Older, Non-Insulated Homes
The first thing to consider is how well insulated your home is. If you have an older home that isn’t properly insulated, you could potentially save a little money by closing off vents on your air-conditioning system.
Some people may even close off all the vents except for the living room or another room where they spend most of their time. The lack of insulation keeps the pressure caused by modern forced-air cooling systems from building up in older homes.
Duct leakage is the main problem associated with closing some of your air-conditioning vents. When parts of your duct system are shut off at the registers, the air continues to blow against the closed vents and high pressure is built up inside the duct system, particularly in the section near the closed vent.
This leads to duct leakage, which results in lost energy and increased cooling costs. Although you are attempting to save money by cooling less of the house, you are actually costing yourself more money by funneling cool air through duct leaks inside walls or ceilings.
Closing Far Registers
If you feel you must close off part of your house’s vents, there is a way to reduce the negative effects. Close vents that are the farthest away from the air handler as possible.
The closer the vents are to the source of the forced air, the more pressure is built up in the ducts and the more air leakage will happen.
Closing Too Many
The effects of closing a couple of vents in your home may be minimal, but the more you close the more negative effect you will experience. These effects are also not limited to your utility bills.
According to the California Energy Commission, closing more than 60 percent of your air conditioning vents will severely restrict the airflow through the system and can have serious implications within your central air conditioner. This airflow restriction could make condenser coils freeze up, leading to a reduction in cooling and possibly complete central air malfunction.
Similarly, closing too many vents during the winter when the central heat is on can cause the furnace to operate on the high-limit switch.
A Better Idea
If you are looking for a safer and more efficient way of reducing cooling costs when you don’t use certain rooms, it’s best to leave vents open and reduce the cooling load on the system in those rooms. Keep the vents open and the doors closed, draw all blinds or curtains and keep the room dark and cool as long as possible, meaning the room will not require much work from the system to keep the temperature comfortable.