Puzzled by wiring to Honeywell ST6400C - replacing with Nest Thermostat

[lowercasename]

I'm trying to replace an ancient Honeywell ST6400C programmer which controls our hot water and heating with a Nest Thermostat Heatlink. When I opened up the Honeywell, I was met with this wiring, which puzzles me greatly - it appears that there's only one wire leading to the central heating, but both the central heating and the hot water do work!

Would anyone know the best way to connect this wiring to the Nest Heatlink? We also have a 240V wiring leading to a Drayton thermostat which I'm planning to disconnect and tie together so that it doesn't interfere with the Nest.

We have a gas boiler and a hot water tank powering the system.

 

[CBW]

You have a Y plan heating system, with a programmer that has a permanent live and permanent neutral. Switch live for heating and hot water and the hot water off which is needed for the 3 port mid position valve to operate correctly. Also you have a wrongly used Earth/cpc as a line/live conductor. You’d be better mounting the heatlink near the wiring center if you plan to use the T1 and T2 to power the Nest thermostat.

 

[ericmark]

Since you have DHW off it would seem you have a Y Plan system. There are some errors, you should not have a green/yellow wire over sleeved, and although the heat link is class II if powering the thermostat from the heat link you need an earth.

The heat link is volt free where the old programmer is not, so it will need link wires L to both Coms.

However first decide where to mount. With the old programmer you needed to access the unit to change settings, but with Nest you can set all from the thermostat or PC, so the heat link does not need to go where the programmer was, this may allow one to address the errors with the original installation.

Motorised valves can be at boiler or near to the water store, so you need to decide where it is all going to go. My programmer was in main house, but one reason for moving to Nest was like of wires between main house and flat where the boiler, pumps and motorised valves are, so Nest was fitted in the flat.

You want all powered from same outlet, I have realised how important that is after fitting solar panels, with battery back up, so if using a USB power supply that supply should come from same as rest of boiler.

Look for a wiring centre, there is not much room in the heat link for cables, so I have a wiring centre directly under the heat link.

 

[lowercasename]

Thank you both, with your help and this great blog post: https://www.urbanplumbers.co.uk/how-to-install-nest-thermostat-on-a-y-plan-heating-system-uk/ I got everything working! The wiring centre was directly below the programmer. And I've replaced that mystery earth wire!