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Topic Title: Heating controls
Topic Summary: Wiring between two circuits
Created On: 10 October 2017 10:43 AM
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 10 October 2017 10:43 AM
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Tradewind25

Posts: 5
Joined: 10 October 2017

Hi,

I have a wet central heating system wired up on the conventional 'S' Plan, with motorised valves for both heating and domestic hot water. All this wiring, including that for the boiler, is fed from a seperate circuit off my distribution board.

I have now installed a fan assisted heater (a Link Removed) in my garage (which happens to be where the boiler is). Power to the garage is supplied by a separate radial circuit. This new heater is plumbed into the heating system and has its own motorised valve designed to open only when the gargae thermostat calls for heat.

My problem is this. It is impractical for me to lead the switched live (orange wire) from the valve through to the 10-way junction box which is in another part of the house. However there is already a second 10-way junction box in the garage, close to the boiler. My CH installer believes it is OK for me to connect the orange wire into this box. This would indeed provide power to the boiler & pump. However my concern is that this would effectively bridge two power circuits: the one feeding the heating system and that feeding the garage.

I am correct to be concerned?

===========
NOTE:
In case you are wondering, the boiler was moved from inside the house to the garage before we bought the house. Hence the reason that the original controls and junction box are some distance from the boiler/garage.
 10 October 2017 12:08 PM
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Zoomup

Posts: 3283
Joined: 20 February 2014

We would not normally bridge two circuits together, as isolation could not be assured under fault conditions or at the time of working on the system. Could a relay be employed to keep both systems electrically separate?


Z.
 10 October 2017 12:16 PM
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Tradewind25

Posts: 5
Joined: 10 October 2017

Hi Zoomup,
Thanks for your comments.
I take your point about isolation. My heating installer suggested a note be fixed to the junction box pointing to the two DP connection units that would need to be switched off to allow safe working on the junction boxes. I guess this is a sub-optimal solution. However I was wondering whether there were any other concerns in bridging these two radial circuits .
I'm not a qualified electrician but I have further doubts given that, for example, the two circuits are protected by the same RCD in the consumer unit. Should this be of concern?
Geoff
 10 October 2017 12:19 PM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

Or just feed the fan-assisted heater completely from the CH wiring? (presuming the 2nd 10-way JB actually has all the connections needed)
- Andy.
 10 October 2017 12:36 PM
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Tradewind25

Posts: 5
Joined: 10 October 2017

Originally posted by: AJJewsbury

Or just feed the fan-assisted heater completely from the CH wiring? (presuming the 2nd 10-way JB actually has all the connections needed)

- Andy.


Andy,
This thought did occur; sadly only after installing the heater.
Given where we are now, I am still wondering whether there are any potential safety/RCD concerns with going ahead and bridging these two mradial circuits.
Geoff
 10 October 2017 12:51 PM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

Given where we are now, I am still wondering whether there are any potential safety/RCD concerns with going ahead and bridging these two mradial circuits.

Well, bare in mind that the wiring regulations explicitly and completely prohibit such a practice (insisting that wiring of each final circuit is kept electrically separate from that of every other final circuit) - you'd be on very soft ground if anything ever did go wrong. In addition to the safety/isolation issues Z mentioned (it's a really awkward situation as someone might isolate and test for dead in the normal way, then half-way through the job have the garage thermostat click on an make everything unexpectedly live again - including any disconnected N conductors), there's the potential risk of overloading neutrals (although probably unlikely in your particular case), and creating EMI issues (e.g. 50Hz hum on audio equipment etc) as L and N will probably now create a large loop. by taking completely different paths though the building. It would also catch you out if you ever wanted to upgrade to RCBOs in future.

This thought did occur; sadly only after installing the heater.

How much extra work can it be? If you are willing to run one wire to the CH JB, is it that much harder to pick up L,N & PE from there too?

- Andy.
 10 October 2017 12:57 PM
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geoffsd

Posts: 1704
Joined: 15 June 2010

Originally posted by: Tradewind25

I'm not a qualified electrician but I have further doubts given that, for example, the two circuits are protected by the same RCD in the consumer unit. Should this be of concern?

Surely, if the interconnected circuits were on separate RCDs, they would not allow the system to work.
 10 October 2017 01:46 PM
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Tradewind25

Posts: 5
Joined: 10 October 2017

Originally posted by: AJJewsbury

How much extra work can it be? If you are willing to run one wire to the CH JB, is it that much harder to pick up L,N & PE from there too?


Andy,
Of course you are right; particularly in view of your comments re: the Regs and possible consequences. I'll get on do as you suggest.
Many thanks for your input.
Geoff
 10 October 2017 02:02 PM
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typiod

Posts: 492
Joined: 09 February 2008

I take it that you only have a 3 core at the boiler then?
 10 October 2017 10:43 PM
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mapj1

Posts: 9371
Joined: 22 July 2004

Please don't bridge circuits that are supplied by different fuses or MCBs, neither live nor neutral conductors should ever meet. Apart from making it quite likley to not be isolated - increasing the shock risk, if someone thinks they have turned it off, also, in a fault situation, currents could flow through the bridging wiring equal to at least the larger of the two circuit breaker short term rating.
If one circuit must 'gate' another, then either have a relay coil powered by one circuit, operating contacts that switch a load on the other, or merge the sources of supply into one, so that it becomes one single but more complex circuit. And, as it is rather non-standard, please label things very clearly, and/or leave a diagram in an envelope at the fuseboard so that when you are not there the next man in has a chance.

-------------------------
regards Mike
 10 October 2017 10:59 PM
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OlympusMons

Posts: 50
Joined: 05 February 2016

is your consumer unit in the garage or nearby? You could come off the heating circuit from there to the fan. Might be easier than bringing one from the cylinder cupboard.
 11 October 2017 01:43 PM
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scottseedell

Posts: 92
Joined: 05 June 2009

I can't help thinking there is a much better solution than bridging across 2x final circuits. Why not use a wireless stat for the garage heating and place the controller in the house? I've used these before and they are very reliable (cost about £30 for a decent one). You can then power the garage fan heater from any available supply via a s/f/spur or even on a plugtop. If memory serves correctly, these just have a heat sensor on the pipework and blow when the pipework is hot (stop when cold) so they don't need any control other than a power supply. Can you locate the garage 2 port valve in the house and keep all control local?

-------------------------
Scott Seedell - MSc IEng MIET
 12 October 2017 07:58 PM
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aligarjon

Posts: 3854
Joined: 09 September 2005

Put the 2 circuits together at the main consumer unit if conditions allow. If the garage is fed with a larger circuit breaker you could maybe fit a switched spur at the CU to downrate boiler supply to protect for overload.

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
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