Central heating programmer cable 3 and earth

[Paul Dodds]

Hi,
Hope you're all doing well. I have a customer who needs a 5 core cable to the boiler from the wiring centre. At the moment the feed is coming from a 13a socket in the upstairs cupboard through the programmer into the wiring centre and then 3 and earth under bathroom floor and into the kitchen. In order to get the 4th cable they were using the earth. The plumber has obviously put this back to its original purpose but now there's 1 cable too few - line, neutral, earth, call for heat and call for hot water.
Under the boiler is a fused spur for the cooker hood. Is it possible to use this as the feed for the boiler allowing the line or neutral cable to be used for the call for heat?

I hope this makes sense. Sorry I should have taken pictures but thought of posting here with this "fix" after I left!

Cheers

Paul

 

[LankyWill]

What’s the heating system? Combi or System boiler? S plan or Y? Are you a spark?

 

[Octopus]

Running a new cable is the only way forward

 

[Paul Dodds]

Is Y plan system boiler. The mains feed for the programmer would still be the 13a socket but the mains feed for the boiler would be the fused spur and not from the wiring Centre. This would also mean that the boiler could be isolated locally, via the sfcu. Yes I'm a sparky.

 

[buzzlightyear]

you can work it out from the plan . you only need one isolating point for the full system .

 

[Paul Dodds]

Yes, this is a cracking diagram that someone has spent time doing. It really helped the first few times I installed nest etc. My post is to try and avoid ripping up tiled bathroom flooring to get to run a cable. If I can use a supply to the boiler from a sfcu that's right next to it then I seem to have fulfilled all criteria in the diagram without the need for a new cable. I just wanted to check to see if I had missed something important!?

 

[buzzlightyear]

Yeah,doing the job.

 

[suffolkspark]

i don't follow,,, are you meaning use a separate supply for the boiler feed, then itl have 2 feeds that need isolation ?

 

[Octopus]

I would not recommend having a central heating system with 2 points of supply ..... And isolation

 

[suffolkspark]

same here, sometimes the way on large commercial jobs but certainly not in domestic but I think that's what he's saying ?

a plan b may be to look at some wireless controlls

 

[Paul Dodds]

Yeah, sorry I thought it would get confusing. So the supply for the programmer would be the 13a plug up in the cupboard. The supply for the boiler in the kitchen would be the fcu. This would then mean that the 3 and earth would no longer be supplying the boiler with power and would only be used for call for heat and call for hot water.
So the line and neutral on the 3 and earth would not be connected in the wiring Centre to line and neutral but instead used for call for heat and call for hot water, plus a spare.

 

[suffolkspark]

So you would need to both unplug the plug from upstairs, and switch off the fused connection unit to fully isolate power from the boiler? I guess you could fit a label saying multiple supplies present but I don't like it.

Is the pump in the boiler?
Is it a combi?

 

[LankyWill]

Cant you use the current cable as draw wire for the new 5 core? You sort of know what you need to do but are putting it off for an easier option, id just bite the bullet and fit a new cable.

 

[MJPD29]

Not really suitable mixing 2 circuits, if they are either side of a split board you'll run into issues

 

[nunthe wiser]

Pull in a new cable. Or fit single channel programmer for hot water. Plus wireless programmable roomstat, for central heating.

 

[Midwest]

Think OP saying he wants to alter use of existing 3c & earth, and then add new supply?

What is the existing 3c & earth cable?

 

[nunthe wiser]

Or spur programmer from a separate source,using outputs to switch relays, live and 2 switch lines. That should be possible. Never interconnect 2 circuits.

 

[nunthe wiser]

Or spur programmer from a separate source,using outputs to switch relays, live and 2 switch lines. That should be possible. Never interconnect 2 circuits.

 

[nunthe wiser]

Presumably 3c and e was used as pl n ch hw. So he’s lost a sw line due to cpc being reinstated

 

[Lucien Nunes]

The nub of this is doing away with the N in the 3c+E that is difficult to replace.

If the programmer has fully isolated volt-free contacts, you can power it from one circuit while controlling another. The programmer L & N would be sourced locally, while the 3C+E brings a PL from the boiler circuit to the contact common and returns the CH and HW outputs. Two circuits would be present in the programmer and there is a risk of later alterations accidentally linking the circuits together. Not sure how you would address those risks in a domestic install.

Or, if the programmer doesn't provide isolation between the contacts and supply L, or to avoid two circuits in the programmer, you would need to add relays as per above. Power the programmer locally, use the 3C+E to send CH, HW & N to near the wiring centre, where two relays control the boiler circuit. Now the two circuits are present in the relay box instead, which can have suitable prominent labelling, and the separation is less likely to be breached.