Control. of minimum outdoor air for multiple space systems

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1 The following article was published in SHRE Journal, October Copyright 2004 merican Society of Heating, Refrigerating and ir-conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of SHRE. Supply ir Control of minimum outdoor air for multiple space systems By David Warden, P.Eng., Member SHRE Most ventilating systems have shortcomings in the control of their minimum outdoor air intake. They do not always deliver the intended ventilation to the occupants, their real performance is hard to check and they do not adjust minimum outdoor air intake as the ventilation demand varies. Demand control ventilation (DCV), based on sensing the rise in space, can address these problems 8,9 but is difficult to apply to multiple space systems. Supply air control (S ) effectively applies technology to recirculating systems. It is inexpensive, saves energy, and helps ensure good indoor air quality. Supply ir Control Description S is a technique for measuring the outdoor air fraction in the supply air and controlling the outdoor air intake, so the supply air always contains a high enough fraction of outdoor air to ventilate any space served by the system. It is applicable to recirculating systems serving multiple spaces where ventilation targets are based on outdoor airflow rate per person. Figure 1 illustrates application of S to a typical VV system. single sensor alternately senses in the supply duct and outdoors. valve switches between sources, and fan suction draws air through the sensor. The outdoor air intake is controlled so the rise in between outdoors and the supply air does not exceed a value that corresponds to the required minimum outdoor air fraction in the supply air. Calculation of this minimum outdoor air fraction and the maximum rise in and the outdoor air intake fraction are illustrated in Sidebars Space Ventilation Calculations, CO2 Calculations and Minimum Outdoor ir Intake Calculation. (Other values in Figure 1 were calculated in a similar manner.) Outdoor ir in Multiple Space Recirculating Systems s the supply air must contain a high enough outdoor air fraction to satisfy the space with the greatest need, most other spaces receive a surplus of outdoor air (a) because they need more supply air to control temperature than to provide ventilation, and (b) because of population diversity between spaces. This surplus unused outdoor air is bout the uthor David Warden, P.Eng., is the principal at Warden Engineering in Victoria, BC, Canada. 2 6 S H R E J o u r n a l a s h r a e. o r g O c t o b e r

2 Relief 43% O ppm O Intake ppm 18% O 82% Recirc. Containing 43% O S NO NC DDC O REF. Supply ir 53% O ppm Relief 51% O ppm O Intake ppm 5% O 95% Recirc. Containing 51% O S NO DDC O REF. Supply ir 53% O ppm to satisfy a fully occupied space System peak overall occupancy 200 cfm/p S 107 cfm/p O ppm Individual office min. supply airflow and max. occupant density 37.5 cfm/p S 20 cfm/p O ppm System 25% overall occupancy 800 cfm/p S 426 cfm/p O ppm Individual office min. supply airflow and max. occupant density 37.5 cfm/p S 20 cfm/p O ppm Figure 1: S system at peak occupancy. Figure 2: S system at partial occupancy. recirculated and often contributes more ventilation than first pass air directly from the outdoor air intake (twice as much in Figure 1). Calculations to account for the effect of unused outdoor air were developed by Kowalczewski 3 more than 30 years ago and SHRE Standard 62, Ventilation for cceptable Indoor ir Quality, has required similar calculations for multiple space systems since (see Equation 6-1 in Standard and system ventilation efficiency E v from Table 6-3, or ppendix G in ddendum 62n. 2 ) Derivations that may be most useful and easiest to find are ppendix G in Standard and SHRE Journal article Outdoor ir: Calculation and Delivery, 4 which expanded the method to handle fan-powered terminal systems and other situations with a secondary source of recirculated air. Supply ir as Occupancy Varies s the total number of building occupants varies, the unused outdoor air content and the in recirculated air varies. The sensor detects the effect this has on the supply air and, unless more outdoor air is needed for makeup or free cooling, the S system adjusts the outdoor air intake to maintain the design minimum outdoor air fraction in the supply air. Storage in the air volume served by the system causes the of contaminants directly emitted by occupants and the to lag the change in occupancy in a similar manner so even under changing conditions, is an indicator of the outdoor air content available for ventilation. Figure 2 illustrates the reduction of outdoor air intake for the previous example under 25% occupancy conditions, assuming that the total supply flow is unchanged. The system still provides enough outdoor air for any fully occupied space but the necessary outdoor air intake fraction drops to 5%. Similarly, if the number of system occupants exceeds the design occupancy, rises, the sensor detects this, and S increases the outdoor air intake to correct the rise in and the outdoor air content of the supply air. O, Sources and Short-Circuits Outdoor air often enters through windows and doors, transfers from adjacent systems with a ventilation surplus or leaks from supply ducts into the return system. When such air is recirculated, the S system detects the drop in supply air and reduces the minimum outdoor air intake accordingly. Similarly, if relief air short-circuits into the outdoor air intake, the S system detects the rise in and increases outdoor air intake as needed. VV Systems and Zone Flow Reset If total system supply airflow decreases (e.g., loaded filters or VV), the return air contains less unused outdoor air and the S system adjusts the O c t o b e r S H R E J o u r n a l 2 7

3 outdoor air intake to maintain the total outdoor air fraction in the supply air. (The outdoor air intake fraction rises, but the outdoor air intake flow falls slightly as the relief air contains less outdoor air and less is carried out of the building before it can be used by occupants.) If the outdoor air content in the supply air is higher than needed (e.g., makeup air or economizer cycle), the S system will detect and measure this. If desired, this information can be used to dynamically calculate and adjust minimum zone supply flow rates to reduce reheat and fan energy. Empty and Nearly Empty Buildings Some contaminants affecting odor, irritation and/or health come from building surfaces and contents, or are temporarily absorbed and reemitted from these surfaces. lthough not all codes require it, it is highly desirable to ventilate the building before scheduled occupancy to reduce contaminant buildup after any sustained period without ventilation (e.g., overnight or on the weekend). Similarly, a limit exists on reducing outdoor air intake during periods of very low occupancy. 62n s area outdoor air rates (0.06 to 0.18 cfm/ft 2 [0.3 to 0.9 L/s per m 2 ]) apply where 62n has been adopted into code. Elsewhere, they could be used as a benchmark, but it is a lot of air for a single occupant and less ventilation may be practical under low occupancy conditions in clean buildings with low emitting materials. In some cases, enough air may enter through general infiltration, makeup for exhausts, suction through closed outdoor air dampers and transfer of air from adjacent spaces with a ventilation surplus. s some of these sources are hard to quantify, to maintain building pressure control it is usually desirable to supplement demand ventilation control with an absolute minimum intake airflow whenever the building is occupied. simple method of providing this absolute minimum intake, such as a minimum signal to the outdoor air damper may suffice, but for large or special systems, more sophisticated methods may be appropriate. New and Existing Buildings Installation of S is simple in either new or existing buildings. In existing buildings, however, establishing the setpoint requires more work because the relationship between existing space supply rates and the design outdoor air intake needs to be checked. s most existing ventilation systems were not designed based on multiple spaces calculations, this analysis often will reveal spaces with ventilation problems. To solve these existing problems, it may be necessary to increase primary supply to these spaces, provide them with a secondary source of recirculated air or increase the outdoor air fraction for the whole system. Compared to an existing ventilation system that is properly designed and operating correctly, S saves energy to the extent that the area it serves is less than fully occupied over the system s operating hours. In practice, existing systems bring in anything from no outdoor air, to a large excess. When this is corrected by the S system, energy use will change accordingly. Cost of Supply ir Control Purchase, installation, programming, calibration and commissioning cost of S in U.S. dollars based on competitive pricing and local installation by contractors familiar with the work is likely to be about $500 plus $1,000/sensor, plus general contractor s margins and taxes. Engineering costs can be expected to be relatively minor on new projects. On existing projects, time will be spent to review existing drawings and reports, visit the site, analyze the existing system, prepare separate documents and check the installation. This will result in costs likely to be on the order of $2,000 plus $600/sensor if the work is done by an Meeting Rooms Fully occupied meeting rooms supplied from an office system require a higher minimum supply flow rate per unit area than office spaces due to their high occupant density. In 62-89, this is exacerbated by an outdoor air per person rate that is slightly high compared to similar spaces. (See the table in sidebar Space Ventilation Calculations. ) This supply rate usually is more than is needed for cooling and unless reheat is provided, the space will be overcooled. Increasing the outdoor air intake for the whole system increases the outdoor air fraction in the supply air, thus reducing the minimum supply rate for the meeting room and saving reheat but increasing initial cost and energy used to condition outdoor air. more efficient approach is to add direct recirculation of unused outdoor air from spaces that have a surplus of outdoor air. Ideally, this second ventilation source is from a central point to take full advantage of occupant diversity across a wide area (e.g., dual fan dual duct). Other alternatives are fan-powered VV terminals or transfer fans arranged to draw secondary recirculation from a location where unused outdoor air will be available (e.g., adjacent to a central return air collection point). n exhaust fan from a meeting room is not an efficient solution because it only increases ventilation to the extent that it is larger than the supply and it is difficult to control the source and quality of any air it draws into the space. The required flow of secondary air can be calculated by rearranging Equation 6 from Outdoor ir, Calculation and Delivery 4 or equation G-2 from ddendum 62n ppendix G 2 and back-calculating based on the outdoor air fraction in the primary supply. The following additional measures can also save energy: 1. button for occupants to change the supply rate or activate a transfer fan. 2. Reduce the minimum ventilation supply rate to the meeting room when the outdoor air fraction in the supply air is high (e.g., outdoor air economizer cycle). This combines well with S. 3. Sense space to reset minimum supply flow based on demand (attractive in large meeting rooms with highly variable occupancy). 2 8 S H R E J o u r n a l a s h r a e. o r g O c t o b e r

4 engineer set up for this work. On the first project, about two extra weeks of engineering time is required to adequately learn the concept and develop appropriate methods, spreadsheets, detail drawings, technical specifications, and check sheets. These costs do not include fixing existing space ventilation problems, faulty dampers or expanding the DDC system capacity (one extra analog input and one extra digital output). Comparison With Other Minimum O Control Strategies new control method is only relevant if it offers benefits compared to existing alternatives. This section is intended to help readers assess this. First, it discusses significant criteria and identifies typical problems that may occur. Second (Table 1), it lists alternative methods and identifies criteria that I expect they may have problems satisfying. Full discussion of these opinions is not possible in this article, but I expect that once alerted to a potential problem, readers will be able to assess this for themselves. Third, other -based methods are discussed in more detail. Function: If a method cannot be counted on to perform the desired function when it is needed, then the method is of little value. One example would be control from temperature balance in warm weather when there is inadequate temperature difference between outdoor and return air. Temperature balance also can be problematic at other times due to inability to accurately read the mixed air temperature. Other examples discussed below are multisensor space control and average return air control. The function of S is discussed previously. Setup: Methods that depend on setting a specific minimum flow, but have no provision to permit accurate direct measurement of that flow by a balancing agent, are difficult to set up. Sometimes they can be set up based on the difference between supply and return flows, but it is often not possible to do this with sufficient accuracy. In this case, they are usually set up by measuring the total flow and the temperatures of return, outdoor and supply air, then calculating the minimum outdoor airflow based on temperature balance. Difficulties in getting accurate temperature measurements sometimes make the results more an expression of hope rather than an accurate representation of what is happening. ssuming that the airsupply system is (or will be) balanced, S can be set up in a just few minutes without requiring balancing equipment or waiting for special weather conditions (see Sensing ). Reliability: Control based on using a fixed signal to place dampers precisely in a slightly open position deteriorates over time as wear increases play in linkages and when the setting is lost during damper or actuator maintenance. My experience is that control from direct flow sensing in the outdoor airstream is subject to dirt blocking the upstream pitot openings or affecting electronic sensing elements in the airstream. Multisensor space control is a concern due to its many potential failure points. S has relatively little that can go wrong. One sensor and one switching valve serve the whole system. S can alarm a fault, and as it is an active method of control, it compensates for play in damper linkages. Energy: Systems that do not reduce the minimum outdoor intake during partial occupancy can waste a lot of energy as most buildings have far less than the peak number of occupants most of the time. -based DCV systems can deal with this issue, the other (non-dcv) systems cannot. S also saves by adjusting for recirculation of outdoor air that has entered through doors or windows or has spilled over from adjoining systems with a surplus. nother energy benefit with S for VV systems is the ability to save reheat energy by resetting the space flow minimums down based on sensing any rise in outdoor air content of the supply air due to makeup air or an airside economizer cycle. Cost: Control from a sensor in each space is extraordinarily expensive. Multipoint space sensors generally will be less costly but are not cheap. separate minimum outdoor air damper and outdoor air duct arranged for flow measurement are more expensive than most other methods and a separate outdoor air fan or direct flow sensing increases this cost. The cost of the remaining methods likely is acceptable for most multiple space systems but, in some cases, may be a burden for very small systems. The cost of S is reasonable as only one sensor is needed for the whole system. The one sensor potentially can handle more than one system, and no special mixing damper or intake arrangement is needed. Maintenance: Systems based upon placing dampers in a precise partially open position should be regularly checked and recalibrated by a balancing agent. Direct flow sensing in the outdoor air intake implies regular and careful cleaning. sensors also need periodic checking and recalibration. With the right sensor, this is simple and takes five to 15 minutes. Some operators do it themselves and some use a specialist at about $100/sensor including a report. This is more than many owners currently pay because at present they are not doing the maintenance needed to ensure their existing system really works. VV: Systems that mix in a fixed minimum percentage of flow from the outdoor air intake have serious problems with VV. If the supply flow drops to 50%, so will the flow through the outdoor air intake. If the damper setting is based on the minimum supply flow then at maximum flow, outdoor air intake may be double what is needed, resulting in high energy use and the cost of larger equipment. If the dampers modulate through a range of minimum positions based on total supply flow, the result will be better, but not particularly accurate. daptability: If the actual number of occupants served by the system is greater or smaller than the number on which the design is based, S and other based systems will automatically adapt. Other systems will deliver the wrong outdoor air quantity unless the intake is recalulated and reset. If open plan office space is changed to a meeting room O c t o b e r S H R E J o u r n a l 2 9

5 Minimum Outdoor ir Control Method Description Function Setup Reliability Energy Cost Maintenance VV daptability Verifiability Record Passive Damper Position Separate Damper Minimum opening position on modulating damper set by damper actuator. No flow measurement provisions. Two position minimum outdoor damper separate from any modulating O damper. O Duct Two position minimum O damper plus separate minimum outdoor air duct arranged for flow measurement during system setup. O Fan Separate constant flow, ducted, fan powered, minimum outdoor air to main system supply. DOV Separate constant flow, ducted, fan powered, minimum outdoor air system directly serving each space. n/a ctive Control Direct Flow Sensing Separate minimum outdoor air duct with flow sensing controlling a minimum O damper. Temperature Balance Control O & R dampers to provide minimum outdoor air based on a temperature balance between O, R and S (and supply airflow for VV). Pressure Balance Separate two-position min. O damper acts as a flow orifice. R damper controls suction pressure in the mixing plenum to ensure the desired minimum O intake airflow. ctive Control with Occupancy Feedback Space Ind. Sensors Control O & R dampers based on a separate sensor for each space. Space Multipoint Control O & R dampers using a limited number of sensors to sample every space. verage Return Control O & R dampers based on the common return air from all spaces or groups of spaces. Supply Control O & R dampers based on sensing in the supply duct. Note: This table is intended to provide a simple indication of issues to watch out for with different methods of outdoor air control. It is somewhat subjective (e.g., what cost is acceptable?) and does not provide absolute ratings that would apply in all circumstances. The darker shading indicates that functional issues are likely to make the method inappropriate in most cases. Table 1: Potential issues for minimum outdoor air control methods. without increase in system-wide population, most systems just require changing the minimum supply rate to the space, but with some systems, more work is required. With room sensing, an extra sensor or sampling point is needed. With DOV systems ductwork changes, rebalancing and increasing outdoor air intake are required. Verifiability: With most systems it is not practical for an operator to personally check if the system is delivering the intended minimum outdoor air. They can check that something is happening, but to check whether it achieves the desired result, they need to call in a balancing agent. This is relatively expensive and rarely done. With S it is easy to check if the supply air readings are as they should be, and checking sensor calibration only takes a few minutes with simple equipment (see Sensing ). Record: Being able to log and chart important aspects of HVC system operation is now the expected norm, yet few systems have provision to do this for ventilation, which is one of the most critical issues in terms of health, occupant satisfaction and protection from litigation. Many outdoor air-control systems have no really meaningful way to do this. With dedicated outdoor air fans, you can record that the fan was pressurizing its duct system. Inlet flow sensing can record intake of outdoor air. S can record the outdoor air content in the supply to the spaces and space can give an indication of outdoor air per person delivered to the space. Direct outdoor air ventilation (DOV) systems that do not recirculate but directly supply each space with enough outdoor air to meet its peak demand are popular in Europe and gaining interest in North merica. Their outdoor air intake is the sum of the peak space requirements; hence, ventilation calculation is simple. Some other differences are: 1) They do not provide airside free cooling or enhanced ventilation in mild weather. 2) They do not recirculate unused outdoor air back to the occupants (e.g., from empty spaces, partially occupied spaces and air that bypassed the breathing zone). 3) Ventilation of over-occupied spaces is less effective as recirculated air from 3 0 S H R E J o u r n a l a s h r a e. o r g O c t o b e r

6 Sensing sensors are sophisticated devices that are better and less costly than in the past. Still, they are not cheap, not precise and their calibration drifts over time. Special attention is needed to how they are applied, how they are calibrated and how accuracy is checked. Sensor Configuration. The sensing confi guration shown in Figure 1 uses a single sensor to alternately sense supply air and outdoor air via a three-way EP valve. Fan suction draws outdoor air samples through the sensor. This confi guration offers the following benefi ts: Outdoor air is measured rather than assumed. This compensates for the variations that occur through the day, during the year, and between sites. single sensor provides far more accurate control than separate supply air and outdoor sensors as error is largely cancelled out when the difference in is calculated. Sensor faults can be alarmed based on outdoor air readings outside normal range. The sensor can be located remotely in a well-lit location at a comfortable height to check the reading on a built in display, verify sensor accuracy and recalibrate the sensor. My experience is that using 0.25 in. (6 mm) pneumatic tube, runs from source to sensor of 33 ft (10 m) work well. The suction run from the sensor could be much longer as a larger tube diameter could be used without affecting the time to change from sensing one source to the other. single sensor can control any system size. Where appropriate, valving can be arranged to permit a single sensor also to sense return air, the second supply duct of a dual fan system, supply ducts from more than one system, or individual spaces. Sensor Calibration. sensors are calibrated using specially formulated test gases (typically nitrogen to set the zero and a /nitrogen mix to set the top of the range). To minimize error, it is important to use calibration gas at a close to that at which you wish to control. If the supply air is going to be at about 700 ppm, calibrating with special 700 ppm ±2% gas is far more accurate than with standard 2,500 ppm ±2% gas. It also makes it easy to check whether the sensor needs recalibrating. Verification and Recalibration. Checking sensor accuracy is simple, quick and can avoid unnecessary recalibration. Provided the sensor has a display, it only requires a bottle of gas at the of interest, a regulator and a plastic tube. Sensors drift most when they are new, so checking three months after installation is a good idea. Thereafter, if the recommended sensor confi guration with outdoor air reference is used, an annual check should be fi ne. If the system has an outdoor air economizer, minimum outdoor air will be seasonal, and it is best to check the sensor at the start of the main season in which the system will operate on minimum outdoor air. uto recalibration is an option on many sensors. It typically resets the sensor based on the assumption that the lowest reading of the week is at the typical outdoor. If this assumption is not true, then auto recalibration can drive the sensor into serious error. The recommended method (Figure 1 ) avoids this risk, is more accurate and offers self-checking. Control Issues. The control system should read the supply air most of the time and periodically sample the outdoor air. The supply air changes quickly as the dampers move but the outdoor air normally is comparatively stable. sensors sample the air in their sensing chamber (e.g., every 10 seconds), and noticeable jitter exists between readings even if there is no change in the gas. For control purposes, the readings should be averaged to improve accuracy and stability. s fast changes are not desirable in minimum outdoor air control, averaging the signal over one or more minutes is not a problem. When switching between sources, the controls should ignore readings until the new sample flows through the sensor, readings stabilize and it has enough readings to average out jitter. Following any extended period when outdoor air ventilation was shut down or the space was unoccupied (night or weekend), a separate control method should be used to bring in suffi cient outdoor air to dilute contaminants that have built up while the system was off. other spaces is not available to help flush contaminants. 4) They must be physically modified and rebalanced if a change is made in the building (e.g., new meeting room). If all spaces are continuously at peak occupancy, a DOV system requires less outdoor air intake than a recirculating system. However, in all buildings with a normal diversity of population that I have checked, DOV systems have required more outdoor air intake than recirculating systems because much of their outdoor air is delivered to spaces that are empty or partially occupied, and is then exhausted. (In a partially occupied building, the DOV outdoor air intake is unchanged and is far greater than for a recirculating system with S.) Individual sensors for each space with minimum outdoor air intake controlled to satisfy the space with the greatest demand is possible in theory, but unattractive in practice. s most spaces could potentially have the greatest demand under some partial occupancy condition, the number of sensors required is generally large. s the number of sensors increases, so does the cost, the maintenance needs and the risk of system failure. This method depends on every sensor working properly. If any sensor reads significantly high, it will increase the outdoor air intake for the whole system, quite possibly to 100%. Limiting sensors to selected typical spaces, or disregarding a certain percentage of high readings, risks ignoring and underventilating the spaces that are most heavily occupied at a particular time. Multipoint space sensing uses one sensor to sample many spaces plus outdoor air (just as one sensor can sample supply air and outdoor air). This would improve cost and reliability, but the number of spaces that can be effectively sensed with one sensor is limited by the sampling time needed to draw air from a space and get a stable reading. verage return air sensing generally is not a satisfactory method of minimum outdoor air control because it does not ensure delivery of the required minimum outdoor air per person to a fully occupied space when the overall system is only partially occupied (see sidebar Why verage R Control Does Not Work ). It is similar to controlling the temperature of a whole building from a single thermostat. Setting the return air control setpoint for the worst partial occupancy condition is no solution, as it would cause corresponding overventilation during full occupancy. 2 6 S H R E J o u r n a l a s h r a e. o r g O c t o b e r

7 dvantages of S Control: 1. Excellent probability of good ventilation because: a. ccurate setup is easy at any time of year. b. S corrects for damper linkage wear and avoids the potential problems of damper position setpoints being changed or lost during maintenance or repair. c. It corrects for short circuiting of relief air back into the outdoor air intake. d. It is quick and easy to check the outdoor air system operation and sensor accuracy. e. The sensor readings can be monitored and charted to demonstrate performance. f. Outdoor air readings can trigger an alarm if they suggest that the sensor needs recalibration or is faulty. g. If the original total building design occupancy is exceeded, S automatically adjusts the outdoor air intake. 2. Energy savings can be large if the average number of occupants is much less than the peak number or if the system replaces one that doesn t operate correctly. 3. Installation and maintenance costs are low, as a single sensor serves one or more entire systems. 4. S is easily applied to new or existing buildings. Space Ventilation Calculations These calculations apply to any multiple space recirculating system regardless of the method of minimum outdoor air control. The example is based on the outdoor air per person rates of SHRE Standard 62 ( editions) rather than the two-part rates of ddendum 62n but refers to ddendum 62n where a concept is not dealt with in the base standard. For convenience, fl ow terms and equations have been calculated on a per person (R terms) or per unit area basis (V V terms). In principle, one could start by choosing a design outdoor air fraction in the supply air (Z s ) and go straight to Step 4 to calculate the corresponding minimum space supply airflow rate for each space type. In practice, Steps 1 to 3 help optimize between outdoor air intake and minimum space fl ow needs: Step 1: Select the space type on which design will be based. Select the common space type with the least supply air per person. Other spaces will be supplied with minimum flow rates calculated in Step 4. In this case, the dominant occupancy type is offi ce space and the offi ces that receive the least supply air per person will be private interior offi ces with low thermal loads. s this is a VV system, space supply will be least when the supply air is coldest. Step 2: Determine the minimum supply flow per person to the space. ssume for this example, a 150 ft 2 (14 m 2 ) offi ce with two people plus three lights emitting 210 W, negligible equipment load (e.g., laptop computers) and a supply air T of 15 F (8 C). The resulting space supply fl owrate R sz without reheat is: R sz = (2P x 250 Btu/h/P W x Btu/h/W)/ 15 F/1.08 Btu/h/ F/cfm/2P = 37.5 cfm/person. Step 3: Calculate the outdoor air fraction (Z ) required in the supply air. From Standard the ventilation target for occupants of offi ce spaces is R = 20 cfm/person. obz The room ventilation effectiveness E = 1 (62 n, table 6.2) and as z E z = R obz /R oz where R is the required space supply rate to achieve oz the required ventilation rate in the breathing zone R obz : R oz = R obz /E z = 20 /1 = 20 cfm/person nd the minimum required outdoor air fraction in the space supply air, Z z = R oz /R = 20 cfm/person/37.5 cfm/person = 0.53 (i.e., 53%). sz s the primary supply air is the sole source of ventilation to the space, the minimum required outdoor air fraction in the system supply air to satisfy this space (Z s ) is also Step 4: Establish minimum supply flow rates for each space type. Based on the above outdoor air fraction, calculate the corresponding minimum supply rate required to ventilate each space type when it is at peak design occupant density as shown in the following table. Space Type Min. O Rate/Person cfm/person R obz Space ir Distribution Effectiveness E z O Fraction In Supply ir Z s Min. Supply ir Rate/ Person cfm/p R sz = R obz / E z Z Design Max. Occ. Density ft 2 /P z / P z Min. Supply ir Rate/ Unit rea cfm/ft 2 V sz = R sz /( z / P z ) Office a Office (heating) b b 0.53 a d 75 d f Meeting Room c Reception c e g 0.5 Notes a. Interior private offi ce; the occupancy type upon which the minimum O fraction in the supply air was based. b. Perimeter offi ce: When heating from the ceiling with air, supply air can bypass the occupants, particularly if VV turndown results in hot supply air and low discharge velocity. This example uses E =0.8 (20% bypass), as per 62n. If supply air is too hot and the velocity is low, E z can be lower than 0.8. In this case however, stratifi caz tion usually causes thermal discomfort and correcting the comfort problem also improves E z. c. Used E = 1.0, even for perimeter meeting rooms, as the high internal heat gain minimizes heating needs. z d. 75 ft 2 /person (7 m 2 /person) for private offi ces because two people routinely meet in a 150 ft 2 (14 m 2 ) private offi ce. (62-89 s default density of 7 people/1,000 ft 2 [93 m 2 ] is insuffi cient for private offi ces in any building I have ever seen. 62n s default of 5 people /1,000 ft 2 (93 m 2 ) is even less realistic for private offi ces but is reasonable for the diversifi ed building density) e. 20 ft 2 /person (1.9 m 2 /person) (50 people/1,000 ft 2 [93 m 2 ]) for meeting rooms as per 62-89, 62n, and personal experience. f. The minimum ventilation supply rate for perimeter offi ces heating with air is rarely a practical concern when they are on the same system as interior offi ces. E may v be less than 1.0 but the minimum space supply rate to achieve comfort, and maintain VV terminal control on turndown, is usually higher than is needed for adequate ventilation. g. Meeting rooms in offi ce buildings generally require reheat to maintain room temperature if they are supplied with adequate ventilation air from the primary system alone. lternative solutions are discussed in the Sidebar Meeting Rooms. Table 2: Minimum supply flow rates for each space type. 2 7 S H R E J o u r n a l a s h r a e. o r g O c t o b e r

8 Calculations Step 1: Calculate the rise in if all outdoor air was used. If the building only was supplied outdoor air at the rate it is used by the occupants, there would be no unused outdoor air remaining in the building return. The rise under this steady state condition can be calculated using an equation similar to that in SHRE Standard , ppendix C. C ru = 1,000,000 N b M /R os = 1,000,000 x x 1.2 / ppm where C ru = in recirculated air if all outdoor air supplied to the building is used = outdoors 1,000,000 is conversion from a fraction to ppm N b = generation rate per person at base metabolic rate = cfm/person ( L/s per person) M = Relative metabolic rate in met units from Standard , ppendix C, Figure C-2 = 1.2 R os = The rate at which occupants use outdoor air, i.e. the average design outdoor air ventilation rate for the area served by the system = 20 cfm/person (10 L/s per person). This was based on the worst occupant distribution (greatest fraction of occupants in spaces requiring the highest ventilation rates). Return air may be regarded as a mixture of outdoor air that has been used and outdoor air that was supplied in excess of requirements and remains unused. In this case, the return air may be regarded as a mix of used outdoor air at 550 ppm above outdoors and unused outdoor air at 0 ppm above outdoors. Step 2: Calculate the supply air rise. Supply air may be regarded as a mix of recirculated used outdoor air at 550 ppm above outdoors and unused outdoor air. This unused outdoor air is made up of recirculated unused outdoor air plus fi rst pass outdoor air directly from the outdoor air intake. If supply air is 100% outdoor air, its is 0 ppm above outdoors, and from Step 1, a of 550 ppm above outdoors corresponds to 0% outdoor air (for this example). Hence, the rise in corresponding to the desired outdoor air fraction in the supply air from the Space Ventilation Calculation sidebar (Z s = 0.53) is: C = Z s 0 + (1 Z s ) 550 = 0 + (1 0.53) 550 = 260 ppm C s Minimum O Intake Calculation (Optional) Calculating the minimum outdoor air intake airflow is not essential to implement S, but is needed for equipment sizing and is useful for optimization as discussed below. Calculation Example For this example, the overall system supply flow rate (V s ) is assumed to be 1 cfm/ft 2 (5 L/s per m 2 ) and the average occupant density across the area served by the system is assumed to be a 200 ft 2 /P (19 m 2 /P ). s in Sidebar Calculations, the average required ventilation rate at peak system conditions is assumed to be 20 cfm/person (10 L/s per person). The outdoor airflow used by all occupants of the system, V ou = 20 cfm/person/200 ft 2 /person = 0.1 cfm/ft 2 (0.004 L/s per m 2 ). The outdoor air used by all occupants expressed as a fraction of the system supply, s = V ou / V = 0.1/1.0 = 0.1. os s the outdoor air fraction in the supply air Z s = 0.53 from sidebar Space Ventilation Calculations, and of this, s = 0.1 is used, the outdoor air fraction remaining in the system return air, Z r = Z s s = = From this, Y, the outdoor air intake flow expressed as a fraction of the supply airflow can be calculated with a simple mixing calculation: Y = (Z Z s Z r ) / (1 - Z r ) = ( ) / (1 0.43) = This means that 18% first pass outdoor air directly from the intake is suffi cient to provide a 53% outdoor air fraction in the supply air. The remaining 35% is outdoor air that has passed through the building without being used, and is then recirculated. Is the nswer Right? Let s check the result against the multiple spaces equation from SHRE Standard Expressing Equation 6-1 in the terms from this article and using the same data: Y = s /(1 + s Z s ) = 0.1/( ) = The result is identical, which is not surprising as the logic is that used to develop multiple spaces equations: 1,3,4,5 1) Calculate the outdoor air fraction needed in the supply air. 2) Calculate the outdoor air used in the whole area served by the system. 3) From 1 and 2, calculate the outdoor air fraction remaining in the return air. 4) From 1 and 3, calculate the outdoor air intake required. (Steps 3 and 4 are usually combined into one more complex equation and this makes the logic less apparent.) Why So Simple? Engineers who have applied Standard 62 s multiple spaces calculations know how much work it can be and may be puzzled that the outdoor air calculation for a large system can be this simple. The answer lies in the way the calculation process is organized. From reading Standard 62, the most obvious process is that outlined in 62n. Calculations are done for each space based upon predetermined minimum space flow rates. System level calculations then are based on adding space values and applying population diversity. If the spaces change, the calculations are redone. While this method is valid, it is akin to weighing a ton of sand a grain at a time then reweighing it if the arrangement of grains changes! It is really only suitable for people who have a lot of free time and love doing calculations! The method used in this article calculates the outdoor air fraction in the supply air and the outdoor air intake needed to satisfy one space by looking only at that space and at the system as a whole. The distribu- tion of area, people and airflows in the other spaces is irrelevant for this purpose. This can be seen from the original derivations of the multiple space equations. 1,3,4,5 Minimum supply rates for other space types are back calculated based on the outdoor air fraction in the supply air being set to meet the needs of the base space. ll of this is done on a per unit area basis, so that it can be done without knowing partitioning details and to establish a simple table showing the minimum supply rate per unit area needed to adequately ventilate each space type. The table provides simple space ventilation rules for both the original designers and those designing future building changes. Optimization Minimum space supply airflows and minimum outdoor air intake flow are interdependent. If the outdoor air intake flow seems excessive, it can be lowered by recalculating based on higher minimum space supply flow rates, or alternatively the higher space flow rates can be directly calculated by rearranging the formulae. Similarly, if the minimum space supply flow rates seem too high, they can be lowered and the effect on outdoor air intake checked. O c t o b e r S H R E J o u r n a l 2 8

9 Limitations of Supply ir Control: 1. For single space systems, S works, but space sensing is more effective. 2. S may not be affordable for very small systems, unless they share a common sensor. 3. s with any minimum outdoor air system, S should be periodically checked and recalibrated. (Having a sensor makes this need more obvious, but makes the task easier.) 4. If no space requires the design minimum fraction of outdoor air in the supply air, S only responds to the overall reduction in occupancy (usually not an issue). 5. fter any unoccupied period, S initially will not bring in enough outdoor air to flush any contaminants that have built up during the unoccupied period. simple flushing cycle should be implemented to provide good air quality following unoccupied periods. 6. S cannot be applied if minimum outdoor air needs are based totally on makeup air requirements or an outdoor air per unit area based rate. 7. If minimum outdoor air needs are based on a combination of airflow per person and makeup air requirements and/or an area-based rate, S cannot be used alone but can be part of a more complex control strategy. SHRE Standard 62 ddendum n 62n changes the ventilation rate calculation section of Standard One signifi cant change is the shift from outdoor air per person ventilation rates, to additive rates made up of a smaller rate per person, plus a rate per unit area. nother is the reduction in net ventilation for most fully occupied spaces. The reduction in ventilation rate varies depending upon occupant density and space type. Fully occupied seating areas of auditoriums drop from 15 cfm/person (7 L/s per person) to 5.4 cfm/person (2.5 L/s per person). 150 ft 2 (14 m 2 ) private offi ce with two occupants drops from 20 cfm/person (10 L/s per person) to 9.5 cfm/person (4.5 L/s per person) though the net reduction in a multiple space system with diverse occupancy is slightly less. Some other space types have much smaller changes. Health and productivity are not discussed in the addendum or its foreword, but there is broad acceptance that worse health, absenteeism and task performance are associated with ventilation rates below 20 cfm per person (10 L/s per person). 6 In view of this, why the reduced rates? Part of the answer is that to the extent that the 62n rates and additive structure were based on scientifi c data, this data was more about perceived odor rather than health. second factor is a change in objectives. Unlike previous versions of , the rates in 62n were chosen with the goal of setting an absolute minimum standard rather than a guideline to what is desirable. One consideration was to change from a ventilation rate at which 80% of people would fi nd odor acceptable when they fi rst arrived, to a much lower ventilation rate at which 80% of people would fi nd odor acceptable after they had adapted to it. While 62n generally reduces ventilation rates under normal operating conditions, it has the reverse effect when the building is almost empty. t night, a system serving 100,000 ft 2 (9290 m 2 ) of offi ce space would have to operate and provide more than 0.06 cfm/ft 2 100,000 ft 2 = 6,000 cfm (2831 L/s) of outdoor air to meet the needs of one or two people who are working late. If a roving security offi cer is entitled to the same ventilation quality, then this would be required all night. While some odor studies are supporting this, the cost and energy consumption seem excessive. My experience suggests that the benefi t is limited and that few owners will run the ventilation systems in nearly empty buildings regardless of what the standard says. Standard 62 s rates always have relied heavily on judgment and a compromise between conflicting views as to whether they should rise or fall. Through the years, the rates have varied widely, and they probably will change again in response to different weighting of health or other issues and more scientifi c information. Most countries base their ventilation standards on rates per person, and the extent to which 62n s rates will be adopted into code in North merica remains to be seen. Even in jurisdictions that adopt 62n s minimum rates, it seems desirable to operate with a higher rate at peak occupancy, except possibly in extreme weather conditions. 62n and Demand Control Ventilation With 62n s two part rates, when the number of occupants in a space changes, so does the net rate per person and so does the rise in corresponding to the 62n requirement. Similarly, when the number of occupants served by a recirculating system varies, so does the rise in supply air corresponding to the outdoor air fraction in the supply air that is required to meet 62n. S can be adapted to handle 62n by adding a sampling point in the return air. The control system calculates the system occupancy based on the rise in between the supply air and the return, then calculates the rise in supply air that corresponds to the 62n requirement. When the occupant density is relatively high, this method of control is satisfactory. However, as the building empties, the ventilation rate per person required by 62n becomes very high and the differences become too low for practical measurement. When this point is reached, the minimum intake flow must be maintained at approximately this level. Sometimes exhaust makeup will provide enough outdoor air to meet this requirement. When it does not, relying on a minimum position signal to the outdoor air damper is one possibility. more accurate option is to use a separate fi xed position minimum outdoor air damper as a measuring orifi ce and control the suction pressure across it by modulating the recirculation damper. n alternative approach is to sense the rise in between outdoors and the return air and measure the minimum outdoor air intake flow. Based on these two values, the control system can calculate the outdoor air intake airflow required to maintain the necessary outdoor air fraction in the supply air and adjust the outdoor air intake accordingly. This method can theoretically track the 62n requirement right down to zero, but it costs more and requires additional maintenance. s minimum outdoor air intake requirements under 62n are less dependent on the number of occupants, why would one wish to use -based demand ventilation control? Possible reasons include: Local code authorities may not have adopted the 62n rates; desire to improve air quality when the building is fully occupied and save energy when it is not; Concern about the performance of other methods of controlling minimum outdoor air; and desire to have a measured record of ventilation delivery. 2 9 S H R E J o u r n a l a s h r a e. o r g O c t o b e r

10 Conclusions S offers significant benefits for systems that recirculate from multiple spaces. These include energy savings, simple maintenance, better assurance of adequate ventilation and the ability to measure and record performance. Benefits are largest where indoor air quality is of concern, there are large variations in the number of occupants, the climate is extreme, or energy rates are high. It is not suited to single space systems, is not needed where makeup needs exceed ventilation needs, and requires modification if used with two-part ventilation rates (ddendum n). cknowledgments The author would like to acknowledge the support of Kevin Jeffries and Jack Meredith of the British Columbia Buildings Corporation in developing S. References 1. NSI/SHRE Standard , Ventilation for cceptable Indoor ir Quality. 2. ddendum 62n, NSI/SHRE Standard , Ventilation for cceptable Indoor ir Quality. 3. Kowalczewski, J Quality of air in air conditioning. I- RH (2). 4. Warden, D Outdoor air, calculation and delivery. SHRE Journal 37(6). 5. Ke, Y-P., S.. Mumma generalized multiple spaces equation to accommodate any mix of close-off and fan powered VV boxes SHRE Transactions 102(1). 6. Seppanen, O.., W.J. Fisk, M.J. Mendell ssociation of ventilation rates and -s with health and other responses in commercial and institutional buildings Indoor ir 9: Krarti, M., et al. 1999, Techniques for measuring and controlling outside air intake rates in variable volume systems. JCEM TR/99/3 (SHRE Research Paper). 8. Emmerich S.J.,.K. Persily State-of-the-art review of co2 demand controlled ventilation technology and application. NIST, NISTIR Persily,.K., et al Simulations of indoor air quality and ventilation impacts of demand controlled ventilation in commercial and institutional buildings. NIST, NISTIR Relief ir Outdoor ir ppm Supply ir ( + 360) ppm max. for 15 cfm/person in a fully occupied classroom. ( + 590) ppm, when partially occupied and under R control. Return ir Sensor Setpoint ppm Classroom 1 Fully Occupied School 33 cfm S/Person 15 cfm O/Person ( + 670) ppm Classroom 2 Fully Occupied School 33 cfm S/Person 16 cfm O/Person ( + 670) ppm Classroom 3 Fully Occupied School 33 cfm S/Person 15 cfm O/Person ( + 670) ppm Classroom 4 Fully Occupied School 33 cfm S/Person 15 cfm O/Person ( + 670) ppm Partially Occupied School 33 cfm S/Person 11 cfm O/Person ( + 900) ppm Partially Occupied School No Occupants in Classroom 2 ( + 590) ppm Partially Occupied School No Occupants in Classroom 3 ( + 590) ppm Partially Occupied School No Occupants in Classroom 4 ( + 590) ppm Figure 3: R control with a simple classroom system. s were calculated based on a met rate of 1.2. Peak occupancy ventilation rates and s are shown in grey. Partial occupancy ventilation rates and CO s are shown in black with problem conditions in red. 2 Why verage R Control Does Not Work This example illustrates R control with a simple classroom system. The outdoor air intake is controlled from a sensor installed in the return air. The return air setpoint is 670 ppm above outdoor, as this is the rise when all classrooms are occupied and receiving the intended minimum outdoor air ventilation rate. During partial overall occupancy, any fully occupied classroom is underventilated. Occupancy patterns like this are common. In a school, it may occur when some classes are at a special event, when some classrooms in a wing are unused due to reduced enrollment or when the facility is used for night school. In an offi ce building it can occur at night, on weekends or when a building is partially tenanted. s were calculated using the method illustrated in Sidebar 2, based on a met rate of 1.2. Peak occupancy ventilation rates and s are shown in grey. Partial occupancy ventilation rates and s are shown in black with problem conditions in red. During partial occupancy, supply air bypassing through empty rooms reduces the rise in between the supply air and the average return air from 310 ppm to approximately 80 ppm. s the return air is controlled at 670 ppm above outdoors and this is 80 ppm above the supply air, the supply air is = 590 ppm above outdoors. This is 230 ppm above the maximum acceptable in the supply air to a fully occupied classroom. The rise in between the supply air and the fully occupied classroom is unchanged, so the in this classroom also is 230 ppm high. ( = 900 ppm above outdoors). The result is that the fully occupied classroom has an outdoor air ventilation rate that is only 11 cfm/person (5 L/s per person) rather than the 15 cfm/person (7 L/s per person) that was intended. 3 0 S H R E J o u r n a l a s h r a e. o r g O c t o b e r