Ac Return Vent Not Pulling Air . The refrigerant flows through the indoor coil and absorbs the heat from the indoor air blown over it. In case only one vent shows the problem, you may be having a problem with a disconnected or leaky duct, a crushed duct flex or a closed damper. AC Main Central Return Duct & Vent 20" 30" Energy Solutions from energysolutions.us.com This is the primary reason behind frozen ac lines. Hvac return air duct leaks & obstructions that reduce return air flow rates. I need to increase air return in ceiling, not large enough for ac unit?