he might just mean 15mm at the boiler.. depends how you read it i guess.
I read it with my experience of old boilers in mind ( no pun intended ). How many boilers do find with a drop of less than 1mb?
surprisingly a few! I know what you mean, but to often on here, people tell OPs "you need this or that" when we don't actually know. It "may" be required, but let the man on the job do his job is what i say