Multiple lives in junction box

[terryhawkins]

I recently had a Nest thermostat, and am having one small issue...

When it was first installed, it was connected to a live that was only live when the heating was off, this resulted in it the heating constantly going on and off in a loop.

The installer then used another live which appeared to be live all of the time, the result that the heating came on as normal when the thermostat clicked on. All good, or so I thought.

It now appears that this second live is only live when the hot water is off. When the hot water comes on it cuts all power to the Nest and I get a warning to say there's no power.

Is this to stop the heating and hot water coming on at the same time? And if so, is it likely that there is a permanent live in there somewhere to use as power only for the Nest?

Thanks for your help but the junction box is a mess and not labelled at all.

 

[ScottishGasMan]

Does your nest control heating and how water? or is it just meant to work as a heating thermostat?

Do you still have a timer that controls either the Heating or how water?

Normally for any control for a boiler, it will be supplied with permanent live, and the controllers switching determines when to let that live signal through to the zone valves/boiler to activate it when required, the nest its self will be fed with a permanent live to keep it active surely? that would normally be bridged over to the signal input live (common) and then you have your heating or hot water demand out, which are switch lives controlled by the nest.

 

[terryhawkins]

It is just meant to work as a heating thermostat.

We still have a programmer near the boiler which controls hot water and the heating part is set to permanently on.

What I didn't want to do was supply a permanent live to the nest (and bridged to common) if that would then allow heating and hot water to come on at once and do something to the system.

Am I correct in saying I just need to find the permanent live and swap it to that?

What is the purpose of the live when hot water off? Is that for some other part of the system?

Thanks for your help.

 

[ScottishGasMan]

If you have a 3-Port Valve (Y-Plan System) then there is often a HW Off Live from the clock to the Cylinder stat and then onto the 3Port Valve to allow power signal through Valve to activate boiler, depending on the set up some 3port setups will demand that the HW is activated With the heating demand (although generally you will see this on the programmer (turn heating on and HW lights up at same time)

If its just for the heating then there should be a signal wire from the Programmer, that will only be live when the heating on the programmer is on, when you turn the heating at programer off this signal wire should go dead, thats the one that Should be feeding the common on the nest. (not the power to the nest but the common)

But if its set on all the time, it really doesnt matter, any permanant live to the Nest should be fine to bridge to common as full control of the CH will be taken care of by the nest anyway

 

[ScottishGasMan]

 

[terryhawkins]

That is inside the junction box.

At the moment the Nest live is connected to the 3rd one down, but that is (as mentioned) only live when hot water is off.

Any idea which one might be permanent live?

P.S. White cable is call for heat which is connected to switched live on Nest (green/yellow on right side).

 

[ScottishGasMan]

For a start they should never use a green yellow earth wire to carry any signal or voltage other than as an earth connection so straight away I would not be happy at that.

 

[ScottishGasMan]

Problem here is that there is not always a permanant live wire in a Y-plan junction box that is remote to the clock. The live can change depending on clock signaling so this may be the issue the installer has had.

You need a multimeter and a bit of know how to go any further and find the switched live from the clock output to heating assuming its still in there somewhere. Thats what will need to feed the common on the nest, or take a bridge from the Perm live from the nest over to the common.

But use a multimeter and dont assume if you turn anyhting off that its dead in there, heating wiring is bad for being taken from different sources and having had many incompetent people adjust it over the years.

 

[D_Hailsham]

When it was first installed, it was connected to a live that was only live when the heating was off, this resulted in it the heating constantly going on and off in a loop.

The installer then used another live which appeared to be live all of the time, the result that the heating came on as normal when the thermostat clicked on. All good, or so I thought.

It now appears that this second live is only live when the hot water is off. When the hot water comes on it cuts all power to the Nest and I get a warning to say there's no power.

I suspect that, in the first instance, the installer used the existing programmer's CH OFF terminal; and in the second case he used the HW OFF terminal.

Your pic has just confirmed my second guess - the grey wire from the motorized valve connects to the same terminal. It's impossible to say from the pic which wire will be permanently live. If you have a multimeter you will be able to find one easily,

If the existing programmer has been set to CH permanently on then you can use the CH ON terminal as the permanent supply to the Nest.

Is the Nest a replacement for an older stat? if so the wire from programmer CH ON will have been connected to one terminal of the old stat and the valve's white wire to the other terminal of the old stat.

I was going to mention the use of the Green/Yellow earth wire as a voltage carrying conductor but SGM has beaten me to it

 

[bernardgreen]

P.S. White cable is call for heat which is connected to switched live on Nest (green/yellow on right side).

Greeen yellow is used ONLY for earth.

Your "plummer" ( dummer ) has used the wrong cable to connect the thermostat. He should have use three core and earth. Not twin and earth.

The blue in the five core cable is ( should be ) connected to the incoming Neutral. The incoming Neutral will also have a blue to the incoming cable which is probably a grey oval twin and earth cable. The brown in that cable will be permanent Live ( or should be )

But as the "plummer" has used green yellow as a live conductor anything is possible.

 

[terryhawkins]

Thanks both of you for your help.

There is a switched live in the top block on the connector block, which connects to nothing.

Although I will test with a multimeter, is it pretty certain that is the live is the top one?

Having looked at an old photo before Nest was installed, there is a red going into the top connector which does appear to be the one that went to the old thermostat.

What you're saying is the old thermostat would have been connected to a live and the white call for heat? In which case the top one is live when the controller heating is on.

 

[D_Hailsham]

If the terminal you mention is live when CH is ON at the programmer and not live when CH is OFF, you have found your live supply for the Nest. It would be sensible to bridge the L and CH ON terminals at the existing programmer to negate the effect of someone accidentally turning CH off.

 

[terryhawkins]

Thanks again everyone, it's now all working properly.

A question about the green/yellow earth cable being used though...

Other than the colour, what is the issue? It's three core flex being used, one the cores just happens to be green/yellow. I know it's probably not the 'right' thing but if I mark it as switched live, will that make it better?

It's not twin and earth cable so the earth isn't bare cable.

 

[simond]

You can use the earth conductor in a flex providing it is marked with the appropriate colour identification for the phase used and is visible at both ends.

This does not apply to cables with a bare ecc, such as twin and earth.

I'm not going to suggest it is good practice or that I would do it myself, but when I did the full scope 17th Edition a few years back this precise question was asked. If there has been any changes wrt earth conductors in flexes since, I'm not aware.

 

[terryhawkins]

Thanks everyone, the yellow/green flex is now marked as live and everything is working as it should do.