Many people have questions regarding the heat function available on some portable air conditioners. This is not a separate space heater built into the air conditioner. Rather, it is basically a reversal of the cooling function.
The cooling function works using a process similar to a refrigerator. Room air is drawn into the unit from the room. Without getting into technical details, in essence the air drawn into the unit is divided into two components, one in which the heat is concentrated, the other cooled by the removal of this heat. The resultant hot air and associated condensate are exhausted from the unit. Whereas a refrigerator releases it into the room and a drain pan, a portable air conditioner blows the warm air out through the window duct, and most now have a mechanism to remove condensate from the unit, either by a drain tube out the window or evaporating it. (If too much accumulates, it will also collect in a pan which must be periodically emptied). The rest of the indrawn air has been cooled and is blown into the box by a refrigerator, and into the room by an air conditioner.
The heat function reverses this process, blowing the warmed air back into the room and the cooled air out the window. Since the majority of the air is cooled, the heat function is generally less efficient than the cooling function, will often have a lower btu rating, and is generally recommended for supplementation when room temperatures are above 50 degrees. Since air is exhausted from the room in either function, less air is returned to the room than was drawn into the unit. This accounts for the negative pressure that single hose portable air conditioners can generate in a closed room. Dual hose units use the second hose to offset this, drawing some air from outside as well as inside. Therefore they do not generate negative pressures, meaning doors do not close, etc.