Posted By Bob I on 06 Dec 2013 12:44 PM
Is the floor insulated under the PEX and insulated from the foundation walls? (so you're not heating the ground under and around the shop) Frankly I wouldn't put water in the system until it can be permanently heated, although you might be able to use it with an antifreeze solution(?) If you can do that, hook it up to a dedicated water heater, making sure that the antifreeze solution is compatible with all components.
You can't use glycol in an open flame water heater. Fire hazard.