Electric Boiler

[sbelec]

Hi,

I am wiring up an electric flow boiler, basically a central heating wet system run from a 9kW boiler. In the instance I have now I would like to install an unmarked cooker switch, main switch for the supply and the socket for the control system (I would like an FCU - but cannot find anyone who makes them). Any issues?

The problem I have had in the past is with the RCD configuration, both the control system and the boiler must run on the same RCD as nuetrals get confused with boiler controlled pump over runs. When over-run comes on the RCD trips.

In this situation I may have to run an RCBO as the boiler and the controls from elsewhere hence my problem. In the past I have run an oversized supply from an MCB to a secondary local "garage" CU with RCD main switch, MCB boiler and MCB controls, discrimination becomes the issue as MCB's go from 40 or 45 to 63 and in this case more of a problem.

With more and more of these about we really need a 45 amp main switch with a built in unswitched FCU - manufacturers?

thanks for your comments

 

[ImpededLoop]

You've basically described a "borrowed" neutral. Surely both the pump live and neutral should be supplied from the boiler? Alternatively you could use a relay to switch the pump so that you overcome the neutral problem.

 

[TJ Anderson]

Why are you supplying the controls externally, you should ideally be taking your control supply from the boiler PCB.

 

[davesparks]

Why are you supplying the controls externally, you should ideally be taking your control supply from the boiler PCB.

Not necessarily, many of these systems have a large supply for the heater which routes straight to a contactor in the unit then to the heater with a second small supply for the control circuitry.

 

[TJ Anderson]

Not necessarily, many of these systems have a large supply for the heater which routes straight to a contactor in the unit then to the heater with a second small supply for the control circuitry.

Granted I have little experience with electric boilers. The ones I have done were for static caravans, had integrated programmer and just used TRV's. Something aint right with what the OP is doing as his controls and main supplies are not separated. As suggested already a relay might be in order here.

 

[ImpededLoop]

As suggested already a relay might be in order here.

It shouldn't even need a relay, but there's no information about the boiler or controls so I mentioned it to cover all the possible scenarios.

 

[TJ Anderson]

True Impeeded. Not enough info. The instructions will show how to wire for external controls.

 

[davesparks]

Granted I have little experience with electric boilers. The ones I have done were for static caravans, had integrated programmer and just used TRV's. Something aint right with what the OP is doing as his controls and main supplies are not separated. As suggested already a relay might be in order here.

If you have limited experience of the subject then why are you offering up advice as if it were facts?

 

[TJ Anderson]

Approx 20 Electric Boilers over last 10 years, of only 2 types, both were controlled from PCB. I have an experience I can draw from. 100's & 100's of gas / oil systems which operate on same. How many electric boilers have you done?

 

[sbelec]

Depends on the boiler, the one in question supplies pump and supplies a live feed for switching through a zero volts contact on a control system it is not sufficient to supply power for anything. To get round this the call return of this leg, simply a 240v switched supply from elsewhere, ie the control pack. In both cases the boiler feed and the control pack are different mcb's.
A recent mod links the incoming boiler supply to the call return to create a timed pump run on from the PCB. This means that the PCB is supplied with voltage from 2 sources and you create the borrowed nuetral in the PCB architecture and it trips when it switches to run on.

I am and have been service for 2 companies supplying these boilers for almost 5 years and spend most of my life correcting installation probelms, even though both electrics and plumbing in the manual are words of one syllable. The last one the electrician melted the PCB through stupidity.

Hence my very simple question about field wiring, and after discussions with a technical helpline since posting and the consensus was it was acceptable to use a correctly marked cooker switch and plug the control system into the socket. It just gives me options on this site.

many thanks
steve

 

[ImpededLoop]

I know a lot less but surely there is a risk of removing the plugtop from the socket and getting a shock from the neutral pin if the boiler is over running the pump?

 

[TJ Anderson]

Depends on the boiler, the one in question supplies pump and supplies a live feed for switching through a zero volts contact on a control system it is not sufficient to supply power for anything. To get round this the call return of this leg, simply a 240v switched supply from elsewhere, ie the control pack. In both cases the boiler feed and the control pack are different mcb's.
A recent mod links the incoming boiler supply to the call return to create a timed pump run on from the PCB. This means that the PCB is supplied with voltage from 2 sources and you create the borrowed nuetral in the PCB architecture and it trips when it switches to run on.

I am and have been service for 2 companies supplying these boilers for almost 5 years and spend most of my life correcting installation probelms, even though both electrics and plumbing in the manual are words of one syllable. The last one the electrician melted the PCB through stupidity.

Hence my very simple question about field wiring, and after discussions with a technical helpline since posting and the consensus was it was acceptable to use a correctly marked cooker switch and plug the control system into the socket. It just gives me options on this site.

many thanks
steve

That is a bad design. Adding a relay (for the pump) could enable the pump to be run from either control circuit or overrun from PCB and keep them separated.

 

[Leesparkykent]

Depends on the boiler, the one in question supplies pump and supplies a live feed for switching through a zero volts contact on a control system it is not sufficient to supply power for anything. To get round this the call return of this leg, simply a 240v switched supply from elsewhere, ie the control pack. In both cases the boiler feed and the control pack are different mcb's.
A recent mod links the incoming boiler supply to the call return to create a timed pump run on from the PCB. This means that the PCB is supplied with voltage from 2 sources and you create the borrowed nuetral in the PCB architecture and it trips when it switches to run on.

I am and have been service for 2 companies supplying these boilers for almost 5 years and spend most of my life correcting installation probelms, even though both electrics and plumbing in the manual are words of one syllable. The last one the electrician melted the PCB through stupidity.

Hence my very simple question about field wiring, and after discussions with a technical helpline since posting and the consensus was it was acceptable to use a correctly marked cooker switch and plug the control system into the socket. It just gives me options on this site.

many thanks
steve

Does it have to be an unswitched socket in this case?

 

[HandySparks]

Why can't you use a FCU for the controls, fed from the load side of the 45A isolator?

 

[sbelec]

I know a lot less but surely there is a risk of removing the plugtop from the socket and getting a shock from the neutral pin if the boiler is over running the pump?

At last a comment about my problem. Hadn't thought of that, don't normal think of neutrals when in houses as it's always potentially carrying but small values. In this case given it's 9kW of heater elements I suspect the neutral could be higher and so your point is more valid. Scrap that till I get an FCU on a switch and back to my original methodology of using a small CU. Which also replaces the need for the switch anyway with better access for 10mm cables.

I don't write the rules on installing these but I do know when people try to use their own methodology that things go very wrong. Cost the last spark £1000 because he ignored the wiring instructions because he knew best, my time plus new boiler!

 

[TJ Anderson]

Sounds like a poor design boiler. It should be redesigned to provide 1A fused terminals on PCB (like a gas boiler does) for external controls.

 

[TJ Anderson]

Cost the last spark £1000 because he ignored the wiring instructions because he knew best, my time plus new boiler!

What did he do exactly?

 

[sbelec]

Why can't you use a FCU for the controls, fed from the load side of the 45A isolator?

Have seen this before, terminals will take 16mm cable and personally, never happy trying to pin small cables with large cables borne out of seeing the insulation burnt on the smaller cable.

Again many thanks to all, but in my mind the best way is local CU and accept that whilst different local MCB's the supply is common.

 

[davesparks]

Approx 20 Electric Boilers over last 10 years, of only 2 types, both were controlled from PCB. I have an experience I can draw from. 100's & 100's of gas / oil systems which operate on same. How many electric boilers have you done?

I have probably dealt with roughly the same number of electric boilers and similar but probably 17/18 different makes/models.

Gas and oil systems can be identical apart from the heat being created electrically instead of through combustion. The control system and it's supply can be exactly the same with a completely seperate supply for the heater.

 

[davesparks]

Can you post a link to the manual for this electric boiler.

 

[Leesparkykent]

Have seen this before, terminals will take 16mm cable and personally, never happy trying to pin small cables with large cables borne out of seeing the insulation burnt on the smaller cable.

Again many thanks to all, but in my mind the best way is local CU and accept that whilst different local MCB's the supply is common.

what about 45amp switch, load side to a dual connection plate and pattress, one side of connection plate connects to electric boiler, other side of connection plate connects to a FCU.

 

[sbelec]

Sorry about this power cuts every few minutes! and I have been out to 9 faulty electric boilers in the last 16 weeks plus 2 installs.
The electrician effectively linked the neutral of the control wiring to the neutral of the boiler through the PCB, he did not follow the manual wiring diagram. The pump is external but fed from a dedicated supply from the boiler, on most gas boilers the pump does not need wiring as it is built in.
His diagram or experience said the pump was fed from control wiring so he linked boiler pump neutral to control neutral. When the main isolator neutral came loose (not tightened) the elements had a 1.5mm neutral through the pump PCB outlet in reverse. And it melted along with the neutral wiring in the control box.
But who wires up a boiler nowadays, my 10 year old combi has a plug on it and 2 terminals for the mechanical thermostat as installed by BG. My mate the gas fitter rarely ever gets me involved and then it's normally a new FCU for him to wire into.

The boiler which had the problem of incorrect wiring is a Trianco Aztec, lovely piece of kit and extremely reliable. I replace PCB's on 10 year old pls machines but rarely anything else, the rest of the kit always tests out perfectly. The issue is more prevalent on fitting new PCB's into old units where the RCD issue comes to light due to the mods on pump over run. Especially where an RCD board has been fitted after the original unit installed.

The boiler that I am to fit is client supply and haven't seen it, I am being naughty and asking for general opinions on the main switch / controls supply just in case. But I will plan for my standard install which has been accepted as appropriate by both manufacturers.

As a very last comment I went out last year after a main fuse went, the electician installed a 9kW or 40amp unit in a property and new tenants moved in and blew the fuse. Ran a few tests whilst waiting for ENW and found the main fuse was 60 amps. Before anyone starts going on about 60 amps not being common thats all I have and I live in house built in 1968. They are not uncommon in the north west.
Do not propose electric heating without checking and not assuming, 9kW is 40 amps for about 60 minutes to heat up a small system and a further 30 minutes or so to heat a small tanks. There is no discrimination so boiler, shower and a couple of appliances and you are above the 100 amp rating of the main switch and I have one in front of me which has melted.

And PS regardless what you read you cannot have a direct water heating supply off these boilers you have to have a hot water tank.

Sorry if this is preaching but look at some of the posts on this website, makes me cringe.

 

[sbelec]

I am a spark but from history prefer control system design, installation and commissioning from industrial control systems hence boilers. Am getting too old for rewires, roof and under floors!

 

[TJ Anderson]

But who wires up a boiler nowadays

I do. Loads of them for the local heating engineers, even combi's often zoned now, S Plans, Y Plans, Thermal Stores etc.

 

[Leesparkykent]

I do. Loads of them for the local heating engineers, even combi's often zoned now, S Plans, Y Plans, Thermal Stores etc.

Same over here. What do you think about #21? This achieves what he wants IMO

 

[TJ Anderson]

I love doing the heating controls. Quite interesting with the different configurations, good money (as a lot of sparks not interested) and most of the time the plumbers have taken over the house, loads of the floors up so makes the wiring easy. Love it!!

 

[TJ Anderson]

Same over here. What do you think about #21? This achieves what he wants IMO

Yeah, that would work and the 45A switch would isolate the whole lot. It is either that or the 2 Way DB as he is doing.

 

[Leesparkykent]

I love doing the heating controls. Quite interesting with the different configurations, good money (as a lot of sparks not interested) and most of the time the plumbers have taken over the house, loads of the floors up so makes the wiring easy. Love it!!

A lot of sparks wouldn't know where to start let alone be interested in them lol.

 

[TJ Anderson]

A lot of sparks wouldn't know where to start let alone be interested in them lol.

I know, good innit (for us!!)