What size of Boiler do I require and why?

in all probability the system boilers expansion vessel is not big enough for 17 rads and cylinder,and as it has got older ( the expansion vessel ) may have scaled or sludged up,thus lowering its capacity this will lead to discharge from prv as water expands and loss of pressure,before you chose on your boiler get a heating engineer to work out the boiler you need.is this pipe external?,discharge from prv you may have problems with expansion vessel,loss of charge/ruptured membrane or as i said before boilers internal expansion vessel capacity may not be adequate for 17 rads pipework and cylinder.

Hi, First a new boiler may not solve the pressure issue, once the basics of checking expansion vessel pressures and a physical check for water leaks then the best way to ascertain where the leak is, is my isolating boiler from system, some boilers have ability to isolate, some do not, how ever isolation valves can be fitted to carry out the test. Increase the system pressure to 1.5 bar, then isolate the valves. The boiler will need to remain off 24hrs, if after this period the boiler pressure remains the same the leak is most likely on the radiators/pipework and a new boiler will not solve your pressure issues. As for sizing your system for a new boiler, heat loss calcutions and correctly sizing the expansion vessel requirement are the next step, an additional expansion vessel can be added to overcome any short fall that a system boiler doesn’t provide for.