Naturally, you should consider booking your annual boiler servicing just before the 12-months period runs out unless you notice some problems with your appliance before that point. And in general terms, it makes sense to service your heating device when you least use it, say, before the start of the heating season. Another important note to make here is that your boiler cover or warranty will become invalid if you fail to service your appliance every year, meaning any future repairs will be at your expense.