I have recently been experiencing very wet and humid air coming from the vents on my 2008 Volvo XC70 when the car is blowing warm air. You don't notice the wet air when you have the AC on blowing cold air however I believe the water is just condensing in the vents instead. I searched the web everywhere and have found people with this problem but no real explanation.
This problem is most likely caused by a clogged drain inside of the evaporator/heater core box which is located inside of the dash. Another symptom of a clogged evaporator drain is the lack of water under the car after the AC has been running for awhile. You can easily check this by driving the car on a hot day with the AC on. Then park the car and check underneath for water dripping and a puddle of water forming. You should get a small puddle under the car. My car did not have this puddle at all.
It turns out the humid air in the vents was just the beginning of the problem, this can rust out your evaporator causing your AC not to work and more importantly it appears the water has made its way to the floor under the carpet which was soaking wet underneath. Upon searching this issue I found some people had clogged sun roof vents, I checked the vents by opening the sunroof and pouring water around the edge then watching and the water come out underneath the car.
Here is what the problem looked like when the vents were on:
You can see water around the vents, this actually came from the humid air.
Here is how I fixed the problem:
On the right hand side (passenger side in North America) I pulled off the bottom trim on the center console:
Exposing the evaporator and heater core unit
Pull back the carpet to expose the drain
Put towels around and pulled off the drain hose. A lot of water came pouring out.
Because water came pouring out of the upper drain hose, I know the clog is in the lower drain portion. So I purged the lower drain out with water in a medical syringe
This took several flushes to get the water flowing and a lot of dirt came out of the lower drain hose
I pulled back the carpet further and noticed everything was soaking wet
These vents under the carpet and driver/passenger seat carry air to the rear of the vehicle, these were sitting in a pool of water
I checked the driver's side and found the same problem, pools of water under the carpet. I then pulled the front carpet up as much as I could to let it air out. I did not pull the back carpet up because it is more difficult however I can tell there is water underneath.
I continued to purge out the drain hole one last time.
Check underneath the vehicle while purging out the line to make sure the water in going underneath as it should
Re-attach the drain hose and dry up everything as much as possible
I then ran a dehumidifier in the car (with the front carpets pulled back) for several days to remove all of the moisture. I ended up renting a industrial dehumidifier to speed up the process (no picture).
I hope this helps someone and thank you to everyone for posting things which have helped me in the past!
