We bought an underground house (bunker). After installing bathroom ceiling vent fan we have condensation dripping from fan. Due to using existing duct work that runs up through 9 inches of concrete with around 4ft of soil on top of that. Is there anything that can be done to stop the condensation? Thank you
I call it the “coke bottle effect”. It’s what happens when you take a coke bottle out of the refrigerator. The cold bottle will produce condensation when it meets with the warm room air.
In your case, the warm (probably moist) air from your bathroom is vented up to the cooler outside air. When the two meet and you reach the Dew Point, condensation is created by turning the warm moist air back to a liquid and runs down into your house.
One way of solving this problem is to insulate the duct and install a backdraft damper at the roof that seals the vent pipe.
When you installed the ceiling fan it probably came with a backdraft damper. This keeps all the air in the vent pipe at the OUTSIDE temperature. You want the air in the vent pipe kept at your INDOOR temperature.
I suspect you live in a cooler climate.
As others have stated, the problem is the cooling effect of the discharge fan's warm moist air being cooled by the surrounding concrete and dirt. The answer to your problem would be to install a double wall pipe similar to class A or class B furnace vent or a similar product. If this can't be done you could experiment with a different piping material that will not conduct the heat as quickly from the exhaust air. You could try PVC plastic type piping but the double wall flue piping would be better. You did not state the size of the existing discharge pipe or the name and model number of the existing fan, which may help with recommending other possible solutions.