On Dec 12, 12:48 pm, wrote:
I am finishing my basement. There was previously one duct to supply
the entire unfinished basement with air, which left it a little cool
in the winter. Now I'm creating a mechanical room of only about 150
sqft, a bathroom of 50 sqft, and a main room of about 700 sqft. I
have added two additional supply lines to heat the basement to a
comfortable temperature, and plan to install the requisite two
combustion air grates to "connect" the mechanical and main room. Do I
need to add a return air duct as well? Is it advisable or not for any
particular reason?

I did the same and put in a closeable return, I only open it when im
working in the basement as it seems to heat faster. If the basement
needs AC it should cool it better as well and circulate air better