Can't find any reference to a Honeywell D200 thermostat is that the right number?
Can't give exact instructions as every stat is slightly different.
The three wires on your old thermostat would be live, switched live to the boiler, and neutral. When wiring to a digital thermostat the neutral is unused. The neutral would have been connected to the ballast resistor on the old stat.
Wire it the right way or it will go bang, and you will probably be buying another stat.
Last edited by plumbbob on Sat Oct 04, 2008 11:20 pm, edited 1 time in total.