I have a Potterton Lynx 2 combi boiler that has had its fair share of problems over the years.It now fires continuously with the result the pressure rises, the prv release a bit and then lasts a couple more days before it hits low pressure and needs topping up. The gas board have replaced the prv , the pcb, the disc thermostat and the thermister , the gas valve , all to no avail .It works until there is a pressure build up .I do not think the PCB is at fault and think the culprit is a faulty thermostat .Any suggestions would be welcome

David Walker, February 2008

sounds like the hot water flow switch is sticking and requires replacing this can be checked by unpluging and seeing in boiler turns off
The thermostates will not cause boiler to fire without signel from heating or hot water side to do so

tony, February 2008