Tested system that is leaking water. Found that the leak is coming from the flue pipe going out of the roof. Customer had the roof checked multiple times and the roof was recently replaced. Found that this flue pipe is condensing water because the pipe is oversized for the furnace. This will create a trap for gases and when they are cooled, they turn into a liquid. Over years of this happening it eventually rust out the flue pipe and begins to leak condensation along with the flue gases. Attempted to search for a different termination point for this flu pipe, but the only option would be to run it back through the wall and out the roof which would include demolition of the wall and the old flue pipe. Inform the customer that the best option may be to look at replacing with a 90% furnace. Also, while looking at the system, I realized the AC is undersized for the house while the furnace and evaporator coil are oversized. The system is mismatched due to poor installation practices.