What happens when a portable air conditioner is not ventilated properly?

Answer 1

Initially portable AC units had 1 hose that ejected hot air outdoors, this meant that the indoor air was being sent outside, as a result hot outdoor air is being pulled into the home in equal amounts to what is being blown outdoors, fighting itself in effect. More modern portable AC units have 2 hoses, 1 is the intake and 1 is the exhaust, both go outdoors so there is no hot air being pulled into the home we are trying to cool down. This is seen in forced air heaters with a supply side leak in the ducting.

While the return air intake is removing , say 1200 cfm per minute from the home, if a supply duct is leaking, this means less than 100% of the air removed will be returned, this creates a negative pressure in the home and can make flues back draft, water heaters too! If you have the fireplace lit and the heater starts up you will notice the fireplace begins to back draft into the home unless you crack a window etc.