British Gas engineer advice was more regarding humidity affecting boiler performance and risk of electrical shock. Mine is not in a cupboard, close to both shower and hand basin. New boiler will be re-sited when my bathroom moves up stairs.
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Open flued boilers are a total no no, Balanced (or room sealed) are safe as long as the controls cannot be touched from the bath as far as I know, but I am out of the industry since retiring. I never liked boilers in bathrooms for the reasons you suggest electrical safety, rust, damp with the ever more complicated electronics of modern boilers. If you plan to have one fitted I would avoid bathroom (and bedrooms for that matter, the noises will drive you crazy) kitchen would be my favourite.
Sound. Mine's in an airing cupboard in the bathroom, well away from the bath. Warm and dry. Only issue could be ventilation, although the door doesn't stay catched.
@stevo_com @hoefla
https://boilerchoice.com/boiler-in-bathroom-regulations/
British Gas engineer advice was more regarding humidity affecting boiler performance and risk of electrical shock. Mine is not in a cupboard, close to both shower and hand basin. New boiler will be re-sited when my bathroom moves up stairs.
from screwfix community forum