Built a 24'x 34' attached garage. Added a gas fired 40 gallon domestic hot water heater, circ pump, baseboard heaters, and filled the system with propoline glycol. The water tank is sealed off from the garage area, has it's own built-in temperature switch so the water stays hot all the time. Any heat lost there just helps heat the garage. A wall mounted thermostat turns the circ pump on/off via a relay. I keep the thermostat at about 52 degrees. Above the tank, higher than where the tubing passes over the garage doors, ia a small, vented two gallon "stand tank". It acts to keep the system full of glycol and provides a place for any trapped air to escape. There is also a manual ball valve above the garage door section of tubing to quickly bleed any air from the system prior to starting it up in the early winter.
It's been working great for 8 years now, and because our winter temperatures typically average in the upper teens at night, and above freezing during the day; it hasn't cost much to run.