Plan Your Fresh Air System Project Carefully
Tighter homes today require mechanical ventilation to maintain good indoor air quality. But how do we know what the air tightness level of our home is? Or how much mechanical ventilation we need? Start with an air tightness or blower door test!
Once you determine the need for mechanical ventilation in your home, consider which whole-house ventilation system you want:
- Exhaust Only Mechanical Ventilation System
- Supply Only Mechanical Ventilation System
- Balanced Mechanical Ventilation System (typically includes heat or energy recovery).
Exhaust Only Mechanical Ventilation Systems
Exhaust ventilation systems are very common and work by depressurizing your home slightly. The system exhausts air, moisture and pollutants from the house while an equal amount of outside fresh air enters your home through leaks in the building shell (or through passive vents).
For example, installing exhaust ventilation systems remain relatively simple and inexpensive. Typically, an exhaust ventilation system consists of a single fan located in a bathroom or kitchen. In other cases, a single exhaust fan connects by ducts to several rooms in the house (preferably rooms where pollutants are generated, such as bathrooms or kitchens). One concern with exhaust ventilation systems is that the fresh air entering your home can be entering anywhere there is a hole or crack to the outside. Common pathways for fresh air entering homes include chimneys, wet crawlspaces, dusty attics or garages with carbon monoxide.
Supply Only Mechanical Ventilation Systems
Supply ventilation systems use a fan to pressurize your home, forcing outside air into the building while air leaks out of the building through holes in the shell. Like exhaust ventilation systems, supply ventilation systems are relatively simple and inexpensive to install. A typical supply ventilation system uses the existing furnace or central AC system to supply fresh air into your home. This approach can raise electric bills if the furnace fan runs continuously to provide proper ventilation. Others only run the furnace fan when heating or cooling is needed. This limits fresh air delivery to the home to only those days when heating or cooling is needed.
Balanced Mechanical Ventilation Systems
Balanced ventilation systems, if properly designed and installed, neither pressurize nor depressurize your home. Rather, they introduce and exhaust approximately equal quantities of fresh outside air and polluted inside air. Most balanced ventilation systems are heat or energy recovery systems. Heat and energy recovery ventilation systems reduce the costs of heating ventilated air in the winter by transferring heat from the warm inside exhaust air to the fresh (but cold) outside supply air. In the summer, the inside air cools the warmer supply air to reduce cooling costs.
There are two types of heat or energy recovery systems:
- Heat recovery ventilators (HRV)
- Energy recovery (or enthalpy recovery) ventilators (ERV).
Both types include a heat exchanger, one or more fans to push air through the machine, and controls. There are some small wall mounted models, but the majority are central, whole-house ventilation systems with their own duct system or shared ductwork. The main difference between a heat-recovery and an energy-recovery ventilator is the way the heat exchanger works. With an energy-recovery ventilator, the heat exchanger transfers a certain amount of water vapor along with heat energy, while a heat-recovery ventilator only transfers heat.
If you feel your home could benefit from an upgraded mechanical ventilation system, it's time to contact a quality energy auditor or quality contractor to help you select the best system type and design for your home.