Mains voltage boiler and voltage free stat: Wiring Question

[kivster]

Hello,

I'd like to fit a voltage free wireless thermostat (Horstmann HRT4-ZW and ASR-ZW receiver) to my ten year old oil combination boiler (Worcester Heatslave 12/14). I do not currently have a thermostat, just a programmer on the boiler and TRVs on the radiators.

The boiler manual says "the thermostat must be suitable for use on mains voltage". From my research on this forum, I think it's okay to use a voltage free thermostat here, I just need to wire it differently to the boiler instructions for wiring a mains voltage thermostat.

For a mains voltage thermostat the boiler instructions say to wire the boiler terminal block as follows:

(What I think is) return live on 2.
Switched live on 3.
Remove the link between 2 and 3
Neutral on 6.

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• kivster
• 10 Jan 2015
• 1

The thermostat instructions say:

Switched live on 2.
Return live on 4
Separate live and neutral

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• kivster
• 10 Jan 2015
• 1

So would I wire this as:

Boiler 3 to stat 2
Boiler 2 to stat 4
Separate live and neutral

If so, what's the best way to get a live to this thermostat? Would I have to run 4-core cable (1.0mm heat protected?) to run another live from the boiler or power the receiver externally?

I appreciate this is what some refer to as "another thermostat wiring question", but I have read a lot of posts on here and I would really appreciate running it past a few of you friendly chaps as a sanity check

 

[D_Hailsham]

So would I wire this as:

Boiler 3 to stat 2
Boiler 2 to stat 4

That's OK, but this is preferable:

Boiler 2 to stat 2
Boiler 3 to stat 4

Separate live and neutral

If so, what's the best way to get a live to this thermostat? Would I have to run 4-core cable (1.0mm heat protected?) to run another live from the boiler or power the receiver externally?

Easiest would be to run a 4-core heat resistant cable from boiler to receiver. The important thing is that the 230V supply to the L and N terminals must be a permanent supply, i.e not turned on and off by the programmer. The mains input terminals to the boiler is probably the best place to connect.

 

[kivster]

Thanks very much for the help, I'll wire 2 <> 2 and 3 <> 4 as you suggested.

You have me curious now, why is that way preferable? Is it because 2 <> 2 and 3 <> 4 makes more sense from an ordering perspective, or is there an electrical reason?

Easiest would be to run a 4-core heat resistant cable from boiler to receiver. The important thing is that the 230V supply to the L and N terminals must be a permanent supply, i.e not turned on and off by the programmer. The mains input terminals to the boiler is probably the best place to connect.

I'll have a look tomorrow and look for the live terminal. I guess it will be on the PCB board somewhere.

 

[D_Hailsham]

You have me curious now, why is that way preferable? Is it because 2 <> 2 and 3 <> 4 makes more sense from an ordering perspective, or is there an electrical reason?

The diagram below shows stat/2 connected to boiler "S", which is terminal 2 on your boiler. Terminal "R" in the diagram is terminal 3 on your boiler, so this should connect to T4 on the stat.

Logically you should connect the input to the Common terminal and have the output from either ON or OFF. You had it the other way round - input to ON, output from Common.

I'm just being fussy, but it does make future fault finding easier if wiring is how you would expect.

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• kivster
• 10 Jan 2015
• 1

I'll have a look tomorrow and look for the live terminal. I guess it will be on the PCB board somewhere.

Just find where the mains cable connects to the boiler.