It's at least remotely conceivable that 147F output is the balance point of the full-output of the ALP-150 with the size of the radiation, and that +5F outdoor temps could be bigger than the output of the unit. At 150K input, assuming ~90% efficiency @122F return water temps you're looking at 135,000 BTU/hr.
That would be a HUGE heat load for a tight 2500' house with at least some insulation, and at least storm windows over single panes. But for a 2-3 story barely insulated air-leaky 4000' Victorian with single pane double hungs it wouldn't be unthinkable.
A two story or taller house with a lot of air leaks ends up with a VERY significant stack-effect air infiltration heat load. A house with a lot of single-pane windows can also have an inordinate heat load. Can you described the house (size/age/windows/R-values/basement)?
Test the ALP-150 against your radiation sizing: Set it up to run 175F fixed-temp output (taking it off ODR control), bump the T-stat up another 5F and see if it EVER hits 175F. If it can get there, the ODR probably needs tweaking. If it can't, and peaks out at 145-150F output, the radiation is delivering as much heat as the ALP-150 can deliver at whatever temp it stabilizes, which means you probably really DO have a heat load in excess of 135K @ +5F outdoor temps.
In a house that's "a little drafty" it's likely you can bring the heat load down significantly and cost-effectively with blower-door & infra-red imaging directed air sealing, and some spot insulation. If you have a lot of single-pane glass, tight low-E storm windows can improve window performance to that of a code-min replacement window at about 1/2-2/3 the installed cost. (It's a pretty straightforward DIY for the handy.) Even though low-E storms cost more than clear-glass, the improved performance makes it a quicker payback. (IIRC in PA there is even subsidy money available for low-E storm windows.)
BTW: Try to refer to a boiler a "boiler", and not "furnace"- a term that explicitly refers to ducted/forced hot air heating (to distinguish it from other heating system types.) It may be regional dialect conflates the two- I've met several people from central PA who always seem to refer to their boiler as "the furnace", but it's sometimes a point of confusion to others.