Gentlemen ,  I would like to get a component layout for a system I am DIYing together.
I just put up a 30X50 metal building near Chapel Hill, NC ,with 14' eaves , 1.5-2" closed cell foam (complete)insulation on a 5"slab on 2"xps, sides also done.  I have two circuits of 3/4"pex , 18" loop spacing, each 480' long.

I have been reading the posts here for some time, that's how I have gotten this far.
Now, the hard part "The Boiler" or heating system.
I have not done any Calculations for heat Load "don't know how"   but  I have come to
a determination based on others posts, that I can heat my building (part time machine shop/ welding fab business) with propane fired water heaters.  

I found two brand new 30 gallon 29,000 btu each  propane fired water heaters at the local Habitat for Humanity.  Assuming that one or both of these together will be enough
horsepower/btus  what would the layout be?  physical,    heater tanks,to pump,to manifold, out to,loops,return to manifold, then  to tank ?  is there an accumulator? how do you keep the water level/ amount constant?    I know there is more to it, thanks for the help. 

I have a two zone manifold with flow controls , and the two heaters 

1500 square feet, large... and your area is somewhat mild, but not that mild. I would not be confident that water heaters are the economical choice for you.

NRTRob , probably not, but because I have budget constraints now ( I've got about 1500$ to set this up.) I will try to make them work until I can make some coin and eventually get a professionally designed system ,maybe wood fired / gas hybrid boiler, or something  to heat my home and this building.  Thanks for the reply
 

Based on last year's weather data for Siler City I'd hazard that the heating design temperature is about 0-5, even if your average winter temps are about 30F:

http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KNCSILER2&day=2&year=2009&month=12&graphspan=year


Assuming your 1.5"-2" of foam is averages about R9 for walls & roof I'm thinkin' (without careful calculation) that you'll be maxed out on both heaters at 6AM on design day, especially if you have much glazed area. Assuming that's 29kbtu/hr-input per heater, at ~80% combustion efficiency you're looking at ~46.5kbtu max possible delivered to the slab. Assuming no windows and a perfectly tight enclosure the heat loss from the shell to 0F outdoor air going to be about 25k. To that add the heat loss out the bottom of the slab, the loss out the windows, plus ventilation/infiltration losses- I'm guessing it'll be a bit north of 50kbtu/hr in the real world, and about half that for your January average. (cood b rong, offen am)

Whether you can get the full 46K from the water heaters both into and out of a slab with that tubing spacing is another factor that I'll let the pros speak to. (Is 30btu/ft with 18" spacing & 3/4" tubing reasonable in a slab?)

The loop length is also a bit on the long side, but probably not a show-stopper...

Dana1 , thanks for the info, I appreciate the calcs you did and your interest. I'm starting to realize that I may fall short of my own expectations, and at this point "it's starting to get cold" I've got to get with the program and put something together even if it's not sufficient. It will be my lab for my hydronic 101 lesson.

I actually went to one of the posters (web sites) and purchased the tubing and got the "free" layout. I just hope the layout is worth more than it cost.  

                                                                                                          Thanks,

3/4" pipe wasn't necessary (or an advantage of any kind, really) but it can handle 500 foot loop lengths pretty easily. however your max temp could range as high as 120 or so for 30 BTUs/sq ft. that woudl be a pretty high load of course.

I see a problem with your layout. On shops or sheds most times have a higher heat load then a house of the same size would. Your pipe layout should not have been over 16inches I recommend 8 inches in high heat loads. Your pipe size is over size it will work how ever. As the use of a tank type water heater to heat the building not a good idea. It will not handle the heat load of 1500sq feet. It will run more then its off and also over time burn the flue out of the water heater. I would go with a small gas boiler my self or a tankless gas water heater that can be used for heating.
Watch on ebay and craigslist.org you can find these units perty cheap at times.
Evan an older boiler will be cheaper to run then a standard tank type water heater.

Snicks , Thanks for the input, I am rethinking my current situation.I did look at the rinnai 2520, looks nice $1300plus shipping This is my Shop and I have to get something going, it has been in the 20s here lately and I took some digital temp readings, of the insulation performance. 26deg outside @ 6am it was 48deg inside with only my fan type 1500watt electric heater on all night. Of course I have no idea what that means except that
all my stuff has not frozen yet. Of which I am extremely happy about. Actually tonight I came to the site to ask for help on the physical layout , what components to use, and how to plumb them or a good site to look at. Thanks