That is a great question, Eva!
You’re heating and cooling system probably uses a lot of energy. Typically heating and cooling make up about half of the total energy use in a home. If you are like most budget-minded people it means finding ways to reduce these costs would be a great benefit. So a common statement we hear is, “I close vents in unused rooms to save money.” I mean since those registers are adjustable that’s what they are meant to do right?? The truth of the matter may surprise you.
Air flow is everything!
The blower in your furnace is the heart of your air distribution system. It pulls air from the house through the return ducts and then pushes it back into the house through the supply ducts. Many newer systems, the blower is powered by an electronically commutated motor (ECM), which can adjust its speed to varying conditions. The majority of blowers, however, are of the permanent split capacitor (PSC) type, which is not a variable speed motor.
In either case, the system is designed for the blower to push against some maximum pressure difference. If the filter gets too dirty or the supply ducts are too restrictive, the blower pushes against a higher pressure.
In the case of an ECM blower, a higher pressure will cause the motor will ramp up in an attempt to maintain proper air flow. An ECM is much more efficient than a PSC motor under ideal conditions, but as it ramps up to work against higher pressure, you lose that efficiency. You still get the air flow (maybe) but at a higher cost.
The PSC motor, on the other hand, will keep spinning but at lower speeds as the pressure goes up. Thus, higher pressure means less air flow, and low air flow can cause some serious problems.
The important thing to remember here is that no matter which type of blower motor you have, it's not a good thing when it has to push against a higher pressure.
Closing vents increases pressures
In a well-designed system, the blower moves the air against a pressure that's no greater than the maximum specified by the manufacturer The ideal system also has low duct leakage.
The typical system, however, is far from ideal. Although most systems are rated for 0.5 iwc, the National Comfort Institute, which has measured static pressure and air flow in a lot of systems, finds the typical system to be pushing against a static pressure of about 0.8 iwc.
Now we're ready to address the question of closing vents!
When you start closing vents in unused rooms, you make the duct system more restrictive. The pressure increases, and that means an ECM blower will ramp up to keep air flow up whereas a PSC blower will move less air. Most homes don't have sealed ducts either, so the higher pressure in the duct system will mean more duct leakage.
The more vents you close, the higher the pressure in the duct system goes. The ECM blower will use more and more energy as you do so. The PSC blower will work less but not move as much conditioned air. In both cases, the duct leakage will increase further.
What about heat?
In addition to moving air, your air conditioner, heat pump, or furnace is also cooling or heating that air that flows through the system. The air passes over a coil or heat exchanger and either gives up heat or picks up heat.
In a fixed-capacity system—and most are—the amount of heat the coil or heat exchanger is capable of absorbing or giving up is fixed. When the air flow goes down, less heat exchange happens with the air. As a result, the temperature of the coil or heat exchanger changes.
If air flow is low, it'll dump less heat into the coil in summer, and the coil will get colder. If there's water vapor in the air, the condensation on the coil may start freezing. You might even end up with a block of ice,. And ice on the coil is really bad for air flow.
It's also bad for the compressor as not all of the refrigerant evaporates and liquid refrigerant makes its way back to the compressor. If you want to have to buy a new compressor, this is a good way to do it.
Same thing if you have low air flow over a heat pump coil in winter. You could get a really hot coil, high refrigerant pressure, and a blown compressor or refrigerant leaks.
Similarly, low air flow in a furnace can get the heat exchanger hot enough to cause cracks. Those cracks, then, allow exhaust gases to mix with your conditioned air. When that happens, your duct system can become a poison distribution system as it could be sending carbon monoxide into your home.
9 unintended consequences of closing vents
Let me now summarize the problems described above that can result from closing vents in your home. The first thing that happens is the air pressure in the duct system increases, which may give rise to these negative consequences:
- Increased duct leakage
- Lower air flow with PSC blowers
- Increased energy use with ECM blowers
- Comfort problems because of low air flow
- 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
- Condensation and mold growth in winter due to lower surface temperatures in rooms with closed vents
- You're not guaranteed to get all the problems that apply to your system, but why take the chance.
The only way closing vents could work
The fundamental problem here is that closing supply vents in your HVAC system changes what comes out in particular locations. It doesn't change what the blower is trying to do. Nor does it change the amount of heat the air conditioner, heat pump, or furnace is trying to move or produce.
It's possible you may be fine closing a vent or two in your home, but it will depend on how restrictive and leaky your duct system is. If it’s a typical duct system with 60% higher static pressure than the maximum specified, closing even one vent could send it over the edge. If it’s a well designed system with low static pressure and sealed ducts, you shouldn’t have a problem as long as you don’t try to close too many. Otherwise you're subject to those 7 unintended consequences, one of them potentially deadly.