Replacing my old Thermostat with a Honeywell DT90E - Thermostat not Switching off Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

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I am also replacing my old thermostat with a Honeywell DT90e. It has 3 wires Black Brown and blue.I connected the brown to A the blue to B and and isolated the black. The thermostat does not switch off when temperature is reached.What have I done wrong please?