Room thermostat replacement issues

[Gorbash12346]

Hi all,

I'm having some issues with replacing our old Honeywell T6160B1004 room thermostat which has just packed in after being a bit intermittent.

I have an old ideal Mexico system boiler

Initially I purchased a 2 wire digital thermostat and attempted to make it work but for some reason it would control the central heating pump but not the boiler which would turn on and not switch off.

I tried a few different things before giving up and swapping it for a new 3 wire rotary stat (Honeywell THR830TUK)

The old stat was wired with red in pin 1, blue in 2 and yellow in three.

So going by the diagrams both are exactly the same but when wired this way again I can control the central heating pump but the boiler stays on.

Old stat wiring diagram

New stat wiring diagram

Please help!

 

[Gorbash12346]

Have I misunderstood the function? Is it just running on because it is heating the contents of the hot water tank?