As far as I know, there are two different types of heaters...hot water heaters and fuel burning heaters. I am not sure of this, but I bet there were different models with slightly different ratings, too. As I said, I'm not sure of this, though. I believe both the hot water and fuel burning heaters are rated at 30,000 BTUs. However, you get much more efficient, intense heat from the fuel burning heaters. The problem is that they are more rare and more expensive when you do find them. With regard to functional differences, if I understand you correctly, the hot water heater takes a while to produce heat (until the motor is good and warm). The fuel burning heater puts out almost instantaneous heat...it takes a minute or so for it to fire up. Then, it is like standing next to a furnace.