New and existing energy-efficient houses require mechanical ventilation to maintain interior air quality. There are four fundamental mechanical ventilation systems for the entire home: exhaust, supply, balanced, and energy recovery.
Exhaust Ventilation System
Exhaust ventilation systems function by reducing the inside pressure. The system exhausts air from the home while make-up air enters via building shell leaks and passive vents.
Exhaust ventilation systems are ideal for colder areas. In regions with warm, humid summers, depressurization can suck wet air into the wall cavities of a structure, where it can condense and cause moisture damage.
Installation of exhaust ventilation systems is straightforward and economical. Typically, a house’s exhaust ventilation system consists of a single fan attached to a central exhaust point. Connecting the fan to ducts from many rooms, ideally, locations where pollutants are produced, such as restrooms, is a superior design. Rather than relying on building envelope leaks to send fresh air into other rooms, passive, adjustable vents can be built through windows or walls. To function correctly, passive vents may require greater pressure differences than those generated by the ventilation fan.
A difficulty with exhaust ventilation systems is that, in addition to bringing in the fresh air, they may also pull in contaminants such as:
- Radon and mold originating from a crawlspace.
- Dust from a loft
- emissions from the adjoining garage
- Flue gases from a fireplace or water heater and furnace powered by fossil fuels.
These pollutants are of particular concern when bath fans, range fans, and clothes dryers (which depressurize the residence while functioning) are used in conjunction with an exhaust ventilation system.
Because exhaust systems do not condition or remove moisture from the make-up air before it reaches the home, they can also contribute to greater heating and cooling expenses.
Supply Ventilation Systems
Supply ventilation systems utilize a fan to pressurize a residence, forcing outside air in while the air escapes through holes in the shell, bath fan ducts, and purposeful vents (if any exist).
Similar to exhaust ventilation systems, supply ventilation systems are easy and affordable to install. A typical supply ventilation system consists of a fan and duct system that delivers fresh air to at least one, but ideally many, of the rooms where occupants spend the most time (e.g., bedrooms, living room). In other rooms, this system may feature adjustable windows or wall vents.
Supply ventilation systems provide superior control over the air that enters a home compared to exhaust ventilation systems. By pressurizing the dwelling, supply ventilation systems reduce outside pollutants in the living area and prevent the back-drafting of combustion gases from fireplaces and appliances. Supply ventilation also permits outdoor air brought into the home to be filtered to eliminate pollen and dust or dehumidified for humidity regulation.
Supply ventilation systems perform well in warm or mixed climates. These systems have the potential to produce moisture problems in cold areas since they pressurize the home. In the winter, the supply ventilation system permits warm internal air to escape through random outside wall and ceiling vents. If the internal air is sufficiently humid, moisture may condense in the attic or cold outer portions of the outside wall, leading to mold, mildew, and rot.
Similar to exhaust ventilation systems, supply ventilation systems do not condition or remove moisture from make-up air before it enters a home. Consequently, they may increase heating and cooling expenditures in comparison to energy recovery ventilation systems. Due to the fact that air is supplied into the home at distinct points, outdoor air may need to be blended with inside air prior to delivery in the winter to prevent cold air drafts. In-line duct heaters are an alternative. However, they raise running expenses.
Balanced Ventilation System
If correctly planned and implemented, balanced ventilation systems neither pressurize nor depressurize a home. Instead, they introduce and exhaust about equal amounts of clean outside air and contaminated inside air.
Typically, a balanced ventilation system consists of two fans and two duct systems. Fresh air supply and exhaust vents can be put in every room, but a typical balanced ventilation system is designed to feed bedrooms and living rooms, where people spend the most time, with fresh air. Additionally, it exhausts air from spaces where moisture and contaminants are often produced (kitchen, bathrooms, and perhaps the laundry room).
Some designs employ a single-point exhaust. Because they feed outside air directly, balanced systems permit the use of filters to remove dust and pollen before bringing it into the home.
Balanced ventilation systems are suitable for all climatic conditions. However, balanced ventilation systems are often more expensive to build and run than supply or exhaust systems since they require two duct and fan systems.
Balanced ventilation systems, like both supply and exhaust systems, do not condition or remove moisture from the make-up air before it reaches the home. Consequently, they may increase heating and cooling expenses, in contrast to energy recovery ventilation systems. Similarly to supply ventilation systems, outside air may need to be mixed with internal air before delivery in order to prevent cold air drafts during the winter.
Energy Recovery Ventilation System
Energy recovery ventilation systems offer regulated ventilation while reducing energy loss. They lower the cost of heating ventilated air throughout the winter by transferring heat from the heated exhaust air inside to the fresh (but cold) supply air outside. In the summer, the inside air cools the supply air, which is warmer, to minimize cooling expenses.
Heat-recovery ventilators (HRV) and energy-recovery (or enthalpy-recovery) ventilators are the two forms of energy-recovery devices (ERV). Each kind consists of a heat exchanger, one or more fans to propel air through the machine, and controls. There are a few tiny wall-mounted or window-mounted ventilation systems, but the majority are central, whole-house ventilation systems with their own duct system or shared ductwork.
The operation of the heat exchanger distinguishes a heat-recovery ventilator from an energy-recovery unit. The heat exchanger of an energy-recovery ventilator transports water vapor together with heat energy, whereas a heat-recovery ventilator just transfers heat.
As a result of an energy-recovery ventilator transferring some of the moisture from the exhaust air to the often less humid entering winter air, the relative humidity of the indoor air remains more stable. This also prevents the heat exchanger core from freezing by keeping it warmer.
In the summer, an energy-recovery ventilator may help to regulate indoor humidity by moving a portion of the water vapor in the entering air to the ostensibly drier air that is leaving the home. If you use an air conditioner, an energy-recovery ventilator is often superior to a heat-recovery system for controlling humidity. However, the use of ventilation systems during humid but not excessively hot summer weather is controversial. Some experts recommend turning off the device during extremely humid conditions to maintain low interior humidity levels. Use pre-cooling coils or configure the system to operate only when the air conditioning system is operating.
The majority of energy recovery ventilation systems can recover between 70 and 80 percent of the energy in the exhaust air and transfer it to the entering air. However, they are most cost-effective in regions with harsh winters or summers and high gasoline prices. In warm regions, the cost of the additional electricity required by the system’s fans may outweigh the energy savings realized by omitting supply air conditioning.
Installing energy recovery ventilation systems is typically more expensive than installing conventional ventilation systems. In general, the key to a cost-effective installation is simplicity. In order to save installation expenses, several systems share existing ducting. Not only are complex systems more expensive to build, but they also often require more upkeep and consume more electricity. Attempting to collect all of the energy in the exhaust air will likely not be cost-effective for the majority of homes. Additionally, these sorts of ventilation systems remain uncommon. Few HVAC contractors have the necessary technical knowledge and experience to install them.
In general, you should install supply and return ducts in every bedroom and common living space. The runs of a duct should be as brief and as straight as feasible. It is vital to use a duct of the proper size to reduce pressure drops in the system and so enhance performance. Insulate ducts situated in unheated areas and use duct mastic to seal any seams (never ordinary duct tape).
In addition, cold-climate energy recovery ventilation systems must be equipped with technologies to avoid freezing and frost development. The extremely cold supply of air can harm a heat exchanger by causing frost development. Frost accumulation also decreases ventilation efficiency.
Energy recovery ventilation systems are more difficult to maintain than traditional ventilation systems. Regular cleaning is required to prevent the degradation of ventilation rates and heat recovery, as well as the growth of mold and bacteria on heat exchanger surfaces.
