Some older systems were actually fairly efficient so it's hard to say how much improvement you might get with a new one. New mod-con boilers can be quite efficient - in the mid-90% range. They also are designed for cold starts, so aren't maintaining a high set point during the summer to keep the indirect hot. The better indirects have exceptional standby efficiency - some only lose 1/4-degree per hour, so may not need to refire the boiler except after a large use. Installation and equipment are moderately expensive. Some of the indirects are SS, and should last for a very long time, but nearly any one will typically last longer than a stand-alone unit. You get the full output of the boiler to keep water in the indirect hot if you set it up as a priority zone, so you can often get by with a smaller tank, which also minimizes standby losses. Payback could take a long time, if it ever happens. Hard to tell, as it depends on the local rates, and the improvement. Keep in mind that often, there is a rebate from the local utility company. You're probably too late (only one day left!) to qualify for the federal energy credit, but that would have given you up to $1500 (30%) of the cost back on your taxes.