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pipe sizing and pipe material
... thing you need to determine is the heat gain and heat loss for the shop. Once you have that, the rest can be calculated.
If you're thinking of adding heating, I wouldn't look at PVC pipe. It won't take the temp. Also, in this kind of building, the piping stands a pretty good chance of being bumped by lift trucks, hit by swinging material, having somebody sling a chainfall off it, etc. I'd use steel. Lots of applications like this go with grooved piping over 2". If you do that, make sure the gaskets are the right ones for both heating & cooling. The supplier - Victaulic, Grulok, etc should have all the info you'll need for that.