Navigating my central heating circuits and fitting 3rd Gen Nest

davemate23

davemate23

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Hi all,

I just bought a house and noticed the central heating wiring was a bit of a mess. I bought a Nest 3rd gen and thought it would be worth trying to sort it whilst fitting it into my new house - everything was wired straight into the old Honeywell ST7100 without any sort of junction box. It’s a Y-plan system with a (very old) Glow Worm 35F boiler, a Honeywell V4073A1039 valve and a UnitFive pump.

First time I wired it - given that it was all a bit ad-hoc, I just switched the wires around but must not have wired the pump properly, so the boiler started and went a little crazy! I’m now adamant it’s wired properly after buying and wiring my own junction box, but the thing seems to trip as soon as it’s turned on. Initially I changed the fuse at the spur and it turned on which worked but as soon as the pump kicked in (or could be the boiler) it shorted. Now even after changing the fuses the power doesn’t seem to kick in at all. The circuit breaker hasn’t tripped so could do with any advice on where to look for the fault and how to fix it!

Any help would be much appreciated!

Dave
 
Ditch your junction box and put everything back as it was before you fiddled with it. If it still doesn't work you've broken something.
 
Don’t think I could if I wanted to now. There were a bunch of wires thrown into the spares block. It’s wired up exactly as is stated in the Nest manual - so guess something must be broken!
 
The Honeywell ST7100 is volt free, the Hive is not, and many old boilers have 24 volt control systems so not suitable for Hive.
The tank thermostat uses three wires with Y plan, it is clearly wrong, but many cowboys have wired them using the yellow/green wire as a line wire, not what we expect to find but common.

The published Y plan is not always followed, because of the boiler having a 24 volt control system, step one is look up boiler and see if 230 volt or 24 volt control.
 
What a mess and waste of money.

You could try disconnecting the nest and motor value.
It should then be possible to have boiler and hot water. And be left with a simpler circuit you can understand.

Then add CH, when you have the rest working

Do you still have a timer in the system ?
 
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@ericmark I’ve checked the mains and it was kicking in at 240v, and I checked the manual for the boiler and it specifies 240v so that shouldn’t be the issue. I disconnected the old thermostat, which was wired in using a 4-core cable, isolated two and have used the old Live and Neutral 0.75mm cables for the 12v DC connections to the thermostat. This seemed to work fine whilst it was in use.

You’re right about the cylinder stat cable. I took the cylinder thermostat cover off and checked the links myself and wired them up appropriately so this isn’t the issue. I think it’s likely to be either the boiler or pump.

@AndyPRK I still have the Honeywell, my plan tomorrow (unless anyone has better suggestions) is to wire in a switch to the live and then connect the pump, then boiler to isolate the individual components and see if they work on their own. Not really sure what else could be the issue.
 
Well the stat or motor could shorting neutral to live.

Got any pics of the wiring center ? Wired up
 
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I’ve attached the photos of the junction box and another with the Nest Heatlink. I do plan to change the wiring colours once it’s all up and running - as mentioned, the cylinder stat needs changing plus a few others.

I have the following:

1 - Live
2 - Neutral
3 - Earth
4 - Blank
5 - CH on (valve white)
6 - HW off (cylinder C)
7 - HW on (valve grey, cylinder 1)
8 - CH on (pump L, boiler L, valve Orange, cylinder 2)
 
@AndyPRK I’ve disconnected the thermostat, only connected the live and neutral wires to terminals 9 and 10 to supply the 12v DC for the Nest thermostat. By motor, do you mean the pump? What would cause this to short the system?
 
The thermostats and programmer does not turn on central heating boiler they turn on the motorised valve then the valve turns on the boiler, so you need to monitor the valve, I found wiring diagrams of the valve are over simplistic and using a multi-meter I could not work out fault as there were three micro switches inside the valve not just one as shown in the plan.
 
@ericmark There’s a 3-core cable for the boiler and the mains though? The instructions for the valve are here and seem to state that the orange wire should be connected to the pump and boiler live as I have done.

@AndyPRK ah ok, how do you think this would be shorting? The live is plugged straight to terminal 8 and the neutral just joins the others.
 
@ban-all-sheds as mentioned, the cylinder was wired with a brown, blue, yellow/green 3-core and those are the cylinder 2 and the orange wire from the 3-port valve. Is that not correct, or are you highlighting the colours? I’ve already mentioned that i’ll be sorting them once the system is actually working.
 
Green/yellow is for earth, not live.

Whilst using the G/Y core of a cable for something other than earth is allowed (as long as the core is oversleeved to identify it's use, which that one isn't), it's regarded by most people as a Very Bad practice, only engaged in by plumbers and others with no interest in doing things properly. You can get cables with more than 3 cores, so re-purposing the G/Y is inexcusable.
 
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