Underfloor heating is without dought the best and most economical way to heat a garage or anything else for that matter. Have installed it in a very large warehouse and the client still can't believe how cheap it is to run, same for houses. The floor only ever gets warm at best but will heat a very large area.
Ray
What you are talking about, Ray, is "radiant heat" and it isn't any more or less costly to run than anything else. BTU's is BTU's and the cost to generate those BTU's is what you need to know. Right now, in many places, a natural-gas fired system is going to be the least costly. Electrically fired systems will always be the costliest to run.
What thelews showed us is the start of a "hydronic radiant heat system". I used this at my car wash (when I owned one) in a snow-melt or de-icing system, and did the same residentially for my driveway. The former was heated by an 82% efficient, 500K BTU boiler; the latter by a 200K BTU on-demand tankless water heater. The heater for my driveway was twice the size of the furnace (in BTU capacity) and when in operation was in full fire mode for 8-36 hours at a pop. That's a lot of gas. My driveway had 60 tons of concrete in it, (over 54K kg) and NOTHING happens fast. It's slow to heat up. On my driveway (see photo in action) I would turn it on 6 hours prior to a snowfall prediction. Most of that weather forecasting was spot on accurate; we knew what was coming to the Detroit area by simply looking west. What's it doing in Chicago? Well, it will be doing that here in a few hours...
Used inside--a home, a garage or warehouse, is different. The heat is "low and slow" and provides a very soft comfortable heat; but it's slow to react to changes. But the amount of heat a home (or garage) will require is based on heat loss calculations, and in order to meet those needs, you will require a certain amount of BTU/hr in order to meet them. HOW you provide those BTUs isn't really relevant--a gas hot air furnace, an oil fired hydronic baseboard, electric, etc. The efficiency of these products combined with the cost/BTU was the determining factor.
The hydronic is great when you have concrete slab for a floor such as a garage. It's also great if you intend to keep it on a low temperature, and a constant temp as well. One thing to consider is the concept of "thermal mass"; if you keep your garage at say, 60 degrees F, well everything IN your garage over time, will settle to 60 degrees. If you have one of those gas fired hot air heaters--common in garages--and you decide your going to work in your garage one night, well it will be easy to heat the air to 60 degrees, but nothing else will be that temperature...the floor, the walls, the tool chest will all be cold. If you have the floor heat on at a constant temperature you'll get a good thermal mass going. Very comfortable!
And by the way, thanks to EVERYONE for such a great array of photos of garage projects. Very cool. Who knew there were so many choices?