The boiler is Trianco Contractor H.E. 50/90, less than a year old, in a fully pumped 2 pipe system using 2 x 2 port zone valves controlled by the room and cylinder thermostats. DHW is supplied via a double feed indirect cylinder.The problem is that the water in the primary system is overflowing via the vent pipe into the boiler feed and expansion cistern. I found the water in the expansion tank to be boiling hot. I thought that the cylinder thermostat and/or boiler thermostat may have failed causing this problem. However, I've been assured that this isn't the problem and that another valve needs to be installed in the system to help dissipate any excess heat from the boiler.
Any suggestions would be greatly appreciated.[/b]