would it be unwise to use a manual j from 1967 to size a furnace? the house has no new windows, and all the wall and other surfaces are older than 1967. it seems if there were new windows i would need a newer manual j. i can get the 1967 one from the library, but i would have to buy a new one.
If you have trouble with any part of the calculation, just phone up any of the major plumbing & heating wholesalers in your area. Each of them will have "heating specialists" on staff whose job it is to size boilers and furnaces for new buildings under construction or additions being added to buildings.
These guys won't be heating contractors. They won't even be plumbers. They're just guys who have worked their way up in the company and have been sent on courses to teach them how to do these calculations. Normally, these courses are put on by the companies who that wholesaler carries, like Slantfin, Weil McLaine, Carrier, Bryant, etc.
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