If your return air duct is in the living room and you are closing any doors that have supply vents with no returns in the rooms it will create a negative pressure in the living room that will suck in attic air through leaks in the attic floor, like shophound stated
Wow a lot of answers thank you everybody.
So let me explain better i smell attic nearby the return in the living room, my duct work are located in the attic, and so the filter, i have just purchased this house and the old AC didn' t pass the inspection so i got a new one for free. I believe is a leak but i dunno where it can be. bedrooms are ok i also have a return in the master and one in the hallway by the guest bedroom.
the return ductwork and the supply (cold air vents) ductwork need to be totally sealed, if you got the system for free, it's possible the company that installed it was a low bidder and they did a SUBSTANDARD job on the install. did the system get a ductwork inspection? was there a permit pulled for the job?
The TRUE highest cost system is the system not installed properly...
The three big summer hearththrobs...
The A/C repairman