[olddog]

My garage is 36 x 50 x 9 tall or 16,200 cubic feet.
Furnace is forced air 100,000 btu/hr 80% effecient so 80K btu/hr goes into garage.
So I have just under 5 btu/ft^3.
garage is insulated and has drywall - finished like inside of a house.
I keep it 35 F in the winter and it will heat to 70 F in ~20 minutes (I never timed it).

A point I will make is that the cement floor is still freaking cold. You crawl under a car and it is cold. You can crank it up to 80 and be sweating when you stand up and it is still freaking cold under a car.

If I built a new garage, I would pour the floor with heating tubing in the floor and use hot water as you describe. I do not know how economical it would be to do this in an existing garage, but if it was me I would ask before I made my final decision. It would be real nice if the heat came up from the floor.

I saved the blower out of an old furnace and plan to duct it to pull heat off the ceiling and blow it under the car. Someday I will get to this, I hope.