Read
the manual. Make it your bedtime story for a week See page 51. This boiler can be set to modulate the firing rate up and down in response to the entering water temperature rather than output temperature. Without the outdoor sensor you can still set it up to control the return water to 95F, 110F, 125F ,or 140F with the DIP switches. At 140F there is effectively zero condensing efficiency, but at 11oF entering water temperature the raw combustion efficiency will be in the mid 90s (as oppposed to mid-80s if blasting away at 180F out.)
Try running it at 110F return water temp, see if it keeps up with the load (it might, it might not.) If it doesn't keep up, set it to 125F- it probably will manage the load at recent If it keeps up and isn't short-cycling, leave it there, but order the
outdoor reset sensor (and the pre-made sensor wire with the connector).
It's not big money, and will enable the system to run in condensing mode
most of the time, only increasing the temperature as-needed when the outdoor temperatures drop.
Even though it only tests in the low 90s on a AFUE test, it's combustion efficiency will be higher than the AFUE efficiency with 110F return water, and most of the time that's likely to still work, without short-cycling the boiler. Tuning the reset curve (K-factor) will take a bit of experimentation to dial it in perfectly, but it's worth it for both efficiency & comfort. As you fix up the insulation & air sealing issues you'll be able to drop the temperatures even lower, for even higher efficiency.
A typical natural gas boiler condensing curve looks like this:
It's actually more complicated than that, since it's also a function of the firing rate- less efficient at the max firing rate than at the minimum at every point on the curve:
Propane has more water in the exhaust than natural gas, and starts condensing a slightly higher temperature than natural gas, so the efficiency curve is slightly higher natural gas at any point on the curve. So with 110F return water at minimum fire it'll most propane boilers will ~95% efficiency or even a bit higher, whereas natural gas will usually be a bit shy of that mark (as seen in the first picture.)