[mockingbird812]

Thanks - my research led me to picking a model that takes its combustion air from the outside. If there are combustibles such as gas vapor, or saw dust in the garage there is a higher likely hood of explosion if combustion air comes from within the building. External combustion air heater requires an extra vent and is a bit more expensive but gives me some piece of mind. My heater uses a concentric vent system which combines the two vents together shortly after the two vents leave the back of the heater. It is something to consider when you are putting your shop togeither.