Make-up Air

How Exhaust Systems Create a Problem

All exhaust systems, from a small bathroom fan to a powerful commercial kitchen hood, work by pulling air out of a space and venting it to the outside. In a modern, tightly sealed home, this creates negative pressure. If a source of makeup air is not provided, the home will struggle to "breathe." This leads to a series of compounding problems:

1. Reduced Airflow: When an exhaust fan (e.g., a kitchen hood, dryer, or bathroom fan) is trying to push air out of a building under negative pressure, it works against itself. The fan's efficiency plummets, and it fails to remove the moisture, odors, or pollutants it was designed to.

2. Backdrafting of Combustion Appliances: This is the most serious and life-threatening issue. The negative pressure created by the exhaust systems can pull air from the flues of combustion appliances (furnaces, water heaters, fireplaces, etc.). Instead of venting to the outside, dangerous gases like carbon monoxide (CO) are pulled back into the home.

3. Infiltration of Pollutants: The negative pressure can also pull unfiltered air from less-than-clean spaces, such as attics, crawl spaces, and wall cavities. This air can contain mold spores, dust, insulation particles, and other contaminants that compromise indoor air quality.

Common Ventilation Systems and Makeup Air

• Dryers: A clothes dryer, especially a gas model, exhausts a significant amount of air (100-200 CFM). To prevent backdrafting and inefficient drying, a dedicated makeup air duct is often the most effective solution in modern homes. This can be a passive damper that opens when the dryer turns on or a fan-powered system that automatically supplies fresh air.

• Kitchen Range Hoods: Powerful kitchen hoods can exhaust hundreds of cubic feet of air per minute. Most residential hoods with a capacity over 400 CFM require a dedicated makeup air system to comply with building codes. This system ensures the hood can vent properly and prevents the depressurization of the home.

• Bathroom Exhaust Fans: While a typical bathroom fan only exhausts 50-100 CFM, in a very tight home, even this can cause pressure imbalances. In these cases, makeup air is often provided through a gap beneath the bathroom door, allowing air to be drawn from the rest of the conditioned space, preventing the fan from straining and ensuring the humid air is properly vented.

The Solution: Balanced Ventilation

The ultimate goal in building science is balanced ventilation. This means that a home's mechanical ventilation systems are designed to exhaust the same amount of air that is being supplied, preventing any significant pressure imbalances. Whether through a combination of spot exhaust fans with dedicated makeup air ducts, or a whole-house system like a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV), the principle remains the same: for every cubic foot of air you exhaust, you must provide a clean, controlled source of fresh air to take its place. This is crucial for both energy efficiency and the health and safety of the building's occupants.

The fundamental principles for effective and compliant make-up air (MUA) system design in building science applications, particularly in relation to high-volume exhaust systems such as commercial or high-capacity residential range hoods.

Core Design Principles for Make-up Air

The following tenets prioritize occupant comfort, building physics integrity, and system efficiency:

• Occupant Thermal Comfort and Draft Mitigation: Airflow, including MUA, should be introduced such that the velocity at occupied zones is minimized to prevent drafts and thermal discomfort. Specifically, the air velocity at occupied zones should be kept substantially below a threshold that causes perceptible air movement.

• Surface Condensation and Material Integrity: MUA should not be directed onto building surfaces or materials. Direct impingement of MUA can induce localized thermal gradients, potentially leading to surface condensation (dew point transgression) and subsequent moisture-related issues, including mold growth or material degradation.

• System Efficiency and Airflow Circuitry: The MUA intake location should be as proximal as practically possible to the point of exhaust (e.g., the hood) to create a short-circuit airflow path. This minimizes the impact of MUA on the building's overall pressurization state and reduces the opportunity for the MUA to disrupt other critical ventilation or air distribution systems.

• Acoustic Performance and Low Velocity: MUA discharge velocity must be maintained at a level that ensures low noise generation. A typical maximum benchmark for discharge velocity at the diffuser face or grille is often specified to be below 250 feet per minute (fpm) to achieve acoustically acceptable operation.

• Air Conditioning and Moisture Management: MUA must be conditioned (temperature and humidity adjusted) to be thermally and hygroscopically compatible with the receiving interior environment. Introducing unconditioned air—such as hot, humid air into a cool, dry space, or cold air into a warm space—poses a significant risk of localized interstitial or surface condensation.

System Filtration and Code Requirements

• Air Quality and Filtration: All MUA must pass through an appropriate filtration medium to remove particulate matter, ensuring the MUA meets or exceeds the indoor air quality (IAQ) requirements of the conditioned space.

• Mandatory Code Compliance: Building codes typically mandate the inclusion of MUA systems when a single exhaust device's capacity exceeds a specified volumetric flow rate. The common threshold requiring MUA is an exhaust rate greater than 400 cubic feet per minute (CFM), primarily to prevent negative building pressurization which can lead to back-drafting of combustion appliances.

Practical Constraints and System Integration

• Dehumidification Limitations: Achieving acceptable dehumidification of MUA in a single-pass configuration is often impractical due to the substantial energy penalty (reheat required after cooling) and the typically high latent loads involved. Heating, however, is often a necessary and plausible conditioning strategy.

• Exhaust and Appliance Airflow Protection: The MUA system must be designed to avoid disruption of the exhaust hood's capture and containment efficiency. Furthermore, MUA should not create adverse flow conditions over the cooktop surface, as this manipulation of local air currents can detrimentally affect cooking appliance performance (e.g., flame stability, heat transfer efficiency).

• Deviation from Principles: While these principles are fundamental to robust MUA design, specific site conditions or operational constraints may occasionally necessitate minor, carefully calculated deviations from these guidelines. Any such deviation requires a thorough engineering analysis to mitigate potential adverse consequences.