Thermostat wiring

propertydev83 · 19 Oct 2015

Maybe I'm misinterpreting the instructions, but to me this appears to be a contradiction.

The internal wiring diagram for the Horstmann Thermoplus AS1 states that "A link L - 2 is required for mains Voltage applications", while the other wiring diagram states that "Terminals L - 2 should not be linked"?

ericmark · 19 Oct 2015

It depends on your boiler. The thermostat is what is called volt free that means the switching contacts and the supply contacts are independent. Where it is used to replace a thermostat which switched the 230 volt supply the link may be required. Where the boiler used a 24 volt control voltage the link would damage the boiler.

propertydev83 · 19 Oct 2015

OK, that makes sense. Where I'm still a bit confused in applying that to what we have. The thermostat is mains powered (230V) but volt free? So does that mean using the mains supply the thermostat's mains supply connections, and the boiler's 24V connectors for the thermostat's comms connections? (Whilst removing any links between the 230V and 24V)

Basically, this:

Boiler is Heatline Capriz 24a. Wiring diagrams below.

Boiler Supply

230V System Controls - NOT USED

External Accessories

EFLImpudence · 19 Oct 2015

propertydev83 said:
OK, that makes sense. Where I'm still a bit confused in applying that to what we have. The thermostat is mains powered (230V) but volt free?

Yes, 'volt free' is a misleading term meaning the thermostat contacts are not connected to 240V supply - unless you connect them.
This means it can be used for 240V or 24V.

So does that mean using the mains supply the thermostat's mains supply connections, and the boiler's 24V connectors for the thermostat's comms connections? (Whilst removing any links between the 230V and 24V)

I think so - a bit unclear.

To use the boiler's 24V you must connect the 24V terminals to the volt free terminals. Remove links.
As per your diagram.

propertydev83 · 19 Oct 2015

Got it.

The wording wasn't very clear. So the thermostat is volt-free, but can alternatively be used for 240V switching if putting the L-2 link in (which would not be the intended use with our boiler).

I was going to use two T&Es for the wiring. The one being used for the 24V connections will have an unnecessary earth wire. Should I still go and connect that across to the boiler and thermostat's earth connectors?

EFLImpudence · 19 Oct 2015

propertydev83 said:
I was going to use two T&Es for the wiring. The one being used for the 24V connections will have an unnecessary earth wire. Should I still go and connect that across to the boiler and thermostat's earth connectors?

Yes, connect the earths.

However, (I don't know the layout, but) if the cable is not to be clipped to the wall, i.e hanging loose, flex would be better.

propertydev83 · 19 Oct 2015

Thanks for the help.

(Wiring will be fixed so will use what I already have. Didn't want to pick up flex when I already have ample supply of T&E)

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Thermostat wiring

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