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Kilowatt Heating, Air Conditioning and Electrical Blog

Don’t Close Vents in Rooms You Aren’t Using!


This is a common mistake homeowners make with their HVAC systems during the summer. When the air conditioner is running, the blower fan sends the cooled air into the ducts so the air travels to all the room vents. Because the vent registers usually have shutters on them that can be moved to change the direction air blows into rooms, people think they can shutter the vent completely, thus stopping the AC from cooling the room.

But does this save money? No! In fact, it can cause serious problems leading to the need for AC repair in Van Nuys, CA. Please always keep the vents open, and don’t obstruct them with furniture or anything else.

Why Covering the Vents Is a Problem

The big issue with blocking off the vents is that it changes the air pressure inside the ventilation system away from what it’s supposed to be. If the blower fan has an ECM (electronically commutated motor) which can work at variable speeds, blocking off the vents will create an increase in pressure in the ductwork as the motor ramps up its power to push back against the resistance to airflow. For the standard PSC (permanent split capacitor) motor that only operates at one speed, the airflow within the system will start to decrease. Neither situation is good: you don’t want the motor to have to work against pressure that it’s not designed for.

So what might happen because of the pressure changes?

  • The ducts will start to develop leaks. A rise in duct pressure is one of the main causes for leakage from ducts, and leakage means a significant drop in cooling power and a rise in costs. You’ll need to have the ducts professionally sealed.
  • The evaporator coil may freeze. If the air pressure drops because the motor slows down or airflow declines, insufficient warm air will pass over the coil, and this leads to the coil staying too cold and freezing water moisture along it.
  • Low airflow means less air is reaching the rooms that do have open vents. Comfort will decline around the house.

There’s more, because each of these problems can cause a chain reaction of other problems, up to having a failed compressor—which often means the AC itself needs to be replaced.

The Solutions

If you want to be able to shut off cooling to rooms that aren’t used and not put the HVAC system in danger, you have options. For a standard central air conditioning system, you can have zone controls installed. These controls allow you to shut off cooling to certain areas of the house using dampers placed in the ductwork and a variable-speed motor that will adjust itself to maintain proper air pressure. Zone control is often done during a new air conditioning system installation, although older systems can be retrofitted.

Another option, if you’re planning on remodeling or you want to cool a specific area of your house that has trouble staying comfortable, is to put in a ductless HVAC system. You can turn off the individual wall-mounted units in each room, and there is no ductwork that can suffer from pressure increases.

Kilowatt Heating, Air Conditioning and Electrical serves Los Angeles and the San Fernando Valley. Call our certified service technicians 24/7 for exceptional customer service!

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