thermostat

[Ne0star]

I live in a new build and I'm currently trying to replace the fitted controller and wired thermostat with a 3rd gen nest. Heating is a system boiler with tribune hot water system and wired stat.

There is plenty of info with regards to the wiring, but I am struggling with the old wired thermostat.

I'm pretty sure I have everything worked out with regards to replacing the current 2ch timer with the nest heatlink, but I'm a bit unsure of the wired thermostat.

The current wired thermostat only has two wires. If I trace those back to the controller one goes to the live terminal of the controller and the other to the neutral. My plan was to move these to the T1 and T2 of the nest thermostat respectively as the thermostat requires 12v not 230. Most other installs have 3 wires to the stat.

So my question is what do I do with regards to wiring to compensate for the lack of wired thermostat? Do I just bridge a couple of terminals on the heatlink?

Any help would be appreciated.

Regards

 

[Newboy]

The current wired thermostat only has two wires. If I trace those back to the controller one goes to the live terminal of the controller and the other to the neutral.

If there are only two cores to the thermostat then they won't be connected to live and neutral - when the thermostat closed there would be a short circuit.

You may have a live (voltage dependant on controller connections) and a switched live.

 

[Ne0star]

Your totally right, I have double checked the wiring and although the white cable is connected to neutral at the controller, its not connected to anything at the thermostat end. Its a black cable and live at the thermostat. However the Black cable isn't connected to anything at the controller end. I have left my multimeter at work so can't do any checking until tomorrow.

I have uploaded a few pics.

My plan was take cable from terminal 4 on existing controller and attach to terminal 3 on the nest (Call for heat). take cable from terminal 3 on existing controller and attach to terminal 6 on the nest (call for hot water). Live and Neutral go to terminals 1 and 2 on the nest respectively. I was then going to use the exiting wired thermostat cabling to power the 12v for the nest thermostat. I just need to figure out how the wired thermostat connects up. There is also the junction box in the airing cupboard.

Thanks for the reply.

 

[Ne0star]

For anyone who might be interested I managed to sort this out. I figured out I have an 'S-Plan' system and after studying the associated wiring guide in the nest manual it was clear my wiring almost matched the diagram pretty much perfectly even down to colours.

I clearly got a bit caught up on the fact I couldn't measure 240v across the thermostat which after some cable tracing it was clear there was no live as upon closer inspection the thermostat is just one of those temperature sensitive disk jobbies that just close the circuit. Using a tone based cable tester I traced the thermostat cables to the junction box in the airing cupboard so I just installed the heat link in there, removed the old controller downstairs and fitted a blanking plate after connecting up any wiring that needed to still connect upstairs. Double checked there was only 12v going to the nest thermostat mount and bobs your uncle.

Only problem is now thats all done the wife wants everywhere decorating to get rid of the awful new build quality paint!