Borrowing a live

[theflyingpig]

I've messed up and only pulled a 3 core & earth to location of wired programmable room thermostats. This means the there is only power to these when the main programmer is calling for heat in that zone... Annoying when you want to make a change at the thermostat when programmer is off.....

My question..... If labelled clearly as to what it is, can I borrow a perm live from a lighting circuit?

Thanks

 

[CountryFan]

Not something I’d recommend, as it should all be fed from the same source. Which thermostats?

 

[flameport]

. This means the there is only power to these when the main programmer is calling for heat in that zone

With programmable thermostats they do both time and temperature.
There is no need for a 'main programmer' - that's the whole point of a programmable thermostat.

 

[ericmark]

Borrowing a line is actually called a borrowed neutral, and that is not permitted.

 

[JohnW2]

Borrowing a line is actually called a borrowed neutral,

I'm not sure how that works in the OP's context?

 

[Murdochcat]

What make and model ?

 

[flameport]

, can I borrow a perm live from a lighting circuit?

Certainly not.

If you really must have a programmer and a programmable thermostat, they can be made to work with a relay.
Permanent power to the thermostat, switched output from the thermostat powers a relay coil, the relay contact switches the output from the programmer.

The real solution is to sling the programmer away and just use the thermostats.

 

[plugwash]

Current taken from the Live conductor of a circuit absoloutely *MUST* be returned to the neutral of the same circuit. Not doing so can be very dangerous to anyone working on the installation.

If the device has "volt free" contacts then technically you could supply the device from a local power source, while using it to switch stuff that forms part of the heating system, but if you do this then you need to fit warning notices to warn people that the equipment is fed from multiple sources. I would not reccomend doing this on a domestic heating system, it goes strongly against the principle of least surprise.

 

[mike42]

I'm not sure how that works in the OP's context?

 

[mike42]

Just to play devil's advocate and further my own understanding, presumably there is nothing wrong from a wiring perspective with say a programmer being on one circuit and a thermostat being on another? Each device would be powered by the live on that circuit and returned to the consumer unit along the neutral of the same circuit. Be a poor design though not being able to turn off all the central heating devices with a single mcb. Although what about the immersion - that would likely be on a separate circuit. Hmmm....

 

[plugwash]

Just to play devil's advocate and further my own understanding, presumably there is nothing wrong from a wiring perspective with say a programmer being on one circuit and a thermostat being on another? Each device would be powered by the live on that circuit and returned to the consumer unit along the neutral of the same circuit.

Right.

But what do this "programmer" and "thermostat" control and how do they control it? If they are switching mains voltage circuits fed from a different source to the one supplying the programmer then there needs to be an appropriate warning notice. Which then leads to practical considerations about where and how to affix said warning notice (and whether people will actually read it).

If they are switching ELV or sending wireless signals this is less of an issue.

Be a poor design though not being able to turn off all the central heating devices with a single mcb.

Agreed.

Although what about the immersion - that would likely be on a separate circuit. Hmmm....

IMO this comes down to "principle of least surprise". People expect all the controls on a domestic central heating system to be fed from the same place, people don't expect imersion heaters to be fed from the central heating system.

 

[theflyingpig]

Thanks for all input. The programmer is a EPH R37 and thermostats are EPH Cp4m.

My heating /DHW system is an S plan +. That is, there are 3 x 2 port valves - one for DHW, one fir radiators and one for UFH. I installed the R37 so that I could control the time of these. I then have, so far, 2 of the Cp4m thermostats....

I completly concur that taking a live from elsewhere is not good practise - when I turn off my fused switch to heating I want it ALL off...

It's not that they won't work as is - the thermostats have battery backup for settings - just that I won't be able to adjust thermostat settings unless main programmer is calling for heat.

You live and learn

 

[CountryFan]

I've messed up and only pulled a 3 core & earth to location of wired programmable room thermostats.

Thanks for all input. The programmer is a EPH R37 and thermostats are EPH Cp4m.

My heating /DHW system is an S plan +. That is, there are 3 x 2 port valves

So permanent live from the grey, neutral, link L to com and switched live. Sounds like you have enough to me. Or swap the programmable stats for battery operated ones.

 

[theflyingpig]

Ah, I can put perm live into grey? I assumed I had to take the switched live from programmer... Otherwise programmer defunct, no?

 

[CountryFan]

Ah, I can put perm live into grey? I assumed I had to take the switched live from programmer... Otherwise programmer defunct, no?

Apologies, I had my train of thought wrong. Yes, you would need the switch live from the programmer. However, you could do it that way as per @flameport suggests and do away with the programmer as you have programmable room thermostats.