When homeowners are trying to make a space hotter or colder, they very often shut off some of their vents. This is also a common practice for unused rooms to save energy; after all, what’s the point in heating or cooling an unused room? Speak to any HVAC contractor, and they will tell you the same thing: don’t do it. But friends, neighbors, and strangers on forums have all done it seemingly successfully, so why are HVAC contractors so against the practice?
To answer this question, we’ll need to look a little deeper and get a little technical. But don’t worry, I will explain all the terminology and give you some handy analogies to help you make sense of it, even if you are not physics-minded.
Closed supply vents in unused rooms increase air volume in the ducts, increasing air pressure, including static pressure (resistance to airflow). The HVAC works harder to overcome the resistance and move air. Closed return vents reduce air volume, so the HVAC works harder to take enough air to other rooms.
Understanding Your HVAC System
Your basic HVAC system is composed of a unit with a motor (blower, heat pump, air conditioner, etc.), ducts, and vents. The terms ducts and vents are often mistakenly used synonymously, so let’s clarify what is what.
The ducts are the tubes through which air moves. They connect the different aspects of your HVAC system. Vents are entry and exit points for air, i.e., your return vents, supply vents, registers, grilles—whatever you want to call them.
You can look at this as being similar to your own circulatory system. You have a heart, lungs, arteries, and veins. Your heart pumps blood into the arteries to supply organs and tissues with oxygenated blood. Deoxygenated blood is returned to the heart via veins. From there, it’s pumped to the lungs, where it is reoxygenated.
The “heart” of your HVAC system is the motor, the “lungs” are the cooling/heating units, “arteries” are the ducts connected to supply vents, and “veins” are the ducts connected to return vents. *Often, the motor and heating/cooling units are included in the same unit.
When something happens to your veins and arteries, it affects the functioning of your heart. Similarly, when something happens to your supply and return vents, it affects the functioning of your HVAC motor.
Understanding the HVAC System’s Pressures
When it comes to understanding how closing vents can impact the efficiency of your system, you need to know about the pressure in the ductwork. There 3 main types of pressures that exist in your HVAC system’s ductwork:
- Velocity pressure = “the kinetic energy of a unit of air flow in an air system.” (source). It’s the pressure of air flowing in a particular direction.
- Static pressure = the air pressure exerted against the duct. It is also considered to be a resistance to airflow.
- Total pressure = velocity pressure + static pressure.
If your HVAC pressure system was represented by a person pushing a box up a hill, then the person would be the velocity pressure, the box would be the air, and gravity would be static pressure.
HVAC Systems are Balanced When Installed
When you have your HVAC system installed, your HVAC contractor takes great pains to ensure that they have designed the ductwork efficiently, installed the correct number of vents in the rooms and spaces that require them, and selected the right-sized motor/heating/cooling unit to supply the whole house with conditioned airflow.
By closing vents, you upset this balance, and the heart of your HVAC system does not function at optimal efficiency. Now that you know the foundational information let’s get to the good stuff; the reason you came here: what are the effects of closing your vents? (Please note that I will be switching between the two analogies based on what I think will make each concept clearest).
Effects of Closing a Supply Vent
The Motor Works Harder
If you have an unused room and you decide to shut the supply vents so that the room is not actively being cooled or heated, then, according to our circulatory analogy, you clog up an artery.
Clogged arteries result in greater blood volume in the other parts of the system, thereby resulting in higher blood pressure. Here, let’s switch to our box-on-a-hill pressure system analogy.
As mentioned, by closing a supply vent, you increase the volume of air in the ducts in all other parts of the system; you make the box being pushed up the hill heavier. Just as gravity exerts a greater force against heavier objects, so too does increased air volume create more pressure (when the size of the container, i.e., the diameter of the ducts, remains unchanged).
Before you think that this means that conditioned air will simply be blown into the other rooms with greater force (which might actually be pleasant), remember that the static pressure has increased, so there is greater resistance to airflow.
So, yes, your velocity pressure increases, but it does so to overcome the greater static pressure. The person has to exert more energy to push the heavier box up the hill at the same rate it was moving before. As the HVAC motor supplies the energy required to create velocity pressure, it has to work harder to supply an equal amount of air to the rooms and spaces to which you still want air to be supplied.
Ducts Leak and Break
When you increase pressure in the ducts, you also increase the chance that air will be forced through cracks and leaks in the ductwork, which exist even in the best of systems. This means that you lose conditioned air to unconditioned spaces. As a result of this, the heating/cooling unit needs to work harder to achieve the heating/cooling/ventilating targets set on your thermostat, AC, etc.
In addition, the greater air volume exerts more static pressure against the ducts. Over time, this can cause leaks and cracks to appear, particularly at joins. So, as time passes, the loss of conditioned air to unconditioned spaces will increase.
While the leaking ducts may alleviate some of the pressure build-up, they will only do so insignificantly.
How Closing Supply Vents Costs You Money
A motor that is working harder is using up more energy, and this will result in an increase in your utility bills.
The unit is going to wear out faster with the increased workload. As a motor wears, its efficiency naturally deteriorates, further increasing electricity/gas usage and cost.
A unit that wears out faster is going to require more maintenance, repairs, and premature replacement, which all take a toll on your wallet.
Ducts that are under strain from increased air pressure are also going to require more maintenance to repair leaks and reseal joins. However, there are only so many repair jobs you can do on ductwork, and the pipes will need to be repaired sooner than normal.
Not closing your vents is one of the main ways to run an energy-efficient HVAC system.
Effects of Closing a Return Vent
The Motor Works Harder
We often think that closing a return vent is going to stop unconditioned air from entering the HVAC system and needing to be heated or cooled, which requires energy. On the surface, this is logical and seems like a really energy-efficient solution. However, I’ll have to disillusion you on this score as well.
Closing off a return vent is like severing a vein. You stop blood in organs or tissues from reaching the heart or lungs again. It cannot be reoxygenated, but it also means that the amount of blood in the circulatory system decreases, and blood pressure drops.
You might be thinking, “great!” considering the trouble that increasing pressure caused for the system. However, while static pressure drops, so too does the velocity pressure. But let’s look at what this means to a circulatory system so that we can see how it negatively impacts the HVAC system.
When blood pressure drops, it does not change the amount of blood that organs and tissues need to survive; thus, the heart must compensate for this. How does it do it? By beating faster. Basically, it’s harder for the heart to do its normal job.
If we look at our box-on-a-hill analogy, closing a return vent is like depriving the person of energy. Yes, the box is lighter, but the person is weaker.
So, when you reduce the pressure in the HVAC system, you reduce the movement of air. In order to maintain its level of function, the motor has to work harder.
Unwanted Air is Pulled Into the System
Those duct leaks that I spoke about before also become problematic in this situation, but in the opposite way.
When you close return vents, dropping the pressure in the ducts, then you increase the likelihood of unconditioned and polluted air being pulled into the system from crawl spaces, ceilings, etc.
Not only does this mean that the air reaching your rooms is probably warmer or colder than you want it to be (depending on if you have your heating or cooling system on because of the season), but it also means that you are potentially supplying the room with dusty, humid, or even moldy air.
How Closing Return Vents Costs You Money
Closing return vents make the HVAC system work harder. This has the same consequences as when it works harder to compensate for the closed supply vents. It uses more energy to run and deteriorates faster, thereby using even more energy, increasing maintenance requirements, and requiring replacement sooner than expected.
Closing vents in unused rooms does not just affect the functioning of the HVAC system and cost you money, it can also negatively impact the indoor air quality in a number of ways.
Will Closing Both Supply and Return Solve Problem?
You may be thinking that by closing both the supply and return vents in a room (something that people often like to do with their basement vents in winter), you solve the issues by balancing out the pressure changes. However, this is not going to work, and I will explain why.
Your supply vents and return vents are located at different points along the system, so they exert their effect at different locations. If anything, closing both vents will have a compounding effect. Think of it in terms of our analogies.
If you both clog an artery and sever a vein, the heart will work harder to supply blood to the rest of the body—that is, until it stops beating entirely due to overexertion.
If you both increase the weight of the box and reduce the person’s energy, it is significantly harder for the person to push the box up the hill.
Closing vents temporarily shouldn’t cause too many issues or lasting effects. For example, if you need to seal some vents to cool your basement down in winter until you can get someone in to fix the leaking furnace, then you can do that.