The newest generation of hydronic equipment-such as those with condensing technology that tenaciously extracts heat from condensate within the system-has pushed combustion efficiency into the 95%-99% range. That's smart use of energy and a key determinant in building a high-efficiency hydronic system. This high efficiency also becomes a very important part when long-term operational cost of the system is examined. When the design engineer can calculate a three- to four-year payback (or less) for new equipment, there's real incentive to install new technology.
So if the spec dictates high efficiency, as many do today, the next question may be: How high? Non-condensing commercial boiler technology offers a lot, with broad capabilities and fuel efficiency into the mid-to-high 80s percentile. If that's not enough, new condensing boilers can deliver up to 99% efficiency.