Went to a house today that complained of water dripping from the furnace. The setup is an older horizontal gas furnace with a evap coil on the return side. The water is dripping from the heat exchanger chambers. There are 6 vertical chambers in the furnace and water is running down the insides of all the chambers. I haven't ran across this before, but my first guess is that warm outside air (90-100 degrees for the past three weeks) is coming down the flue pipe and condensing as the heat exchanger is in the cold air stream and around 55 degrees.
Does this sound reasonable to you guys? Would it help to add a damper to the flue to stop the warm outside air from traveling into the heat exchanger? Or do you think I have something else causing the condensation problem?