providing insights for today’s hvac engineers newsletter system designer
managing the ins and outs of… Commercial Building Pressurization
from the editor… the pressure inside is greater than the Providing energy-efficient thermal Why Pressure Matters pressure outside. comfort without adversely affecting in Commercial Buildings indoor air quality or violating codes Untreated outdoor air leaks into— During the summer, exfiltration of cool, poses a considerable design challenge. infiltrates—the building when indoor dehumidified indoor air benefits the Yet something as subtle as air pressure is less than the pressure building by keeping the envelope dry. movement through the building outside. Control strategies typically But excessively positive pressure envelope can determine whether an strive to limit or eliminate infiltration as makes opening and closing doors otherwise well-designed HVAC system a means of minimizing HVAC loads and difficult and creates noisy high-velocity performs effectively. related operating costs. Infiltration isn’t airflow around doors and windows. It always bad, however. During the can also wreak havoc with temperature This EN reviews the importance of heating season, for example, a small control by impeding supply airflow into controlling building pressure. It amount of dry outdoor air leaking into occupied spaces. identifies the effects of indoor–outdoor the building envelope discourages air pressures on building performance, moisture from condensing there. During the winter, even slightly positive and then evaluates two common pressure forces moist indoor air into methods for directly controlling But excessively negative pressure the building envelope. Moisture may pressure in commercial buildings. causes problems. Uncomfortable drafts and stratification interfere with temperature control and may Ducted or Plenum Return? encourage odor migration. Outward- Ducting return air from a building’s occupied swinging doors become difficult to spaces back to the air handler increases the initial open, and inward-swinging doors fail cost of the system, so why do it? Is it to meet to reclose, compromising security. local codes? Sometimes. Is it to facilitate return- path cleaning? Maybe. Is it to avoid moisture problems in the ceiling plenum? Probably not. To Any amount of infiltration during the understand why, consider the following cooling season can raise the dew point example… within the building envelope, which increases the likelihood of microbial Suppose that the pressure in the return air plenum must be approximately 0.03 in. wg less than the growth and structural deterioration. pressure in the occupied space to overcome the Infiltration of warm, moist air also return-grille pressure drop. If building pressure is affects occupied spaces by increasing controlled to 0.05 in. wg, then the plenum latent loads. pressure will be +0.02 in. wg with respect to outdoors. This slight difference between the indoor and outdoor pressures will induce very Conditioned indoor air leaks out of— little infiltration. ■ exfiltrates from—the building when
© 2002 American Standard Inc. All rights reserved Volume 31, No. 2 ■ condense on cold surfaces inside walls, hastening structural deterioration. Semantics of Building Pressure Control True or False: “Relief” and “exhaust” intake airflow and maintain proper building Ideally, the net pressure inside the are interchangeable descriptors for air pressure. building relative to outside should range removed from a building. from slightly negative or neutral during Intake airflow describes the rate at which the cold weather (minimizing exfiltration) to Although it’s true that both relief air and air handler brings air into the building. Local exhaust air leave the building, their purposes codes or industry standards require a slightly positive during warm weather (and definitions) differ. minimum amount of intake airflow for proper (minimizing infiltration). Excessive ventilation and to dilute and remove general building pressure, whether negative or Exhaust airflow—which may be central or contaminants. Additional outdoor air positive, should be avoided. local, constant or variable—carries is brought into the building during economizer contaminants from the building. Local codes operation to provide “free” cooling. or industry standards define how much exhaust air must be removed from specific In the absence of infiltration and exfiltration, types of spaces (rest rooms, for example), negative building pressure results when Forces that regardless of pressure-related concerns or exhaust-plus-relief airflow exceeds intake Affect Pressure operating mode. airflow. Conversely, positive building pressure Preventing extreme building pressures results when exhaust-plus-relief airflow is less ■ is much easier said than done. In most Relief airflow removes air from the building than intake airflow. (again, either centrally or locally) to balance structures, the indoor–outdoor pressure difference results directly from the combined effect of weather, wind, and operation of the mechanical ventilation system. barometric pressure, temperature, and When indoor air is warmer than outdoor humidity ratio. air, the less dense column of air inside Weather. Like a column of water in a the building results in a net negative pipe, the weight of a column of air Temperature-related differences in pressure below the neutral pressure results in a “head” pressure that indoor and outdoor air density create level (NPL) and a corresponding net increases from the top of the column to differences in pressure that can affect positive pressure above it. Because all the bottom. Described as hydrostatic infiltration, exfiltration, and the direction building envelopes contain unavoidable pressure, but more commonly known of air movement within shafts and cracks and openings, this pressure as “stack pressure,” the weight of the stairwells (Figure 1). difference induces outdoor air to enter air column is affected by local the lower floors and indoor air to leave the upper floors. These leakage characteristics also encourage upward airflow—normal stack effect—within Figure 1. Stack effect and building pressure shafts and stairwells.
When indoor air is cooler than outdoor air, the column of air inside the building is more dense. The result is a net negative pressure at the top of the building and a corresponding net positive pressure at the bottom. Unless building pressure is controlled, outdoor air will infiltrate the upper floors while indoor air exfiltrates from the lower levels. The pressure difference also induces downward airflow in stairwells and shafts—reverse stack effect.
■ 2 Trane Engineers Newsletter — Vol. 31, No. 2 The 2001 ASHRAE Handbook– throughout the building envelope, the Because it’s impossible to control Fundamentals provides an equation to neutral pressure level exists at the weather, stack-effect pressure quantify the stack-effect pressure mid-height (HNPL = 25 ft). differences are best addressed using difference:1 vertical compartmentalization to
On a hot day (To = 95°F = 555°R, reduce the height of the indoor air T – T ⎛⎞o i ρo = 0.0715 lbm /ft³), the difference in column. (See “What’s the Right Δps = C1 ⋅⋅ρo ⎜⎟------⋅⋅gH()NPL – H ⎝⎠Ti indoor–outdoor air densities produces Setpoint for Building Pressure?,” p.6.) a +0.013 in. wg pressure difference at Vestibule-type entries and revolving where, the ground floor. Without building doors (which can accommodate a = difference between indoor and outdoor Δps pressure control, conditioned indoor air high pressure difference without stack pressures, in. wg exfiltrates from the lower floors compromising door operation) will C1 = conversion factor, (H = 0 ft), keeping that part of the minimize stack-induced air currents at 0.00598 in. wg · ft · sec²/lbm envelope dry. A less desirable condition entrances, elevators, and stairwells. ρo = outdoor air density at outdoor air temperature, lbm/ft³ exists at the top floor (H = 50 ft), To reduce air currents at the building g = gravitational constant, 32.2 ft/sec² however, where the –0.013 in. wg envelope, use a well-designed, well- H = height above reference plane, ft pressure difference encourages entry constructed air barrier.
HNPL = height of neutral-pressure plane above of hot (possibly humid) outdoor air. reference plane, ft Note: A building actually consists of Ti = indoor air temperature, °R On a cold day (To = 0°F = 460°R, many interconnected “containers” To = outdoor air temperature, °R ρo = 0.0863 lbm /ft³), the density of with differing pressures. Unless strict the indoor air column produces net contaminant control is necessary The following example illustrates the pressures of –0.058 in. wg at the (hospitals, clean rooms), however, most magnitude of stack effect in a four- ground floor and +0.058 in. wg at the building-pressure control schemes treat story, air-conditioned building (H = 50 ft, top floor. This time, lack of building the building as one “equi-pressure
Ti = 75°F = 535°R). Assuming that cracks pressure control permits warm, moist container” to regulate envelope airflow and openings are evenly distributed indoor air to exfiltrate from the upper and permit proper door operation. floors of the building, while cold air infiltrates the lower floors. The Wind. Wind pressure “pushes” substantial pressure difference outdoor air into the windward side of 1 American Society of Heating, Refrigerating, created by wintertime stack effect the building and “pulls” indoor air from and Air-Conditioning Engineers, Inc., 2001, ASHRAE Handbook: Fundamentals (Atlanta, GA: may make it difficult to open outward- the leeward side (Figure 2). The ASHRAE), 26.5–Equation 17. swinging doors at building entrances. differential pressure exerted on building
Siting Pressure Sensors Figure 2. Wind and building pressure Regardless of whether the relief system uses a locations that can be influenced by wind-induced return fan or a relief fan, direct control of pressure fluctuations. building pressure requires a differential pressure sensor to monitor the indoor–outdoor pressure Outdoor sensor. Again, placement of outdoor difference. sensors typically follows one of two common practices. Many designers place the outdoor Indoor sensor. Two schools of thought exist pressure sensor on the roof-mounted air handler. regarding placement of the indoor sensor. Some Others use multiple sensors (one at each corner designers place it near the door, where over-/ of the building, at least 15 ft above the roof) and under-pressurization effects are most noticeable; average their signals to cancel the effect of wind others isolate the indoor sensor from the door to pressure. In any case, select sensors that will dampen the effect of rapid pressure changes minimize wind effect. caused by door operation. For applications in which the building is tall and In either location, the indoor pressure sensor compartmentalized for pressure, position an should include sufficient signal filtering to outdoor pressure sensor at the elevation of each minimize the effects of high-speed pressure compartment. ■ changes. It is also important to avoid perimeter
“providing insights for today’s HVAC system designer” 3 ■ surfaces as a result of wind velocity is effective wind speed (UH) is 25 mph between intake airflow and relief calculated as:2 and that the wind-surface-pressure airflow determines whether the net
coefficient (Cp) is 0.50 at the ground building pressure is positive or 2 2 ()U floor (0 ft), then the windward pressure negative. (See “Semantics of Building Δp = C ⋅⋅s C ⋅ρ ⋅------H w 2 p o 2 difference will be 0.023 in. wg on a hot Pressure Control,” p. 2.) Excess intake day and 0.028 in. wg on a cold day. airflow pressurizes the building by where, creating a net positive pressure = wind surface pressure relative to Δpw At the top floor (50 ft), where the wind- (Figure 3); excess relief airflow outdoor static pressure in undisturbed flow, in. wg surface-pressure coefficient may reach depressurizes the building by creating 0.80, the windward pressure difference a net negative pressure. Cp = wind-surface-pressure coefficient, dimensionless (0.5 to 0.8 for this rises to 0.037 in. wg on a hot day and example) 0.044 in. wg on a cold day. Depending on the system type, control
C2 = conversion factor, strategy, and operating schedule, intake 0.0129 in. wg · ft³/ lb m · mph² Arguably, the best way for designers and relief airflows may vary during ρo = outdoor air density at outdoor air to mitigate the effect of wind is by normal system operation. As examples, temperature, lb /ft³ m maintaining a very negative building the relief fan may operate intermittently = shelter factor (0.40 for this example) s pressure during the winter (deterring on demand, and intake (outdoor) airflow = effective wind speed, mph UH leeward exfiltration) and a very positive will modulate with airside economizer building pressure during the summer operation. Like stack effect, wind pressure (deterring windward infiltration). varies with outdoor air density and However, successful implementation building height; wind pressure also also requires the use of a vestibule to varies with the shelter provided by the prevent interference with door Controlling immediate landscape, including nearby operation, and an effective air barrier in Building Pressure the building envelope to prevent large trees and buildings. To illustrate the Depending on the season and the indoor–outdoor pressure differences magnitude of wind-effect pressure, let’s height of the structure, the preferred from creating undesirable airflows. revisit our example building at the hot building pressure may be positive or and cold weather conditions described negative. Meanwhile, the actual Mechanical ventilation system. previously. If we assume that the building pressure can be positive or Some fans force air into the building; negative due to the combined forces of others force it out. The balance wind, weather, and operation of the 2 Ibid., 26.5–6. mechanical ventilation system. (See “What’s the Right Setpoint for Building Pressure?,” p. 6.) Figure 3. Effect of fan operation on building pressure Establishing the preferred building pressure in the face of continuously changing conditions usually requires an automated control scheme. For VAV applications with airside economizers, designers typically modulate relief airflow to directly or indirectly maintain building pressure within an acceptable range.3
3 Noteworthy articles include “Comparing Economizer Relief Systems” by Steven Taylor, PE, (2000) ASHRAE Journal 42 no. 9: 33–42; and “The Impact of Airtightness on System Design” by Wagdy A.Y. Anis, AIA (2001) ASHRAE Journal 43 no. 12: 31–35.
■ 4 Trane Engineers Newsletter — Vol. 31, No. 2 Return fan with direct control. Alternatives for Return-Fan Control Figure 4 illustrates a typical VAV system ASHRAE Guideline 16P recommends using because of the disparity between the with a return fan and direct control of the pressure in the return air plenum as the fan curves. building pressure: basis for controlling a return-fan relief system. (See “Return fan with direct control,” pp. 5– Flow tracking indirectly controls building 1 A flow sensor (in this case, a flow- 6.) This simple, direct approach is pressure by monitoring both supply and return measuring damper) monitors intake inexpensive to implement; it’s also more airflows. The capacity of the return fan airflow to maintain the proper volume reliable than the following commonly used “tracks” supply airflow to maintain a fixed control schemes: signal tracking and flow differential between the two. of outdoor air for ventilation. tracking. Flow tracking works best in applications with The pressure in the mixed-air plenum Signal tracking monitors supply-duct constant local exhaust. Successful changes as the actuator modulates the pressure to regulate the speeds of the supply implementation also requires expensive, well- linked intake and recirculating dampers. and return fans. Building pressure is calibrated flow sensors because the difference controlled indirectly (and ineffectively) between supply and return airflows can be a Note: An intake airflow sensor controls very small fraction of the sensed airflow. ■ ventilation more accurately than supply/ return airflow tracking. Monitoring intake airflow also costs less and is Note: Intake airflow is usually pushes air out of the damper and usually more reliable than using an modulated to control ventilation and prevents overpressurization. injection fan, pressure sensor, and economizer cooling. variable frequency drive. Relief dampers are often inexpensive to For controls to maintain either positive install; however, susceptibility to wind 2 A pressure sensor monitors or negative building pressure by and stack effect often limits their use to supply-duct static pressure and adjusts modulating relief air, the minimum small HVAC systems and single-story supply-fan capacity accordingly. intake airflow must exceed maximum buildings. exhaust airflow; relief airflow can then Note: For DDC/VAV systems, ASHRAE be used to “trim” the difference Depending on the configuration of the Standard 90.1 requires duct-static- between airflow into and out of the air distribution system, schemes for pressure reset based on VAV-box building. active building pressure control position to minimize supply-duct static involve either a return fan or a relief fan. pressure and, therefore, supply-fan Minimum relief airflow yields positive Table 1 (p. 8) compares the pros and horsepower. building pressure, while maximum cons of these two approaches. relief airflow yields negative building pressure. For systems with airside- economizer operation, minimum relief Figure 4. Typical VAV system with central return fan airflow may be zero, while maximum relief airflow may approach maximum intake airflow.
Building pressure can be regulated passively or actively. Passive building pressure control most commonly consists of a gravity-operated damper in the occupied space. When the economizer operates, the positive pressure that develops inside the space
“providing insights for today’s HVAC system designer” 5 ■ 3 The thermostat in each occupied 5 A pressure sensor in the return air the return fan. This strategy modulates space detects the dry-bulb temperature plenum adjusts the capacity of the the plenum-pressure setpoint, and and appropriately modulates the supply return fan. (See “Alternatives for therefore return-fan speed, based on airflow. Return-Fan Control,” p. 5.) the positions of the relief and recirculating dampers. 4 Local exhaust fans (in rest rooms, for The return fan, which operates example) remove some of the air from whenever the supply fan does, pulls 6 A differential pressure sensor the occupied spaces. return air from the occupied space and monitors the indoor–outdoor pressure pressurizes the return air plenum at the difference. Its signal modulates the The remaining air either exfiltrates, if air handler. Air from this plenum either position of the relief damper, directly the spaces are pressurized; or, it passes through the recirculating controlling building pressure. returns to the air handler, accompanied damper into the mixed air plenum or by “infiltrated” air, via a return duct or exits the building through the relief The outdoor intake and recirculating ceiling plenum. damper. dampers can share the same actuator. However, the relief damper must be An energy-efficient enhancement to controlled separately to accommodate return-air-plenum control, optimized varying building-pressure setpoints. damper control, keeps one damper nearly wide open to ease the burden on
What’s the Right Setpoint for Building Pressure? The answer depends on a number of factors: minimize or eliminate infiltration at the top building of only four stories. To cope with Where is the sensor? What is the building’s floor by pressurizing the building as much as winter stack effect, tall buildings typically geographic location? Is it sheltered? What’s possible without interfering with proper door include vestibules with revolving doors. This the climate like? operation. arrangement effectively separates the indoor air column from outdoor air, enabling proper Conventional practice is to situate the indoor A similar tactic is employed for winter door operation despite the large indoor– pressure sensor on the ground floor. A control operation to discourage exfiltration of moist outdoor pressure difference. scheme is then devised to maintain a slightly indoor air near the top of the building positive building pressure (0.00 in. wg to envelope. But this time, the target pressure is “Compartmentalizing” tall buildings—that is, +0.05 in. wg or more) during summer neutral or slightly negative (0.00 in. wg to – vertically dividing the building into separate operation. In hot humid climates, a setpoint 0.02 in. wg or less). In cold climates, where air distribution systems—can help minimize greater than +0.10 in. wg may be appropriate, winter stack effect exerts considerable force, the effect of stack pressure differences by especially for tall buildings and multistory further depressurization is often necessary… reducing the height of the indoor air column. pressure compartments. The objective is to perhaps –0.10 in. wg on a windy day for a (Elevators usually require a separate vertical compartment.)
Compartmentalization and winter stack effect Without compartmentalization, minimizing air leakage through the entire building envelope due to stack effect would require a very positive setpoint during the summer and a very negative setpoint during the winter.
Ultimately, the extent to which a building can be pressurized depends on its construction (whether it is “tight,” “leaky,” or something in-between) and its operation. A well-sealed building must include controls to avoid developing excessive pressure, while even marginal pressurization may be unachievable in a “leaky” building. ■
■ 6 Trane Engineers Newsletter — Vol. 31, No. 2 Relief fan with direct control. Figure 5. Typical VAV system with central relief fan Figure 5 illustrates another typical VAV system, but in this case, a relief fan relief fan and relief/exhaust damper directly controls building pressure: RELIEF RA 1 The flow sensor (a flow-measuring return air local exhaust fan damper, as in the previous example) plenum EA monitors intake airflow to maintain proper ventilation. RRA occupied space infiltration exfiltration 2 Supply-fan capacity is based on recirculating the signal of a pressure sensor that variable-speed damper supply fan VAV box monitors supply-duct pressure. SA INTAKE MA 3 Airflow from each VAV box is flow-measuring mixed air modulated based on the dry-bulb intake (OA) damper plenum filters coils temperature detected by the thermostat in each occupied space.
4 Local exhaust fans and exfiltration Capacity control can be accomplished configuration is preferred. Use a return remove a percentage of air from by “riding the fan curve” as the relief fan if the ducted return adds more occupied spaces; the rest (including damper modulates or by equipping the pressure drop than a reasonably sized “infiltrated” air) returns to the air relief fan with speed control. supply fan can handle. handler, usually by way of a ceiling- plenum return. Air from the return air plenum either passes into the mixed air plenum 5 The relief fan only operates when through the recirculating damper, or Closing Thoughts necessary to relieve excess building exits the building through the Building pressure control is important pressure. With minimum intake airflow, relief damper. for two reasons: relief airflow is seldom necessary during mechanical cooling and heating Why pick one relief method versus ■ It is fundamental to attaining the operation. Note, too, that without a another? Choosing a relief system design targets for infiltration and return fan it is unnecessary to monitor based on the unique requirements of exfiltration. and control the static pressure in the each application lets you optimize the return air plenum. costs of installing and operating it. ■ It enables proper door operation by preventing excessively positive or Although the relief fan (when running) Generally, the relief-fan configuration negative pressure. relieves the burden on the supply fan, works best in VAV systems designed the latter still must be sized to handle with a ceiling-plenum return. Relief fans Both factors affect the performance the pressure drop of the entire system effectively control building pressure; and longevity of the building and its at design airflow. For this reason, the they are also easier to control, less systems. Direct control of building relief-fan configuration is typically expensive to install, and less costly to pressure, whether with a return fan or a applied in ceiling-plenum-return operate than return-fan configurations. relief fan, best manages the combined systems. effects of weather, wind, and When a ducted return is necessary (see mechanical ventilation. 6 As in the return-fan example, a “Ducted or Plenum Return?” p. 1), be differential pressure sensor monitors sure to evaluate both relief- and return- Return fan control works. Although and directly controls building pressure fan configurations: If the supply fan can it requires at least one additional by adjusting the relief fan’s capacity. handle the pressure drop from the air- pressure sensor (which can be difficult handler’s discharge opening to its to situate) and continuous operation of return opening, then the relief-fan the return fan, it also permits a smaller
“providing insights for today’s HVAC system designer” 7 ■ supply fan. Use this type of relief if the air distribution system includes return ducts.
Relief fan control works, too. And it’s often less costly to install and operate Table 1. Pros and cons of return fan vs. relief fan in VAV systems than a central return fan. The fan Return fan Relief fan horsepower necessary to overcome Advantages Advantages the pressure drops of the relief damper ■ Lower differential pressure across the ■ Lower operating cost. Typically, the relief fan and return path is about the same as for supply fan if the pressure drop of the return air can remain off during “non-economizer” hours path is greater than the pressure drop of the and operate at low airflow during many return-fan relief. But unlike the return outdoor air path. “economizer” hours. Also, the recirculating fan, the relief fan only operates when damper can be sized for a lower pressure drop. needed (usually during the economizer ■ Potentially lower initial cost for systems ■ Greater layout flexibility. The relief fan can be mode). with ducted returns. Less supply fan pressure positioned anywhere in the return path (lower can mean lower fan horsepower and/or a initial cost) because the supply fan draws the smaller fan and smaller variable-speed drive. return path negative (relative to the occupied Use relief fan control when the spaces) during modulated economizer operation. supply fan can overcome the combined A ground-floor air handler with top-floor relief can take advantage of winter stack effect to lower pressure drop of the supply and return operating cost. paths—usually in systems with ceiling ■ Simpler control scheme. One less sensor and plenum return or with very short one less actuator simplifies installation and return ducts—or when the system can balancing. Applications with a low return-path pressure drop can accommodate lower-cost fans benefit from locating the relief fan as well as fewer (less costly) controls. remotely from the air handler. ■ Disadvantages Disadvantages ■ Higher operating costs, especially in climates ■ Negative building pressure at low loads. This By Dennis Stanke, applications with extended hours of economizer cooling condition can occur when a variable-speed drive (The return fan must run whenever the supply controls relief-fan capacity, the supply airflow rate engineer, and Brenda Bradley, fan operates.) is low, and required relief airflow is less than the information designer, Trane. minimum airflow at lowest fan speed. (Using a constant-speed relief fan with a modulated relief damper avoids this disadvantage.) You can find this and other issues ■ More complex (expensive) fan-speed ■ Greater likelihood of outdoor air leakage at of the Engineers Newsletter in the control. Return-air-plenum pressure control the relief damper. The return air plenum commercial section of www.trane.com. requires an additional pressure sensor and operates at negative pressure whenever the To comment, send a note to Trane, modulating device (either a damper actuator or relief fan is off. (Using low-leak relief dampers variable-speed drive). can minimize air leakage from outdoors.) Engineers Newsletter Editor, ■ Difficult to situate the return-air-plenum ■ Higher differential pressure across the 3600 Pammel Creek Road, La Crosse, pressure sensor (in systems with return-air- supply fan than in return-fan systems. When the WI 56401-7599, or e-mail us at plenum pressure control) because the return air relief fan is off, the supply fan must overcome [email protected]. plenum is usually small and turbulent. the pressure drops of the return path as well as the supply path. (For this reason, relief fans may ■ Requires more fan horsepower at part load. or may not be appropriate for systems with Return-air-plenum pressure must always be ducted returns.) positive enough to establish relief airflow; the recirculating damper must therefore drop significant pressure between the negative mixed air plenum and positive return air plenum. (Optimized damper control can reduce part-load fan horsepower.)
■ Limited layout flexibility. The return fan must be situated between the air handler and the closest leg of the return path (usually near the air handler) because it must draw the entire return path negative relative to the occupied spaces. It must also discharge into the return air plenum during modulated economizer operation. An American Standard Company www.trane.com
For more information, contact your Trane believes the facts and suggestions presented here to be accurate. However, local district office or e-mail us at final design and application decisions are your responsibility. Trane disclaims any [email protected] responsibility for actions taken on the material presented.
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