A few moments with the Google Search Engine (it's really jolly good) got me to this page...
http://www.worcester-bosch.co.uk/installer/literature/gas-boiler-literature/greenstar-28i-junior-literature
The PDF document Tech Spec for Wall Mounted Gas Combi Boilers may answer the questions you have - I hope so.
On the last page (31) there's a telephone number of a technical help line...
0844 892 3366
Good luck...
Peccavi, February 2013
