The more strenuous rule will take presence.
Remember that the NEC is a minimum safety standard so to do anything less than what it calls for leads to an unsafe installation.
The True rating is in BTU, Then when you do the math, their own figures don't add up.
It works just like a water heater, Well it is one, and all Heating elements will not be in operation simultaneously.
How do you figure? At 24KW, I get ~81,900 BTU (thermal) per hour, which agrees with the 82,000 BTU/hr listed earlier. Maybe you are looking at a different unit?
Even if it doesn't pull 100A 100% of the time, the wiring needs to be sized for a 100A load (really sized for 125A).
As I (and others) brought up earlier, we don't know that this house even needs a 82k BTU/hr unit. We also don't know anything about the "new house" either. One of the smaller units might get the job done and will cost less for the unit as well as the wiring, breaker, etc.
The NEC requires that the KW rating of the boiler to be multiply by 125% for sizing the conductors. If the name plate on this boiler says 24KW then it is 24,000 times 125% for sizing conductors.
You guys are exactly correct.
When you look at the units Controller Sequencing a different story is told.
The link you posted on the cover of the manual is calls it a boiler