Odd Central Heating Wiring

I've just moved into a new place and decided to fit a Nest thermostat, but the Central Heating wiring doesn't make any sense.

The Cylinder Thermostat Call wire isn't connected to anything, and neither is the 3-Port valve HW Off wire

The old programmer is a British Gas UP1, and is configured for independent (Not linked) Hot Water / Heating timers. Turning the hot water on does start the boiler and pump. After a while, the Boiler will cycle turning on for a minute and then turning off for a minute. The heating work fine, and I do get hot water.

The Wiring Centre and the Hot Water Cylinder have been moved at some point up into the attic. The colour of the wires that go from the room thermostat and the Programmer do not match up with the colours that come into the Wiring Centre (e.g. Thermostat Call Yellow -> Grey)



Based on the Wiring Centre, the hot water shouldn't be coming on at all if the heating is off, due to the 2 missing links. What should I check next?

Hello and Welcome to the Forum. If I understand correctly the CH and HW works, but there is no HW thermostat connection? Have you traced that connection ? Please be aware that boiler systems have permanent live terminals and you need to safely isolate the circuit before opening them up.

The hot water will be heated whenever the boiler fires and the grey to the valve is not live.

Hi Wilko. Yes, the CH and HW both work but the HW Thermostat call isn't connected to anything, and I've traced those wires into the HW thermostat to make sure they are correct and run to the correct terminals in the wiring centre.
There is no potential path between the Programmer HW ON and the Boiler/Pump that I can see in the wiring centre, but turning just the hot water on will still start the boiler and pump.
I've been turning off the main breaker while I've been working on it.
I'm starting to suspect that there's some additional connections somewhere where the old and new cables join when they moved the system to the attic, I'll get back if I find anything.

If your adamant your competant to do this yourself and not get an electrician in.... Dissconnect ALL of the wiring centre and download a copy of the appropriate honeywell wiring plan - if youve just got the 1 3port valve thats a "y-plan".... Connect back up to that its easy to follow then insert your nest bo11ox where appropriate

So the system is working fine, but just not terminated as you think it should be?
Do not take everything as red in those wiring centers, I've seen many where the electrician has 'done his own thing' and not necessarily followed the terminations within the center.
If its working fine my advice would be to leave it alone, alternatively take Suffolks advice and re-terminate as you think.

Lol it's blue which means it's the BG version, from personal experience of working on heating systems with blue components it is likely that it is wired wrong.

If the grey wire on the 3 port valve is not connected to anything the valve can't move to Heating only, it can only move to Hot Water only or both CH and HW together.