Published: august 13, 2015 - by ray wohlfarth. This is the amount you pay if you make a claim the higher your excess, the cheaper your monthly premium. Before you apply please make sure you have the following available. Celebrating british citizenship. The three things to think about when it comes to cost are: the purchase price of the new boiler system; installation by a qualified engineer to get everything connected and running perfectly; and the overall running cost i.e energy.