Forced air heating systems have two main components for airflow. The heater vents are the vents in your floor or wall that have a cover or grate and damper to control heat flow. Cold air return vents, also either on your wall or floor, are just like the heater vents except they do not have a damper. Since it seems as though cold air return vents do nothing, many people wonder if they can cover them.
About Cold Air Return Vents
A forced air furnace heats air and forces it through the ducts and out the heater vents throughout the house. In order to work properly, these types of furnaces need air fed into them in order to heat it up and warm the house. Cold air return vents look like holes in the floor covered up by a decorative vent because that is exactly what they are. Forced air heating systems work by the simple physics fact that warm air rises and cold air falls. Basically, the cold air in your home falls down into the furnace via the cold air return vents. The furnace then heats the cold air to the temperature at which you set your thermostat and forces it into the duct work throughout your home.
If you are thinking about whether you can cover a cold air return vent, technically you can, but you shouldn't.
Effects of Covering Cold Air Return Vents
When you cover your cold air return vents, your furnace cannot breathe. When this happens, it has to work harder for its cold air supply. When your furnace has to work harder to get air in, it makes it significantly less energy efficient. The main effect of this is that your furnace kicks on more often because it is heating the air less efficiently. When your furnace has to kick on more often, it increases your heating bills.
Another effect of covering cold air return vents is that by your furnace not having a constant stream of air flow, it creates a sort of vacuum in your home. This leads to a "stuffy" feeling that is uncomfortable for most people and extremely uncomfortable for those whose ears are sensitive to air pressure.