Discuss Advice boiler wiring in the Central Heating Systems area at ElectriciansForums.net

Trcurt

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Hello all
Looking for some electrical advice if possible. We had a boiler and complete new central heating system installed in our house 3 years ago. This has been working fine until recently when we had a new consumer unit with rcd protection installed to upgrade.
From the day this was installed we started getting multiple intermittent rcd faults affecting the 4 circuits the rcd covered. By process of elimination we narrowed the fault down to the central heating system, by this time we were unable to heat water without rcd tripping.

We called an electrician to investigate and he tested the central heating system for faults but said he couldn't find any. However what he did find was that he said the boiler had been unsafely wired. He said there are 2 main power supplies into the boiler on 2 different circuits which he says is very dangerous. Essentially the boiler was relocated to the garage and had been wired to a permanent live supply from the garage sockets on one mcb circuit, and then another source of power from a different circuit leading to the airing cupboard upstairs where the timer and water cylinder etc is. He told me having 2 power supplies into the boiler on 2 different circuits was a safety hazard and couldn't be left like that. He disconnected the frost protection which was wired into the permanent live garage socket circuit and left the boiler with no permanent live supply and just running from the timer etc in the airing cupboard on that one separate circuit. He said to contact the installer and ask them to run a permanent live wire from boiler to airing cupboard. The installer is completely ignoring all of my communication so I am planning to contact oftec for advice.

Just wondering really, is this correct that you cannot have 2 power supplies to the boiler on 2 different circuits?

Since the frost protection and boiler was disconnected from the permanent live garage circuit we have not had any further issues with the rcd tripping. Would this wiring be likely to cause rcd faults?

Any advice would be greatfully received as nobody at the moment seems very willing to help and I'm concerned about the safety of a central heating system without a permanent live connection.

Thankyou
 

brianmoooore

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There's no problem with two supplies to a device per se*, as long as there is a prominent label in place warning of this, BUT in your case, I suspect you have a 'borrowed neutral'** scenario, which is causing the RCD problem.

* Modern storage heaters, for instance, have a 7 hour supply and a 24 hour supply.

** Or, thinking about it, more likely a 'doubled up' neutral scenario.
 
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brianmoooore

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The switching for both the frost protection control circuit and the heating/water control circuit is all done in the live wire. Both circuits feed the same devices in the boiler, so there is no provision for separate neutrals for each circuit, and I expect that you have two neutral feeds, effectively in parallel.
RCDs compare the current in the live wire with the current in the neutral wire, and if they differ by more than a specified small amount, they switch off. The neutral current in your set up has a choice of two wires to flow along, so will be less than that in the live, tripping the RCD.
Your electrician is correct that there is danger associated with this, since the neutral from the frost circuit (classified as a 'live' wire, even though it's a neutral), is back fed to the heating control circuit, and will still be 'live' even when the main heating isolator switch is off.
The cable from the main connection box in the airing cupboard to the boiler should have been three core and earth, but I suspect twin and earth has been used.
 

James

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I suspect the electrician that has fixed the problem is on the right tracks, there may well be a problem with 2 x supplies and current flowing between them or sharing each others N conductor under certain circumstances.

I would be inclined to get him back to fit the extra wire he is suggesting, write a report for you about the incorrect and dangerous fault he discovered with the installation and then go back to the original installer with the bill.

however, to complicate matters, who is to say that the original installation was wrong when it was first installed, could they have originally been installed on the same circuit and the new distribution board installer split the supplies from a single fuse and they then became 2 separate circuits on the new board.

unfortunatley I think because of the timescale and number of people involved so far, the best option for you may well be to just accept the cost and get it done by the guy who has identified the problem.

p.s. he may be able to use a wireless thermostat to free up a spare core in the original wiring so he dosnt need to pull in an extra cable, but this is something you would need to discuss with him.
 
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Trcurt

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There's no problem with two supplies to a device per se*, as long as there is a prominent label in place warning of this, BUT in your case, I suspect you have a 'borrowed neutral'** scenario, which is causing the RCD problem.

* Modern storage heaters, for instance, have a 7 hour supply and a 24 hour supply.

** Or, thinking about it, more likely a 'doubled up' neutral scenario.

The switching for both the frost protection control circuit and the heating/water control circuit is all done in the live wire. Both circuits feed the same devices in the boiler, so there is no provision for separate neutrals for each circuit, and I expect that you have two neutral feeds, effectively in parallel.
RCDs compare the current in the live wire with the current in the neutral wire, and if they differ by more than a specified small amount, they switch off. The neutral current in your set up has a choice of two wires to flow along, so will be less than that in the live, tripping the RCD.
Your electrician is correct that there is danger associated with this, since the neutral from the frost circuit (classified as a 'live' wire, even though it's a neutral), is back fed to the heating control circuit, and will still be 'live' even when the main heating isolator switch is off.
The cable from the main connection box in the airing cupboard to the boiler should have been three core and earth, but I suspect twin and earth has been used.
Thankyou for your reply, appreciate it.

Is it likely that a modern consumer unit with rcd protection would pick this up when an old fuse box didn't? The original installer has got back to me basically uninterested in this as it's all been fine for 3 years until we upgraded the Consumer unit
 

brianmoooore

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however, to complicate matters, who is to say that the original installation was wrong when it was first installed, could they have originally been installed on the same circuit and the new distribution board installer split the supplies from a single fuse and they then became 2 separate circuits on the new board.
Still not acceptable, unless the neutrals went through the same isolator.
 

brianmoooore

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Is it likely that a modern consumer unit with rcd protection would pick this up when an old fuse box didn't? The original installer has got back to me basically uninterested in this as it's all been fine for 3 years until we upgraded the Consumer unit
The RCD consumer unit would pick it up, where the old one wouldn't, BUT the circuit testing that is required to be done at the time of installation would have picked it up.
He did supply an electrical test certificate for the heating system??
 

James

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Still not acceptable, unless the neutrals went through the same isolator.
Agree, however it is so long ago and it has now been changed by 2 different contractors, the chances of pinning the mistake on the original installer who probably made the mistake is virtually impossible.

get it put right by the last guy on site as he seems to know how to diagnose a difficult to find fault.
pay his bill and then you can try to recover the costs from either or both of the other contractors.
However, I would be amazed if you could get anything from either of them.
 
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Trcurt

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The RCD consumer unit would pick it up, where the old one wouldn't, BUT the circuit testing that is required to be done at the time of installation would have picked it up.
He did supply an electrical test certificate for the heating system??
Nooooo, no electrical test certificate 😬
Just an oftec building regulations compliance certificate
 
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Both the circuits that the central heating was wired into were covered by the rcd that kept tripping
Borrowed neutral wouldn't trip it then. As the line and neutral for both circuits are after the same RCD the current would still be balanced, thereby, the RCD wouldn't trip.
Possible you may have a N-E fault on the circuit that has been disconnected.
 

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