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Do I need two mains supplies for boiler and wiring centre?

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Do I need two mains supplies for boiler and wiring centre?

Posted by simon201 on Nov 6, 2019 5:52 pm

Good evening, Just like to say hello and that I am not a gas engineer or an electrician, but I would just like to understand what I need to ask for. I have a question about an old y-plan central heating installation dating back to 2000. I have an old Gloworm micron 50ff boiler situated in the consumer unit cupboard. However, it is connected to the wiring centre situated in the hot water cupboard 6 meters away via 3 wires, switched live, neutral and earth. I also have an old Grundfoss pump connected into the wiring centre that has only 4 wires, Blue, orange, brown-white, and grey, (I notice new pumps have 5 wires). The pump also has 3 settings using 40,65 & 95 watts. I also have 3 port mid-position valve, room stat, hot water stat and 2 channel programmer, (all from Danfoss heatshare pack).

 

Due to limited access in consumer unit cupboard I am limited to fitting either a Worcester greenstar ri series boiler or a Baxi ecoblue advance. Looking at the wiring diagrams, with my limited understanding, does fitting the Bosch require separate mains supplies in each cupboard, one for the boiler and one for the wiring centre and will the pump kill the boiler using an extra 5 watts as the worcester boiler is rated at 90 watts, and could I still use it with only 4 wires or would I need a new 5 wire pump too.

 

Or would it be better to go for what seems to be the simpler option of the Baxi as it requires no pump overrun or permanent live and looks as though it is fed with just live, neutral and earth from the wiring centre as I have now.

 

Sorry about the long question, and many thanks in advance. Simon.

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Kelly Marie on Nov 7, 2019 11:08 am

Hi Simon you really need to let a professional heating engineer sort this for you  it's a bit of a minefeild trying to sort all this out  and unless you are quilified to do it it can quickly get very complicated. If you have any choice in the matter go for the Worcester boiler they are generally very reliable  the one here at Kelly towers runs very well even after 5 years hope this helps a bit

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by alanblaby on Nov 7, 2019 1:15 pm

No, you dont put two supplies in, for boiler and wiring centre. One supply feeds it all.
And most importantly, one switch turns it off and can isolate it all.
I dont know why you think new pumps have 5 wires, all the ones I've fitted have 3, LNE.
Pump overrun is done at the wiring centre, with an extra wire from the boiler.
Don't choose  a boiler by how many wires it has going to it (most have - permanent LNE, Switched live, and pump overrun).

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by burn on Nov 7, 2019 2:18 pm

alanblaby:
No, you dont put two supplies in, for boiler and wiring centre. One supply feeds it all.
And most importantly, one switch turns it off and can isolate it all.
I dont know why you think new pumps have 5 wires, all the ones I've fitted have 3, LNE.
Pump overrun is done at the wiring centre, with an extra wire from the boiler.
Don't choose  a boiler by how many wires it has going to it (most have - permanent LNE, Switched live, and pump overrun).

Not quite Alan, I would agree that is the most common way, but have a Worcester Bosch Greenstar and the pump overrun is at the pump itself.

Regards.
burn

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by simon201 on Nov 7, 2019 4:31 pm

Good Afternoon, Thank you for your replies to my completely messed up question, much appreciated. I feel like a right dummy!  What I meant to say was that my old Danfoss 3 port mid-position valve has only 4 wires coming out of it, and would I still be able to use it with a new boiler?  Everyone is right, my pump only has 3 wires. I must admit I'm inclined towards Worcester as they seem to have good reliability scores, It was the potential problems, as I saw them, with installation that worried me.

Kind regards, Simon

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by mapj1 on Nov 7, 2019 5:43 pm

If the valve switches from hot water to rads properly, then you can re-use it - the boiler does not care, so long as it has power and a line that goes live to say 'light the fire now'.
The valve and various thermostats are interlinked to provide that signal, so if the water is cold, and the valve is in that position, then fire, or if the room stat is cold and the valve is in the other position then fire...

One or other of your thermostats will need a back contact, so that it can report 'demand for heat' and also 'no demand for heat = satisfied' to force the changeover.
This is normally best explained in the instructions for the combined heat and water timers.

It is just mains level logic, but it can be quite confusing, as there as the equivalent of a not and = or not.
As a personal opinion, I'd go with the Bosch if you can afford it, I've had one for ~ 15yrs now and it has just had its first fault.

 
regards Mike

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Chris Pearson on Nov 7, 2019 6:58 pm

mapj1:
As a personal opinion, I'd go with the Bosch if you can afford it, I've had one for ~ 15yrs now and it has just had its first fault.

All that shows (he says mischievously) is that Bosch we're good 15 years ago. 🙂

Some maintenance has been required, and I don't want to tempt fate, but our Ideal boiler (which doesn't actually boil anything) has been going for 37 years. Floor-standing cast iron and none of that condensing nonsense! 😎

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Typiod on Nov 7, 2019 8:10 pm

burn:

alanblaby:
No, you dont put two supplies in, for boiler and wiring centre. One supply feeds it all.
And most importantly, one switch turns it off and can isolate it all.
I dont know why you think new pumps have 5 wires, all the ones I've fitted have 3, LNE.
Pump overrun is done at the wiring centre, with an extra wire from the boiler.
Don't choose  a boiler by how many wires it has going to it (most have - permanent LNE, Switched live, and pump overrun).

Not quite Alan, I would agree that is the most common way, but have a Worcester Bosch Greenstar and the pump overrun is at the pump itself.

Regards.

 

burn how would the pump know when to overrun or not, all the ri's that i have wired require a connection from the boiler for the pump thus a 5 core between the boiler and the wiring centre.
Back to the op question, is it hard to get a new wire from the wiring centre to the boiler? if so then go with the Baxi, if not go with the Worcester.

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Sparkingchip on Nov 7, 2019 8:21 pm

Worcester Bosch specifically tell you to take the mains supply into the boiler then take the supply to the wiring centre from the boiler,  so that when you turn the boiler off if disconnects the whole of the control wiring. 

Safe and simple wiring.

Andy Betteridge 

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Typiod on Nov 7, 2019 8:39 pm

Sparkingchip:
Worcester Bosch specifically tell you to take the mains supply into the boiler then take the supply to the wiring centre from the boiler,  so that when you turn the boiler off if disconnects the whole of the control wiring. 

Safe and simple wiring.

Andy Betteridge 

New one on me Andy I have never seen that in the installation manual, all it says is
The mains supply to the appliance and system wiring centre must be
common, and through either a fused double pole isolator or a fused
three pin plug and unswitched socket.
The isolator/socket, where practicable, should be situated next to the
appliance.

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Sparkingchip on Nov 7, 2019 10:16 pm

That came from conversations with their technical helpline and service engineers.

On the board within the boiler there are a permanent live, neutral and earth connections to supply an external programmer and for an external circulation pump, if required because a prewired internal circulator has not been installed within the boiler.

The off switch built into the boiler should disconnect the power supply to the boiler along with the external controls and any circulation pump.

Andy Betteridge 

 

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by hertzal123 on Nov 7, 2019 10:23 pm

Thought with the w/Bosch.mains into boiler,then live output from boiler feeds all controls inc stats and clock,then orange on threeway valve returns to boiler to fire it. Hz

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Sparkingchip on Nov 7, 2019 10:33 pm

That sounds right.

Wiring instructions page 30.

Terminals 9- 230 V A.C. mains output to wiring centre

Andy Betteridge 

 

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by simon201 on Nov 8, 2019 1:55 pm

Good afternoon,

I would like to thank everyone who has kindly shared their expertise relating to my question, it is greatly appreciated. I now have a much clearer understanding of what can and cannot be done and what I should do.

1. to ensure there is only one mains supply to the entire system, which can be isolated at a single double pole switch, preferably near the boiler. Or a Plug.

2. That I can still use my existing Danfoss 4 wire 3 port valve as it is still operating correctly.

3. That there seems to be a bias toward Worcester as a brand.

4. Connecting the Worcester via a multi-core cable to the wiring centre is relatively easy as it can route through the loft space as I live in a bungalow.

5. To employ an accredited installer who is fully conversant with the chosen brand.

Kind regards, Simon

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Typiod on Nov 8, 2019 4:06 pm

Sparkingchip:
That sounds right.

Wiring instructions page 30.

Terminals 9- 230 V A.C. mains output to wiring centre

Andy Betteridge 

 

Andy that is the installation instructions for a system boiler (which is basically their combi, but without the hot water). If your were going to use a Worcester as a replacement and allready had a 4 core between wiring centre and boiler position are you really going to insist that the customer has further expense and upheavel and wire it up exactlly as in the instructions?
The op was thinking about an Ri and if you look at page 37 of the installer instructions they have the boiler feed spured from the supply going to the wiring centre.

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Sparkingchip on Nov 8, 2019 4:52 pm

Did I say that we would not work around what is already installed?

As the the original poster wants to keep the existing three port valve, pump and controls I assumed it would need a system boiler without all of those built into it in the factory.

😋

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Typiod on Nov 8, 2019 10:52 pm

Sparkingchip:
Did I say that we would not work around what is already installed?

As the the original poster wants to keep the existing three port valve, pump and controls I assumed it would need a system boiler without all of those built into it in the factory.

😋

Andy no you did not but I did not want people doing an internet search and finding this thread and think that the boiler must be wired as the installer instructions of the Worcester boiler alude to. However I can not see why you would think that the op would require a system boiler if they wanted to keep their existing pump.

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Sparkingchip on Nov 9, 2019 12:27 am

Sparkingchip:
Did I say that we would not work around what is already installed?

As the the original poster wants to keep the existing three port valve, pump and controls I assumed it would need a system boiler without all of those built into it in the factory.

😋

Quick posting whilst really concentrating on other things leads to posts that may not make sense as there is a clear contradiction in that post!

However I wasn’t particularly trying to recommend any specific boiler, this is the second discussion lately where there has been a proposal to have two electrical supplies to a heating system.

The topic heading is “Do I need two mains supplies for boiler and wiring centre” and my answer is no and actually it would be extremely dangerous, ideally take the supply into the boiler then connect the wiring centre and everything else so that when the boiler is disconnected everything else goes dead as well.

There are some central heating systems around with extremely dangerous wiring, don’t assume that what is already there is safe and reusable.

Andy Betteridge 

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by kfh on Nov 9, 2019 8:31 am

Today I am going to replace a faulty programmable room thermostat/water timer with a remote sensor.  The clever bit of the thermostat is under the wall mounted boiler adjacent to a SFCU which controls the power to the thermostat and the thermostat had got a bit wet. 

On my first visit having determined it was broke, with the SFCU turned off and the clever bit in my hand, I was pondering how to give him hot water until I could get a new one. Then I realised the boiler had just fired up. Then I am told there is another clever thermostat on the first floor and a heat store for the water which are both still working. This answered the question why one of the terminals on the thermostat backplate was live when there was no incoming power.

So this installation has more than  one point of isolation and the most obvious SFCU by the boiler does not. I am going to stick a nice label on it in to warn any unsuspecting plumber or electrician. To misquote  from Monty Python "Always expect the unexpected"

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Sparkingchip on Nov 9, 2019 8:24 pm

If I unplug a boiler supply from socket outlet I always test for voltage on the pins of the plug, numerous times I have found the line plug pin has mains  voltage on it because it is being back fed from another supply.

Treat all central heating control wiring as if it has been done by an idiot, because a fair bit of it has and doing so will help keep you alive.

Rightly or wrongly I also generally exclude central heating control wiring beyond the SFCU from EICRs to limit my liability as an inspector. There’s one for you to debate if you want to.

 Andy Betteridge 

Re: Do I need two mains supplies for boiler and wiring centre?

Posted by Chris Pearson on Nov 9, 2019 9:40 pm

Sparkingchip:
Rightly or wrongly I also generally exclude central heating control wiring beyond the SFCU from EICRs to limit my liability as an inspector. There’s one for you to debate if you want to.

Not a lot to do with BS 7671!

It is yet another matter of contract and who does what. In a new boiler installation, it seems perfectly reasonable to me for the sparks to provide a safe supply to a FCU full stop; plumbers seem to think otherwise. So be it.

I think that Sparkingchip is correct: the limit of the inspection is the fixed wiring, i.e. the supply side of the FCU. (No more than I would expect to inspect an immersion heater, for example, but yet again a matter of contract as to the extent of the PIR.)

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