This is so called the Boiler failure chain reaction.. You’ve got it because your neibour had it , and if you have it your friends will get it…Ha ha… seriously.. If you dont get the annual service.. then the expansion vessel pressure will come to and end one day.. in this case it was like 10 years.. ( Lucky ) then the pressure will cycle from min to max.. pressure relief valve will cycle till it fails.. more air in the boiler will ruin almost everything.. ! ta ra..surprise…Your boiler is broken beyond cost effective repair.. because .. now you need to replace.. expansioon vessel ( need to take the boiler off the wall in most cases) , PRV, Automatic air vent.. and due to unrelated reason in this perticular case .. the plate heat exchanger is packed and all! Lovely… We will repace it with an ecoTEC PLUS 831.. Happy? To see the other problem with the same boiler see links youtu.be youtu.be