If you included the kind of car you have,I could be more specific but all cars have a vacuum canister under the hood. some are round plastic and some are a tin can and most are black. There shuold be a vacuum line going from the intake to one end and a vacuum line out of the other that supplies vacuum to the interior of the car, There should be a one way valve in the canister or the line from the intake. This valve is not functioning and loosing vacuum in the resirvoir. If it is in the resirvoir which most are, replace the resirvoir.
Sounds like vents are in open position where it passes through based on speed with or without fan on
A/C and Heater duct controls are often operated by vacuum chambers with spring return, under acceleration...wide open throttle...manifold vacuum goes to zero!
I have the same problem, and looking for answers. Please help!
The heater core is leaking coolant.
Your AC might be blowing out of dash vents on the windscreen as you might have turned the knob to that particular mode. It is also possible that the vent mechanism might be out of order or broken.
Low speed relay bad Blower fan resistor Defective winding in blower motor
If central air conditioner turns on but is not blowing through the vents, check the filter. If it is dirty, change it. Change the thermostat into cool, this will defrost the air conditioner.
Call someone to repair the outdoor unit and the vents will blow cool air.
you may just be low on freon
your cabin filter might be dirty