Originally Posted by Rev1

The most cost-effective way to heat with gas is - "properly sized and installed" infrared heaters. Roberts-Gordon makes a unit that is UL listed for residential use, but you have to look at "distance to combustibles" on all sides of these units. These distances to combustibles is not as great as you might think, thus rated for residential use. The other way is to use a standard gas fired unit heater. Many manufacturers make them approved for residential use. Sterling makes a model called a TF/RF that is a low profile design and comes with inputs ranging from 30, 45, 60, 75 and 90k btu. These look like your typical unit heater with propeller fan. Keep in mind that you will have to vent the flue gases for both options to the outside.

The advantage to infrared (forgive me if you already know this) is that you use the concrete floor as a heat sink. The heater heats the floor up slowly over time. The floor re-radiates this heat back up to approximately 6 feet above the floor (above the height of the average person) thus keeping you comfortable. You can typically run your thermostat an average of 10 - 15 degrees less than the setpoint of unit heaters with the same comfort level. This saves gas consumption over the unit heater option.

Another advantage to the infrared is that each time the door opens and closes, it does not take long to re-heat the "new cold" air (recovery time). Unit heaters have to heat the cold air each time this happens. The recovery time is longer than infrared and consumes MUCH more gas during this time.

By the way, both of the above are also available in electric type units (which may actually prove less expensive to operate these days). A good manufacturer of the electric type is Qmark. Sorry for the long post, just my .02. Good luck.

What is the square footage of you garage? And what are the walls constructed of? Insulated?