Ventilation for Changeover-Bypass VAV Systems

Transcription

1 Copyright 2004, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. This posting is by permission of ASHRAE Journal. This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission. Contact ASHRAE at Standard Addendum 62n Ventilation for Changeoer-Bypass VAV Systems By Dennis Stanke, Member ASHRAE A NSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality, 1 prescribes entilation rates for commercial and institutional buildings. Historically, Standard 62 required both one- and system-leel calculations for the design of multiple-one entilation systems, such as single-path constant olume and VAV systems. Unfortunately, without clear calculation procedures, system-leel calculations frequently were misinterpreted or ignored by designers. As a result, many multiple-one systems were underentilated. In an effort to aoid underentilation and increase calculation consistency among designers, Addendum 62n 2 updates prescribed entilation rates and the calculation procedure for one entilation airflow and for system intake airflow for different entilation systems. A preious article 3 introduced Addendum 62n rates and described the design of single-one and 100%- outdoor-air entilation systems. This article discusses the 62n-compliant design of a ery specific multiple-one entilation system: changeoer-bypass VAV, also called ariable olume and temperature (VVT. A changeoer-bypass VAV system (Figure 1 uses a constant-olume air handler (often a packaged rooftop unit or split DX system to entilate and cool or heat a large area within a building. The area sered comprises many comfort ones or HVAC ones, def ined by ANSI/ASHRAE/IESNA Although Addendum 62n shows entilation rates in both IP and SI units, this paper uses IP, except in selected specifi c calculations. This is because 62n uses rational conersions, not mathematical. Standard as areas with heating and cooling requirements that are sufficiently similar so that desired conditions can be maintained throughout using a single sensor. Each comfort one has its own thermostatically controlled VAV damper and ideally, all comfort ones associated with an air handler hae similar thermal loads (that is, all ones need some leel of heating or some leel of cooling, like interior spaces or southern perimeter spaces. In practice, howeer, many systems include comfort ones with dissimilar loads; some ones need heating at the same time that other ones need cooling. In either case, each comfort one usually is considered as a entilation one, defined by Addendum 62n as an area with similar occupancy category, occupant density, one air distribution effectieness, and one primary airflow per unit area. About the Author Dennis Stanke is a staff applications engineer with Trane, La Crosse, Wis. He is ice chair of Standing Standards Project Committee A S H R A E J o u r n a l a s h r a e. o r g N o e m b e r

2 The changeoer-bypass VAV system includes a central bypass damper that modulates open as the one VAV dampers modulate closed. The air handler deliers approximately constant primary airflow, while each comfort one receies a ariable air olume to match its thermal load. Air-handler mode (heating or cooling is usually based on one-by-one polling to determine the number of ones calling for heating or cooling, for instance, and the strength of the calls. Air-handler capacity may be controlled by the largest load, or by simply maintaining primary air temperature at a heating or cooling setpoint. As the one dampers close and one airflow drops, the bypass damper opens to maintain a constant differential pressure between the primary duct and the return plenum, and consequently, relatiely constant airflow through the fan and coils. This results in relatiely constant outdoor-air intake flow (ignoring wind effects. Note: Depending on building construction and the operation of relief (exhaust fans and/or dampers, mixed-air pressure may actually rise relatie to outdoor pressure as the bypass damper opens. If the minimum outdoor-air-damper position is fixed, it should be set to ensure at least minimum outdoor-air intake fl ow with the bypass damper at both extremes (open to maximum and fully closed. A single duct deliers air for thermal comfort and entilation to all comfort ones. When the system is occupied, the outdoor air damper at the air handler opens to proide minimum outdoor-air intake flow. This outdoor airflow helps determine design cooling and heating capacity. Designing to Comply with Addendum 62n Changeoer-bypass VAV systems can be designed (and operated to comply with Addendum 62n,* thereby ensuring proper entilation in each one. To comply, the system must delier sufficient outdoor-air intake fl ow at the air handler as well as sufficient outdoor air to the breathing one within each entilation one. These flows, which are inextricably linked, must be determined for design cooling and design heating conditions. Because this is a multiple-one recirculating system, wherein the air handler supplies a mixture of outdoor air and recirculated return air to more than one entilation one, Equations 6.5 through 6.8 of Addendum 62n must be used. In the following paragraphs, we consider two approaches to entilation system design for an area within an example office building (Figure 2. In the first approach, we work from the ones to the air handler, selecting minimum one primary airfl ow settings and then calculating the outdoor-air intake fl ow needed for proper entilation of all ones. In the second approach, we work from the air handler to the ones, selecting * Along with many other Web-published addenda, Addendum 62n will be incorporated into an updated ersion of Standard and published as ASHRAE Standard later this year. the outdoor-air intake fl ow and then calculating the required settings for minimum one primary airfl ow. Based on thermal loads, our example building area is diided into eight comfort ones, each with its own thermostat and VAV damper. We consider each of these comfort ones as a separate entilation one. The following one-leel calculations must be performed, regardless of which approach to system design is used (Figure 3: 1. Look up the minimum people outdoor air rate (R p and the minimum area outdoor air rate (R a in Addendum 62n, Table 6.1, for each one. Using peak one population (P and one fl oor area (A, find the design breathing-one outdoor airflow using Equation 6-1 ( V b = R p P + R a A. (We could hae used aerage one population for P, but then we probably could not hae taken credit for occupant diersity. See Step 6. For example, the south offices in our example building require V b = = = 220 cfm. 2. For each one, look up one air-distribution effectieness (E based on the air-distribution configuration and the default alues in Table 6.2 (see excerpt in Figure 4. In our example, all offices use oerhead diffusers and ceiling returns, and receie 58 F (14 C primary air during cooling and 95 F (35 C during heating. Effectieness changes from 1.0 when cooling to 0.8 when heating, demonstrating how operating mode can affect one entilation performance. 3.Find one outdoor airflow using Equation 6-2 (V o = V b / E for both cooling and heating operation. For our example south offices, V o = 220/1.0 = 220 cfm during cooling, and V o = 220/0.8 = 275 cfm during heating. Therefore, the system must be designed to delier at least V o = 275 cfm, either continuously or on aerage, to ensure adequate entilation in both modes. (During operation, the example VAV dampers may be allowed to close to delier less than 275 cfm when the primary air temperature doesn t match the one thermal requirements, proided system controls are designed to ensure that aerage minimum primary airflow is at least 275 cfm. See Design for Short-Term Conditions. Addendum 62n doesn t explicitly require calculation of V o at both cooling and heating design, but it is implicit in the requirements for system entilation efficiency and in the ariable load conditions section, which states that required entilation rate must be deliered under all load conditions. Each entilation one in our example is assigned to one of two air handlers, forming two multiple-one entilation systems (Figure 2. The interior system includes all interior ones and the north ones, which are largely adjacent to a conditioned warehouse space; these ones usually need some leel of cooling, regardless of outdoor conditions. The perimeter system includes ones that need some leel of cooling in warm weather and some leel of heating in cold weather. N o e m b e r A S H R A E J o u r n a l 2 3

3 EA RA Warehouse Bypass Damper OA V ot Supply Fan SA V ps North North Conf. Room Constant-olume supply fan ariable air olume to spaces VAV Damper V p V p Northeast Interior Interior RTU Perimeter RTU Figure 1: Changeoer-bypass system. The following procedures describe the two approaches to entilation system design mentioned preiously. Each approach can result in systems that comply with Addendum 62n. Design Procedure 1: Choose Minimum Zone Settings, Find Intake Airflow Using this approach, we first select minimum settings for one primary airfl ow and then determine the minimum outdoor-air intake fl ow at the air handler. We assume continuous primary airflow (non-ero minimums to illustrate the calculations. In some cases (as in our example perimeter system, continuous primary airflow can quickly oercool or oerheat some ones, making them uncomfortable or causing frequent system changeoer (heating to cooling or ice ersa. While continuous primary airflow works well for entilation, it may not proide acceptable comfort and/or energy use. Lower or een intermittent primary airflow (ero minimums can be used to design changeoer-bypass systems but only if operation results in an aerage primary airflow that equals or exceeds the minimum chosen for continuous primary airflow. Aeraging is discussed in more detail later (see Design for Short-Term Conditions. The entilation rate procedure in Addendum 62n prescribes the steps for calculating both one-leel entilation airflows (described earlier and systemleel intake airflow. Before we examine these steps, though, we must take care of some preliminary calculations and settings. First, based on design cooling load and primary air temperature, determine design one primary airfl ow (V p for each one. We use cooling airflow because it s usually much West South Conference Room Southwest Interior North East Figure 2: Changeoer-bypass office building. Adertisement formerly in this space. 2 4 A S H R A E J o u r n a l a s h r a e. o r g N o e m b e r

4 Cooling Heating Procedural Step Variable R p P R a A V b E V o E V o Ventilation Zone cfm/p p cfm/sf sf cfm cfm cfm South , West , South Conference Room , East , Southwest Interior Northeast Interior , , , ,060 North , North Conference Room , Figure 3: Zone entilation calculations, which must be performed regardless of chosen approach. higher than heating airflow. (This example ignores one-toone load diersity for simplicity, but as in other VAV systems, primary airflow at the changeoer-bypass air handler often is less than the sum of the peak one airflows. Next, establish a minimum primary airflow setting (V V p-min for each one. In this case, we set the one minimums to 30% of design airflow. We know that Standard 90.1 allows reheat (if we need it with 30% minimum settings, and although we might prefer lower settings, 30% minimums allow us to use default alues for system entilation efficiency (see Step 5. (With lower minimum settings, Standard 62 may hae required us to calculate E, rather than use default alues. Depending on the VAV damper controls, it may be possible to set two minimums one for cooling and another for heating but we considered only the simple case of a single minimum setting for each one. For design, we assumed that these are aerage alues for minimum primary airflow. For operation, the system controls must enforce these aerage minimums. Zero minimums are acceptable only if the system controls are designed to delier, on aerage, the design minimum primary flow. (See Design for Short-Term Conditions. The following system-leel steps (Figure 5 build on the one-leel steps coered preiously, so we begin with Step Using the highest one outdoor airflow (V V o-clg or V o-htg and the lowest primary airflow (V p = V p-min, find the minimum primary outdoor-air fraction for each one using Equation 6-5 (Z p = V o /V p. For example, for our south offices, Z p = 275/540 = Addendum 62n doesn t explicitly differentiate between heating-mode and cooling-mode Z p alues, but Table 6.3 (next step implies that the highest V o must be used because we must find the largest alue of Z p among all the ones sered by the system. Note: To aoid a design that requires 100% intake airflow, minimum (or aerage V p settings must exceed one Adertisement formerly in this space. N o e m b e r A S H R A E J o u r n a l 2 5

5 outdoor airflow ( V o. As a result, some systems may require duct reheat or one heat to aoid oercooling at low load; depending on thermal oning, systems without local heat may be less comfortable and may change oer more frequently. 5. Look up system entilation effi ciency (E in Table 6.3 (Figure 6. Use the largest primary outdoor-air fraction ( max Z p because it represents the critical one the one that needs the highest percentage of outdoor air in its primary airstream. Adertisement formerly in this space. For our example, max Z p occurs in the south offices (Z p = 0.51 and the north offices (Z p = Interpolating in Table 6.3, we find that minimum system entilation effi ciency E = 0.64 and 0.63 for each system, respectiely. Alternatiely (or if max Z p exceeds 0.55, we could determine E using the equations in Addendum 62n, Appendix G,** but most changeoer-bypass system designers likely prefer the simpler default-table approach. Note: Lower minimum settings, though perhaps preferred for one operation, would result in max Z p greater than 0.55 and require use of the equations rather than the default alues from Table 6.3; reduced E alues and increased outdoorair intake fl ow would result. 6. Find occupant diersity for the system using Equation 6-7 (D = P s /ΣP. Use a reasonable estimate for the expected system population (P s, along with the sum-of-peak one population (P. In this example, we assumed that the conference rooms would only be populated by people from the nearby office spaces. So, system occupant diersity is D = 60/90 = 0.67 for the perimeter system, and D = 100/136 = 0.74 for the interior system. 7. Find the uncorrected outdoor-air intake flow for the system using Equation 6-6 (V ou = D Σ(R( p P +Σ(R a A. For our perimeter system, V ou = = 840 cfm; for the interior system, V ou = ,440 = 1,950 cfm. 8. Finally, find the design (minimum outdoor-air intake fl ow using Equation 6-8 (V V ot = V ou /E. In our example, the perimeter system needs V ot = 840/0.64 = 1,310 cfm (about 14% outdoor air, while the interior system needs V ot = 1950/0.63 = 3,100 cfm (about 18% outdoor air. As is always the case in multiple-one recirculating systems, high one minimum settings lead to low intake airflow requirements. This design approach (one-to-system results in a relatiely low outdoor airflow percentage, but it requires high minimum settings for one primary airflow. This may mean reheat in many ones or frequent heating/cooling changeoer. ** In some cases, calculated E alues may be lower than default alues, but Appendix G calculations are left to future articles. 2 6 A S H R A E J o u r n a l a s h r a e. o r g N o e m b e r

6 Design Procedure 2: Choose Intake Minimum, Find Zone Minimum Settings Using this second approach, we select the minimum outdoor-air intake flow at the air handler and then determine the required minimum one-primary-airflow setting for each space. In this case, we start with more intake airflow than we found using the first approach. As we will see, more intake airflow results in lower one minimum settings (or aerages. Lower minimums mean less oercooling and/or oerheating and less frequent changeoer, which in turn may mean increased occupant comfort and lower energy use. Air Distribution Confi guration Although you won t find the following calculation steps (Figure 7 detailed there, this approach is based on the principles of Addendum 62n. The calculations build on the one-leel calculations (Figure 3 coered earlier, so we start with Step 4: 4. Establish the design (minimum outdoor-air intake fl ow Ceiling Supply of Cool Air 1.0 Ceiling Supply of Warm Air and Floor Return Ceiling Supply of Warm Air at Least 8 C (15 F Aboe Space Temp. and Ceiling Return Ceiling Supply of Warm Air less than 8 C (15 F aboe Space Temperature and Ceiling Return, Proided That the 0.8 m/s (150 fpm Supply Air Jet Reaches to Within 1.4 m (4.5 ft of Floor Leel (V V ot within the constraints of the air handler (such as primary airflow per ton of cooling capacity or unit heating capacity. We assume that each packaged unit in our example can handle 30% outdoor air, so V ot = 9, = 2,730 cfm for the perimeter unit and V ot = 17, = 5,220 cfm for the interior unit. Again, we ignore load diersity in this example by assuming that primary airflow is simply the sum of the peak one airflows. 5. Find occupant diersity using Equa- tion 6-7 (D = P s /ΣP. For the perimeter and interior systems, respectiely, D = 60/90 = 0.67 and D = 100/136 = Find the occupant portion of breathing-one outdoor airflow (R p P for each one using the people outdoor-air rate (R p from Addendum 62n, Table 6.1, and the design one population (P. 7. Using the area outdoor-air rate (R a from Table 6.1 and one fl oor area (A, find the building portion of breathingone outdoor airflow (R a A for each one. 8. Find the uncorrected outdoor-air intake fl ow for each system using Equation 6-6 (V ou = D Σ(R( p P + Σ(R( a A. In our example, the perimeter system needs V ou = = 840 cfm, while the interior system needs V ou = 0.74 Figure 4: Zone air-distribution effectieness for seeral entilation-one configurations. Not all Table 6.2 configurations are listed ,440 = 1940 cfm. E 9. Using Equation 6-8 (E = V ou /V V ot and the V ou and V ot alues already established, find the lowest allowable system 1.0 entilation effi ciency for proper entilation. For the perimeter system, lowest 0.8 allowable E = 840/2730 = 0.31, and for the interior system lowest allowable E = 1,940/5,220 = Use Table 6.3 (Figure 6 to find the maximum primary outdoor-air fraction ( max Z p for any one based on the lowest allowable system entilation effi ciency. Our minimum E alues are off the chart, but because we found the lowest allowable E for each system, we can use any alue aboe 0.31 or 0.37, respectiely, to find max Z p. For simplicity (that is, to stay in the default table, we used the lowest E alue in Table 6.3, E = 0.60, for both systems; that alue corresponds to max Z p In other words, no one can require a primary outdoor air fraction (Z p greater than Gien V o ( Figure 3 and using Equation 6-5 (V p = V o /Z p, find each one s minimum setting for one primary airfl ow. Adertisement formerly in this space. N o e m b e r A S H R A E J o u r n a l 2 7

7 Figure 5: System entilation calculations for Design Procedure 1 (fixed minimum one settings. For the south offices in our example, minimum V p = 275/0.55 = 500 cfm. This design approach yields somewhat lower minimum settings (or aerages for one primary airflow than the first procedure. Raising outdoor-air intake flow allows lower minimum primary airflow settings and therefore, less reheat or local heat (if reheat is used and less frequent changeoer, which may increase comfort and reduce energy use. Many changeoer-bypass system designers are likely to use the default table to find max Z p because it s relatiely simple, een though it yields conseratie results; that s why we used it in our example. But, the equations in Appendix G can result in a much lower max Z p alue when using a high intake-airflow alue, so minimum one primary airfl ow settings can be much lower (almost one half in some ones with only a little more work. Designers may want to consider using the equations to comply with the standard, rather Cooling Design Min. From (SAT = 58 F Setting Fig. 3 Procedural Step Outdoor Ventilation Zone Clg Load V p Maximum Z p * > 0.55 Use Appendix G * Largest one primary outdoor airfl ow fraction, calculated using Equation 6-5, among all of the entilation ones that the system seres. than the default table; calculations are not as simple, but results are more accurate and less conseratie. Min. Primary V p -min V o Z p E D V ou V ot cfm/sf cfm % cfm cfm (Tbl. 6.3 cfm cfm % South 0.9 1, * West 1.0 2, South Conference Room 1.1 3, East 1.0 2, Perimeter System 9, ,310 14% Southwest Interior 0.7 7, ,100 1, Northeast Interior 0.7 7, ,100 1, North 0.8 1, * North Conference Room 0.9 1, Interior System 17, ,950 3,100 18% * Ventilation-critical one. Design for Short-Term Conditions Regardless of the design approach used, Addendum 62n recognies that it may be reasonable to base design airflow alues on aerage conditions oer a finite period, rather than peak conditions. Addendum 62n list three acceptable design adjustments : It specifically allows use of an aerage one population (P in Equation 6-1 for calculation of the design breathing-one outdoor airfl ow (V b. (This seems to preclude the use of occupant diersity (D at the system leel, to aoid double credit at both one and system population; we didn t use aerage one population in our examples. It allows interruptions in deliered breathing-one outdoor airflow (V b proided that the aerage airflow oer aeraging time period T equals or exceeds the minimum V b alue found using Equation 6-1. (This short-term condition design option may be key for changeoer-bypass systems with minimum primary airflow settings of ero, but System Ventilation calculating aerage V b requires detailed Effi ciency, E knowledge of system control operation, which aries greatly by manufacturer and designer. Due to this ariation in system operation, detailed aeraging calculations are beyond the scope of this article. It allows interruption of outdoor- Figure 6: E defaults from Addendum n to ANSI/ASHRAE Standard , Table 6.3. Air Intake air intake fl ow (V V ot, proided that the aerage intake airflow equals or exceeds the alue found using Equations 6-5 through 6-8. Changeoer-bypass VAV systems with ery low or ero-minimum settings for one primary airfl ow tend to interrupt one entilation for short periods, making them candidates for design based on primary airflow aeraging. Before we aerage anything, we need an aeraging time. For a gien entilation one, Equation 6-9 (T = 3/V b from Addendum 62n defines required aeraging time as the ratio of three space olumes to the continuous breathing-one outdoor airflow. This equates to three space time constants. In response to a step change in airflow, space conditions approach steady-state alues after three time constants, so this seems to be a reasonable aeraging time. Assuming a 10 ft ceiling, 2 8 A S H R A E J o u r n a l a s h r a e. o r g N o e m b e r

8 Setting Zone Minimums For Design Procedure 1, our rule of thumb set the minimum alue for one primary airfl ow (V p at about twice the minimum one outdoor airfl ow (V o needed for entilation. This results in a one-air-distribution effectieness (Z p of about 50%, so we can use Table 6.3 default alues rather than Appendix G calculated alues. Lower minimum settings (like ero could be selected, proided system controls are designed to delier at least V o on aerage. Minimum settings also may be limited by diffuser selection and location. When the damper is open, minimum primary airfl ow should result in a diffuser elocity that will ensure good air distribution in the space (without diffuser dumping when cooling, and good air mixing in the space (without stratifi cation when heating. If electric reheat is used, minimum primary airfl ow may be based on the minimum airfl ow needed for safe heater operation. Standard 90.1 requirements also may impact minimum primary airflow settings. To comply using primary air reheat, the olume of air reheated must be no more than the highest of: The minimum olume required for entilation (V o. An airflow rate of 0.4 cfm/ft². An airflow equal to 30% of design peak airflow (V p. An airflow of 300 cfm, for small ones. Any higher rate that reduces oerall energy use by reducing intake airfl ow (V V ot, een though it increases local reheat energy. In our example ones, all minimum primary airflow settings are 30% of peak design airflow and all are lower than 0.4 cfm/ ft², so reheat would be allowed in any of the ones. our south office, for example, has an aeraging time of T = 3 2,000 10/220 = 270 minutes in cooling mode and T = 3 2,000 10/275 = 220 minutes in heating mode. Depending on load, any one in a changeoer-bypass VAV system may require either heating or cooling at any time. For a gien one, heating or cooling mode either matches the sys- tem mode or opposes it. An opposing one will be more comfortable with ery low primary airflow. A matching one must hae enough airflow to ensure that, aeraged oer time T,, the one receies at least the minimum required breathing-one outdoor airfl ow. A matching one must be oerentilated to compensate for those times when it becomes an opposing one. A system could be designed to integrate primary airflow oer the aeraging time (T for each one, allowing ero opposing minimums and resetting the matching minimums as high as necessary to ensure that the required continuous breathing-one outdoor airfl ow is proided on aerage. In any case, Addendum 62n specifically allows designs based on aeraging for short-term conditions, but leaes implementation details to the designer. one airflow settings with minimum intake airflow settings, allowing proper equipment selection. But Addendum 62n addresses entilation system operation as well as system design. It allows the system controls to reset both minimum one primary airflow and/or outdoor-air intake flow, matching current entilation capacity to current entilation require- Adertisement formerly in this space. Operational Considerations The preceding design procedures result in proper matching of minimum We didn t consider the ero-minimum case in the preceding design calculations because actual design depends on the control operation specified for a system. N o e m b e r A S H R A E J o u r n a l 2 9

9 Procedural Step From 11 Minimum V p Fig. 3 Setting* Ventilation Zone V ot D V b V b V Procedure Procedure ou o p (At 30% (Occ. (Area E Z p (Min. 1 2 cfm cfm cfm cfm cfm cfm % % South West South Conference Room East Perimeter System 2, Southwest Interior ,060 1, Northeast Interior ,060 1, North North Conference Room Interior System 5, ,440 1, * To aid comparison, minimum primary airfl ow settings (V p are shown as percentages of design primary airfl ow. Figure 7: System entilation calculations for Design Procedure 2 (fixed minimum intake setting. ments. As we saw when we considered short-term conditions, control design can be crucial for both the energy-conscious, comfortable operation of changeoer-bypass systems and for proper design. The dynamic reset section of Addendum 62n allows the use of system controls that reset the design outdoor air intake fl ow (V V ot and/or space or one airflow as operating conditions change. It lists three specific operating conditions, allowing a system-leel reset control strategy that: Uses CO 2 - or schedule-based, demand-controlled entilation (DCV to reset one-leel V o and system-leel V ot in response to one population changes. Seeral DCV approaches indirectly can determine or estimate the actual airflow per person and reset either V p or V ot (or both in response to the current requirement. Uses one airflow to find the currently required intake airflow (V V ot based on recalculation of system entilation effi ciency (E a dynamic ersion of Design Procedure 1. This includes resetting V ot as one air-distribution effectieness (E aries due to heating/cooling operation. Adertisement formerly in this space. Lowers the minimum one primary airfl ow during economier operation, when V ot exceeds the minimum required intake a dynamic ersion of Design Procedure 2. Addendum 62n allows these dynamic reset approaches. It gies no implementation details, but the requirement is clear: Any system control approach that responds to arying conditions must be capable of proiding the required breathing-one entilation airflow wheneer the ones sered by the system are occupied. Dynamic reset approaches and results can differ widely. Regrettably, they are outside the scope of this article. Summary Addendum 62n calculation procedures can be applied to changeoer-bypass systems with both ero and non-ero minimum settings for one primary airflow. Design calculations can proceed from the one minimums to find the air-handler intake flow, or from intake flow to find the one minimums. As we e shown, the default table for system entilation efficiency can be used (and is likely to be common practice simple design for simple systems. As with other VAV systems, dynamic reset approaches are allowed by Addendum 62n, but operational controls for changeoer bypass systems are beyond our present scope. References 1. ANSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality. 2. ANSI/ASHRAE Addendum n to ANSI/ ASHRAE Standard Stanke, D Addendum 62n: single-one dedicated-oa systems ASHRAE Journal 46(10: ANSI/ASHRAE/IESNA Standard , Energy Standard for Buildings Except Low-Rise Residential Buildings. 3 0 A S H R A E J o u r n a l a s h r a e. o r g N o e m b e r