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Should I close the air conditioner registers in my unused rooms to save energy? Jimmy in Prescott Valley

Jimmy, it seems like common sense to seal off a room you rarely use so you do not waste money cooling this area.  But it is not the best idea.  If you seal off a room’s air conditioning registers and close the door, you can create extra pressure on your air conditioning system.  That can actually cause air to leak through your ducts and that is a waste of air conditioning, energy and money.  A study by the Lawrence Berkeley National Laboratory proved that closing registers in unused rooms actually increased energy use throughout the home.  Plus a hot room with too little air flow is a breeding ground for mold and mildew, which can ruin your furniture and create health problems.

I have two registers in the basement from the central A/C.  Since I do not use the basement, should I close these registers?  There is still cold air coming out from the registers when I close them.  Why is cold air seeping through the registers?  If I want to close them tighter, what should I do?

It may be just that simple to close off the registers in the basement; however, you must be aware that you may create a pressure imbalance in the heating system for the rest of your home.  If there is a door that blocks off the natural flow of air from the main level to the basement and you close off the registers and there is no return air vent in the basement, you are forcing the remaining return air upstairs to take in more air which may throw off your main level into negative pressure.  A forced air heating system is expected to be a “closed” system which means that whatever air goes through the return air grill is the same air that flows out the supply air grills.  If you restrict the air supply the return will still attempt to move the same amount of air and will find the air from somewhere else.  The furnace blower does not recognize this and will attempt to move the same amount of air and you will have issues, possibly a life safety issue.  

Supply diffusers (registers) are not made for being closed off.  If you do not want air coming out of those registers the easiest and most affordable fix would be to remove the register, cover with a piece of sheet metal and seal the hole.  The return will not recognize air and you should not have any issues.  However, you should call in a HVAC contractor to confirm your system and verify that it will continue to operate efficiently and safely for you.

Shutting off a register is never recommended for a properly designed duct system.  Registers are never air tight so some leakage is to be expected.  By closing off the register, you will run the risk of “freezing up” the system of proper air flow is hampered.  Also, back pressure can actually create an air foil back to the main trunk line and slow down air flow to all registers.  The proper way to handle this situation would be to make sure there is enough ducting to allow proper air flow for the system with these two registers shut down and then provide dampers at the trunk line and shut off the air at the main line.  This would prevent any back pressure from occurring and you would achieve more air in the other areas of your home and you would not have any issues in the system.

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