Alan, if you use any type of unvented heater, kerosene, radiant propane, etc, beware that the byproduct is CO and WATER. If your equipment is cold, when you heat up the barn/garage really quickly, everything gets wet from condensation. You may want to look at this in two parts. Many folks here are putting PE tubing or copper pipe in the floor, (with insulation under it like many new house constructions) and then heating the water (preferably antifreeze) with a regular residential water heater (read cheap) equipped with a pump. They're setting these so that the garage stays well above the outside temp but not necessary as warm as you might want for extended work. THEN, when you want it really warm, add your forced air, LP or kerosene, vented or not, to get the final heat boost. Only problem is, you have to put the tubing or pipe (tubing is easier and cheaper) and insulation in before you pour the concrete floor.
