If the PRV (Pressure relief valve) is releasing a constant flow of water, and your pressure gauge is above 3 bar,...you've answered your own question.
The safe working temperature on your boiler is roughly 1.5bar (when hot), anything above 3bar will release pressure to the outside to prevent your pipework/boiler exploding.
To fix this, simply shut off your filling loop which feeds water into your boiler. This should only be opened to top up the water pressure, and should never be open all the time as it will be feeding a constant flow of cold water water into the boiler.
Quick fix,...look for a silver flexible braided hose (maybe even listen for water flow),...find the little plastic lever,....turn the lever off,...the water flow should stop,...then when boiler runs out of pressure, the prv should close. if it dosn't, you'll need a new prv. Usually about £8 to £12 at any Parts/Plumb Center in the UK.
Northern Knight, February 2010
