I bought my new house 3 years ago with a potterton boiler and I have been told I need a new one. When I switch on the heating, the boiler bangs and water/steam spurts out the top, radiators heat up ok. I have had people out already cause of the low pressure light coming on.
Could be one or more of various problems.
Ch thermistor not reacting causing overheat.
PCB not responding to (correct sensing of thermistor), adjusting gas valve accordingly causing overheat.
Pump not running quickly enough to clear water from boiler causing overheat.
Gas valve modulator not responding to (correct voltage from pcb) causing overheat.
All overheat situations are likely to spit through aav as described, also possibly operating prv which would cause loss of pressure
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