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engineers newsletter volume 35–2 ● providing insights for today’s hvac system designer

handler with controls has keeping cool with outdoor air … four operating modes. Airside Heating mode. During very cold weather, the brings in minimum outdoor airflow (for many jurisdictions, by the building ventilation) and mixes it with return from the editor … code. For the denizens of commercial air. The mixed air is then heated as buildings, comfort cooling is one of life’s Let’s take a high-level look at airside necessary to maintain the desired basic necessities. Without it, we quickly supply-air (or space) . become irritable, lethargic, and unable economizing: what it is, what it to concentrate. But the cost of keeping requires, and how it’s done. ASHRAE Standard 62.1–2004[2] us comfortably cool rises with each passing day … and that cost isn’t (Standard 62) or building code confined to the person or organization requirements determine the minimum who pays the building utility bill. intake of outdoor air. This minimum How does economizer may be reset downward from the Using outdoor air to help cool a building isn’t a new concept. Thanks to energy cooling work? design value (to save heating energy) standards and local building codes, it’s using various dynamic reset no longer optional in many areas, either. Most commercial buildings have some approaches. But when it’s cold outside, spaces that need cooling all year long. no more than minimum outdoor air In this EN, Dennis Stanke (Trane staff If it’s colder outdoors than indoors, it enters the building, whether or not the engineer and chair of ASHRAE SSPC often makes energy-sense to 62.1) reviews ASHRAE Standard 90.1’s system includes airside economizer requirements for airside economizers “economize” by bringing in more-than- controls. and discusses the underlying design minimum outdoor air to reduce the decisions and benefits of effective airside hours of mechanical cooling system Modulated economizer mode. economizer systems. operation. During “cool” weather (30°F to 55°F A typical “mixed-air” air handler [1°C to 13°C], for example), the You’re in your kitchen on a beautiful, includes dampers for outdoor air, return required space temperature can be breezy fall day, with a pie baking in the air, and relief air (Figure 1). These maintained without any mechanical oven. Would you rather cool off the dampers can be controlled to provide cooling or heating by simply adjusting kitchen by opening your windows or by airside economizing. Usually, an air the mix of outdoor air and return air. turning on your central air conditioner? By opening the windows, of course! Why? If the outdoor air is cool and dry, Figure 1. Typical air handler it can cool the space nicely without using for and fans, and it ventilates the space with fresh outdoor air in the bargain. The same logic holds in non-residential buildings. In fact, it not only makes sense to introduce outdoor air when conditions are “right” but it’s also required by ASHRAE Standard 90.1–2004 [1] (Standard 90) and, in

© 2006 Trane All rights reserved ● 1 In this mode, the economizer system Figure 2. U.S. climate zones adjusts both the outdoor- and return-air dampers, modulating these airflows to match cooling capacity with cooling load—without mechanical cooling. Intake airflow varies between the minimum setting and a maximum value (100% of supply airflow) to maintain the supply-air (or space) temperature at setpoint.

Integrated economizer mode. During mild weather (55°F to 75°F [13°C to 24°C], for example), outdoor air can provide some cooling capacity, but not enough to satisfy the load, so mechanical cooling supplements the economizer cooling provided by the wide-open outdoor-air . We refer to this mode as integrated economizer because it combines “free” cooling (100% outdoor air) downward from its design value to Standard. Figure 3 shows regional with mechanical cooling to meet the save cooling energy. But, when it’s hot economizer requirements, which aim required cooling capacity. The system outside, no more than minimum to minimize the ratio of economizer- stays in integrated economizer mode outdoor air enters the building related costs to energy-related savings. until outdoor conditions reach the high- whether the system includes an These requirements are based on limit shutoff setting (discussed in more airside economizer or not. cooling system capacity and the detail on p. 5), or until the outdoor expected number of hours with conditions fall to the point where Note: Specific control sequences for outdoor conditions that are appropriate modulated economizer operation the preceding operating modes may for economizing. can handle the cooling load. vary with equipment configuration, type of economizer control, and high- Potential energy savings are highest limit settings. where the weather is dry or marine, so Mechanical cooling mode. systems with capacities larger than Economizer operation is disabled ≈5 tons (65,000 Btu/h [19 kW]) in during warm weather, when outdoor these locations must include conditions exceed the high-limit economizer cooling. Moist, cool shutoff setting. Minimum outdoor air What does climates provide fewer opportunities for ventilation (determined by Standard Standard 90 require? for “free” cooling, so the economizer 62 or the local building code) mixes requirement only applies to systems with return air. The mixed air then is According to Standard 90, with capacities larger than ≈11 tons mechanically cooled as needed to Section 6.5.1, economizer cooling (135,000 Btu/h [40 kW]). maintain the supply-air (or space) systems (either airside or waterside) temperature at setpoint. must be used in all cooling systems with fans. There’s also a long list of Potential benefits are lowest where the weather is moist and warm. As in the heating mode, the exceptions to this requirement (see Economizer cooling is not required in minimum outdoor airflow may be reset inset, p. 3) because the value of economizer cooling depends on these climates (although it is allowed) climate, building type, system type, because ASHRAE studies indicate that and control settings. Nevertheless, the potential savings in mechanical Standard 90 requires economizer cooling in many locations for many * buildings and many different systems. Appendix B of Standard 90 also identifies climate zones for various cities elsewhere in North America and around the world. Figure 2 shows the U.S. climate zones* defined in Appendix B of the

2 ● Trane Engineers Newsletter volume 35–2 providing insights for today’s HVAC system designer Figure 3. Standard 90’s regional economizer requirements based on cooling system capacity Figure 4a and Figure 4b (p. 4) show typical economizer sequencing for constant- and variable-volume systems. Standard 90 does not specifically require these operating modes, but they result from logical sequencing of mechanical and economizer cooling. These diagrams expand on the single diagram presented in the Standard 90.1–2004 User’s Manual.[3]

Figure 4a: Constant-volume systems. In heating mode, minimum outdoor-air intake flow enters the system. Heating capacity decreases as the outdoor-air heating load decreases.

As the weather warms, when the cooling energy may not be sufficient to mechanical cooling to maximize energy system no longer needs heating, it justify the additional cost of savings while avoiding wasteful enters the modulated economizer implementing it. simultaneous cooling and heating. mode. Outdoor air and recirculated To help assure proper sequencing, return air modulate to maintain space Section 6.5.1.1.1: Design capacity. control of the economizer dampers (or supply-air) temperature at setpoint. Airside-economizer systems must shouldn’t be based on mixed-air Outdoor air provides the needed include outdoor- and return-air conditions alone. cooling capacity without any dampers that are sized and modulated mechanical cooling. The black area in so that up to 100% of design supply Figure 4a represents the mechanical airflow can be outdoor air. In other † Warmer supply air increases the delivered airflow cooling energy that’s saved during words, the system must be designed (therefore energy) and space levels modulated economizer operation or while reducing both mechanical cooling and local to allow outdoor airflow ranging from reheat energy. Thoroughly analyze the effects of “.” the minimum required for ventilation to a supply-air-temperature reset strategy before the maximum delivered by the making it part of the system design. As the cooling load increases, the supply fan. outdoor-air damper eventually opens

For VAV systems, the supply fan usually delivers less than cooling- Exceptions to Standard 90’s economizer requirement design airflow during the integrated economizer mode—even though the Section 6.5.1 requires economizers for (d) Systems with condenser all systems in all locations. But it also recovery. outdoor-air damper is wide open. Of identifies the nine exceptions listed below. course, supply fan airflow (and (e) Any residential space system with a therefore, intake airflow) increases if Note: Economizer systems may be capacity that’s less than five times the the control resets the supply-air used even if not required, provided that applicable limit listed in Exception (a). temperature upward at part load.† the economizer system conforms to the requirements in Sections 6.5.1.1 through (f) Systems with space sensible cooling 6.5.1.4. loads (excluding transmission and Section 6.5.1.1.2: Control signal. loads) equal to or less than Airside economizer operation must (a) Systems using fan-cooling units transmission and infiltration loads at 60°F. with individual capacities less than be appropriately sequenced with 65,000 Btu/h (19 kW) in dry climates, less (g) Systems that are expected to operate than 135,000 Btu/h (40 kW) in cool-moist less than 20 hours per week. climates, and with any capacity, large or small, in warm-moist climates (Figure 3). (h) Supermarket systems where outdoor air for cooling affects open refrigerated (b) Systems with gas-phase outdoor air cases. cleaning to meet ASHRAE Standard 62. (i) Systems with high mechanical cooling (c) Systems that deliver more than 25% efficiencies (equal to or exceeding the of the supply air to spaces humidified requirements of Table 6.3.2). • above 35°F dew point for process needs.

providing insights for today’s HVAC system designer Trane Engineers Newsletter volume 35–2 ● 3 to 100% and the return-air damper Figure 4a. Typical economizer control sequence for constant-volume systems closes completely. The system enters integrated economizer mode, where 100% outdoor airflow provides part of the required cooling capacity and mechanical cooling provides the balance, modulating or cycling as necessary to maintain the required space (or supply-air) temperature. The red area in Figure 4a represents the mechanical cooling energy that’s saved during integrated economizer operation.

The system stays in integrated economizer mode until the outdoor-air condition reaches the high-limit shutoff setting. At this point, the controls disable economizer operation and the system enters the mechanical cooling mode, where a water valve modulates or a cycles to provide all cooling capacity needed to maintain reheat terminals need less airflow The system enters mechanical cooling space (or supply-air) temperature. In when cooling than when heating. mode when outdoor air reaches the this mode, the outdoor-air damper When the cooling load starts to rise, high-limit shutoff condition. Intake closes to allow only minimum the modulated economizer mode airflow drops to the minimum intake airflow. begins. Supply airflow increases (that requirement, and supply and return is, intake airflow increases while return airflows increase while the cooling coil In some locations, direct expansion airflow decreases) to maintain the provides the required cooling capacity. (DX) systems may be designed to required supply-air temperature enter the mechanical cooling mode without mechanical cooling. In the Both constant-volume and VAV directly from the modulated integrated economizer mode, the systems use linked outdoor- and economizer mode. If 100% outdoor air outdoor-air damper stays wide open return-air dampers, which are operated is unable to provide the required to provide some cooling capacity by a single actuator or by multiple cooling capacity, then the outdoor-air while the mechanical system coordinated actuators. However, as damper closes to its minimum position modulates to provide the balance. mentioned in the Standard 90 user’s and mechanical cooling modulates to provide all of the needed cooling capacity. This “non-integrated Figure 4b. Typical economizer control sequence for variable-volume (VAV) systems economizer” approach avoids unstable system operation and coil frosting, which can occur when a DX system cycles at low loads. But it also reduces the potential savings in mechanical cooling energy represented by the red-shaded area in Figure 4a.

Figure 4b: Variable-volume systems. Consider a single-, VAV system with reheat terminals: In heating mode, minimum outdoor airflow enters the system and recirculated return air provides the balance of supply airflow. Supply airflow usually decreases as the heating load diminishes because the

4 ● Trane Engineers Newsletter volume 35–2 providing insights for today’s HVAC system designer manual and in Guideline 16, Selecting high-limit shutoff. In other words, the might actually differ from the Outdoor, Return and Relief Dampers prescribed high-limit shutoff setting prescribed limit. for Airside Economizer Systems [4], acts as an economizer enable/ sequential or “optimized” damper disable setting. The prescribed high-limit setting operation can save supply fan energy depends on location and the type of by reducing mixing box during Above the prescribed high-limit shutoff high-limit that’s used. Table 6.5.1.1.3A the modulated economizer mode.[5] —a “best compromise” established by in the Standard allows six different ASHRAE using computer simulations control types: fixed dry bulb, for various control types in various differential dry bulb, fixed , Section 6.5.1.1.3: High-limit climates—Standard 90 assumes that electronic enthalpy, differential shutoff. Integrated airside-economizer the system needs more mechanical enthalpy, and dew point–and–dry bulb. operation must be disabled (that is, the cooling energy to condition 100% outdoor-air damper must reduce intake outdoor air than to condition mixed air, In dry and marine climates and in very airflow from maximum to the minimum so economizer operation must be cold climates, fixed enthalpy control is airflow required for ventilation) disabled. Below the high-limit shutoff, not allowed (Figure 5, p. 6). Why? whenever outdoor air exceeds a the Standard assumes that the system Because it could result in 100% prescribed high-limit condition. This needs less mechanical cooling to outdoor air during many hours when makes sense: If it starts to rain or if it condition 100% outdoor air. Of course, outdoor conditions are dry but very gets really hot outside, you want to since the prescribed limits are based warm. If the cooling coil is dry, too, close the kitchen window, right? on broad assumptions, the “best” both sensible load and mechanical Conversely, economizer operation economizer enable/disable condition cooling energy increase rather than must be enabled when outdoor for a given building in a given location decrease. In cold climates, fixed conditions are equal to or below the

Decisions, Decisions

Standard 90 includes prescribed minimum High-limit shutoff type? Depending on • Others: Standard 90 doesn’t requirements for economizer systems. But location, designers may choose from five permit other control types under the within these requirements, designers still or six types of high-limit shutoff control, prescriptive approach to compliance. must make several economizer-related ranging from lowest cost (fixed dry bulb) decisions in addition to choosing the HVAC to perhaps lowest energy (differential Each permitted approach has a unique first system type (CV-basic, CV-reheat, VAV) enthalpy): cost, operating cost, maintenance cost, and size: to economize or not to and space relative-humidity performance. • Fixed dry bulb disables economizer economize, whether to use an integrated To properly weigh the tradeoffs and make operation when outdoor air exceeds a or non-integrated economizer, which of six an informed choice, designers should use fixed temperature. types of high-limit shutoff control to an economic/performance analysis implement, and perhaps an optimum high- • Differential dry bulb (not allowed in program to compare at least fixed dry limit shutoff setting as well. some climate zones) disables bulb, fixed enthalpy, and differential economizer operation when the enthalpy, since these are common and Economize? Even though economizer temperature outside is warmer than widely applicable. cooling may not be required (because the the return air. system is small or because it’s in a warm, High-limit shutoff setting? These moist climate), a thorough engineering • Fixed enthalpy (not allowed in some values are prescribed for different control analysis may show that it reduces climate zones) disables economizer types in different climates. An economic/ operating costs enough to be worthwhile. operation when outdoor air exceeds a performance analysis may show that fixed enthalpy (28 Btu/lb [65 kJ/kg]). higher or lower values offer better energy Economizer type? Even though it’s not • Electronic enthalpy disables economizer savings or better relative humidity required in most locations, a designer may operation when outdoor air exceeds a performance for a particular building choose integrated economizer control selected dry-bulb/dew-point curve, system. The authority having jurisdiction anyway. Why use it? It always increases defined by electronic sensor may agree to approve a variance if such an economizer hours and it doesn’t increase manufacturers. analysis shows that a “custom” shutoff first cost in chilled water systems. Why not setting results in lower energy use than the use it? In simple DX systems, operating • Differential enthalpy disables “generic” prescribed setting. with 100% outdoor air may result in coil economizer operation when outdoor-air frosting at low loads. And, at low outdoor enthalpy exceeds return-air enthalpy. Decisions about economizer systems can air (low loads), it causes • Dew point–and–dry bulb disables be made spontaneously based on first-cost compressor short-cycling, which may lead economizer operation when outdoor air considerations (for example) … or they can to refrigerant system instability and exceeds a fixed dry-bulb (75°F [24°C]) be made deliberately based on an analysis possible reliability problems. The common OR a fixed dew-point (55°F [13°C]) of both life-cycle cost and relative-humidity “fix” for low-load cycling usually involves temperature. performance. Many designers and their hot gas bypass, which increases the first customers prefer the latter. • cost of integrated economizer control and decreases the benefit.

providing insights for today’s HVAC system designer Trane Engineers Newsletter volume 35–2 ● 5 Figure 5. Standard 90’s regional limitations for economizer control types building. Excessive building pressure can cause doors to stand open, creating a potential security risk.

Also, in small constant-volume systems, relief airflow from a space affects intake airflow. Imagine a classroom that has an unlouvered door and is served by a 1200-cfm (560 L/s) unit ventilator. About 800 cfm (380 L/s) leaks from the room at 0.3 in. wg (75 Pa). With the outdoor-air damper wide open, and assuming that the unit fan can raise space pressure to 0.3 in. wg (75 Pa), the economizer introduces only 800 cfm (380 L/s) of outdoor air, not 1200 cfm (560 L/s). This reduction of intake airflow also reduces economizer cooling capacity and enthalpy control could disable airflow to/from the building when fans mechanical cooling energy savings. economizer operation during cool, rainy are off or when they operate during weather and miss valuable hours of unoccupied hours. Less obviously, but Building pressure can be controlled integrated economizer operation and sometimes more importantly, return-air (for example) using barometric relief forgo mechanical cooling savings. dampers also should close tightly to dampers, modulated relief fans, or reduce return-air recirculation during return fans with modulated relief In moist climates with warm to very integrated economizer operation. dampers.[6] Each approach has its hot temperatures, differential dry-bulb Recirculating return air reduces the advantages and disadvantages, but control is not allowed because it could cooling capacity of the outdoor air, using an airside economizer result in many hours of economizer increasing mechanical cooling hours necessitates some method of building operation when outdoor air is cool but and energy use. pressure control. very damp. This would increase latent load and mechanical cooling energy, Section 6.5.1.1.5: Relief of excess Section 6.5.1.3: Integrated and would result in an unacceptably outdoor air. Airside economizer economizer control. To comply high relative humidity in the space. systems must be designed so that with Standard 90, economizer The choice of control type depends excess building pressure can be operation must be integrated with largely on the tradeoff between first relieved. When more-than-minimum mechanical cooling operation (as cost and operating cost. However, outdoor air enters the building during discussed above), so that when space relative humidity performance economizer cooling modes, more-than- conditions permit, outdoor air provides also should be considered. In constant- minimum relief air must leave the part of the required cooling capacity volume applications, it may rise significantly during part-load cooling, Figure 6. Standard 90’s requirement for integrated economizer operation depending on both control type and system type. (See the “Decisions, Decisions” inset, p. 5.)

Section 6.5.1.1.4: Dampers. Airside systems must use return- and outdoor-air dampers selected for low leakage. Obviously, outdoor-air dampers should close tightly to reduce

6 ● Trane Engineers Newsletter volume 35–2 providing insights for today’s HVAC system designer while mechanical cooling provides the because outdoor air enters the balance. In other words, economizer system via the cooling fan (rather References systems must include both a than the heating fan). [1] ANSI/ASHRAE/IESNA Standard modulated economizer mode AND an 90.1–2004. Energy Standard for integrated economizer mode. One important exception to this Buildings Except Low-Rise requirement allows VAV systems to Residential Buildings. Atlanta, GA: ASHRAE. The integrated economizer mode must use airside economizers even though be used for all systems, with these lowering the supply-air temperature [2] ANSI/ASHRAE Standard 62.1–2004. exceptions: may increase zone-level heating. This Ventilation for Acceptable Indoor Air exception is justified because the fan Quality. Atlanta, GA: ASHRAE. (a) some unloading DX systems that uses less energy to deliver cooling air reduce outdoor airflow to prevent [3] ASHRAE. 2004. Standard 90.1–2004 to those zones that need cooling. The coil frosting; User’s Manual. Atlanta, GA: fan energy saved by lowering the ASHRAE. (b) DX units with cooling capacities supply-air temperature offsets the less than 65,000 Btu/h (19 kW); reheat energy used by those few zones [4] ASHRAE Guideline 16–2003. and, Selecting Outdoor, Return, and that need heat. Relief Dampers for Air-Side (c) systems located in climates Economizer Systems. Atlanta, GA: with limited hours of integrated ASHRAE. economizer operation (Figure 6). [5] Trane. 2000. “Advanced System Ultimately, you have Control Strategies.” Engineers Ironically, since Exception (c) covers choices … Newsletter Live broadcast (APP- most climates in the continental United APV004-EN, VHS format). Available at www.trane.com/bookstore. States, Standard 90 only requires Should your kitchen window open? If integrated economizer operation for so, when should you open it? When [6] Stanke, D. 2002. “Managing the Ins systems in the Southwest and on the should you close it? and Outs of Commercial Building West Coast. Even so, integrated Pressurization.” Engineers economizer systems should still be For a given building in a given location Newsletter 31-2. La Crosse, WI: Trane. considered because they do reduce with an established system cooling mechanical cooling energy. Energy and capacity, Standard 90 either requires [7] Trane. 2006. “HVAC Systems and economics should be analyzed for each an economizer system or leaves that Airside Economizers.” Engineers specific building and its HVAC system decision to the designer. The type of Newsletter Live broadcast. Check and economizer control type. After all, www.trane.com/bookstore for high-limit control may be somewhat availability. • just because you’re not required to restricted, but many options remain. open the kitchen window doesn’t Although high-limit shutoff settings are mean that it’s a bad idea. prescribed, the designer might still be able to justify a different high-limit Section 6.5.1.4: Economizer heating setting—one that saves more energy system impact. Economizer system for the project at hand. controls must not result in increased building heating energy. This Altogether, the combination of a few requirement may limit system choices. restrictions and so many options can For instance, an airside economizer complicate the design of an effective used with a single-fan, dual-duct VAV airside economizer system. Designers system or multizone system would would do well—for their clients, for lower the air temperature entering the building occupants, and for Earth—to heating coil and increase the required thoroughly analyze the alternatives and heating energy. On the other hand, an make economizer system choices airside economizer does not impact using sound engineering judgment, heating energy when used with a available systems, and accurate dual-fan, dual-duct VAV system modeling tools. •

By Dennis Stanke, staff applications engineer, and Brenda Bradley, information designer, both of Trane. You can find this and previous issues of the Engineers Newsletter at http://www.trane.com/ engineersnewsletter. To comment, e-mail us at [email protected].

providing insights for today’s HVAC system designer Trane Engineers Newsletter volume 35–2 ● 7 Tr a n e www.trane.com

For more information, contact your local Trane office or e-mail us at [email protected]

Trane believes the facts and suggestions presented here to be accurate. However, final design and application decisions are your responsibility. Trane disclaims any responsibility for actions taken on the material presented.

8 ● Trane Engineers Newsletter volume 35–2 ADM-APN020-EN (May 2006)