Live in nepa, have goodman 4 ton heat pump w/propane back up. Foam insulated about 1200 square feet that I want to heat. I am at the end and ready to put up the dividing wall.I have 2 returns and 3 exhausts, I'm going to add 2 more exhausts, I get plenty of push from the system now, my question is do I open the now closed returns year round to pull the air out and if so should I add another? I ran 8 inch metal to 6 and with the air on you can feel the cold air immediately cool off the shop even with 90 degrees out.
The best systems have a return in every room that has a supply register, but a lot of systems work just fine with only a central return (usually located right at the furnace). To achieve the highest efficiency, you're going to have to do a heat load calculation, however it sounds like it's working fine.
A good system will have balancing dampers placed throughout the duct work, but you can also balance the system by e.g., half closing all of the registers upstairs, to force more air into the basement.
Beautiful & open or close whatever works. It's the only way if you're not going to have programmable motorized dampers, like most everyone doesn't...you can outdo the Jones' & not just try to keep up with them. But, you'll likely have to re-experiment for the cooling & heating seasons to figure out what works for each.
All returns can & should usually stay open & don't come as adjustable for that reason, they're just "grills" not "registers". You'd only add a return(s) if they're noticeably needed. If a room's damp as you discovered or cold instead of warm or vice versa a return is needed to suck out that offensive air & suck in good air for "conditioning" at the Air Conditioner...everyone's HVAC system.
Of course, typically in cooling you fully open top floor registers & mostly close bottom floor registers, heating is then just the reverse. Since, cold air drops & hot air rises. But, then you'll have to take it further for building entry points, picture windows, dog doors & daughters kissing boyfriends through a window.