I'm having problems with condensation damaging my ceiling, and I'm wondering if there are simple/cheap solutions I could try. I think the overall problem is poor home architecture, but "redo entire 2nd story of home" is not simple or cheap. ;)

My home was built in 2011 in Columbus, OH - 1,600 square foot - Basically like an oversized detached townhome with an open plan downstairs.

My AC is downstairs and outside but my furnace and air blower are upstairs in a closet near the center of the home. My thermostat is downstairs. (I do have mild issues with upstairs/downstairs being different temperatures).

And here's the problem - I have water damage in my ceiling below the furnace/blower unit. It literally traces the shape of the unit upstairs in one part.

There's actually only been 3 specific incidents where I've noticed water or noticed the water stains spread. It's a combination of it needs to be really hot outside and really humid, which doesn't happen often. Usually I remember to turn up my thermostat a bit and be miserable when it gets hot like that (to avoid the problem). And yet.

So is there anything DIY I should do to combat this?

More details:

- There's a vent right by/below the blower unit. It almost looks like instead of adding ducting, it's just bare wood. There's a big hole in the wood below the blower unit, which opens out into my main vent for downstairs. When the AC is running a lot (hot outside), that metal gets noticeably cool to the touch. And it does start to sweat (that's some of the water damage).

- It looks like there are metal braces locking the furnace/blower into the floor upstairs. The part of those I can reach seem to get cool too. I think they come way down and that's what creates the square-shaped water stain in the downstairs ceiling.

- The vent area below the unit is L shaped, with the vent itself on the front of the L. It looks like water drips down the shaft and pools on the bottom of the L, then seeps through the ceiling beneath it.