The type of WH you have are typically not spec'ed to produce much volume - to maintain the temperature rise and provide a reasonably hot supply of water, they ususally have a flow restrictor in them so that the water doesn't move too fast. Forcing it to move slower through the heat exchanger means that it can absorb enough heat to be useful. Think your hand through a candle flame...do it fast, and you don't notice, do it slowly, and you'd burn yourself. Same idea with producing the hot water. the only way you'll get more volume is to store up some hot in a large tank, or buy a different setup to produce more hot 'instantly'. Most of those 'tankless' are designed for NG, not oil. So, find room for an indirect WH tank, or live with what you've got. An indirect WH is functionally installed as a separate zone off of the boiler. You can place it away from the boiler, if that would help, but running the new lines to and from it might be a problem and additional expense. Since there's no burner, the indirect WH could be placed anywhere since there's no flue...as long as you can get the boiler zone heat to it, and can connect to the hot and cold lines to feed the rest of the house.