Just pressed the button on a Nest and contemplating installing myself instead of getting a pro in. Seems simple enough in principle, but would like a check on my logic and understanding for those in the know or with experience.

My entire system is actually controlled by a wiring centre attached to the hot water cylinder in the airing cupboard upstairs (boiler, timer and thermostat all located downstairs). This wiring centre takes the thermostat as an input then when it is live it is demanding heat, the logic in the wiring centre circuit board then outputs a signal to the motorised valve to actuate, the circulation pump to turn on and the boiler to turn on.

HW demand is similar, 240v input from the timer, through the HW thermostat into the wiring centre, which will then switch the HW motorised valve and turn on the boiler when hot water is demanded.

The whole thing is controlled by the timer…so the whole circuit is electrically dead when the timer is off, but when the timer clicks on it provides 240v to the system then the two stats (room stat and cylinder stat) then act as on/off switches to the wiring centre which then does all the magic.

So to install the Nest all I need to do is to remove and ditch the timer altogether. Remove the room stat. Connect the Nest Heat Link CH and HW demand outputs to the relevant inputs of the wiring centre (actually in the case of HW to the cylinder stat). And use the wire between the old downstairs room stat and the wiring centre to provide power to the Nest thermostat from the power terminals on the Nest heat link. Seems simple enough…all the wiring is already in place.

Am I missing something?

Thanks.

Your boiler instructions will have the wiring.
Often (meaning ours) there is a separate circuit and switch so you can have a timer or ‘switched’ and select the switch between the two.

They’re easy enough to do, but the Nest is 5V not 240V. I’d be removing the wiring centre altogether and putting the Nest heat link in its place, then reusing the old room stat wires to connect up to the Nest using the 5V terminals inside the heat link.
You’ll want to make sure you know which wire does what on the old wiring centre though. Are you S-plan or Y-plan?

It’s S-Plan and only one zone so only two motorised valves – one for the CH and one for HW. The boiler is downstairs so all the wiring between the wiring centre and boiler and motorised valves and pump is already established so just working on the basis that I leave all that alone and all the Nest is now doing is providing the demand signals for HW and CH to the wiring centre. That would be the simplest from a layout as the boiler is downstairs and the rest of the gubbins is all upstairs in the airing cupboard.

Why would I discard the wiring centre. The ‘intelligence’ of the system is in the thermostat and all that is ultimately doing is providing the demand signals for HW and CH. The wiring centre can then sort out the scheduling of all the motorised valves and pump and boiler. Unless its not intended to be used in conjunction with a wiring centre.

I appreciate the thermostat runs off 12 v (or whatever) and not 240 which is why I was going to utilise the current wire that runs between the current thermostat and the wiring centre as the 12v power supply from the heat link (terminals T1 and T2 on the heatlink are 12v supply for the thermostat). I’m keeping the thermostat located in the same place

The rest of the wiring between the current timer and thermostat will then be redundant.

You’ll have to forgive me. I mean remove the timer and put the heat link in its place. I blame the sun and sangria.

You’ll still need to rewire the CH zone valve in the wiring centre as it will currently be:
240V demand at timer -> wiring centre -> room stat -> wiring centre -> zone valve -> wiring centre -> boiler.
You need to cut out the room stat bit. Easiest way would be remove room stat wires (and use them for powering the nest) and put a link between the two terminals you removed them from.

Mr Ox,

You’re right and think you’re supporting my view. I think the only slight difference is that in my system the wiring centre is actually a printed circuit board so not possible to do any rewiring of the valves. All the logic is done in the circuitry according to all the input states.

The PCB has an input side where the HW (from room stat then timer) and CH (from timer on a separate wire) demands are connected as well as the hot water cylinder stat…all on separate inputs.

On the output side there are all separate outputs for the two zone valves, boiler on, pump on.

So all I need to do is to bypass the HW and CH demand on the input side and connect to the corresponding terminals on the heat link. The wiring centre should sort everything else out just as it does currently.

I can then utilise the HW demand wire from the current timer (a single uninterrupted wire from the timer to the wiring centre) to provide the 240v supply to the heat link (obviously ditching the timer and connecting that end to the mains), and the wire from the current room stat to the heat link to provide the 12v supply to the new Nest thermostat once I’ve ditched the old thermostat.

Think I’ll give it a go. At last I know I won’t need to mess with the boiler wiring. I’d not be keen to do that so at that point I’d be calling in a pro.

Thanks.

You shouldn’t need to touch the wiring centre. As others have said, the Heat Link replaces the traditional time switch. The wiring centre just sounds like a neater way of doing S-plan without using a choc block.

It’s quite fiddly to install (form over function, I think they assumed the UK uses thin wires like the US low voltage systems) but straightforward. The wiring diagrams cover most scenarios.

Depending on whether your boiler uses volt-free terminals to call for heat you may need to remove a wire link. Boiler installation manual will have more info. It sounds like it’s volt-free though.

My personal opinion tends to be that if you’re looking at this job and don’t know in advance exactly what you’re going to be doing, get a professional. If you do this wrong you’re unlikely to hurt yourself but you could zap your boiler.

They really are the crappest of crap to wire up. Whoever designed the wiring centre for them should be made to lick live terminals.

I wouldn’t embark on the job unless I’d have investigated ad infinitum, and not too proud to get a pro in…but even if I get a pro in I like to have a good idea as to what’s involved so I’m not relying not he pro as too many times you get a pro in and they do a crap job. I drive my wife up the walls…announce I’m going to do something, buy in all the bits which clutter up the house then take 6 months pondering how to do it including several exploratory semi dismantling tasks and eventually I’ll crack on and get the job done. I’m always paranoid about getting halfway through and getting stuck or realise I’ve bitten off more than I can chew.

Anyway I’ll double check the volt free thing….the wiring centre is basically electrically dead until it starts receiving demand signals from the HW and CH demand feeds which are at full 240v, and power everything. The wiring centre has no separate power supply, so I’ll check that the heat link terminals are dead until heat is demanded.