I have the following problem. When the hw system is switched on (either manually or at programmed times), the mcb trips. However if switched on when I have the cylinder stat down on it's lowest setting, and then I turn the stat up, the hw system doesn't trip and the boiler lights up. What does this indicate ? I have already replaced the programmer and cylinder stat, so it can't be a fault with them...or deos this belong to the plumber forum ?