LEED AP Building Design + Construction (LEED AP BD+C)

Correct: In air balancing and HVAC fundamentals, a fan’s performance is dictated by its fan curve. As the external static pressure (resistance) increases beyond the manufacturer’s design specifications, the fan’s ability to move air decreases. Therefore, if the measured ESP is higher than the rated TESP, the system will fail to meet the required CFM, potentially compromising the business continuity of climate-sensitive operations.

Incorrect: High static pressure is generally an indicator of restriction or undersized ductwork rather than efficiency or seal integrity. Airflow volume is not constant; it is highly dependent on the pressure the fan must work against. Velocity pressure is a function of air velocity and density; if static pressure increases due to a restriction, the airflow velocity (and thus velocity pressure) actually decreases.

Takeaway: Total delivered airflow is inversely related to the external static pressure of the system based on the equipment’s specific fan performance curve.

Correct: In air balancing, the stability of the system depends on the components remaining in their commissioned state. Manual balancing dampers must be equipped with a positive locking mechanism, such as a locking quadrant. This hardware ensures that the force of the air (velocity pressure) and the mechanical vibrations from the AHU do not cause the damper blade to migrate or drift from its set position, which would otherwise lead to an unbalanced system and performance issues.

Incorrect: While multi-blade opposed dampers offer better control characteristics, they are not a requirement for basic residential balancing stability. Insulation is important for thermal efficiency and preventing condensation but does not address the mechanical stability of the airflow setting. Using stainless steel might prevent corrosion, but it does not solve the primary issue of the damper blade shifting due to air velocity and vibration if a locking mechanism is absent.

Takeaway: Manual balancing dampers must have positive locking hardware to prevent airflow settings from drifting due to system vibration and air pressure.

Correct: Pressure pan testing is a diagnostic procedure used in conjunction with a duct leakage tester (Duct Blaster). By covering a register with the pressure pan while the duct system is depressurized or pressurized, the technician can read the pressure at that specific branch. A high pressure reading indicates a significant leak in the ductwork leading to that specific terminal, allowing for targeted sealing rather than a ‘blind’ approach.

Incorrect: Measuring total external static pressure (TESP) is useful for determining the overall resistance the fan is working against, but it cannot identify the physical location of leaks. Systematic application of mastic is a corrective action, not a diagnostic one, and may miss hidden leaks in unconditioned spaces. Airflow velocity mapping at the return grille identifies intake issues or obstructions but does not isolate leakage points within the supply or return duct runs.

Takeaway: Pressure pan testing is the most effective diagnostic tool for isolating and locating specific leaks within a duct system by measuring the pressure at individual terminal outlets.

Correct: When measuring exhaust airflow with a capture hood, the hood itself can create significant backpressure, which restricts the flow being measured and leads to an inaccurate reading. Professional air balancing standards (NCI) require that the technician use a hood capable of backpressure compensation or apply specific manufacturer-provided compensation factors to ensure the calculated exhaust airflow reflects the actual system performance without the hood in place.

Incorrect: Using fan nameplate data is an estimation of potential performance, not a field measurement of actual airflow. Assuming a universal discharge coefficient (K-factor) is incorrect because these factors are specific to the grille manufacturer and model. Measuring plenum pressure is a diagnostic step for system pressure but does not provide a direct volumetric airflow calculation for a specific exhaust terminal.

Takeaway: Accurate exhaust airflow calculation requires accounting for the backpressure effects introduced by the measurement equipment itself to ensure reported values reflect true system operation.

Correct: When a filter’s pressure drop is too high, it increases the Total External Static Pressure (TESP) of the system. If the TESP exceeds the blower’s capacity, the volume of air (CFM) delivered decreases. In heating mode, lower airflow results in a higher temperature rise, which puts excessive thermal stress on the heat exchanger. Additionally, the blower motor must work harder against the resistance, often leading to premature failure.

Incorrect: Increasing resistance does not naturally increase velocity pressure; in fact, as total airflow drops, velocity pressure typically decreases. Return static pressure becomes more negative (not positive) as the blower struggles to pull air through a restrictive filter. While a motor moving less air might see a slight drop in amperage in some specific PSC motor scenarios, the overall system efficiency drops significantly due to longer run times and potential component damage, and it does not improve the SEER rating.

Takeaway: Excessive filter pressure drop reduces system airflow, leading to equipment overheating and potential mechanical failure of the blower and heat exchanger.

Correct: In air balancing, an increase in Total External Static Pressure (TESP) beyond design specifications indicates an increase in resistance to airflow. Internal duct liner delamination (where the insulation peels away from the duct wall) or physical crushing of the ductwork creates a restriction that forces the blower to work against higher pressure. The correct remediation is to identify the specific location of the restriction through visual inspection or pressure segment testing and replace the damaged ductwork to restore proper flow.

Incorrect: Air leakage at the plenum would typically cause a decrease in static pressure because air is escaping the system before it encounters the resistance of the duct runs. Oversized supply registers actually reduce static pressure by providing a larger opening for air to exit; replacing them with smaller units would further increase the TESP. A failing capacitor is an electrical component issue that affects the motor’s ability to turn the fan, but it does not cause the physical resistance (static pressure) within the ductwork to increase.

Takeaway: High static pressure in a previously balanced system is a primary indicator of physical airflow restrictions, such as collapsed liners or crushed ducts, rather than leakage or electrical failure.

Correct: In the context of air balancing and ventilation, CO2 is utilized as a surrogate or proxy for occupancy. Because humans exhale CO2 at a relatively predictable rate, monitoring the concentration of the gas allows a ventilation system to adjust the volume of outdoor air intake to match the needs of the people currently in the space, ensuring that bioeffluents and other occupancy-related contaminants are sufficiently diluted.

Incorrect: CO2 monitoring does not provide a direct measurement of airflow volume (CFM); that requires physical measurement tools like flow hoods or anemometers. It cannot be used to locate duct leaks, as leaks are identified through pressure testing or visual inspection, not gas concentration gradients. Furthermore, CO2 levels have no relationship with the thermal calibration or high-limit safety setpoints of a heat exchanger.

Takeaway: CO2 monitoring is used in demand-controlled ventilation as a proxy for occupancy to ensure outdoor air intake meets the physiological needs of the building’s occupants.

Correct: Measuring the pressure drop across individual components like the filter and coil allows the technician to isolate the specific location of the restriction. By comparing these measured values to the manufacturer’s specifications or the total External Static Pressure (ESP), the technician can determine if the high pressure is caused by a dirty or restrictive component rather than undersized ductwork, which is a fundamental step in NCI air balancing protocols.

Incorrect: Increasing the blower speed may temporarily increase airflow but ignores the root cause of high static pressure and can lead to premature motor failure or excessive noise. Duct leakage testing identifies air loss but does not diagnose the cause of high resistance or low total airflow at the equipment. Adding return grilles is a potential remediation step for high return-side pressure, but it is not a diagnostic action used to identify where the restriction currently exists.

Takeaway: Systematic isolation of component pressure drops is the essential first step in diagnosing the cause of high static pressure and low airflow in residential systems.

Correct: From an audit and technical perspective, air stratification is a failure of the air distribution system to achieve proper mixing. In heating, the primary risk is buoyancy; if the discharge velocity (throw) is too low, the warm air cannot penetrate the denser, cooler air at the floor level, regardless of the total CFM delivered. This results in a layer of warm air trapped at the ceiling and cooler air at the floor.

Incorrect: Return air systems (Option B) have a limited pull effect and do not significantly contribute to room air mixing or the breakdown of stratification layers. High static pressure from damper settings (Option C) may reduce total airflow but does not inherently cause stratification if the remaining air has sufficient velocity. A poorly placed thermostat (Option D) leads to short-cycling or incorrect run times, but it is not the physical mechanism that causes air to separate into thermal layers.

Takeaway: Effective air mixing and the prevention of stratification require supply air velocity sufficient to overcome the natural buoyancy of heated air.

Correct: ASHRAE 62.2 focuses on indoor air quality and mechanical ventilation. When an exhaust-only ventilation strategy is used, it creates a negative pressure within the building. If the home contains naturally aspirated combustion appliances, such as a standard water heater or fireplace, this depressurization can cause backdrafting, where toxic combustion byproducts like carbon monoxide are pulled into the living space instead of exhausting through the flue. Assessing and preventing excessive depressurization is a critical safety component of the standard.

Incorrect: Maintaining a specific positive pressure percentage is more common in commercial balancing (ASHRAE 62.1) and is not a core requirement of the residential 62.2 standard, which often allows for balanced or slightly negative pressures. Occupancy sensors are an energy-efficiency measure rather than a primary safety or ventilation rate compliance measure. While rigid ductwork is durable and can reduce leakage, the standard focuses on the delivered airflow at the terminal regardless of duct material, making pressure-related safety a higher priority than material selection.

Takeaway: Mechanical ventilation design must account for the potential impact on combustion appliance venting to prevent hazardous backdrafting conditions caused by house depressurization.