The Effect of Ventilation on The Indoor Environment

Effective ventilation is a cornerstone of modern building science, influencing everything from occupant health to energy efficiency and structural integrity. For building scientists, understanding these principles is key to proper design, while for indoor air quality technicians, it's essential for accurate assessment and remediation. This guide will explore the various aspects of residential and commercial ventilation, from common exhaust systems to advanced heat recovery technologies.

Fundamentals of Residential and Commercial Ventilation

Ventilation is the process of intentionally introducing fresh air into a building and removing stale or contaminated air. This is a critical process for controlling moisture, odors, and indoor air pollutants. In both residential and commercial settings, ventilation falls into two main categories: local ventilation (focused on removing air from a specific area) and whole-building ventilation (designed to provide a constant exchange of air for the entire structure).

Common Exhaust Systems

Local exhaust systems, while simple in design, are crucial for managing moisture and contaminants at their source.

• Bathroom Exhaust Fans: These fans are designed to rapidly remove moisture and odors generated from showers and baths. If the fan duct is poorly or improperly routed—for example, if it terminates into an unconditioned attic space instead of outside—it can deposit a large volume of humid air directly into the building's structure. This can lead to condensation (the process of water vapor turning into liquid water) on cold surfaces, providing a food source for mold growth and causing decay of wooden framing and insulation.

• Kitchen Exhaust Fans: These systems are designed to remove cooking fumes, heat, grease, and moisture. A poorly sealed or improperly routed kitchen exhaust can similarly introduce these contaminants into wall cavities or attic spaces, leading to grease buildup, fire hazards, and moisture problems.

• Vented Dryers: The exhaust from a clothes dryer is filled with hot, humid air and lint. If the duct is improperly sealed or becomes detached, this air can escape into the home or a wall cavity. This introduces significant moisture and a potential fire hazard from the lint buildup.

The Hidden Dangers of HVAC Ductwork

Even in a well-engineered HVAC system, duct leaks are a common and significant problem. These leaks are often invisible but can have a profound impact on energy consumption, indoor air quality, and the building's overall performance.

• Supply Duct Leaks: Supply ducts (the part of the duct system that delivers conditioned air from the HVAC unit to the living space) are pressurized. During the cooling season, cool, conditioned air can leak out of gaps in the ductwork and into unconditioned spaces like attics or crawlspaces. When this cold air meets the warm, humid air of the attic, it can cause condensation on the outside of the ductwork, saturating surrounding materials like insulation. This leads to wasted energy and a risk of moisture damage. In the heating season, the same leaks waste valuable heated air, significantly increasing energy bills.

• Return Duct Leaks: Return ducts (the part of the duct system that brings air from the living space back to the HVAC unit to be reconditioned) are under a negative pressure, or vacuum. Leaks in these ducts can draw in unconditioned, and potentially contaminated, air from unsealed crawlspaces, dirty attics, or wall cavities. This not only forces the HVAC system to work harder, increasing power consumption, but also directly affects the indoor air quality for the occupants by introducing dust, mold spores, and other pollutants into the home's air supply.

The Critical Risk of Combustion Equipment

Combustion appliances, such as stoves, water heaters, and boilers, are essential parts of many buildings. However, if improperly designed or ventilated, they can become deadly. These appliances rely on combustion air (the air needed to fuel a flame) and a safe method to vent exhaust gases. An improperly designed venting system can fail to remove toxic gases like carbon monoxide (CO) (an odorless, colorless, and poisonous gas produced by the incomplete burning of carbon-based fuels). This can lead to the buildup of CO in the living space, resulting in serious health effects or even fatalities.

The Stack Effect: A Historical and Modern Perspective

The stack effect (the movement of air into and out of a building through unsealed openings, driven by differences in air density resulting from temperature differences between the indoors and outdoors) is a natural ventilation force that can be both beneficial and detrimental. Historically, this effect was a primary source of ventilation in many buildings. For example, during the Spanish Flu pandemic, early homes with oversized heating systems and open floor plans had improved natural airflow driven by the stack effect, which helped to improve indoor air quality.

Today, while the stack effect is still present, its uncontrolled nature can be a problem. In winter, the warm indoor air rises and escapes through high points in the building, like the attic. This creates a negative pressure at the lower levels, pulling in unconditioned, and potentially contaminated, air from the foundation and crawlspace.

Balancing Ventilation: The Modern Challenge

The challenge for modern construction is to find the right balance between energy efficiency and indoor air quality.

• Not Enough Ventilation: Modern houses are built with much tighter tolerances, creating a highly insulated, sealed structure. While this is excellent for energy efficiency, it also traps pollutants, moisture, and odors inside, leading to a decline in indoor air quality (IAQ) (the condition of the air inside a building, especially as it relates to the health and comfort of the occupants).

• Too Much Ventilation: Conversely, excessive ventilation can also be a problem. Forcing a large amount of unconditioned air into a building, either through leaks or poor design, places an enormous strain on the HVAC system. The building must constantly heat, cool, or dehumidify this new air, which drastically increases energy consumption and cost.

Advanced Ventilation Systems: ERVs and HRVs

To overcome the challenges of a tight building envelope, modern building science relies on advanced ventilation systems to provide controlled, balanced ventilation.

• HRV (Heat Recovery Ventilator): An HRV brings in fresh outdoor air and exhausts stale indoor air. Inside the unit, a heat exchanger (a device that transfers heat from one fluid to another without the fluids mixing. The fluid in this case being air) allows the heat from the outgoing air to pre-warm the incoming fresh air. This process recovers a significant amount of the heat that would have been wasted, making the system highly energy efficient in cold climates.

• ERV (Energy Recovery Ventilator): An ERV works similarly to an HRV but with an added benefit. It not only transfers heat but also transfers a portion of the moisture. In the cooling season, an ERV recovers the cool, dry indoor air's energy to pre-cool and dehumidify the warm, humid incoming air. This makes ERVs highly effective for both hot and cold climates, as they help manage both temperature and humidity levels.

ERVs and HRVs are the primary tools used by building scientists to maintain a healthy indoor environment in modern, tightly-sealed homes without sacrificing the energy efficiency gained from a well-sealed building envelope.

A bathroom exhaust fan can contribute to a home's ventilation. However, for it to meet ASHRAE 62.2 standards, it must be designed and operated as a continuous mechanical ventilation system.

ASHRAE 62.2 and Exhaust Fans

ASHRAE 62.2 allows for the use of a continuous exhaust fan to meet the required ventilation rate. However, a typical bathroom fan that's turned on and off with a light switch doesn't qualify. To meet the standard, the fan must be:

• Rated for Continuous Operation: The fan's motor must be designed to run 24/7 without overheating or failing prematurely.

• Wired for Continuous Operation: It needs to be wired to a switch or controller that allows it to run continuously at a low speed, providing the minimum ventilation rate calculated by the ASHRAE formula. A common setup uses a fan with a dual-speed motor or a smart switch that allows for both continuous low-speed operation and a boost function.

Limitations

While a continuous exhaust fan can satisfy the CFM requirement, it has limitations compared to a balanced ventilation system like an HRV or ERV:

• Unbalanced Pressure: An exhaust-only system creates negative pressure in the house, which can pull in unconditioned air from unwanted places like attics, crawlspaces, or gaps around windows and doors. This can introduce contaminants and moisture and also make the home less comfortable and efficient.

• No Heat Recovery: An exhaust fan simply pushes heated or cooled air out of the building, taking all the energy with it. It doesn't recover any of that energy, so it's far less efficient than an HRV or ERV in managing heating and cooling costs.

So, while a bathroom exhaust fan can be used for ASHRAE 62.2 compliance, it's typically used in situations where energy efficiency isn't the primary concern, or in conjunction with a passive fresh air intake to balance the pressure.

For commercial spaces, ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) provides ventilation standards in ASHRAE 62.1, "Ventilation and Acceptable Indoor Air Quality." Unlike the residential standard, which uses a simplified formula, commercial ventilation rates are more complex and depend heavily on the type of space and its intended use.

ASHRAE 62.1 Commercial Ventilation Rate Procedure

ASHRAE 62.1 uses a Ventilation Rate Procedure (VRP) that is based on both the number of occupants and the floor area of the space. This is a dual approach to ensure sufficient ventilation for both the people in the room and the building materials.

The formula is:

Outdoor Airflow Rate = (Ventilation Rate Per Person x Number of People) + (Ventilation Rate Per Area x Floor Area)

The specific values for "Ventilation Rate Per Person" and "Ventilation Rate Per Area" vary significantly depending on the application, as detailed in ASHRAE 62.1's tables. For example:

Other Important Considerations

• ASHRAE 170: Healthcare facilities, like hospitals and clinics, are governed by a separate standard, ASHRAE Standard 170, which specifies much higher ventilation rates and HEPA filtration requirements for critical areas like operating rooms.

• MERV Rating: ASHRAE 62.1 and other standards are increasingly focused on improving filtration. A minimum MERV (Minimum Efficiency Reporting Value) rating of 13 is often recommended for commercial HVAC systems that recirculate air.

• Air Changes Per Hour (ACH): While not the primary metric in ASHRAE 62.1, ACH is still a useful measure for understanding ventilation. The ACH in a commercial space will be much higher than in a residential home due to the higher occupancy and different use of the space. For example, a classroom with a high occupancy rate might see an ACH of 6 or more, while a low-occupancy office space may be lower.

• IAQ Procedure: In addition to the VRP, ASHRAE 62.1 also includes an Indoor Air Quality Procedure (IAQP), which allows for the use of air cleaning and filtration devices to achieve acceptable indoor air quality even with lower ventilation rates. This procedure is complex and requires engineers to perform detailed mass-balance analyses.