Hi, and yes you have a short in a componant such as a blower motor has quit or shorted out, it could be a number of things. If these units are fairly new, the furnace will have a led light on it that will tell you if you have a control voltage problem, but in this case it trips the 15 amp main. This happens quite a lot. You will need to get to this heater and start looking for anything out of the unusual, burned electrical smell from the windings on the blower motor. You can unplug the blower leads at the circuit board and have someone up top reset the breaker to see if it holds. If so, its the motor that's shot. I know they are hard to work on under the house but you have no choice. You will have to do a process of elimination. It really should be easy to find as it is a dead short on a high voltage part, blower motor, circuit board. Just have a good strong light and have a 5/16 and 1/4 " nut drivers with you. Take a screw driver of each with you also, and have someone up top to work with you. It will save you a lot of time. Keep me posted, please.
Shastalaker7