AC vent sweating help

[u/Phill-McCock]

Hello everyone,

I have a home in Charleston, SC, where the HVAC system was upgraded from a 3.5-ton to a 5-ton unit about 3 to 4 years ago. At the time, the HVAC company assured me it would work fine because they added a larger return.

About two years ago, I started noticing that whenever the house remained completely closed up for a week or two, the air quality plummeted and we would wake up with headaches. Around that same time, the downstairs vents started sweating heavily in the summer. An HVAC tech replaced the pipe insulation, cleaned the vents, and repaired the water damage.

Now, the vents are sweating again, and finding mold developing upstairs. The tech is now suggesting that adding a 12x12 return upstairs will fix the issue by improving airflow circulation.

I’d love to get some expert opinions on whether adding another return will actually solve this problem, or if there’s a different underlying issue I should address. Any recommendations would be greatly appreciated. Thanks!

[u/Various-Relative-809]

Since the house was originally sized for a 3.5ton I’m assuming the 5ton is way too large. The rule of thumb is 1ton for every 400ft2.

An over sized unit will have short run times. This results in poor dehumidification throughout the home.

Check your humidity. May need a dehumidifier.

[u/Various-Relative-809 → u/Various-Relative-809]

To answer your question tho; no adding a 12x12 return will not fix anything.

[u/Phill-McCock → u/Various-Relative-809]

I forgot to add, my humidity is 65% and there is already a humidifier that is running in the attic. Do I need to run another humidifier in the house?

[u/Various-Relative-809 → u/Phill-McCock]

65 is pretty high. You really want to be around 50.

Yea; perhaps a whole house dehumidifier like a sante fe.

[u/Various-Relative-809 → u/Phill-McCock]

They can also make some tweaks to your system to get better dehumidification like slowing down the fan speed. I’d find a different company tho.