Boiler ch water pressure relief valve blown off

[pika_pika_pika]

We have a Chaffoteaux Britony SE combi boiler and the central heating water pressure relief valve has literally blown off, with loads of water falling out off the bottom. Up to now the boiler has had trouble keeping up sufficient pressure and we've had to top it up with water regularly (every few months or so) - does anyone have any idea why the valve might have blown off and whether it's worth repairing? All help much appreciated.

 

[canalman]

"Up to now the boiler has had trouble keeping up sufficient pressure..."
I'm speculating here, but if your expansion vessel has lost its (air) 'pre-charge', you will continually lose water as the system warms up and expands, having nowhere else to go. When the system cools, the pressure will fall a lot (though not to zero). The lost water will then be replaced when you top up, but this doesn't solve the problem.
It is possible to check the expansion vessel 'pre-charge' pressure with a car tyre pressure gauge (should normally be about 1 bar or 15psi), and to repressurise it with a car foot pump to that pressure, but you need to take the system pressure off the expansion vessel first, by draining down.
I think unless you feel confident to do this, get a heating engineer in.