Modulating condensing boilers usually hit their max efficiency at about 1/4-1/3 of full fire, and efficiency ALWAYS increases as the temp of the water entering the boiler returning from the radiation/heat-emitters falls. When using deep setbacks with a programmable T-stat you have a couple of things working against you-
A: The firing rate on the boiler is higher during the recovery ramp from setback, reducing combustion efficiency during the recovery
and
B: In order to recover from setback at a reasonable rate, the outdoor reset curve has to be set to a higher temperature, which increases the return water temps, reducing combustion efficiency across the entire operating range.
With a non-modulating boiler you might save 5-7% with setback, maybe 10%-15% if you live in the leakiest & least insulated house in Massachusetts. But with a mod-con setting it back 9F would increase, not decrease fuel use.
With a modulating-condensing boiler running constant temp, but with the outdoor reset curve finely tweaked to the load it will use LESS fuel than using it with even 5F overnight setbacks. Read the manual, learn how to adjust the boiler's reset curve, and keep lowering it until it doesn't quite keep up, then judiciously raise it.
BTW: The -105 is oversized for most mid-sized houses in MA, which may lead to short-cycling efficiency losses (and boiler wear & tear) when running at low temp, depending on the amount & type of radiation you have, made worse if it's broken up into many zones. These are issues that should have been addressed in the re-design of the system for the low-mass modulating boiler with outdoor reset control, but all too often isn't. (With 96,000BTU/hr peak output it's nearly 3x oversized for my ~2400' house + 1500' of semi conditioned basement in Worcester- a 1920s antique with R-values & U-values way sub-code for newer construction.) The minimum-fire output of this beast is well over half my heat load at 0F outdoor temps, and while it would modulate OK for the coldest weeks of January it would be cycling the rest of the time, and with insufficient thermal mass or radiation to not short-cycle when it's 30-40F outside the condensing efficiency of the lower temp could get chewed up by cycling losses. It's possible to fix these issues to save the boiler and increase efficiency if that turns out to be the case with your house.