Save Money By Closing HVAC Vents In Unused Rooms

Posted by kingofseo on June 13th, 2021

How Does HVAC System Works?

Your air conditioning equipment, heat pump, or furnace probably uses plenty of energy. Heating and cooling make up about half the total energy use in an average house. For air conditioners and heat pumps using electricity generated in fossil-fuel-fired power plants, the amount you use in the home may be only a third of the total. I get asked frequently if it's OK to close vents in unused rooms to save money. The answer may surprise you.

The photo above shows an average vent for a ducted HVAC system (air conditioner, heat pump, or furnace). You'll typically see plain grilles on the return side, but on the supply side, where the conditioned air gets blown back to the home, most HVAC services install registers like the one above. It includes a lever of some sort that lets you adjust the louvers behind the grille.

You'd genuinely believe that since it's adjustable, it must be OK to open or close it to suit your needs, right?

The blower and the blown

The blower in your HVAC system is the heart of air distribution. It pulls air from home through the return ducts and then pushes it back to the home through the supply ducts. In high-efficiency systems, the blower is powered by an electronically commutated motor (ECM), which could adjust its speed to varying conditions. However, the majority of blowers are of the permanent split capacitor (PSC) type, which will be not just a variable speed motor.

Either way, the system is designed for the blower to push against some maximum pressure difference. That number is usually 0.5 inches of water column (iwc). If the filter gets too dirty or the supply ducts are too restrictive, the blower pushes against increased pressure.

In the case of the ECM, a maximum pressure may cause the motor to ramp up to keep up proper airflow. An ECM is a lot more effective than a PSC motor under ideal conditions, but since it ramps up to work against higher pressure, you lose that efficiency. You still obtain the ventilation (maybe). However, it costs you more.

On the other hand, the PSC motor can keep spinning but at lower speeds while the pressure goes up. Thus, higher pressure means less ventilation, and, as we'll see below, low ventilation may cause some serious problems.

The important thing to keep in mind listed here is that whichever form of blower motor your HVAC system has, it's not the best thing when it has to push against increased pressure.

Closed vents increase pressure.

In a well-designed system, the blower moves the air against a force that's no greater than the maximum specified by the maker (typically 0.5 iwc). The best system also has low duct leakage.

The normal 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 ventilation in plenty of systems, finds the conventional system to be pushing against a static pressure of approximately 0.8 iwc. Now we're ready to handle the question of closing vents.

Whenever you start closing vents in unused rooms, you make the duct system more restrictive. The pressure increases, which means an ECM blower will ramp up to keep ventilation up, whereas a PSC blower will move less air.


About the Author

Joined: December 7th, 2017
Articles Posted: 81

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