How do i wire a Siemens RDD10 thermostat?

Question:I have the unit which is powered form the mains. My old unit had 3 wires, red-live, blue-neutral, and yellow-sun sign (not sure what they does). Which coloured wire goes where? I am lead to believe that this unit is a 2 wire, but have tried just using the live and neutral but that only powers the unit, it does not start the central heating.

Answers:
Well your link points to no-where.
Given more information I can help, been in the HVAC trade for only 30 yrs. It would be helpful to know the terminal designations of the unit as well as the thermostat