I would check your outside vent cap to make sure that it is loose enough to open when there is air pressure. Also get rid of the flexible duct and use rigid for the bulk of the run. Home Depot sells small rolls of duct wrap insulation and metal foil tape.

John Grabowski
http://www.MrElectrician.TV


I don't see what good sloping the duct would do, there will still be a low spot and water will still collect there.

Maybe the tenant needs to let the dryer run longer at the end of the cycle when the clothes are almost completely dry so any water that collects in the duct can be evaporated and expelled.

And route the duct not so close to the cold outside wall?

Does the problem occur in summer as well?

m