When were manual J's and D's first used

Missouri Bound

I remember years ago, mostly when spcae heaters were the thing to use that a HVAC guy would measure the room, take a look around at the windows and insulation and come up with a suitable btuh number for a home or a room. When did that become such a science? Btuh's haven't changed although construction methods have significantly improved...and are still improving. Personally I think it's over rated since equipment sizes aren't continually variable, although two stage units are somewhat along that line............just askin'

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how

Hhhmmm.
On this site, thats like shaking a hornets nest to see if anyone's home!