Phil, I assume the radiant floors in your basement and garage are well insulated & drained? Ground water can really suck the btu's out of a radiant slab.
It sounds like you have done your homework, and your numbers seem reasonable to me. I know 60,000 btu's/hr seems low for the shop...but the rule for a radiant slab with tubing on 12" spacing is 25 btu's per square foot of floor space. If the shop is 2400 sq. ft, than 60,000 btu's is right in line. The indirect hot water heater probably requires 75-100,000 btu's per hour to achieve max output. If you have it set as a priority zone, it isn't entirely in "addition" to the heating load.
I think your biggest sources of heat loss are likely the overhead/sliding doors in the garage & shop. Do your best to make sure they are well-sealed.
Phil May wrote:It really sound like a 520 can do the job? Is it economical to run it all summer just for domestic HW.
In warm weather my 520 uses 20-25 lbs of coal per day to provide DHW for a family of three. With the current price of fuel oil, it is cheaper for me to run the 520 for DHW than the oil boiler. As for the total capacity...I wouldn't be scared to put that load on my 520, but that is because I already own it. It would be a good idea for you to talk to an EFM dealer that has experience with both units. Even better, get the dealer to visit your property and evaluate it before making a recommendation.
Phil May wrote:I have a very tall 8" square masonry chimney that is built with 8" blocks and the cores are filled with vermiculite. It has lots of draft it will suck a piece of news paper up from the ash clean out door.
How big is the thimble? A 520 has an 8" flue outlet, a 700 is 9". I would use no less than 8" on either one if you were going to be running it hard.