Optimising air tightness in air-handling units

One of the key performance factors for air-handling units (AHUs) is their air tightness. While it may seem like a minor aspect, the air tightness of an AHU significantly impacts energy efficiency, Indoor Air Quality (IAQ) and the overall performance of the HVAC system. In this article, we will explore the definition of air tightness, its role in AHU performance, its impact on energy efficiency and IAQ, and how to measure and ensure proper air tightness.

Air tightness in AHUs

Air tightness refers to the ability of an AHU to prevent air leakage through its casing. It measures how well the unit can retain the air it is handling without unintended loss or infiltration. In an ideal

scenario, an AHU should only allow air to enter or exit through the intended openings, such as supply and return ducts, and not through gaps or seams in the unit’s casing.

The air tightness of an AHU is quantified by the amount of air leakage per unit surface area under a specific pressure differential. This is usually expressed in terms of litres per second per square metre (l/s/m²) at a given pressure. While 1,000 Pascals (Pa) is a recommended value for testing comfort application AHUs, special applications, such as hospitals or pharma/drug manufacturing require units to be tested at higher pressures, such as 2,000 or 2,500 Pa.

The role of air tightness in AHU performance

Air tightness plays a fundamental role in the overall performance of an AHU. A well-sealed AHU ensures that the air flows as intended, without any loss or contamination. Some key roles that air tightness plays in AHU performance are:

i) Maintaining airflow balance: Air tightness helps maintain the balance between supply and return airflows in the HVAC system. If an AHU is not airtight, it can disrupt this balance, causing pressure imbalances, which can affect occupant comfort.

ii) Preventing air leakage: Air leakage can lead to the mixing of outdoor and indoor air in unintended ways. This can compromise the temperature and humidity levels maintained by the HVAC system.

iii) Reducing contaminant infiltration: An airtight AHU prevents unfiltered air from entering the system. This is crucial for maintaining good IAQ by ensuring that all air passes through the necessary filters before entering the building.

iv) Improving system longevity: By preventing air leakage, an airtight AHU reduces the wear and tear on system components, such as fans and filters, which can extend the lifespan of the equipment.

1. Impact of air tightness on energy efficiency

Air tightness has a direct impact on the energy efficiency of HVAC systems. Here’s how:

i) Reducing Energy Loss: When air leaks from an AHU, the system needs to work harder to maintain the desired indoor conditions. The fans must run longer and at higher speeds to compensate for the lost air, leading to increased energy consumption. Furthermore, research indicates that for every one per cent increase in air leakage, there is a corresponding one per cent increase in energy consumption. This is because the system must condition additional air to compensate for the leakage, leading to higher operational costs.

In addition, the U.S. National Renewable Energy Laboratory (NREL) notes that air leakage in AHUs causes fans, pumps and chillers to work harder, increasing energy consumption. Leaks can also result in heat loss, further elevating energy usage.

ii) Minimising heating and cooling load: Leakage of conditioned air results in an increased heating or cooling load. For example, if warm air escapes in winter or cool air escapes in summer, the HVAC system must operate longer to maintain indoor temperatures. For instance, reducing cabinet air leakage from five per cent to 1.5 percent of system airflow can result in an overall reduction in energy use for space conditioning by about five per cent, and a similar reduction in peak heating and cooling loads. Ensuring air tightness minimises this load, reducing energy usage and utility bills.

Additionally, a study by the Florida Solar Energy Center measured air handler leakage in 69 Florida houses. The findings showed that air leakage from air handler/furnace cabinets averaged 70 cubic feet per minute (cfm) at estimated operating pressure, representing 24% – 76% of total system air leakage.

iii) Optimising fan performance: An airtight AHU ensures that fans operate at their intended capacity, enhancing overall system efficiency. Air leakage can cause pressure drops, forcing fans to work harder to maintain desired airflow, which leads to increased energy consumption. Therefore, maintaining AHU air tightness is crucial for optimising fan performance and reducing energy usage.

iv) Lowering carbon emissions: By reducing energy consumption, airtight AHUs contribute to lower carbon emissions, making the building more environmentally friendly. This is particularly important for buildings aiming to achieve green certifications.

2. Impact of air tightness on IAQ

Air tightness is critical for maintaining good IAQ. Here’s how it influences IAQ:

i) Preventing contaminant ingress: Air leakage can allow unfiltered outdoor air to enter the HVAC system, by passing the filtration process. This can introduce pollutants such as dust, pollen and microbial contaminants into the indoor environment, affecting occupant health. Poorly sealed AHUs can lead to increased pollutant ingress, compromising air quality in sensitive environments.

ii) Ensuring proper filtration: An airtight AHU ensures that air entering the unit upstream of filters passes through the filters, removing particulates and allergens. This helps maintain a healthier indoor environment. Effective filtration combined with airtight AHUs can reduce airborne particles, significantly improving IAQ.

iii) Controlling humidity levels: Air leakage can cause unwanted humidity fluctuations in the building. For instance, in humid climates, air leaks can introduce moisture into the AHU, leading to condensation and potential mould growth. Consistent humidity control in airtight AHUs reduces the risk of microbial growth, creating safer environments for occupants.

iv) Reducing odours and fumes: Leaks can allow odours, fumes and other undesirable gases to enter the HVAC system. This can be particularly problematic in buildings with specific air quality requirements, such as hospitals or laboratories. Airtight AHUs in hospitals significantly reduce the infiltration of external odours and pollutants, enhancing patient recovery environments.

3. How to measure air tightness in AHUs

Measuring the air tightness of an AHU is essential to ensure optimal performance. The measurement process typically involves the following steps:

i) Preparation: The AHU is prepared by sealing all intended openings, such as duct connections, to ensure that the only potential points of air leakage are through the casing.

ii) Pressurisation: A pressure differential is created across the AHU casing, usually using a fan or blower to pressurise the unit to a specific level, such as 1,000 Pa or higher, depending on the application type.

iii) Leakage measurement: The amount of air required to maintain the pressure differential is measured. This value represents the air leakage rate.

iv) Calculating air leakage rate: The leakage rate is calculated by dividing the measured air leakage by the surface area of the AHU casing. The result is expressed in litres per second per square metre (l/s/m²).

4. How tight is good air-tight AHU?

A good air-tight AHU typically achieves a leakage rate of less than 1.0 l/s/m² at 1,000 Pa, according to AHRI Standard 1350. For higher-performance units, the leakage rate can be even lower, with some units achieving rates as low as 0.12 l/s/m² at 1,000 Pa. Slightly higher leakage rates are allowed for greater test pressures such as for 2500 Pa, it is 0.22 l/s/m².

The table, below, summarises the CL1 rating (highest class) for various test pressures as per AHRI 1350 standard.

5. Calculation of air leakage rate for any test pressure value

To calculate the allowed leakage rate for different classes at any test pressure values, the formula used is:

Where:

CL = Casing Air Leakage Rate, L/s/10m2

CLm = Measured leakage, L/s/10 m2at Pm

Pm = Absolute value of test differential pressure, Pa

Pr = Reference pressure, 250 Pa

The rating class is determined from the AHRI 1350 rating chart, shown here.

6. Compliance verification

AHRI 1350 Standard The measured air leakage rate is compared against standard AHRI 1350. The standards provide acceptable limits for air leakage rates based on the application type and size of the AHU.

The AHRI 1350 Standard is a globally recognised test standard for mechanical performance testing of AHUs. The standard focuses on the performance criteria listed below:

a. Casing air leakage

b. Casing strength

c. Thermal transmittance

d. Thermal bridging

e. Filter bypass leakage (being added currently)

The standard specifies allowable leakage rates for AHUs based on their design and application categories at varying pressure values starting from 1,000 Pa to 2,500 Pa. The standard ensures that

AHUs meet stringent air leakage requirements to maintain system efficiency and performance. The tests are conducted under standardised conditions to ensure consistent and accurate results.

7. Conclusion

Air tightness is a critical factor in the performance, energy efficiency and IAQ-enhancing capability of AHUs. By minimising air leakage, building operators can reduce energy consumption, improve occupant comfort and maintain a healthier indoor environment. Measuring and ensuring proper air tightness for AHUs per AHRI standard 1350 is recommended.