Boiler sizing

Peteys

Pecmsg

It seems it would be to account for piping to radiators, which I’m sure you know already.  So in other words, if there was no piping as in my hypothetical, the boiler would be able to hypothetically produce 600 ft.² of steam. In real world scenarios there’s always piping to radiators and the boiler should be sized to the square feet of those radiators.  The pick up factor, as the name implies, should be able to account for the piping alone. At least that’s the way I understand it.  

Jamie Hall

Sounds like you've got it. And as others have said, provided the piping is insulated the 33% is very conservative, to put it mildly...