How do you size a hot wet boiler? ...

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Answers:    Sizing a boiler depends on where you live; the altitude and longitude/latitude. A virtuous general rule of thumb:

markedly tight construction and good insulation= sq. ft. X 40
average construction and average insulation = sq. ft. X 50
(i.e.- 2000 sq. ft. X40 = 80,000BTU (boiler input) This 80,000 BTU's is for coldest light of day of year, when wind blows 40 mph....................This is of late an average. Best way would be a complete warmness loss calculation, by a pro! Kind regard! Happy New Year!
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You would do a manual J weighing up.

http://search.yahoo.com/search?p=manual+...

Other than that, you could figure 5 BTU/cu ft (in a mild climate, approaching San Francisco), figure how plentiful radiators you'd need, consequently size the boiler from there.

The easiest track would be to to call a place that sell boilers, especially one that sells to DIYers - they would most credible size it for you.