Honeywell T3 thermostat seems to require two lives and no neutral. I have one live and one neutral.

[Michael F H]

Hi,

I'm trying to fit this Honeywell T3 thermostat in place of an older Honeywell rotary thermostat that's died. The instruction sheet (photo attached) has a wiring setup for a regular boiler and a wiring setup for a combi boiler. I have a Worcester Green Star which I've been told by several heating engineers, is not a Combi. So then it has to be the diagram on the left. This has a Live from the mains into connector A and a live from the boiler into connector B. But what I have hanging out of the wall behind the old thermostat is one live and one neutral. A red and a black. Not two lives.

Moulded into the plastic case of the thermostat's back-plate are a L into A and a N into B, contradicting the instructions.

Any ideas what it all actually means?

Thanks,

Michael.

 

[EFLImpudence]

This has a Live from the mains into connector A and a live from the boiler into connector B.

No.
It has a live from the mains to B and then, when on, that live from A to boiler.

But what I have hanging out of the wall behind the old thermostat is one live and one neutral. A red and a black. Not two lives.

Any ideas what it all actually means?

It means you do not understand what you are doing.

 

[Michael F H]

It means you do not understand what you are doing.

That much I do know But I also have a broken thermostat so needs must..

As for practical advice - should I therefore put the red wire into A and the black wire into B, and that is correct?

 

[EFLImpudence]

should I therefore put the red wire into A and the black wire into B, and that is correct?

If the Red is the mains 230V then it goes in B and if the Black goes to the Boiler L, then it goes in A.

In your case it doesn't actually matter which way round - A or B -they go but if there were a wire in C as well, then it would matter so copy the diagram.

 

[ericmark]

As far as a thermostat goes, it does not care if combi or not, diagram to left shows a boiler with no run on, rather rare, diagram to right shows boiler with a volt free thermostat which means the switched wires could be 24 volt or 230 volt, it does not matter.

Most boilers today have a permanent line, (you may call it live, but technically line and neutral are both live) this means frost stats can still work even when thermostat not calling for heat, and they often have an internal fuse, and since we don't want that fuse by-passing, the permanent line comes in the main from the boiler.

But where the boiler also heats the domestic hot water (DHW) with an external hot water store, the thermostat normally connect to the motorised valve, and then the valves turn the boiler on.

Worcester Bosch do not seem to use OpenTherm, only their own special version, so in the main the wall thermostat is a simple on/off, and the old dial type is often used missing out the neutral, this gives a larger gap between turn on and turn off, and this works well, as wall thermostat just turns off the boiler in warm weather, the TRV's control room temperature. In theory at least.

 

[Michael F H]

Well, it all works now which is the main thing. Red is in B, black is in A.