There’s a short and simple answer to this question, and that answer is no, you should not close air conditioning and heating vents in your house, even in rooms that you’re not using. Just don’t do it.
But while the answer is simple, the explanation why is a bit more complicated. After all, it seems like common sense that you wouldn’t want to waste conditioned air on rooms that no one is in, but this is one of those situations where what seems like common sense isn’t sensible at all.
The problem here has to do with air pressure. You see, your HVAC system is designed to push air through the ducts at a particular level of pressure, and that design takes into account the number and placement of vents throughout your house. When you close those vents, you increase the air pressure in the ducts, sending it higher than what the system is designed for.
This increase in air pressure leads to a cascade of unintended effects. First of all, the ducts in most homes are unsealed, so when the air pressure rises, more conditioned air leaks out of those ducts into your walls, attic, and crawl space. In other words, instead of making the system more efficient, you’re now wasting energy heating or cooling those totally uninhabited parts of your home.
Next, depending on the type of blower fan you have, your fan will either begin running faster, to push against the pressure, or going slower, decreasing the total airflow through the system. Running faster uses more energy—again, wasting money instead of saving it—but the worst trouble comes if the airflow decreases. If the air conditioner is running, decreased airflow can lead to condensation building up and freezing on the condenser coils. Ultimately, this can damage the coils and lead to them having to be replaced.
On the other hand, if the furnace is running, decreased airflow can lead to the heat exchanger overheating. And when it overheats, that can lead to cracks. A cracked heat exchanger is not only expensive to replace, but can also allow odorless, invisible, and extremely dangerous carbon monoxide gas to leak into your home, posing a serious threat to the health and safety of your family.
Ultimately, there are many safe and effective ways to improve the energy efficiency of your HVAC system, but closing vents is NOT one of them. Keep those vents open and unobstructed so your HVAC system can function the way it’s designed to, keeping you comfortable all year long.