Why You Shouldn’t Close Off Vents In Unused Rooms

11/30/2014

It seems logical that cutting off the heat flow to unused areas of the house will increase heat flow to other areas of the home, but this is a common misconception that could actually cause damage to your HVAC system and increase energy use.

Most registers that blow conditioned air into the home are installed with some sort of lever that allows you to open or close it. This must mean that it is ok, maybe even recommended that you use it, right? Wrong!

Why It Is Not A Good Idea

When you close off vents, you increase the air pressure in that room, causing your blower to work harder to keep up airflow in order to reduce the room’s pressure.

This means that there will be more forced air into your duct system, and if you have duct leaks, this increased pressure in your ducts will go straight for the leaks you have in the ductwork. The more vents you close in your house, the higher the pressure in your ducts.

Cutting off airflow from a room also raises the risk for mold and mildew production. It is best to leave all vents open and allow your conditioned air to circulate properly.

For a more thorough explanation of how your HVAC system works and how closing off registers is not a good idea, read this article by Energy Vanguard.

Unintended Consequences of Closing Vents

• Increased duct leakage
• Lower airflow with PSC blowers
• Increased energy use with ECM blowers
• Comfort problems because of low airflow
• Frozen air conditioner coil
• Dead compressor
• Cracked heat exchanger, with the potential for getting carbon monoxide in your home
• Increased infiltration/exfiltration due to unbalanced leakage, as I (Allison Bailes) described last week
• Condensation and mold growth in winter due to lower surface temperatures in rooms with closed vents

If you have been closing off the registers in your house, don’t panic! Most of these things will not happen, especially not all at once.

If you don’t want to take the risk, however, go around your house and make sure all of your vents are open and unblocked. Sometimes vents are blocked unintentionally by rugs, beanbag chairs, cabinets, bookcases, etc.

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