Room thermostat wiring. (Updated 03 Oct)

[hollyt9]

Hi guys, I hope someone can help me out.

I'm replacing an old analogue room thermostat with a new one.
The current thermostat is wired with an live, neutral and earth (which is connected to a blank/unused terminal) all the terminals are unmarked or are so old the markings have disappeared.

The new thermostat is a "Wexcel" branded "ET39" and has three terminals labelled "L", "N" and "1 (N.C Heating)"
Am I right in assuming the L is live, N is neutral and the earth is 1?
Many thanks,
Holly

 

[BingoBongo]

dont assume because its yellow its earth

most likely SL

can you post a pic?

 

[BingoBongo]

swap most likely for "could be"

 

[hollyt9]

dont assume because its yellow its earth

most likely SL

can you post a pic?

I'm pretty sure it is earth, it's a yellow and green stripe and it isn't used on either thermostat. On the old thermostat it's just connected to an unused terminal on the backplate.
The wiring I have coming out the wall is Blue (neutral), Brown (live) and Yellow/Green stripe (earth).

In the new thermostats leaflet it states "You no longer require the earth wire, because these thermostats are double insulated & have no requirement for the Earth."

 

[BingoBongo]

you will only use the L and N thenas the nc isnt used snip the earth away

 

[hollyt9]

thanks!

 

[MANDATE]

If the yellow/green wire is earth as you say. Then it seems to me that you don't have a neutral wire in the old set up. Sounds to me you have a 'live' wire and a 'switched live' wire.
In the new set up you will still need a 'live' and a 'switched live' [NC], but you will also need a neutral because modern room stats incorporate a tiny heater, which wont work without a neutral.

The old stats did not incorporate a 'load' (cause its just a switch) so no neutral required.

 

[BingoBongo]

might be a 24v jobbie that some1 connected with twin and earth

what kind of boiler is it OP

 

[hollyt9]

Here are some photo's...

Old thermostat

New thermostat

The boiler is a Potterton Kingfisher MF gas boiler, which is controlled by a Danfoss PF715 digital timer and the room thermostat is located in the middle of the house.
Hope this helps?

 

[doitall]

You need to trace that earth back, as it looks like a switch live to me.

It should have a red sleeve on it.

 

[picasso]

your old stat looks like its wired like this

brown......switched live

blue.........live

g/yellow........neutral

so the new one should be

blue......L

brown.....1

g/yellow....N

if you look carefully you can see the numbers between the
terminals on the old stat, its still a good idea to check both enda before connecting.

 

[bengasman]

Lots of roomstats have been wired up as complete cowboys jobs. Y/G green is often used as live despite the fact that it should only ever be used for earth/cpc.
Ignore whatever colour a wire is, and identify by measuring.

 

[TicklyT]

The more I look at it, the more it looks like the G/Y has been used as the Neutral. The earth terminal on your old thermostat is right in the middle of the row of connections, just underneath the inverted 'earth' symbol in your piccie, and not connected to anything

As Bengasman and Doitall said, the only way to be sure is to check what is connected where at the other end of that cable.
Testing with a meter could be misleading uless you have a known reference point to compare the other connections with.

The correct way to do the job would be to replace the cable with a 3 Core & Earth cable.
Even if the earth isn't required by your new thermostat, there should be a CPC (Circuit Protective Conductor or 'earth wire') present as part of the protection of the cable run to the thermostat.

Needs must, though .... As Doitall said, put some sleeving over the green/yellow and blue wire to wires to show how they have been (mis)used.
I suspect the blue wire is a switched live, which should be sleeved brown, and the green/yellow is Neutral, which should be sleeved blue.

 

[bengasman]

...Even if the earth isn't required by your new thermostat, there should be a CPC (Circuit Protective Conductor or 'earth wire') present as part of the protection of the cable run to the thermostat...

Not quite, perfectly ok to run class 2 on a 2-core.

 

[hollyt9]

Thanks for the replies...
There's no realistic way I can replace that wire back to the boiler timer as it looks like runs from the dining room, up inside the wall to the ceiling and then through a boxed in 12ft steel roof/wall support, through the kitchen and then into the laundry.
I did have the house professionally rewired by a group of electricians last year, my hunch is they left that thermostat wire as it was as it would have been near impossible to replace.
I'll trace the wire back to the boiler timer and post a photo of what I find.