T6360 to digital thermostatTHR840D

I am replacing my old T6360 for a digital one. However there are two sets of cables wired to the old thermostat. In the past I have only seen one live an neutral.

On my new Honeywell THR840 I have terminal A - B - C
I have wired them as per image inside casing for the new digital thermostat - Lives to A and C with both neutrals to B

It seems that when I switch the boiler on the room thermostat has no control with temperature. I set it to a temperature way below room temperature but it stays on. To make sure everything with programmer and boiler is still ok I put the old thermostat back on and it controls the boiler firing up based on temp of the room and thermostat.

I've replaced an old T6360 before in my old home with a modern digital one and it only had one set of wires which is all I was expecting here.

Any thoughts on why there would be two and other than leave the old one in or call a heating engineer what else could I do?

Thanks
Lee

What boiler/system is it connected to? I think the confusion is Honeywell’s diagram on the back. You have in the old thermostat a live in (1), a switched live out (3) and a neutral (2). The neutral is to heat a resistor for less of a temperature swing, and because your new thermostat doesn’t need this, you don’t need the neutral (from what I can gather). So put black wires in a terminal block, put 1 from old stat into live (A) and 3 from old stat into B.

Aquarius said:

What boiler/system is it connected to? I think the confusion is Honeywell’s diagram on the back. You have in the old thermostat a live in (1), a switched live out (3) and a neutral (2). The neutral is to heat a resistor for less of a temperature swing, and because your new thermostat doesn’t need this, you don’t need the neutral (from what I can gather). So put black wires in a terminal block, put 1 from old stat into live (A) and 3 from old stat into B.

Click to expand...

That's exactly what I did.
I checked out thermostat with my meter and the switch went from (A) & (B), so this endorsed your comments.
Thanks for your advice all seems to be working fine.