I've wondered for years why our 1st floor bathroom is so hot. I got a temperature gun yesterday and checked, after the heat being on for 30 minutes the inside of the tub read 120. I had to figure it out.
In the basement is a duct trunk with several round 5-6 inch branches coming off. Measuring the distance from an outside wall and looking in the thin space between duct and basement ceiling, as well as first floor layout, I think one ends behind the tub.
If I pull off the access panel in another room that leads to the motor behind the tub there's a lot of air moving out of the access panel and I'm pretty sure I can see the vent opening.
The house has an addition, this would have been an outside wall. Here's my dilemma. I could disconnect the branch from the trunk and close off the whole in the trunk. However, on the other side of that original outside wall is a family room on a slab, which is cooler than the rest of the house. The heat coming out of the ducts in there travels farther and through the slab and is no where near as hot, according to my temp gun.
So, I'm thinking of just locating the location on the family room wall where the duct is (behind a couch, sadly) and breaking through, closing the duct from entering the bathroom and instead have it enter the family room. There is already another vent in the bathroom.