UK Wiring a new smart thermostat

[TheJay]

Can someone please tell me why there new thermostat isn't powering up? I copied the wiring from the old one, with the exception of the earth wire, which I have capped in a wago. Thank you.

 

[SparkySy]

Looks like you need a permanent live in terminal 4, a neutral in terminal 3 then 1&2 are the switch contacts for the heating system. Do you have a neutral at that stat? It looks like there may be but would need to test to be sure!

 

[telectrix]

your new stat requires a L to terminal 4 and neutral to terminal #3. the relay is 1& 2. depending on whether you want to switch 24V or 240V, you may not have enough conductors.

 

[TheJay]

Thanks for your replies. Not sure how to check on the Bosch boiler.

 

[TheJay]

There are four cables in coming through the wall on the thermostat end, how much help is that? How can I test?

 

[123]

I'd assume you have a central heating programmer somewhere as well?

 

[SparkySy]

Most stats are wired like that, it looks like you have enough cores there if the boiler control circuit is 230v! You will need to test to find which is live, neutral and switch live to fire the boiler, and make sure the control circuit is definitely 230v!
If you have a tester and know how to use it then I would proceed but with caution. If you don't then walk away now and call a pro before it potentially costs you an boiler PCB!

 

[UNG]

Can someone please tell me why there new thermostat isn't powering up? I copied the wiring from the old one, with the exception of the earth wire, which I have capped in a wago. Thank you.

The swap is not just as simple as copying the existing wiring, the existing wiring looks like it is just a live and switched live and your new stat would need a neutral, copying the wiring it looks like you might have the live in the neutral connection
The live would also need a link to the switch contact

There are four cables in coming through the wall on the thermostat end, how much help is that? How can I test?

What test equipment do you have
Unless you are familiar with heating control wiring you may be better getting someone in
If you post your location there may be someone on here that is close to where you are

 

[TheJay]

Thank you for your reply. I am in Worcester.

I have a multimeter, what would normally be used for testing?

 

[TheJay]

Can someone please respond? Thank you.

 

[UNG]

You need to confirm that you have 230 / 240v across the brown and the black that has the wago fitted to it if it has then the brown should be connected to terminal 4 with a link connected to terminal 2, the black connects to terminal 3

 

[littlespark]

Yes, a multimeter can test for voltage on the right scale setting.

Wrong scale setting will

A word of caution however, that the spare wire may indeed be a neutral, or it may not be connected to anything at the other end.
Testing between your live and this mystery wire may give the impression the circuit is dead when it is not.

The other end of the cable may be in a wiring centre (joint box) rather than straight back to the boiler.

 

[TheJay]

Yes, a multimeter can test for voltage on the right scale setting.

Wrong scale setting will

A word of caution however, that the spare wire may indeed be a neutral, or it may not be connected to anything at the other end.
Testing between your live and this mystery wire may give the impression the circuit is dead when it is not.

The other end of the cable may be in a wiring centre (joint box) rather than straight back to the boiler.

Thank you. I was thinking of unscrewing the fused switch from the wall behind the boiler to see what wires are in there. Will all the wires be there or will some go straight to the fuse box?

Any recommended setting?

 

[UNG]

Thank you. I was thinking of unscrewing the fused switch from the wall behind the boiler to see what wires are in there. Will all the wires be there or will some go straight to the fuse box?

Any recommended setting?

It would be unlikely to go to the switched spur, the thermostat could go to the heating controller, the wiring centre or direct to the boiler depends on what type of system you have

 

[Lucien Nunes]

To consolidate some of the above:

• Terminal numbers on heating components are not standardised and there is no guarantee of correspondence or compatibility between one product and another. The schematic diagram and technical specs must be checked before use.
• The original stat behaves like a switch. It does not require power to operate itself.
• The smart stat also behaves like a switch but requires a continuous supply of 230V AC power for its electronics.
• If the heating control circuit being switched operates at 230V then the existing cable with three cores plus earth may be adequate for the new stat.
• The connections at the system wiring centre / boiler might need to be changed, so that the line feed is always live (rather than only being live when demanded by the programmer.) Whether this is possible will depend on the programmer and its wiring. The black core (at present unused) might need to be connected to neutral if it isn't already.
• If the heating control circuit being switched operates at extra-low voltage, then the existing cable is not sufficient as there would be a need for two power cores and two control cores in addition to the earth.

Most of the information needed can be gleaned from studying the connections in the wiring centre or at least the other end of the stat cable which you need to find. Be prepared for it to be a spaghetti junction.

 

[Markyd]

Get somebody who knows what they are doing to wire it for you...will be cheaper than a new boiler

 

[TheJay]

Hello, I am really grateful for all your replies. I didn't receive a notification for all of them and missed the really important information you have left.

I wanted to confirm what wiring is visible behind the fused spur for the boiler.

 

[Lucien Nunes]

OK, but what about the wiring centre? Is this a combi or system boiler? Do you have motorised valves?

What I see in the pics is a bit odd. It looks like there's a 5-core flex with the green/yellow core cut off, brown / blue to FCU L & N out and grey / black joined through to red / blue of an old red/white/blue flat triple with the white core sleeved as CPC. If that is the stat cable there must be a joint to the harmonised cable we see at the stat, but if that really is r/w/b triple there aren't enough cores for the smart stat. Also, if it is the stat cable, and the 5-core flex really does have the g/y core cut, how is the boiler earthed?

I think this needs looking at by a professional.

 

[TheJay]

Thank you all for your replies. I am sure that you are right, I have emailed our old electrician as I don't want to get this wrong and end up with serious problems.

 

[James]

Good plan, I have in the past been called by a builder who used a Newley qualified spark to fit a nest stat to a new boiler In a new build.
bottom line was he mixed up the 230v and 24v control wires and blew the main board in the boiler.

mistakes can be made and they can be expensive.