A bit of re-thinking here:
If the three homes are seeing water that (on average) is 4 degrees lower than what it was when it left the boiler, and the homes baseboards are then achieving the typical 20 degrees of drop from 340 feet of HW baseboards on top of this, then the 4 degree heat loss from the underground travel alone is roughly the equivalent of 4/20th's of your 340 feet of HW baseboards. That comes to 4/20 x 340 = 68 feet of "baseboard equivalent" for the underground passage. Call it 70 feet.
70 feet of "baseboard equivalent" + 340 feet of "actual baseboards" = 410 feet of "effective baseboards" that the boiler is actually seeing.
410 ft. x 550 BTU's/ft. = 225,500 BTU's of boiler output per hour required to max out the system from a home heating only perspective.
Add to this that each home requires about 32 gallons of hot water per day, for 96 total gallons of DHW per day (call it 100 gallons). Due to the brief nature of DHW demand lets say that on an average day this quantity of DHW is used over a total of only 2 hours. Thats two hours each day where 50 gallons of water must be heated from 50 degrees to 130 degrees. 50 gal. x 8.33lbs/gal. x (130-50) = ~33,000 BTU's of "peak" DHW demand.
Summing it all up I get 225,500 BTU's + 33,320 BTU's = 258,820 BTU's of required boiler output.
At 80% efficiency when firing this would require 322,900 BTU's of input for the boiler. This need for roughly 320,000 BTU's of input would put you squarely into the EFM 700 arena, and would be out of the nominal rated reach of an AA-260 or AHS 260.
This points once again to the EFM 700 as your choice without having a professional heat loss calculation done.
Last edited by lsayre
on Sun Nov 11, 2012 5:46 pm, edited 1 time in total.