Yes it is IMPORTANT to make sur there are no open RETURN ducts in your
basement. These will casue negative pressure in the basment that can
pull radon in and distribute it in your home. Close off all RETURN
ducts in the basment and open one output duct in the basment to
pressurize the basmement a little. With the furnace blower running,
stand at the door to the basement with the door open a crack, if you
feel air being pulled into the basement from the upstairs, you have an
open return duct and need to close it.

Mark