I buying a new oil boiler and trying to determine whether an indirect fired water heater really saves on fuel and how much.

Currently I have a very old tankless unit. We have a family of four and we have been satisifed with the amount of hot water we get out of it. So, the only reason to get an indirect fired water heater would be to save on oil.

On the one hand, it would seem to me (a rank amateur) that a tankless unit might burn less fuel. In the winter, obviously, the boiler is hot anyway. In the summer, the boiler will have to stay hot, but the indirect fired tank would need to stay hot too.

On the other hand, I spoke to Weil Mclain's customer service and they assure me that the indirect will use less oil, but they cannot give me any data to show that, nor can they give me any idea how much less oil it will use.

Can anyone give me a definitive answer why the indirect uses less oil and how much less oil it will use?