I have a 40X50 (38.5X 48.5 interior) building 2x 6 X13h walls on a insulated 10" poured foundation w 2" foam on outside. The exterior walls are1/2 osb and have 1 1/2 foam under metal. They will be stuffed with R 19 I have heat loss data showing a total heat loss of 4845 btu for the office area (11" X 18") and a second zone (shop) 45,991btu for the balance. The Office will be kept at 70 and the shop will be cooler at about 60, I have 2 of everything (pumps, manifolds 8 port and 2 port, thermostats) fromPEXsupply. How large of hot water supply is needed? I have cheaper electricity than propane so I am considering a electric hot water tank or electric  boiler. I had a 70,000 btu gas boiler given to me so I am unsure what to use?

I am getting the floor ready for pouring  hopefully on 6/20/09 we are using about 8" power tamped sand over well drained clay. How flat does the sand have to be? We are shooting for less than 1/8. screeding off any excess. then vapor barrier and 2" pink dow board with ThermaPEX clipped to foam then wire mats laid over top.

We are planning on using a conveyor truck to reach the back areas of the shop.     The slab will be 5" thick, How many yards is that? I have 2 different calculations by 2 different guys one says 30-32 yds the other says 37-40 yds whats up with that?
I want a smooth floor finish so I will seal it after 28 days but should I use fiberglass in the concrete? 
                                                                  Any Tips Any Ideas
                                                                     Thanks dkor

I calculate 31 cu yards @ 5" thick. Fiberglass will reduce shrinkage cracking.

The 70K gas boiler would get you there and is nearly perfectly "right sized" (assuming 70k is the input w/ 80% steady-state thermal efficiency, but you don't have gas supply? (or did you mean it was a propane boiler?) Are propane & electricity your only options?

There are decent multi-stage ~50KBTU/h electric boilers out there with integral outdoor-reset controls that would work. (If you trust your heat-load numbers, that is.) To be sure an electric hot water heater would be cheaper up front, but also bigger & less-reliable.

If the shop is 40'x50' and the slab is 5-6" thick and there are no footings this calculates out to 40x50= 2,000 s.f. x .5' = 1,000 cu. ft. / 27 cu. ft. per yard = 37 cu. ft. with no waste.