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I think a 30 kw system boiler will be perfect , your original thought of 3kw per rad is way off as many rads you'll find might below 1kw but depends on size and single or double , I was always taught to allow 13kw for hot water but I was using a boiler calculater this morning ( The energy saving Trust heating calculator ) and it stated allow 2kw for small unvented or 3kw for big so I'm gonna have to have a wee check as unvented does use less to heat and have massively less heat loss so quite possible ,,, But back to your boiler , 30kw won't be that far off the mark I don't think , too many people oversize boilers thinking oh it will modulate down but it doesn't get maximum efficiency / cond and there in lies the problem ,
2kw / 3kw is way too small for an unvented your right in saying 13kw but some have a coil cap of around 20kw