My RH inside is around 20% on cold days, I am in Upstate SC, where most people have a high humidity problem.

My house is ~30 years old. 1650 sq ft. on a slab. Windows replaced 3.5 years ago with mid-upper grade vinyl, doors are at least a little leaky. The heat pump air handler seems to be original. The outside unit is ~16 years old, replaced by previous owner.

If I could afford to, I would have the system replaced, ducts checked/repaired/updated as necessary. I am hoping to get a couple/few more years out of this system.

Option 1, is to install a humidifier. But it seems there is more likely a problem with my system causing the air to be so dry.

HVAC repair tech looked at the system to quote installing a humidifier, and says the air ducting is wrong and likely the culprit. The air handler is laying horizontal, the plenum has three lines, one on each side, and one coming out the end. HVAC tech says the one on the end is wrong, the plenum should be longer and the line should come off the side like the other two. The current config causes too much air moving across the coils, causing it to dry the air.

His recommended option is to put a mechanical adjustable damper on the duct line coming off the end of the plenum to equalize the pressure there. He felt this would probably alleviate the dry air problem.

Does this sound accurate? After reading a bit here, it seems I need to also have the ducts checked for leaks. What else should I get checked?

Thank you.