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Hi all, how do you replace this beast? is it a simple swap? do I need to do anything complex like bleed pipes etc? thanks. pic attached

Hello and happy new year. I'm not a plumber, I know you need an unvented ticket to do this work, I've done all I can, I'm just looking to clue up so I know the job will be done right. Megaflo 210i Unvented Indirect cylinder installed in new build house 7 years ago. Problems just started in the...