Our HVAC unit is in the attic. It has a PVC pipe that goes down the wall to the bathroom sink, draining into the P-trap under the bathroom sink.
Since last year, during the summer when the AC is used heavily, the drain hose under the sink from the HVAC unit is icy cold and causes the black rubbery hose to sweat. This causes a smell of mold and requires a bucket to placed under the hose to prevent further cabinet damage:
I have had plumbers and HVAC pros come out and they have replaced all the plumbing under the sink, and inspected the HVAC, but the problem persists.
In general, our house is often humid. There is mold around the exhaust vents. Often times it is more humid in the house than out of the house (Dallas, TX area).
If it gets over 90 degrees outside, the A/C never turns off and cannot cool the house below 75 or 76 degrees or so. It is during this time the cold water pours down the pipe and fills the P-trap with icy cold water.
Lastly, I had a general contractor suggest that my evaporator coils may be dirty. Since I've lived in the house (~15 years) I have never had them cleaned. Also, I have had problems with the filters being sucked in because the plenum was so tight, it bent the sides if the filters (even the 3" ones) and after that, they loose all structural integrity. You don't know until you go to change the filter.
What could be causing this sweating and humidity in our HVAC system?
Best Answer
The problem could be the high humidity in the house. Here, north of Pittsburgh, Pa. I run a dehumidifier any time I am using the A/C unit to help reduce the humidity. Most A/C units are oversized not allowing the unit to run long enough to reduce the humidity. If you live in a high humidity area the A/C unit will not remove enough of the humidity in your house to allow you to feel comfortable. I set my dehumidifier at 40% and allow it to run automatically. I bought a GE unit rated at 70 pints per day from Sam's club and it keeps the humidity in check. One more thing, the drip line should have it's own trap close to the unit to keep cold air from blowing down the pipe causing the pipe to become cold which in turn coupled with a high humidity in the house causes it to swet. I would probably allow the condensate drain to run outside the house if that is legal in your area.