We had a TRANE XL 1400 installed about 1.5 years ago. We have discovered black mold in all of the downstairs registers. The company that installed it has said that it's probably the speed of the exhaust fan needs to be increased. That didn't sound right, so I had another company come and inspect the system. They measured the humidity level inside each register, both with the system turned on, and with it off. With it off, it was close to 100%. With it on, it was 86% humidity. The inside house humidity was about 50%. He said that he believes that the distribution box under the house is too small, because it was sweating. The upstairs registers I don't see any mold.
Should I have all of the duct replaced? The original installer said they would run through some substance that will kill the mold. I'm not comfortable with that.
Thanks in advance for any advice.