There are two major types of air conditioning in a home. One is a unit that is incorporated into a forced air furnace. This is called an HVAC system, which stands for heating, ventilation and air conditioning. The other major type is a portable A/C, which classification contains both window and stand-alone portable units. Depending upon which type is used, bedroom doors can be effective opened or closed.
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A Balanced HVAC System
Installed HVAC systems initially are balanced for overall heating and air conditioning by leaving the doors open. In this way, airflow is monitored and balanced to prevent any hot or cold spots when the HVAC system is running. Regardless of anything else, the most efficient and effective way for heating and cooling a home with an HVAC system is to leave all of the bedroom doors open.
Return Vents in Rooms
Return vents in bedrooms work better for heating than air conditioning. Since return vents are almost universally near the floor, when the heat from an HVAC system is turned on, the heat rises while the return vents collect the cold air. The cold air is sent back to the furnace to be heated and distributed back into the bedrooms. With HVAC air conditioning, when the doors are closed, all of the collected cold air drains down the return vents. This results in chilly air at and below knee level while the comfort zone of your body, the torso, still is exposed to hot air. Keeping the bedroom doors open in this instance better allows cooler air from all parts of the home to flow evenly and cool off the entire bedroom.
No Return Vents in Rooms
If there are no return vents in rooms, even if there are return vents in other parts of the home, there is no way for the heated or cooled air to get out when the doors are closed. This leads to over-pressurization and puts a strain on the HVAC blower motor, which may cause the motor to fail well before it is meant to. The only cure for this dilemma is to leave the doors open and allow normal airflow throughout the home.
Portable Air Conditioners
If the heating and cooling aspects of a home are not connected, like a situation where there is a forced air furnace and each bedroom has its own air conditioner, adequate heating and cooling is applicable when the bedroom doors are closed, as long as there are heating return vents. The return vents effectively circulate the heated air back to the furnace in a bedroom with a closed door when it is running. Conversely, since air conditioning units, either window-mounted or stand-alone types, exhaust heated room air through their bodies -- by blocking or closing the return vents -- the cold air cannot escape, and the blower motor of the portable A/C unit effectively distributes the cool air throughout the entire bedroom space.