The 28 HE cannot use
plastic drain piping for the flue.

Very interesting but not relevant as original poster hasn't got a 28HE

At the risk of supporting IMM on this one, AIUI, the flue options are the
same across the Greenstar range, so the exact model isn't particularly
relevent. They share the same flue part numbers and the same possibilities.

I believe the 29HE is a new heating only model, not a system type. The 28HE
is the system type. Not much difference between them, really, apart from
some missing parts. Combis are the 25HE, 30HE and various "PLUS" models.

Unfortunately, Worcester-Bosch persist in only publishing their 2 year old
manuals on the web.

Christian.