I'm in the middle of an install of a new system that started yesterday. They finished the furnance/heat pump combo for the first floor. Seems like they're doing a good job so far. They used the existing ductwork, but I got them to mastic seal everything up for me, since it wasn't done back in 87. The old return was 20 by 8 rectangular sheet metal, and it was very loud. I asked what they could do to quiet it down. They replaced the basement portion of it with 16 inch flex duct. Pics below. Yesterday, they only had the low stage on the heatpump hooked up, but I couldn't even hear it running, which is nice. Any upside or downside to flex return done in this manner?
The original return made a couple of 90 degree turns to get around the water heater. The flex just bends around it, but goes up and down as you can see in the pics.