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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

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    Does it drip while the fan is on or only after the fan is turned off? If you let the fan run for 30 min after the bathroom is used for showering, does it still drip? – Jim Stewart Nov 15 '18 at 10:21
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    What is the material of the existing duct work? Is it a straight vertical path from the vent fan to the surface? Would it be possible to insert another vent inside the existing one? Some high efficiency furnaces have systems to deal with water condensing in the flue. – Jim Stewart Nov 15 '18 at 10:34
  • spray the inside of the pipe with spray-on tool handle coating (ruberizing) to increase the temp differential between room temp and ground temp. – dandavis Nov 15 '18 at 21:37
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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.

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    I have seen trays to catch condensation, but running the fan longer may also help dry off the coke bottle.+ – Ed Beal Nov 15 '18 at 12:30
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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.

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