In heat mode, portable air conditioners use heat-pump technology. Heat pumps operate in a similar fashion to an air conditioner, but in reverse. The unit pulls heat from outside and transfers it inside. The more outside heat available, the more effective this will be.
The heat output drops as the outdoor temperature drops. A heat pump doesn't have the capacity to quickly bring a cold house up to temperature. Heat pumps tend to be somewhat ineffective in any climate where the outdoor air temperature falls near or below freezing on a regular basis. This is because moving heat from a very cold area to a hotter one takes more energy than moving heat between two areas with a more moderate temperature difference. There is also more heat available outside in a moderate climate than in a cold climate.
It is important to note that even in a cold climate, there is still heat in the outside air to be pumped indoors, but the unit needs to work harder to extract the heat that's available. Supplemental heating may be required to make the produce enough warmth to comfortably heat your home when the temperature falls below freezing.
Note: All of the Haier Portables need to be exhausted outside in heating mode. Units with 2 ducts(dual duct units) are recommended to only use the exhaust/discharge hose and disconnect the intake air hose.