Iv'e just deciphered your words Steve, you say water is coming out of the big exhaust, this "big" exhaust as you call it is used when the machine is in Heat mode i.e. undivert mode or full heating is being produced, if this is the case you probably have a pin hole leak in the coil within the heat exchanger and it's being blown out of the exit pipe.
This is sometimes caused by freezing in the past. In your case with that machine it will be almost if not impossible to repair due to the arrangement of the coil which is within the casing.