Whole-house ventilation systems are appealing to many homeowners, simply because the importance of ventilation is something that most people recognize. While we generally think of our homes as being a safer and more controlled environment than the outdoors, that environment has its problems—namely, interior air quality. According to the EPA, indoor levels of air pollutants are typically between two and five times higher than outdoor levels. In extreme cases, pollutant levels can be 100 times higher indoors than they are outside. Proper ventilation is vital in any home because it helps expel these pollutants out of the indoor environment rather than recirculating them repeatedly. Whole-house ventilation systems are appealing because, as the name implies, they take a more comprehensive approach to ventilation.
Whole-House Ventilation Systems and Why They Are Useful
The National Renewable Energy Laboratory does a good job of defining whole-house ventilation systems—both in terms of how they work and why they are useful. These systems use multiple fans and ducts to exhaust stale air or supply fresh air to the home. They also offer a better exchange of indoor air with outdoor air than the average HVAC system. This ongoing exchange of air helps keep the air in the home fresher and avoids the buildup of pollutants that can cause serious health problems if circulated throughout the home. Of course, other steps—such as regular air duct cleaning—can also help combat contaminants in the home.
Describing the benefits of whole-house ventilation beyond these basics can get complicated, in part because this type of ventilation system doesn’t fall into just one category of HVAC design. Instead, there are four different types of whole-house systems: exhaust, supply, balanced, and energy recovery. See below for a brief description of how each of these types of ventilation systems works.
Exhaust ventilation systems are designed to “depressurize” your home or to drop the air pressure inside your home so that it is at or lower than outdoor air pressure. This process uses exhaust fans to propel used, stale air out of the house through vents. Usually, exhaust systems are simple, made up of just one fan and one exhaust vent. The reduced air pressure then leaves a vacuum to be filled, which allows the home to draw air naturally in from the outdoors. This process is accomplished by passive vents located throughout the house, windows, or other small leaks in the building shell.
This type of system is attractive because it is simplistic, efficient, and inexpensive. By relying on the concept of pressure equilibrium, it provides a natural way to draw fresh air into the house replace recently exhausted air. Unfortunately, it also has its drawbacks. There is no control mechanism to ensure that the system is only drawing in fresh air, which means pollutants can easily make their way into the home following depressurization. In hot or humid climates, moisture tends to get pulled in, which can damage window frames and wall cavities over time. There is even a risk that the depressurization process can cause backdrafts with combustion appliances.
For all these reasons, exhaust systems are not typically the top choice for architects thinking about whole-house ventilation. If they are used, it’s usually in colder climates where humidity concerns aren’t an issue.
Supply systems are the opposite of exhaust systems. Instead of using an exhaust fan to depressurize the home and expel stale air, these systems use supply fans to pressurize your home and force outside air into the dwelling. In turn, the air that is already in the house is forced out, leaking through windows, vents, ducts, and small holes in the building shell.
As with exhaust systems, a major benefit of supply systems is their simplicity. They also offer numerous advantages over exhaust ventilation, though. For one thing, they can better regulate the air that is being pulled into the house, filtering out pollutants such as pollen or dust. For another thing, they don’t risk the backdraft problems that exhaust systems can cause. Unlike exhaust systems, though, supply ventilation designs are not ideal for colder climates. They can lead to considerable condensation during the winter, eventually causing water damage.
As the name implies, balanced systems offer something of a happy medium between exhaust and supply systems. Rather than pressurizing or depressurizing the home, these systems seek to pull in as much fresh outside air as they are exhausting stale inside air. As such, these systems are a bit more complex, utilizing two different fans and two different duct systems—one each for exhaust and supply. These systems strategically exhaust air from rooms where pollutants tend to be most concentrated (kitchens, bathrooms, and laundry rooms) and to supply fresh air where inhabitants spend most of their time (bedrooms, particularly, as well as living room spaces).
Energy recovery systems (also known as heat recovery systems) are technically a type of balanced system. They work in much the same way, plus an additional component: a heat recovery or energy recovery ventilator. A big shortcoming of exhaust, supply, and balanced systems is that they don’t temper the temperature of the air entering your house. As a result, they can bring hot air into the home during summer, or cold air in the winter—thus requiring extra activity from your HVAC system. An energy recovery ventilator helps by using the exhaust air to either heat or cool the supply air, depending on the season. This process reduces the impact of ventilation on HVAC efficiency.
As you can see, balanced and energy recovery systems are the best all-around whole-house ventilation systems in most climates, but some homes can work well with exhaust or supply systems. Consult with your HVAC technician to determine which system is best for your home.