Over the summer I purchased a new oil furnace & AC for a small ranch home. After the install in early June it was working great, now since I have started using the heat I have been getting water building up inside the chimney and leaking out through the cleanout door. It isn’t the chimney leaking because it fills with water on sunny days. The company has come to check it out, and changed the nozzle to a hotter one which made less water but it’s still creating about two gallons a day. While they was here they turned up the heat to 80 and let it run for about 20 min then took a 500deg stack temp but it normaly only runs for about 5 min. I have been researching the problem and have a few ideas. It will run for about 3-5 min per cycle then turn off for about 10-15 min then come back on (is that a short cycle)? They installed an 8” relief bypass duct because of a small return duct could that be a problem (the house is only 1000sf)? The flue from the furnace is 5 in and the chimney is 7”x11” is that oversized? Is there any other possibilities, the company hasn’t been much help and are trying to say the furnace or install is not the problem. Today, I noticed the inside of the flue from the furnace is showing some rust on the metal. Also, when the water leaks out its black and when it dries it has a crystal look to it. Any help would be greatly appreciated.