my alpha cb24 boiler looses pressure every day?

Expansion vessel or pressure relief valve. Drop water pressure from system and check that the air pressure in the vessel is 0.8 bar (you can do this with a normal car or bike pump) repressurise water in system back to about 1.2bar and then test over next few days - is the air pressure still the same or has it gone? If it has then it's the vessel that's damaged and you need a new one or an external one. If not then its time to change the prv.

Ben
February 2013

pressure relief valve

hi-spec plumbing
October 2006