pics to come. town house built in 1979 - 1981 a friend needed a new system here we go with helping a friend. duct looked good until I set up the system. Return plenum started to collaps, traced return to one joyce,thought I would find a large return up stairs,checked for return and found one on the second floor with 12 x 6 grills. at least they were high and low. Now don't start a discussion on the high low comment. Note the system I replaced was put in around 1980 and worked all that time. I still need to get some pic of the large duct going to one 3.5 x 12" hole.
