If an air conditioner is on in a house, but all the vents are closed, the conditioned air produced by the AC has nowhere to go, and it will face a few potential outcomes depending on the HVAC system's design:
Increased Pressure: Closing all the vents in the house creates a higher pressure zone within the ductwork and the air handler (the unit that circulates conditioned air throughout the house). As the AC continues to blow conditioned air into the ducts, the pressure inside the system will increase. Modern HVAC systems often have safety mechanisms that will detect this increased pressure and may shut off the AC to prevent damage to the system.
Reduced Airflow: Since the vents are closed, the conditioned air cannot escape into the rooms. As a result, the air's circulation within the HVAC system becomes restricted. This reduced airflow can lead to inefficiencies and strain on the air conditioner's components, potentially causing the system to work harder and use more energy.
Ice Formation: If the airflow is significantly restricted due to all vents being closed, the evaporator coil (the part of the AC that cools the air) can become too cold. This can lead to condensation freezing on the coil's surface, forming ice. The presence of ice further reduces airflow and may eventually damage the AC unit.
Overheating: Closing all vents can also lead to the AC's evaporator coil getting too cold and the compressor working excessively. If the compressor overheats due to continuous operation, it may shut down automatically to prevent damage.
Short Cycling: The AC may go through a cycle of turning on and off more frequently than usual. This is known as short cycling, and it can occur when the AC is unable to maintain proper airflow and temperature due to all vents being closed. Short cycling is inefficient and can lead to increased wear and tear on the system.
It is not advisable to close all the vents in a house while the AC is running. Air conditioners are designed to function optimally when the air can flow freely through the system and reach the various rooms. Closing vents can cause problems with the AC's performance, efficiency, and may even lead to costly repairs. If you need to control the airflow in specific rooms, it's better to use adjustable vents or invest in a zoned HVAC system that allows you to regulate airflow to different areas of the house more effectively.