CH Boiler isolator switch wiring problem

Postby DIYdunce » Sun May 27, 2007 3:08 pm


I disconnected the isolator switch for our Potterton Suprima 40 boiler in order to fit some hardboard behind it to neaten up a messy installation. Even though I thought I'd noted the wiring & replaced it correctly the boiler is now dead.

The switch has 3 connections on the top and 3 on the bottom. The top 3 seem to come from the house electrics (house built in 2002) and the bottom 3 go to the boiler. The house electrics have 2 reds, 1 yellow, 1 blue, 1 black & 1 earth. The boiler side has 1 black, 1 black+red tag, 1 brown, 1 blue & 1 earth. I think they were wired together in the switch like this...

Terminal 1 2 3
House side: Red Yellow Red
Boiler side Black Brown Black

The house black and the boiler blue are connected directly together with a "chocolate block" type connection. The house blue was just loose inside the switch box, with no exposed conductor showing. The earths from both sides are connected to a pin on the switch box cover.

I've checked the 3A fuse inside the boiler and it seems OK. I'm wondering if the house blue that was loose inside the switch box should be connected somewhere and maybe came adrift when I opened the switch front? But I'm no electrician and don't want to "suck it & see!!" All help gratefully accepted - thanks in advance.


ps - we've hot water from our immersion heater but no boiler and thus no central heating.
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Simply Build It

Postby sparx » Sun May 27, 2007 9:43 pm

check type of wiring from house, is it a twin Rd/BK plus three core R/Y/BLU, (plus earths of course)?
Also is pump remote from boiler?
where is programmer ,in boiler or remote?
It is quite likely the blue from house not used, put in b4 type of system confirmed so there in case needed .
you will need to look in boiler at connections of flex to see what each does, 1 will be N
1 will be perm. L,
1 will be 'call' sw.L
1 will be pump L if remote p/p used
More info please regards SPARX
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Postby DIYdunce » Mon May 28, 2007 10:57 am

Thanx for the reply, here's the info you asked for:

There are 2 holes in the top of the switch box that have cables entering. The one on the left contains the 2 reds, 1 yellow, 1 blue, 1 black and 1 earth. When I've opened up other house switch boxes they've always only contained 1 red, 1 black & 1 earth - so all these new colours leave me utterly baffled! The hole on the right contains the cables that go to the boiler.

The pump and the programmer are both remote from the boiler

I opened up the boiler case and found the flex connections marked as follows..

Black = Pump Live
Black+red tag = Mains SwL (whatever that means)
Blue = Mains Neutral
Brown = Mains Live
Green/Yellow = Mains Earth

Thanx again for the help - I know I can just call an electrician and get this done properly but I'd much prefer to do it myself with some help from an expert!

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Joined: Sun May 27, 2007 10:51 am

Postby sparx » Mon May 28, 2007 2:30 pm

OK Don,
First a run down on what you have there;
the cables from house are a twin/e RED,+ BLACK +E, this is the supply to boiler.
The pump Live comes from boiler so that when boiler finished 'firing' the pump 'runs-on' (like a bathroom fan should, only to take away residual heat rather than smells!).
There is a 'call/demand' wire to boilers 'SwLive, from control system valve when either C.H. or H.W. selected and 'stat is calling for heat, this starts blr/pump.
The other 3 core blue may not have been connected.
UNfortunately there is no standard colour scheme for boiler wiring & near 3 port v/v. will be rectangular connection box full of wires from v/v, pump, programmer, cylinder & room 'stats, plus your 2 cables to boiler, this realy isn't a DIY job as u need to work out what does what,
a doddle for a sparkie,
recommend Get One IN
regards SPARX
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Postby DIYdunce » Mon May 28, 2007 4:10 pm

Thanks for the good advice. From what you told me I was able to identify the wires properly and managed to reconnect them. The boiler is now working fine again and the house is warm! I've checked the programmer works properly and the pump starts and stops when it should. Having said that I'll get an electrician to take a quick look (I've got one booked to come in later in the week to fit some new lighting).

The problem was that I had transposed the 2 black wires. Many thanks again for the information, it's much appreciated.

Posts: 5
Joined: Sun May 27, 2007 10:51 am

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