New C/H room thermostat

[mr_chaela]

Hi all
Just bought a new room stat. Old one was battery powered with volt free switch, the new one, Salus SQ610, is mains powered with volt free switch. I have 2 options, either borrow a neutral or power the stat from a seperate supply (lighting circuit).
Existing cable is 1mm T & E, ie permanent live from programmer and switched return to zone valve which in turn powers the boiler. Unfortunately there is no way I can get a supply (neutral) from the programmer to power the stat, all cabling is hidden in the walls. The only acces I have is from Stat to Lighting Circuit or Ring Main.
Just wondered which option you would recommend. From my point of view powering it from the lighting has the advantage that if the C/H is isolated for service the stat wont lose its settings but I'm not sure if there are any regs that forbid this.

 

[winston1]

Don’t borrow a neutral. Your RCD will trip if you do.

 

[oldbutnotdead]

Take the mains thermostat back to where you bought it & buy something you can use instead.
Borrowed neutral is a bad idea for many reasons. Powering the stat from a different circuit to the boiler is another very bad idea (gives you an appliance with no effective single isolation point).
As for retaining the settings, anything of that ilk built in the last 10 years will almost certainly hold programs- my old Sunvic wireless thermostats were in the bits box for 5 years, no battery, still had my settings in when re batteried last year.

 

[CBW]

I would take it back and swap for a 2 wire thermostat, battery or dial type. Alternatively, change it for a wireless type and wire the receiver near the boiler/wiring center.

 

[flameport]

Just wondered which option you would recommend.

Both options are entirely unacceptable.

 

[Taylortwocities]

Both options are entirely unacceptable.

Any and all items attached to the boiler must be electrically isolated by a single double pole switch. This includes all pumps, valves, programmer and thermostats.
It is a basic safety rule that stops plumbers and others being electrocuted.

As above, you need a different thermostat that does not require a neutral.

 

[ericmark]

I will admit some times it seems daft, I have 10 independent supplies to my central heating system, 9 of them are pairs of batteries one is from the mains, and we are told we should have a single point of isolation and clearly I have 10.

Manufacturers do seem to say must be same supply

Not well written as most homes only have one supply, sure they mean circuit, but it seems only worried about low voltage supply, any extra low voltage supply does not seem to count.

I can see the problem, this

is it, the same back plate is used for 1, 2, and 3 channel control, with one channel we have volt free, and with two and three channel it is not volt free, however with the volt free version we can have 24 volt and 230 volt in the same enclosure so the danger of selecting the wrong supply still exists. If anything worst than two 230 volt supplies.

So we look at other house hold appliances, TV with a set top box, for example, computer with printer is another, and with the latter there was a point where we did have outlets from the computer to monitor, printer etc, but this is no longer the case. It may well be that we do have a common multi-outlet extension lead, but it seems only the central heating wants use to use one circuit.

Even the central heating, my immersion heater is not on the same circuit as my boiler, when I moved into this house not even on the same distribution unit. It would clearly make sense for the immersion heater and central heating to be on a change over switch so only one used at a time, but this is not the case.

So when I moved into this house my central heating had at least three supplies, from two independent distribution units, and was a nightmare to fault find with, so I did a rewire and got all 230 volt from the same fused connection unit, so I do agree better if all is from one plug or fuse connection unit, and a borrowed neutral has been outlawed for as long as I can remember.

However if we look as Nest Gen 3 as an example, it is common for the heat link and the thermostat to have independent 230 volt supplies, OK the thermostat has some power supply to drop the supply to 5 volt and uses a USB system, but at the end of the day, both the heat link and the thermostat are supplied from the mains and there is a unit sold specially designed so the thermostat and boiler can have independent supplies. They can also have the same supply, using 12 volt DC from the heat link, but I can't see how you can have two independent 230 volt supplies when using Nest Gen 3, and then say you can't have two independent 230 volt supplies?

So I have my hard hat on and am waiting for the flack, but I personally can't see the difference to having a step down transformer or power supply built into the thermostat or have it as a independent plug in unit, if one system can have two independent supplies so can others. Can't swap and change rules because one just feels like it.

Personally my Nest Gen 3 is powered from the heat link, and this has caused problems, when the plumber was due to fit motorised valves he asked for heating to be switched off day before so it was cold to work on, which I did, the result was the back-up battery in the Nest thermostat became discharged, so on turning the power back on, nothing worked until the thermostat had recharged its batteries, had I been using the Stand with a USB supply then I would not have needed to wait for battery to recharge.

I can't see how using two AA batteries is OK and using USB is OK, but direct to mains with the power supply built into the thermostat is not? I look forward to seeing the arguments.

 

[oldbutnotdead]

@ericmark , let us assume that the rest of your heating system is supplied by a single double pole isolator. You want to do something to one of your thermostats- you isolate supply to heating system. Your thermostat continues to work cos it's on batteries or USB power- neither will bite you, the USB one will have a manky dangling cable which you can unplug if you want.
If your thermostat was on a different mains power circuit via a plug and trailing cable it could be argued that it's obvious that that plug should be removed as part of isolation- the risk is someone assuming that removing that plug will isolate all mains voltages in the device.
If your thermostat was powered from a different power circuit by concealed wiring then you, the installer, might remember you'd done a bodge but anyone else working on the system might not be aware. DIYers and plumbers are not known for scrupulous adherence to proving dead, that wiring regulation is a reasonable bit of hazard mitigation.

 

[BS3036]

I thought it was volt-free thermostat contacts - not likely to cause a separate issue! Might be hard to demonstrate, but not dangerous.

 

[ericmark]

that wiring regulation is a reasonable bit of hazard mitigation.

What wiring regulation? I only have BS7671:2001 and BS7671:2008 so if new please show regulation not just number.

A thermostat can be mechanical or battery powered and still activate a central heating system in exactly the same way as any volt free 230 volt powered system.

DIYers and plumbers are not known for scrupulous adherence to proving dead

neither do they tend to check if a ring final is still a ring final, or many other safety items. Even seen green/yellow wires connected to line.

However the problem with safety items is they can back fire, so armaco barrier to stop a car leaving the road, makes the road more dangerous for cyclists.

But I can't really think of a scenario where a mains powered thermostat with volt free contacts is more dangerous to a battery powered thermostat. As long as suitably marked

.

 

[oldbutnotdead]

@ericmark apologies- didn't realise (a) that doc was Mfr instructions (b) it was you who posted it . I have no idea which particular regulation (if any) specifies single point of isolation, it's one of those things I've always worked to.
Yeah marking would help, I've done that in commercial settings (where 'competent persons' rather than anyone wanting a go were the only persons allowed to unscrew panels).
Your comment about Armco improving crash survivability for car and lorry drivers at the expense of reducing survivability of motorcyclists doesn't wash here. It's a potential nuisance to system designers ensuring that all bits of the heating system are on one circuit but I can't see the additional risk of injury caused by it.

 

[Harry Bloomfield]

Take the mains thermostat back to where you bought it & buy something you can use instead.
Borrowed neutral is a bad idea for many reasons. Powering the stat from a different circuit to the boiler is another very bad idea (gives you an appliance with no effective single isolation point).

Plus if you have an RCD fitted or were contemplating having them fitted, it would cause it to trip.

 

[oldbutnotdead]

Plus if you have an RCD fitted or were contemplating having them fitted, it would cause it to trip.

If there was 1 RCD covering the whole house, no problem. Multiple RCDs or RCBOs or some circuits not protected and y s it could all get messy. Borrowed neutrals are never a good idea.

 

[BS3036]

Don’t borrow a neutral. Your RCD will trip if you do.

Borrowed neutrals are never a good idea.

He wasn't proposing that. He was suggesting powering the thermostat (with volt-free switching) from the lighting circuit. RCD will not trip.

 

[Taylortwocities]

True, providing the L from the boiler is not used to power the stat.
The OP only mentioned borrowing a neutral from elsewhere.