All of them? Yes, all of them. Even the one in the bathroom that is always freezing cold? Okay, you can close that one.

Depending on many factors, the cost of heating and cooling your home can be responsible for as much as half of your electric bill. At Mode Comfort & Air Quality, we are often asked by homeowners what they can do to lower their energy costs and increase the efficiency and lifespan of their comfort systems. The first response to this question is another question:

Are all of your vents open?

You likely have a return register that is non-adjustable (just a grille), but your supply registers (through which air gets pushed through the house) will have a means to adjust them. It seems perfectly logical that if a grille is adjustable (like window blinds), then you should be free to adjust it, and many people do just that.

Wait, are you saying I shouldn’t touch the lever on the vent cover once it is opened?

Closing even a few vents can cause problems for your system. Issues can be minor, such as low air flow or condensation, or major, such as a dead compressor or a cracked heat exchanger.

Many homeowners tell us that they thought that by closing a vent or two in an unused room they were asking the unit to “work less.” The marvelous world of HVAC does not work this way. By closing vents, you are actually restricting the duct system by increasing the pressure in the air ducts, which could cause leaks. Believe me, no one wants leaky ducts. Energy Star reports that “In a typical house, however, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts. The result is higher utility bills and difficulty keeping the house comfortable, no matter how the thermostat is set.”

Here is the technical part, in case you like numbers.

Air conditioners have to move a certain amount of air all the time. For instance, a 3-ton air conditioner is designed to move 400 cubic feet of air per minute (“CFM”). This flow can be raised or lowered 25 CFM without much impact on the unit (such as closing the vent in the bathroom that is always freezing…but just that one). A typical residential 6-inch metal vent moves 100 CFM. If you have a 3-ton AC and close off four vents, you just turned your 3-ton unit into a 2-ton unit. Thus, losing a third of the effectiveness (having a really dirty air filter can do the same thing).

Worst case scenario

Closing vents also causes the blower to slow down by restricting where the air can go. This can cause the evaporator coil to freeze, which will eventually destroy the compressor in the outdoor unit. Low airflow may also cause the heat exchanger to overheat, which could lead to a crack that potentially releases carbon monoxide into your house.

Final words

We suggest that you open your vents (but you probably figured that out by now), open your interior doors, change your air filters regularly, and let your HVAC unit work as it was intended. We go on many service calls that could have been avoided. At Mode, we are always happy to answer your questions and help you avoid costly repairs to your system.

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