Mechanical Ventilation Types: Exhaust, Supply, Balanced & Energy Recovery

By HVI

Historically, structures were ventilated using natural means, such as opening a window or door to let fresh air into a space. However, with greater air-sealing methodologies on the rise, this method is insufficient. Enter mechanical ventilation, which offers several options for moving stale indoor air out and fresh outdoor air inside. The different types include exhaust, supply, balanced and energy recovery.

In this brief white paper, we’ll take a look at each mechanical ventilation option, and discuss how they work, their benefits and any concerns that exist. We’ll get to the “how and what” of ventilation, but let’s start with the “why.” Why do we even need ventilation in the first place? The answer is to support the health and wellbeing of indoor occupants by improving the quality of the air they breathe.

Increased Air Sealing and Deficient IAQ

As buildings become more airtight to save energy, an unintended consequence is an accumulation of internally generated contaminants that cause deficient indoor air quality (IAQ). Deficient IAQ is a serious problem in all buildings since it negatively impacts indoor occupants’ health, cognitive function, productivity and wellbeing.

Indoor air in residences can be quite unhealthy. In fact, the U.S. Environmental Protection Agency (EPA) states that:

• The average person receives 72% of their chemical exposure at home.[1]
• Indoor levels of pollutants may be two to five times – and occasionally more than 100 times – higher than outdoor levels.[2]
• High amounts of indoor air pollutants are of particular concern because most people spend about 90% of their time indoors.[3] 
• Indoor air pollution is a top-five environmental risk to public health.[4]

Deficient IAQ has many adverse effects. They include short-term health problems, such as allergies, headaches and asthma, as well as long-term ones, such as cancer, liver disease and kidney damage. Harvard and Berkeley Lab also determined that deficient IAQ can cause cognitive impairment. In one of their studies, they found that Carbon Dioxide (CO₂) can negatively impact thinking at levels most Americans ?are routinely exposed to indoors.[5]

Mechanical Ventilation is the Solution

The best way to enhance IAQ is via increased and balanced ventilation. As long as enough controlled fresh outdoor air is coming in and stale indoor air is exhausted out, a high-quality indoor environment will be achieved. The American Lung Association supports this notion and states that proper ventilation is essential for keeping the air fresh and healthy inside homes.[6]

Ventilation is so vital for ensuring acceptable IAQ for homes that the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) created Standard 62.2, which sets ventilation requirements for removing indoor air contaminants from homes. ASHRAE sets the ventilation rate at 7.5 CFM per person, plus 3 CFM per 100 square feet,[7] and Standard 62.2 has been adopted by, as well as influenced, local codes across the U.S.

Mechanical Ventilation Types: Exhaust, Supply, Balanced and Energy Recovery

As the U.S. Department of Energy (DOE) asserts, energy-efficient homes – both new and existing – require mechanical ventilation to maintain IAQ. Consequently, there are four mechanical whole-house ventilation systems to choose from: exhaust, supply, balanced and energy recovery.[8] Note that continuous “whole-house” ventilation systems were developed in the 1980s to meet the IAQ needs of the well-insulated homes of the time.[9]

Let’s now take a look at each mechanical ventilation type, which are also outlined in a Whole-House Ventilation factsheet compiled by the DOE. Here are the four options:

Exhaust Ventilation

System overview and benefits: 

• Exhaust ventilation systems work by depressurizing a structure. The system exhausts air from the house, thus causing a change in pressure that pulls in make-up from the outside through leaks in the building shell and intentional, passive vents. Exhaust ventilation is most appropriate for colder climates, since in warmer climates, depressurization can draw moist air into wall cavities where it may condense and cause moisture damage.[10]
• Exhaust ventilation systems are relatively simple and inexpensive to install. Typically, an exhaust ventilation system consists of a single fan connected to a centrally located, single exhaust point in the house. A better design is to connect the fan to ducts from several rooms, preferably rooms where pollutants are generated, such as bathrooms and kitchens.[11]
• Adjustable, passive vents through windows or walls can be installed in other rooms to introduce fresh air rather than rely on leaks in the building envelope. Passive vents may, however, require larger pressure differences than those induced by the ventilation fan to work properly.[12]

Concerns:

• One concern with exhaust ventilation systems is that, along with fresh air, they may draw in pollutants. These can include: radon and molds from a crawlspace, dust from an attic, fumes from an attached garage and flue gases from a fireplace or fossil-fuel-fired water heater or furnace. These pollutants are a particular concern when bath fans, range fans and clothes dryers (which also depressurize the home while they operate) are run when an exhaust ventilation system is also operating.[13]
• Exhaust ventilation systems can also contribute to higher heating and cooling costs compared with energy recovery ventilation systems because exhaust systems don’t temper or remove moisture from the make-up air before it enters the house.[14]

Supply Ventilation

System overview and benefits: 

• Supply ventilation systems use a fan to pressurize a structure, forcing outside air into the building while air leaks out of the building through holes in the shell, bath and range fan ducts, and intentional vents (if any exist).[15]
• Like exhaust ventilation systems, supply ventilation systems are relatively simple and inexpensive to install. A typical supply ventilation system has a fan and duct system that introduces fresh air into usually one, but preferably several, rooms that residents occupy most, such as bedrooms and the living room. This system may include adjustable window or wall vents in other rooms.[16]
• Supply ventilation systems allow better control of the air that enters the house compared to exhaust ventilation systems. By pressurizing the house, supply ventilation systems minimize outdoor pollutants in the living space and prevent back drafting of combustion gases from fireplaces and appliances. Supply ventilation also allows outdoor air introduced into the house to be filtered to remove pollen and dust or dehumidified to provide humidity control.[17]
• Supply ventilation systems work best in hot or mixed climates. Because they pressurize the house, these systems have the potential to cause moisture problems in cold climates. In winter, the supply ventilation system causes warm interior air to leak through random openings in the exterior wall and ceiling. If the interior air is humid enough, moisture may condense in the attic or cold outer parts of the exterior wall, resulting in mold, mildew and decay.[18]

Concerns:

• Like exhaust ventilation systems, supply ventilation systems don’t temper or remove moisture from the make-up air before it enters the house. Thus, they may contribute to higher heating and cooling costs compared with energy recovery ventilation systems.[19]
• Because air is introduced into the house at discrete locations, outdoor air may need to be mixed with indoor air before delivery to avoid cold air drafts in the winter. An in-line duct heater is another option, but increases operating costs.[20]

Balanced Ventilation

System overview and benefits: 

• Balanced ventilation systems, if properly designed and installed, neither pressurize nor depressurize a structure. Rather, they introduce and exhaust approximately equal quantities of fresh outside air and polluted inside air.[21]
• A balanced ventilation system usually has two fans and two duct systems. Fresh air supply and exhaust vents can be installed in every room, but a typical balanced ventilation system is designed to supply fresh air to bedrooms and living rooms where occupants spend the most time. It also exhausts air from rooms where moisture and pollutants are most often generated, such as the kitchen, bathrooms and the laundry room.[22]
• Some designs use a single-point exhaust, and because they directly supply outside air, balanced systems allow the use of filters to remove dust and pollen from outside air before introducing it into the house. Balanced ventilation systems are also appropriate for all climates.[23]

Concerns:

• Like both supply and exhaust systems, balanced ventilation systems don’t temper or remove moisture from the make-up air before it enters the house. Therefore, they may contribute to higher heating and cooling costs, unlike energy recovery ventilation systems. Similar to supply ventilation systems, outdoor air may need to be mixed with indoor air before delivery to avoid cold air drafts in the winter.[24]
• Because they require two duct and fan systems, balanced ventilation systems are usually more expensive to install and operate than supply or exhaust systems.[25]

Energy Recovery Ventilation

System overview and benefits:

• Energy recovery ventilation systems provide a controlled way of ventilating a home while minimizing energy loss. They 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.[26] 
• There are two types of energy recovery systems: Energy Recovery Ventilators (ERVs) and Heat Recovery Ventilators (HRVs). Both types include a heat exchanger, one or more fans to push air through the machine and controls. There are some small wall- or window-mounted models, but the majority are central, whole-house ventilation systems with their own duct system or shared ductwork.[27] 

• The main difference between an ERV and an HRV is the way the heat exchanger works. With an ERV, the heat exchanger transfers a certain amount of water vapor (latent) along with heat energy (sensible), while an HRV only transfers heat.[28]
• Because an ERV transfers some of the moisture from the exhaust air to the usually less humid incoming winter air, the humidity of the house air stays more constant. This also keeps the heat exchanger core warmer, minimizing problems with freezing.[29] 
• In the summer, an ERV may help to control humidity in the house by transferring some of the water vapor in the incoming air to the theoretically drier air that’s leaving the house. If you use an air conditioner, an ERV generally offers better humidity control than an HRV.[30]
• Most energy recovery ventilation systems can recover about 70-80% of the energy in the exhaust airstream and deliver that energy to the incoming air for conditioning purposes.[31]

Concerns:

• Some energy recovery ventilation systems can cost more to install than other ventilation systems. In general, simplicity is key to a cost-effective installation. To save on installation costs, many systems share existing ductwork. Complex systems are not only more expensive to install, but they are generally more maintenance-intensive and often consume more electric power.[32]

In Summary

Deficient IAQ is threatening the health of indoor occupants in every type of home and building, and the problem is worsening with growing structural airtightness. That’s the bad news. The good news is that we have a solution, mechanical ventilation, and that four different types exist: exhaust, supply, balanced and energy recovery. By implementing one of these systems, the results will be enhanced IAQ and improved occupant wellbeing.

For more information on the adverse effects of deficient IAQ and the benefits of mechanical ventilation, visit the Home Ventilating Institute at www.hvi.org.