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Hard to believe, in desperation I replaced the old programmer for a ST9400, put back the CM907 and bingo it all worked perfectly.

Hi, The one by the HW tank is as per your top picture and the one by the boiler is also similar to the top one i.e. on both the flow is in/out. Regards.

Thanks for your interest. We have Honeywell Motorized valves, 1 is by the HW tank I presume it controls when hot water flows into the loop to heat the water. The other is close to the boiler which I guess is controlling the CH.

I recently replaced an old Honeywell T6360 thermostat with a CM907 programable stat. I have followed the instructions connecting the old 1 & 3 wires to A & B and isolated the old No 2 and earth wires, however with the programmer (ST6400) set to CH Constant and HW Auto the heating performs...