Sounds to me like the duct work feeding the ceiling registers are not insulated and are chilled down sufficiently to condense water from the heated air. With my HP the temperature is too low to use an evaporator humidifier in the forced air, such as is common with gas and oil fired furnaces where the plenum temperatures run well over 100 degrees. But I understand you to say you have humidifiers in the heat pump air stream. Perhaps I misunderstand, and that may be all my fault, it is late and I opened this thread seeing no one had responded. I haven't given your post the careful reading needed I'm sure, but offer the above with the suggestion, by all means, lower you humidity setting.

I know what you mean about them being hard to find - I've had three open ducts for EIGHT years because wife could not find anything acceptable to cover them. I finally found a place with a lot of patterns for metal grille covers at steelcrestonline.com. I've ordered three but have not received them yet. Lead time is about six weeks right now. Price was around $70 for a 14"x6". To add a damper to each is another $50.

Sue36--You're not understanding Buehl's duct. The opening is in the floor, next to the wall. It's not spaced out from the wall where she could use a floor duct. The duct is not coming up into the stud cavity. This style of register is quite common here in the upper Midwest though they were new to me when I moved here from the south central part of the United States. My house has them & is 30+ years old. And it is the overwhelming choice for forced air systems in new builds. They looked weird to me for a while, until I got used to them. Now, I don't notice them because they're 'normal'. We've owned houses with both styles of registers. I prefer these because less dirt & stuff gets into the duct than what does with floor registers. And, there's no need to attach a deflector to them, either. Moving that opening out from the wall is a big deal if she has rigid metal ductwork. I can guarantee blood loss & depletion of one's French vocabulary. :) And for no good reason other than, to a few people, a minor aesthetic one. No one is going to notice that duct except for Buehl and only for a few days and that's because she is admiring her beautiful new floors. Everyone else is going to see the gorgeous floor, the pretty windows & what's outside the windows and the duct is going to disappear in the same way that outlets & switches do.

Yeah, we have one and it just holds the shallow roasting pans that used to be under the broiler in our old range (now we have two ovens, so no drawer there). It's barely worth including, let alone rearranging important stuff for!