As others have pointed out, the outdoor temps have been very low, and the output temp of a mod-con exhaust is low enough that it doesn't take much time/space for the remaining water in the jet of exhaust jet to condense or even freeze when the outdoor temps are in single-digits. The dew point of typical mod-con exhaust right at the burner is ~125F, but the heat exchanger can only lower it to a few degrees above the temp of the return water from radiation entering the boiler. The water condensing in the heat exchanger lowers the dew point of the remaining exhaust that is exiting the heat exchanger on it's way outdoors. With 130F return water very little water condenses in the heat exchanger (maybe even none, since the "max efficiency" indicator goes out when the return water hits 127F) but it's a low enough temp that it's 10F outside it doesn't take long for that exhaust to hit the ~125F (or is it 127F for this one?) dew point- you'll see quite a bit of condensation forming in the air a few inches to a few feet off the vent cap as it hits the chilly air.
On a warmer day you'd have cooler radiation return which results in more condensation inside the boiler, with less left in the exhaust products, and a much smaller delta between exhaust temp and outdoor temp, so the exhaust quickly dilutes with the outdoor air as it cools resulting in little to no visible condensation plume.
Contrary to what Jim stated, the outdoor dew points have little bearing on whether you see a plume or not, but the lower the outdoor dew point the LESS plume you'd see at a given outdoor temperature, since the diluting air is much drier. It's the dew point of the exiting exhaust gas relative to the outdoor temps that is the primary determinant of how much plume you'll see, but when the outdoor dew point is higher, closer to the outdoor temp, the more you get. (When the outdoor temp equals the dew point temp you have fog and rime-icing, and anything coming out the vent will be condensing instantly into a very thick fog.)
Jim is dead on when he says it's probably oversized, but not necessarily correct about the efficiency consequences. The ALP150 would be a
monster-sized boiler for most houses in Newton, unless yours is an uninsulated leaky a 5000' 19th century type of antique. But the high thermal mass of the radiators should keep it from short cycling if the radiation is so oversized that it's able to actually deliver the heat with sub-140F water when it's 10F outside (which is about your 99% outside design temp.) If the burns are typically 5-10 minutes or longer it's a good sign, and properly tweaked you can probably get long efficient low temp burns out of it at the boilers minimum-modulated output even with 100F water, and 90F returns, temps at which there isn't much more efficiency to be had by going lower.
And yes, lowering the speed on the secondary loop increases the delta-T, lowers the return water temp, both of which would seem in order here. (A 10F delta is pretty small, but not ridiculously small.)
Key to maxing out the efficiency is to set up the outdoor reset curves to the absolute lowest temps that actually meets the heat load, which can be figured out by programming it ever lower until it doesn't keep up. The particular steps and user interface will vary somewhat from manufacturer, but all outdoor reset controls have adjustable curve parameters. Read up on it in
the manual.