Physics. Hot air rises, cold air falls. This is why ACT recommends separate systems for upstairs and downstairs or zone controls. It is impossible to have a balanced, comfortable system in a two-story house with one unit and no zone control. Many HVAC contractors wrongly install one unit on a two-story house as a way to save money. While this does indeed save on the initial cost, it will lead to an uncomfortable home that is impossible to balance. We are only interested in happy and comfortable customers. This is why we will not install a single unit on a two-story application unless it is equipped with a temperature zoning system.