I have an addition on the house, 150 sq ft, with two supply vents and no return vent. The adjoining room's, 100 sq ft, return vent basically handles both rooms efficiently when the door is open. Close that door and you will see a 2 to 3 degree drop in temperature to the addition on the house. I am considering tying in a new return air duct, utilizing an 8" saddle tap, from the addition to the existing return air duct of the adjoining room. How does my plan sound?