how do i replace batterys in my greenstar 28i boiler?

You can find manuals here: https://www.worcester-bosch.co.uk/installer/literature/discontinued-product-literature/discontinued-gas-boiler-literature/greenstar-24-28i-junior-discontinued-july-2013

I do not think the boiler uses batteries (or even batterys).

Are you looking at the timer itself? WHat is the make and model of that? Just do a Google search

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August 2015