3 If you're central AC and heating system has two intake ducts in each room high and low which should be open in the winter when you are heating the house?

Answer 1

First, I will assume that the two "intake" ducts deliver air from the furnace or air conditioner depending on the season. These are usually referred to as "supply ducts." Since air warmer than the surrounding air rises, which would be true if it was coming from the working furnace in winter, the maximum transfer of heat from the air entering the room to the air in the room (by mixing) will take place while the air rises from a supply duct at the low point in the room. Hot air entering from the upper supply duct would stay at the ceiling of the room and only transfer heat to the other air by radiation. The room would be heated mainly by replacement as the cooler air below was drawn to the return duct (usually just one on the main floor in a large common room) which returns the air to the furnace. If the two "intake" ducts come from the furnace, then the other duct should be shut unless it can act as a return to the furnace for heating. Either way, the warmer air should enter as low as possible and the cooler air be drawn out from as low a position as possible. The logic of reversing this system in the summer (cold air entering in the upper ducts and exiting through the [doorway to the one] return duct should be obvious.