My wife noticed we have what looks like water damage/stains around a couple of furnace vents in the ceiling. They are at separate ends of the house. Our furnace is a heat pump and we are not using it in the winter. It is up in our attic so the duct work is up in the attic as well. We are postulating that condensation is occurring in the vents and then dripping down. Does anyone have any insight on this? We do run humidifiers in the kids bedrooms at night and in the room the stove is in. I have a cheap humidity gauge and try to keep the humidity around 45%. Thanks in advance!