On Dec 12, 1:48 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?

no do not add a return in the basement..

you do not want to lower the air pressure in the basement because this
is bad for combustion air supply and can pull in radon.

Mark