I have a 6 year old green star 24i junior boiler. Put heating on but unfortunately radiators cold but hot water works. I noticed my pressure was on the red so I bled radiators n put pressure back in system which is now on 1. I turned system on but I've noticed it fires up but after few seconds the green light goes out but blue light stays on. I have no fault errors, pushed reset button n still nothing. Please can you tell me what the matter could be but will only understand simple terms ha. If I need to call somebody out will it cost a lot of money to fix? Thank you