Return air surface area should be like 125% of total output surface area -
more if the return path is long .
^^thanks Terry

Wow, I'm surprised my ducting hasn't buckled! I will add some returns in the basement since there's only 1 for the Air Exchange (4"). The kids bedrooms are also a bit cold so I might add some returns there as the hallway air return doesn't seem to be enough.


(found my scrap paper; Rough measurement is 527sq hot air supply and 331sq for return)