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I replaced my old clothes dryer hose and when I pulled it out of the hole in the floor, a strong gust of continuous cold air came up from the hole. the air conditioner (with the metal grill door in front) is right next to it and the air was running at the time. i've read that the main connection can come apart and leak air out and affect the conditioned air supply , but the force of air coming from my vents is good.

I would bet that there is leakage in the ducts somewhere. It really doesn't take much to pressurize the belly and when you pulled the vent tube out the pressure started to equalize.

Greg