Best practices for infiltration and building pressurization

Design assumptions regarding infiltration and mechanical building pressurization have long played a role in air handling system design. Infiltration, a fickle heating and cooling load, must be accounted for and a mechanical building pressurization strategy must be included to combat infiltration.

Given the demands of energy code compliance and the increasing popularity of dedicated outside air system and dry coil system types, infiltration and building pressurization design details and assumptions remain as important as ever in the design of a successful air handling system.

Infiltration is a natural phenomenon that occurs when the pressure on the exterior of a building is higher than the interior, causing outside air leaks through the building envelope from outside to inside. This can lead to increased peak heating and cooling loads and potential indoor air quality concerns because the infiltrating air is unfiltered and unconditioned.

This differential pressure is created by three elements: stack effect, wind and mechanical building pressurization. Each of these three elements vary from building to building and even moment to moment, resulting in a heating and cooling variable that is challenging to calculate, measure and control.

One may think that if a building is mechanically pressurized, infiltration will not occur; however, that is not true for the following reasons:

• Pressure within a building is nonuniform from room to room and floor to floor.
• Wind pressures on the exterior of a building are highly nonuniform.
• In most applications, a mechanical pressurization strategy cannot and should not attempt to offset high wind pressures experienced by windward exterior walls on windy days.

A calculation procedure to quantify infiltration loads can be found in the ASHRAE Handbook – Fundamentals. The procedure was used to generate the data presented here.

Stack effect

A common culprit in cold lobbies of high-rise buildings throughout the world, stack effect is the industry description for when a building becomes a large, unintended chimney. The process is driven by interior heating, ventilation and air conditioning systems altering the density of air within the building compared to the outside air.

This density differential creates an upward chimney effect in the winter as cool, dense outdoor air enters at lower floors, rises through the building and exits at the upper levels. In the summer, the process is reversed. As a result, lower floors see the largest wintertime infiltration loads due to stack effect and upper floors see the largest summertime infiltration loads due to stack effect.

The magnitude of stack effect is positively correlated with building height, density differential between the interior and exterior air and ease of air travel between floors. Compartmentalization is the designer’s best weapon to combat stack effect. Measures including well-sealed shafts, floor openings and stairwell doors play a major role in preventing vertical air travel through the building, as well described by in ASHRAE documentation.

In addition, multifloor HVAC systems can be designed to prevent unintended air travel between floors by adding airflow measurement and motorized dampers to the return air system on each floor (see Figure 2).

Wind

Wind is perhaps the most easily understood contributor to infiltration, but can be the most difficult to quantify and predict. The pressure created is a function of wind speed and direction with respect to each portion of the building’s exterior envelope.

However, the pressure created will differ significantly even for portions of the same façade due to the dynamic flow of air around the building. Friction from the wind traveling across the surface of the earth results in the formation of a boundary layer leading to higher wind speeds for upper levels of buildings.

Wind is also a highly localized phenomenon. Dense urban centers will have higher boundary layers and have the potential for wind-tunneling between buildings. Exposed sites will have lower boundary layers and more predictable wind speeds and directions.

Weather data files typically are based on measurements at an airport featuring relatively open terrain at an elevation of 33 feet. The designer should determine if those conditions apply to the local site. If not, wind speed adjustment procedures are detailed in the ASHRAE Handbook. Where a higher-level of analysis is needed or desired, wind tunneling, computational fluid dynamics modeling or the use of a portable weather station can be applied.

Mechanical pressurization

Mechanical pressurization is the only contributor to infiltration loads that designers have dynamic control over. It occurs when the building’s air handling system is controlled in a manner that intentionally introduces a higher quantity of outside air into a building than the quantity of air exhausted and relieved. Mechanical pressurization is one of the more debated and written about topics in the HVAC industry — most agree that buildings should be positively pressurized in the summer to prevent the infiltration of hot, humid air into the building and exterior wall assembly.

There is not a simple consensus regarding building pressurization in the winter. In some cases, positive pressurization to prevent the infiltration of cold air is desirable. In others, the need to prevent moisture migration through the exterior wall assembly from inside to outside results in a neutral or slightly negative wintertime pressurization strategy.

This topic is well covered in ASHRAE Humidity Control Design Guide for Commercial and Institutional Buildings. The correct solution is unique to each project and should be determined in coordination with an expert on building envelope technology. Factors to be considered are: exterior wall construction, vapor barrier status, outdoor temperature conditions, indoor temperature and relative humidity conditions, outdoor air quality, sensitivity of building occupants and building height.

The changeover from positive pressurization to neutral/negative pressurization can occur when the outside air dewpoint falls below the indoor dewpoint set point.

The manner of building pressure control is also a subject of frequent debate. Four methods are typical in the industry:

• Manually balanced — no dynamic control.
• Fan speed tracking.
• DP-based control.
• Airflow offset based control.

In new buildings, DP-based control and airflow offset based control are the most common. In DP-based control, the quantity of relief air removed from the building is varied to maintain a fixed DP setpoint between the interior and exterior of the building. The setpoint is typically in the range of 0.001 to 0.05 inches w.c.

When employing this strategy, it is important to specify a sensor that is highly accurate at low DPs. Typical duct DP sensors will not be sufficient. The placement of the outdoor pressure port should negate any wind effects. Avoid vertical exterior walls where possible. A static outside air probe mounted 15 feet above the roof of the building will provide the best results. Consideration should be given to the design and placement of the DP sensor and its associated tubing.

Airflow offset-based control is popular due to its simplicity and ability to maintain generally positive pressurization. The air handling system is controlled to maintain a fixed airflow differential between the outside air intake and relief/exhaust air. The strategy also eliminates what some perceive as an overreliance on a single building DP sensor that may be correctly designed, placed, installed and calibrated. Airflow offset based control typically requires more airflow measuring stations than DP control, increasing system cost.

Equally important to the pressurization strategy is that the return air system design does not create highly negative pressure in ceiling plenums. In ducted return air systems, return air ductwork must be well sealed. Plenum return systems must be carefully designed to avoid negative pressure in exterior plenum spaces. Alternatively, through coordination with the architect, return plenums can be terminated a few feet from exterior walls to avoid the problem altogether.

Summation of pressures

Whether airflow travels into or out of leaks in a building’s envelope is determined by the summation of the stack, wind and mechanical pressurization effects (see Figure 3). Table 1 contains data for average winter conditions in a standard nine-story office building. Note that the wind pressure is highest for the windward wall and higher for upper floors than lower floors. Note that the pressure due to stack effect is highest for the lowest floor.

Table 2: Note in this sample summer infiltration and building pressurization characteristics for a nine-story office building that the values represent average conditions and not peak conditions. Courtesy: SmithGroup[/caption]

The quantity of infiltration and its inverse, exfiltration, is determined by the magnitude of the pressure difference across a wall assembly, the leakiness of the envelope and the amount of exterior wall/roof square footage. A popular practice in the industry has been to calculate infiltration based on an arbitrary air change rate; however, calculating infiltration based on cubic feet per minute per square foot of exterior wall area is more in line with the physics of the situation.

In reality, air leakage mostly occurs at building joints, window frames and where grout has worn away, making cubic feet per minute per square foot of exterior wall area an imperfect but correlated metric.

Determining the leakiness of a building envelope has been a challenge for decades. Fortunately, modern codes and standards require minimum envelope air leakage performance. ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings Sections 5.8 and 5.9 contain requirements for minimum envelope tightness when tested at a pressure differential of 0.3 inches w.c. Building pressure differentials during typical operation will be much lower and the quantity of infiltration to expect can be roughly calculated using the exponential relationship between pressure and flow.

For existing buildings, ASHRAE has published typical infiltration rates as a function of DP for tight, average and loose constructions. The rates were determined through pressure tests of existing buildings. The assumption made for envelope tightness is by far the most important variable in an infiltration calculation. Figure 4 depicts the infiltration airflow for a wall subject to a -0.025 inches w.c. pressure differential for tight, average and loose categories.

Controlling building pressure

Careful design of the building pressurization control system will play a significant role in the operational efficiency and maintainability of a new building A successful mechanical pressurization strategy for most applications is to provide only sufficient pressurization to offset stack effect on the worst-case floor and no more. Any additional pressurization results in little to no reduction in infiltration and increases exfiltration.

The increase in exfiltration beyond what is required represents a missed opportunity for relief air energy recovery. It is important to understand that in positively pressurized building, a significant quantity of air will exfiltrate through the building envelope. The quantity of exfiltrating air is a function of the exterior envelope surface area, DP and envelope tightness.

The amount of air required to exfiltrate from the building to positively pressurize the worst-case floor can be as high as 50% of the design ventilation airflow rate. In systems designed with relief/exhaust air energy recovery, this can lead to significant flow imbalances between the relief/exhaust air and outside air sections of the energy recovery device and a reduction in overall energy recovery effectiveness, potentially resulting in capacity issues if the system/coils are designed to account for full energy recovery.

The location of the indoor pressure port(s) should be an area that represents typical building pressure. Avoid intentionally pressurized spaces, ceiling plenums, exterior zones, locations near building entrances or exits and locations that may be influenced by diffuser jets. Consider a cold winter day and refer to Table 1. Note that stack effect causes an internal pressure difference between the first and ninth floors of 0.104 inches w.c. w.c.

If the indoor pressure port were located on the first floor, a DP setpoint of 0.053 inches w.c. would be required to offset stack effect and keep the building mostly positive. If the indoor pressure port were located on the ninth floor, any DP setpoint below 0.104 inches w.c. would result in negative pressurization of lower floors.

In the summer, the reverse occurs (see Table 2). If the indoor pressure port were located on the first floor, a DP setpoint below 0.01 inches w.c. would result in negative pressurization of the top floors. In taller buildings where summer humidity control is especially important, multiple indoor pressure ports may be warranted — one port on the first or second floor and one port on the top floor. During summer months, the reading from the pressure port on the top floor would be used to control the system and during winter months the reading from the pressure port on the first or second floor would be used to control the system. This strategy eliminates DP setpoint guesswork and allows for a reduction in the setpoint because the system is actively controlling to the worst-case floor.

For tall buildings with air systems serving multiple floors, a hybrid, cascading approach between airflow offset and DP can be used. In this approach, supply/outside air to each floor is measured and a modulating damper in the return/relief air system maintains an active airflow offset setpoint. The active airflow offset setpoint for all floors can be increased or decreased in unison based on the reading of the building DP sensor.

This approach allows for active DP control and reduces the potential for the air handling system to create unintended pressure differentials between floors that may exacerbate stack effect. High- and low-limit airflow offsets can also be programmed to allow the building to stay generally positive should an issue occur with the building DP sensors.

Accounting for building pressurization and infiltration in the design of air handling systems is a challenging task. An understanding of the forces that drive them and the use of the latent infiltration design day method — as highlighted in the case study — can lead to significantly improved component sizing and system performance.

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