If you read any of the other threads, the first step is to get a proper manual-J heat loss calculation done on the house. Most all 20-year old boilers are 2-4x oversized. IF you want to maximize efficiency, this is REQUIRED, not just a suggestion. If you read through here, you'll find lots of people that were talked into oversized units, thus spending more upfront and then having issues with longevity (short-cycling is almost certain and highly destructive to the long-term life of a modern boiler when oversized) and comfort. You can get a wag on your heat loss by matching up your oil use verses heating degree days (that calc is discussed many times - use search). You'd want to subtract say a summer month from a winter month to get an idea of how much you used for heating water for showering, etc. Since it's almost never cold enough to use the full output of the boiler, it is often the best way to use an indirect WH to store what you need for domestic use. Also, if you have some vertical space, a drainage water heat recovery system should allow you to take unlimited length showers with a smaller tank. Doesn't help on things like baths, but works wonders with showers.