Interesting "problem". Had a super hi-tech 96% AFUE boiler installed (Burnham Alpine 105K BTU). Boiler was sized for the house, even though there are many rooms we don't normally use in the Winter. Sizing was the only reasonable thing to do - would hate to sell the house and tell someone the furnace is undersized if all rooms are heated.

With the old traditional boiler, we shut off the radiators in the rooms we didn't use (including our bedroom as we like it cold). This saved money. The guy who installed it as well as the Burham rep said this isn't a good idea now - because it causes the return water to be warmer than design (makes sense, not enough of the boiler output water is being transferred to the air), and the 96% boiler is most efficient when run in "condensing mode" which can only happen if there is sufficient differential between the input and output temperatures.

But there is math here that doesn't seem to add up. The efficiency of the furnace at its worse is still better than any traditional boiler, let's say it's 80%. It seems to me if you are only heating half the house, you still win because you must be using a lot less gas at 80% than with all rooms on at 96% (and it's actually better than that - due to the fancy digital readout on the unit I can see it stays in max efficiency (condensing) mode for quite a while when heat is first called for since the temp differential is high, but then it switches to non-condensing mode until thermostat is satisfied.

The installer and the Burham rep seem quite convinced I should leave all radiators on all the time. They seem very knowledgable - what am I missing here?

It will take a very long time to compute the actual fuel cost savings, made even more complicated by the fact that this was also an oil to gas conversion, and we've had a warm Winter. Need to satisfy my gut feeling in the short term.