Hi- I am trying to get to the bottom of a problem with a central heating Y-plan system. Basically, when CH and HW are both on at the programmer, the ch temperature rises uncontrollably, with the room stat displaying eratic behaviour. I have checked the voltages being output from the various components and have observed 186V on the orange wire from the mid-position V4073 valve, which is also present at the cylinder stat (terminal 1, I think). I will be going back to make a more thorough check today, but am I right in believing that the grey wire outputs 230V and the orange wire outputs around 100V when both CH and HW are on at the programmer but not calling for heat, in order to hold the valve in its last position? This voltage of 100V on the orange wire is also going to be present on the switched live to the pump and boiler, but I am assuming (?!) that this voltage is too low to operate either. However 186V, being significantly higher, is enough to operate both, hence the uncontrolled temperature rise. If this is the case, then it seems that I need to replace the V4073 power head. Basically, I would like some explanation of why 100V is designed to be present on the orange wire when 230/240V is on the grey alone, why this voltage does not affect boiler and pump on/off, and what the effect of a faulty 'too high' voltage of 186V might be. Many thanks!