CO2-based demand-controlled ventilation with ASHRAE
Contents
1. zone population P p eople Returning to our example and assuming the same CO2 generation rate N 0 0105 cfm of CO2 person the indoorto outdoor difference in CO2 concentrations is 1250 ppm at design occupancy But as the number of people in the space decreases the desired indoorto outdoor difference in CO2 concentrations changes because the effective outdoor airflow rate on a cfm person basis is no longer constant With 20 occupants the 2004 version requires 210 cfm of outdoor air This equates to 10 5 cfm person compared with 8 5 cfm person at design occupancy At 10 5 cfm person the desired difference in indoorto outdoor CO2 concentrations drops to 1000 ppm when the zone population is 20 Figure 4 In ASHRAE 62 1 2004 the effective cfm person ventilation rate Vo varies with population Therefore the desired difference in indoorto outdoor CO2 concentrations Cs Co also varies Controlling to a constant differential that s based on design occupancy will underventilate the zone at partial occupancy Bottom line ASHRAE 62 1 2004 makes it more difficult to implement CO2 based DCV because the effective cfm person and therefore the desired difference between indoor and outdoor 4 e Trane Engineers Newsletter volume 34 5 CO2 concentrations vary with occupancy Figure 4 More difficult but not impossible CO2 based DCV ina single zone system In an application where the ventilation system2. lt gt Soe g Vot design P2design 0 0105 550 cfm 65 people 350 ppm 1600 ppm 4 Set the target indoor CO2 concentration at Vot min equal to the outdoor CO2 concentration Co Comin 350 ppm When the indoor CO2 concentration equals Cs design 1600 ppm for our example Vot should equal Vot design 550 cfm When the concentration of COz2 indoors equals Cs min 350 ppm Vot should equal Vot min 60 cfm When the indoor CO2 concentration is providing insights for today s HVAC system designer between Cs min and Cs design a controller should adjust outdoorair intake flow Vot proportionally between Vot min and Vot design C C F s actual s min Va eet SON canes Vii E Wea ot ot design 7 ot min ot min design s min J As Figure 6 shows the proportional control approach yields an outdoorair intake flow Vot that equals or exceeds the requirement of the 2004 standard This control strategy is easy to implement but it does overventilate the zone somewhat at partial occupancy It requires a modulating outdoorair damper and a controller with two CO2 limits Cs design Cs min and two OA damper limits that correspond to intake airflows Vot design Vot min Note A simple improvemert to this approach is to use a value other than zero for minimum population Pz min In most cases this results in actual intake values that more closely approach the minimum values required
3. s floor area cfm ft drops to 210 cfm 75 cfm person x the zone under the 2004 standard Therefore ASHRAE 62 1 prescribes 20 people 0 06 cfm ft x 1000 ft versus 15 cfm person under ASHRAE A E en people The comparison in Figure 3 reveals two 62 1989 through 2001 related sources Rp and another for important effects of the 2004 standard For these two reasons CO2 based building related sources Ra First the required design ventilation DCV under ASHRAE 62 1 2004 Ve Rp P Rax Ay fall cera ge lower _ provides less potential energy savings for most space types Figure 3 where Rp required outdoor airflow rate per person cfm person Table L Minimum ventilation rates in ASHRAE 62 1 2004 versus ASHRAE 62 1989 thru 2001 Pz Poe population number of people i Required ventilation cfm 1000 ft Change Ra required outdoor airflow rate per unit ge area cfm ft2 Occupancy category 62 1989 thru 2001 62 1 2004 2004 1989 1989 Az zone floor area ft Education Art classroom 300 380 27 Let s revisit the lecture classroom in SSe POmS 09S SA dk a our example ASHRAE 62 1 2004 Classrooms ages 9 and up 525 470 10 requires 7 5 cfm of outdoor air per Hoge Gk ajecu 315 250 Se person plus 0 06 cfm of outdoor air per Multi use assembly 1500 810 46 square foot of floor area Figure 3 Science labs 500 430 14 With a design population of 65 and a Food beverage
4. by the standard less overventilation than the approach described in the ASHRAE 62 1 2004 user s manual Single setpoint Following is an alternative control strategy that may result in less overventilation for some occupancy categories 1 Pick a reasonable value other than zero to represent the minimum occupancy for the zone Pz min and find the required intake flow of outdoor air for that population Pomin 25 people v _ 7 5 x 25 0 06 x 1000 omn io 250 cfm 2 Find the target indoor CO2 concentration at Vot min C N_ Vot min Pin 0 0105 250 cfm 25 people C s min 350 ppm 1400 ppm Intake flow Vot is adjusted to maintain the indoor CO2 concentration at Cs min 1400 ppm for any population If the OA damper reaches Vot min and the population in the zone continues to drop the OA damper remains at Vot min This overventilates the zone so the indoor CO2 concentration drifts downward Conversely as the current population nears design the zone will be overventilated As Figure 7 p 6 shows the single setpoint approach results in an outdoorair intake flow Vot that equals or exceeds the ventilation rate required by ASHRAE 62 1 2004 It s simple to implement and depending on the characteristics of the zone it may result in less overventilation at partial occupancy than the proportional control method It also requires a modulating outdoor air damp
5. 400 D 5 ke 200 3 0 40 50 60 70 zone population P p eople 2 e Trane Engineers Newsletter volume 34 5 Figure 2 CO2 based DCV under ASHRAE 62 1989 thru 2001 assumed to be constant CO generation rate OA flow rate person Ret ENCO E Space Coutdoors Controlling this to a constant differential keeps this constant will correspond to 15 cfm person of outdoor air delivered under steady state conditions Figure 1 ASHRAE 62 1989 through 2001 required that the breathing zone receive the same rate of outdoor airflow per person regardless of the number of people actually in the space 15 cfm person in our classroom example Therefore the desired differential between indoor and outdoor CO2 concentrations remained constant too regardless of how many people actually occupied the zone Figure 1 By controlling to this constant differential Cs Co CO2 based demand controlled ventilation maintains the same per person ventilation rate Vo to the space during periods of reduced occupancy Figure 2 Note Assumptions simplify DCV but they also introduce inaccuracy Remember that the CO2 generation rate N varies with occupant activity level diet and health the required ventilation rate Vo differs by space type under ASHRAE 62 1989 through 2001 and the outdoor CO2 concentration Co can vary from location to location t ASHRAE 62 1 2004 The 2004 standard changes the method for de
6. Bars cocktail lounges 3000 930 69 floor area of 1000 ft the 2004 cervice standard requires delivery of 550 cfm Cafeterias fast food dining 2000 930 54 of outdoor air 75 cfm person x65 Restaurant dining rooms 1400 705 50 people 0 06 cfm ft x 1000 ft And General Conference meeting rooms 1000 310 69 with only 20 people in the classroom Corridors 50 60 20 Lodging Barracks sleeping areas 300 160 47 Office Office space 100 85 15 Reception areas 450 210 53 Public assembly Auditorium seating area 2250 810 64 Retail Sales 300 230 23 Supermarket 120 120 0 Sports and Gym stadium play area 600 300 50 entertainment Disco dance floors 2500 2060 18 Gambling casinos 3600 1080 70 a Change compares ASHRAE 62 1 2004 with ASHRAE 62 1989 through 2001 using the default occupant densities in the 2004 standard providing insights for today s HVAC system designer Trane Engineers Newsletter volume 34 5 e 3 S TRANE Figure 4 Implementing CO2 based DCV under ASHRAE 62 1 2004 1400 an 1400 ACO varies with population making DCV more difficult g 1200 3 1200 5 At design occupancy 65 2 5 Cs Co 0 0105 550 cfm 65 p 1000 0 00125 3 E 1250 ppm A pan At reduced occupancy 20 2 e 2 ee Cs Co 0 0105 210 cfm 20 p a 8 a 0 001 S O o 600 1000 ppm 600 8 2 i S 2 400 400 5 Ext ee ane 5 200 62 1200 200 7 7 pSnRA 0 0 0 10 20 30 40 50 60 70
7. S TRANE engineers newsletter volume 34 5 from the editor Demand controlled ventilation or DCV can reduce the cost of operating the HVAC system which is bound to appeal to property owners in light of the recent surge in energy costs But implementing DCV based on indoor levels of carbon dioxide isn t quite as straightforward under ASHRAE Standard 62 1 2004 Ventilation for Acceptable Indoor Air Quality as it was under previous versions The good news is that DCV remains do able and practical especially for spaces like gyms and meeting rooms where people and their activities are the main sources of indoor contaminants In this article author and Trane application engineer John Murphy reviews ASHRAE 62 1 s requirements for dynamic reset and then outlines several methods for using CO2 sensors to successfully implement DCV In Section 6 2 7 Dynamic Reset ASHRAE Standard 62 1 2004 hereafter referred to simply as ASHRAE 62 1 explicitly permits an HVAC system to reset the design outdoor air intake flow Vot and or space or zone airflow as operating conditions change The standard doesn t give details for implementation but any system control approach that responds to varying conditions must be capable of providing at least the required minimum breathing zone outdoor airflow whenever the zones served by the system are occupied The standard goes on to list three examples of dynamic reset str
8. amper accordingly to bring in the required amount of outdoor air Figure 8 In DDC VAV systems this strategy is fairly easy to implement because all of the necessary real time information is already available digitally so no new sensors are required All of the equations are defined in Appendix A of the standard and can be solved dynamically to find the outdoorair intake flow that s currently required CO2 based DCV combined with ventilation reset In most multiple zone VAV systems the best approach often combines CO2 based DCV with ventilation reset Using this strategy Figure 8 Control points for ventilation reset in a multiple zone VAV system with controls central station air handler e Reset outdoor airflow Vot DDC VAV terminals e Required outdoor airflow Vz e Actual primary airflow Voz Current outdoorair fraction Za Vor Vee communicating BAS e Totals Vou Vos e System ventilation efficiency E e New OA intake flow V providing insights for today s HVAC system designer S TRANE CO2 sensors are installed only in those zones conference rooms for example that are densely occupied and experience widely varying patterns of occupancy The sensors in these zones are used to reset the ventilation requirement Voz for their respective zones The other zones which either are not densely occupied or do not experience significant variations in occupancy are assumed to
9. ategies e Reset based on occupancy First ASHRAE 62 1 lets you reset intake airflow in response to variations in zone population This control strategy often 2005 American Standard All rights reserved e providing insights for today s hvac system designer CO2 Based Demand Controlled Ventilation with ASHRAE Standard 62 1 2004 called demand controlled ventilation DCV responds to the actual need or demand for ventilation by regulating the rate at which the HVAC system brings outdoor air into the building There are several ways to assess ventilation demand Occupancy schedules which allow a building automation system to predict the current population based on the time of day Occupancy sensors which detect the presence or number of people in each monitored zone Carbon dioxide CO2 sensors which monitor the concentration of CO that is produced continuously by the occupants and diluted by the outdoor air Regardless of which method is used DCV strategies attempt to vary the outdoorair intake in response to the current population e Reset based on ventilation efficiency ASHRAE 62 1 also lets you reset intake airflow in response to variations in ventilation efficiency In amultiple zone VAV system system ventilation efficiency Ey depends on the current zone and system level primary airflows and it s always higher at part load than at design worst case conditions This control str
10. ategy which we Call ventilation reset dynamically resets the system s outdoor air intake based on this changing efficiency e Reset based on economizer operation Lastly the standard lets you reset the VAV minimum primary airflow settings at each box in response to variations in intake airflow For example when a system is in the economizer free cooling mode the content of the primary air is richer in outdoor air than is necessary to meet minimum ventilation requirements so the minimum primary airflow settings on the VAV boxes can be reduced and still allow the zones to be properly ventilated If any zones require reheat during economizer operation this strategy can reduce both fan and reheat energy Let s take a closer look at what may be the most common application of dynamic ventilation reset that is demand controlled ventilation based on CO2 readings to understand how it works and how ASHRAE 62 1 will affect its implementation Applying CO2 based DCV CO2 based demand controlled ventilation refers to the practice of using carbon dioxide concentrations as an indicator for the perperson ventilation rate In this context CO2 is monitored as a byproduct of respiration rather than as an indoor contaminant The rate at which people produce CO2 varies with diet and health as well as with the duration and intensity of their physical activity The more exertion an activity entails the more carbon dioxide
11. delivers fresh outdoor air to a single zone the CO2 sensor typically is installed on the wall in the breathing zone just like the thermostat Figure 5 It s usually expedient to assume that the outdoor CO2 concentration is constant so the indoor concentration rather than the difference between indoors and outdoors is measured and used to modulate the position of the outdoor air damper and thereby provide the space with the proper amount of ventilation air on a perperson basis Compared to previous versions of the standard ASHRAE 62 1 2004 requires amore complex control strategy for CO2 based DCV Following are two possible strategies the proportional control approach that s described in the ASHRAE 62 1 2004 user s manual and an alternative that requires fewer setpoints Proportional control Appendix A of the ASHRAE 62 1 2004 user s manual discusses a method for implementing CO2 based DCV ina single zone system A paraphrase of that method follows 1 Find the required intake flow of outdoor air for the design zone population Pz Ry x P3 Ra x A Vot design Voz E _ 7 5 x 65 0 06 x 1000 p 1 0 550 cfm The ASHRAE Standard 62 1 2004 User s M anual is scheduled for release in October 2005 Visit ASHRAE s web site http Awww ashrae org for availability and pricing Figure 5 CO2 based DCV in a single zone HVAC system OA ni MA alternate Sensor location hea
12. er but the controller needs only one OA damper setpoint Vot min and one CO2 setpoint Cs min rather than two limits for each Trane Engineers Newsletter volume 34 5 e 5 S TRANE Figure 7 Single setpoint control strategy for CO2 based DCV per ASHRAE 62 1 2004 1200 O O oO 800 600 400 outdoor air intake flow Vst cfm Vo t min 0 10 20 30 1800 1500 Canin m lt pei 1200 amp l Q 290 g 600 Z cimes 3 300 40 50 60 70 zone population P p eople CO2 based DCV ina multiple zone VAV system In a multiple zone VAV system the ventilation system delivers fresh outdoor air to several individually controlled spaces CO2 based DCV alone One approach for implementing CO2 based DCV in multiple zone VAV system is to install a CO2 sensor in every zone A building automation system BAS monitors all the sensors determines how much outdoor air must be brought in at the air handler to satisfy the critical zone and thus overventilate all other zones and then repositions the outdoor air damper accordingly However it s costly to install a CO2 sensor in every zone Especially when you consider that most of the zones will always be overventilated regardless of operating conditions Installing a sensor in those non critical zones offers no added value In some applications you may know that only a handful of zones will ever be critical and y
13. es two benefits e It can assure that each zone receives the proper amount of ventilation without requiring a CO2 sensor in every zone CO2 sensors are used only in those zones where they will bring the most benefit When the other zones are unoccupied time of day schedules or occupancy sensors are used to reduce ventilation e It enables documentation of actual ventilation system performance by communicating the ventilation airflows for every zone to the BAS Table 2 Effect of ventilation control strategies in a single duct VAV system at part load 1 E aG Total OA intake Zonel Zone2 Zone3 flow Vot Ventilation reset only Primary airflow cfm Vpz 1000 3000 3000 7000 cfm Zone outdoor airflow cfm Voz 500 600 700 1800 cfm 2370 cfm OA fraction Za 0 50 0 20 0 23 Zone ventilation efficiency Evz 0 76 106 103 Xs 1800 7000 0 26 Ey 0 76 Vot 1800 0 76 2370 Ventilation reset plus CO2 based DCV in Zone 1 Primary airflow Vpz 1000 3000 3000 7000 cfm Zone outdoor airflow Voz 200 600 700 1500 cfm 1530 cfm OA fraction Za 0 20 0 20 0 23 Zone ventilation efficiency Evz 101 101 0 98 Xs 1500 7000 0 21 Ey 0 98 Vot 1500 0 98 1530 providing insights for today s HVAC system designer Closing thoughts Demand controlled ventilation can reduce the cost of operating the HVAC system especially in applications where contaminant levels result primarily from people or
14. ntial inaccuracy of an outdoor sensor Impact of ASHRAE 62 then and now ASHRAE 62 1 1989 thru 200L In the 1989 through 2001 versions of ASHRAE Standard 62 the required ventilation rates were based on either the number of occupants in the zone cfm person or the floor area of the zone cfm ft As an example let s consider the ventilation rate for a lecture classroom with a design population of 65 Figure 1 ASHRAE 62 1989 through 2001 required 15 cfm of outdoor air per person in this space type To comply our example classroom must receive 975 cfm of outdoor air 15 cfm person x 65 people If the population drops to 20 the required quantity of outdoor air drops too to 300 cfm 15 cfm person x 20 people Assuming that the CO2 generation rate N of the occupants who are seated doing light desk work is a constant 0 0105 cfm of CO2 person the mass balance equation establishes that a 700 ppm difference between the indoor and outdoor CO2 concentrations Figure L CO2 based demand controlled ventilation under ASHRAE 62 1989 thru 2001 1200 600 At reduced occupancy 20 Cs Co 0 0105 15 cfm p 0 0007 400 700 ppm breathing zone outdoor air Vpz cfm 0 10 20 30 1200 ACOz2 remains constant despite a drop in zone population Ce eerereserseeseseeeessseeseeeTe 1000 2 io 800 3 o K OQ a At design occupancy 65 600 F Cs Co 0 0105 15 cfm p O 0 0007 700 ppm
15. ou could choose to locate CO2 sensors only in those potentially critical zones The user s manual for ASHRAE 62 1 2004 discusses this approach further Alternatively some designers opt to install a single CO2 sensor in the retum air duct of a multiple zone 6 e Trane Engineers Newsletter volume 34 5 system and then use this single sensor to vary the amount of outdoor air brought in at the air handler However this CO2 sensor measures the average CO2 concentration so it s likely that some spaces will be underventilated while others are overventilated Whether this approach provides adequate ventilation is a matter of debate among designers Ventilation reset alone Another control strategy for multiple zone VAV systems called ventilation reset resets intake airflow in response to variations in system ventilation efficiency Each VAV box controller senses the current primary airflow Vpz and calculates its outdoorair fraction Za The building automation system totals the primary airflows and required outdoor airflows from all boxes and determines the highest outdoorair fraction reported Then it solves the equations from Appendix A of ASHRAE 62 1 2004 calculating the system ventilation efficiency Ey and the system level intake flow of outdoor air Vot that s required at the current operating condition The new intake flow setpoint is communicated to the air handler controller which then adjusts the OA d
16. require their design ventilation rates whenever they re occupied The BAS then uses the ventilation reset equations to determine how much outdoor air must be brought in at the air handler to satisfy all of the zones served For the example VAV system represented in Table 2 Zone 1 is a conference room which is densely occupied and has a widely varying population and Zones 2 and 3 are general office spaces which are more sparsely and more consistently occupied The top section of the table shows the system operating at part load when ventilation reset is used to reduce the outdoorair intake flow Vot and thereby account for the current system ventilation efficiency For this case it is assumed that all zones including the conference room require their design zone outdoor airflows Voz regardless of actual population The lower section of Table 2 represents the same system but a COz2 sensor is installed only in Zone 1 to reduce the required zone outdoor airflow Voz from the design value of 500 cfm to 200 cfm when the actual population in the conference room is less than design Zones 2 and 3 still require their design outdoor airflows While sensing CO2 and finding the current value for Zone 1 Voz lowers the average outdoorair fraction Xs it increases system ventilation efficiency Ev and lowers the required intake airflow Vot from 2370 cfm to 1530 cfm Combining CO2 based DCV with ventilation reset provid
17. termining the breathing zone ventilation rate Vbz Now the required rate is based on the number t A 2002 Engineers Newsletter volume 31 3 Using CO2 for Demand Controlled Ventilation provides more detail on the mass balance equation and on implementing CO2 based DCV to comply with ASHRAE 62 2001 providing insights for today s HVAC system designer S TRANE Figure 3 Comparison of potential DCV savings ASHRAE 62 2001 versus ASHRAE 62 1 2004 accounting for people and building related sources separately described by some as additivity ASHRAE 62 1 2004 results in lower breathing zone ventilation rates for most occupancy categories than ASHRAE 62 1989 through 2001 see Table 1 In densely occupied spaces those that cena aee e E EA g AEE E historically benefited most from CO2 P Phen ea caer based DCV such as auditoriums gyms ee f conference rooms lecture classrooms and cafeterias shaded in the table the rates dropped dramatically 1200 1000 Potential DCV savings under ASHRAE 62 1989 800 through 2001 600 breathing zone outdoor air Vpz cfm 0 10 20 30 40 50 60 70 Second Figure 3 shows that as the zone population P p eople zone population decreases the required breathing zone ventilation rate Voz drops less rapidly in this case of occupants in the zone cfm person the required quantity of outdoor air by 7 5 spt for ante n Who leaves and the zone
18. their activities and where population varies significantly The most common applications include gymnasiums meeting rooms and auditoriums ASHRAE 62 1 2004 explicitly allows the use of demand controlled ventilation based on CO2 to reset intake airflow in response to variations in zone population However it also reduces the value of implementing CO2 based DCV in most space types by reducing the required design ventilation rates For densely occupied spaces those that historically benefited most from using CO2 based DCV the ventilation rates are dramatically lower The 2004 standard also complicates implementation of CO2 based DCV That s because the effective cfm person and therefore the desired indoorto outdoor difference in CO2 concentrations vary as the zone population changes CO2 based DCV is most commonly used in single zone systems that serve densely occupied spaces with varying populations In multiple zone VAV systems combining CO2 based DCV with ventilation reset using CO2 sensors only in densely occupied zones with widely varying populations provides a cost effective reliable and energy efficient system By J ohn Murphy application 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 commercial library newsletters asp To comment e mail us at comfort trane com Trane Engineers Newsle
19. ting and cooling coils RA SA Because the recirculated air returns from only one zone some designers prefer to place the CO sensor in the return air RA duct But if any supply air bypasses the breathing zone the sensor in the RA duct may register a less than actual indoor CO3 concentration providing insights for today s HVAC system designer S TRANE Figure 6 Proportional control strategy for CO2 based DCV per ASHRAE 62 1 2004 1200 1000 800 600 400 outdoor air intake flow Vst cfm o OS Votmin 0 10 20 30 1800 Cs design 1500 1200 S wdd 59 2Q9 J00pul 40 50 60 70 zone population P p eople Outdoorair intake flow Vot and CO2 are proportional or linear to each other but neither is linear with respect to zone population The controller adjusts intake airflow Vot in proportion to the percentage of the CO2 signal range But when the controller changes outdoor airflow the indoor CO2 concentration changes too So the controller must adjust Vot in small increments until the indoor CO2 reaches a stable value When plotted in relation to zone population the results of these control actions are curves for both Vot and indoor CO2 2 Find the required intake flow of outdoor air when the zone is unoccupied that is Pz 0 v 5x0 0 06 x 1000 ot min 7 T9 60 cfm 3 Find the target indoor CO2 concentration at Vot design N Codesign Co
20. tter volume 34 5 e 7 S TRANE Trane A business of American Standard Companies www trane com For more information contact your local Trane office or e mail us at comfort trane com 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 e Trane Engineers Newsletter volume 34 5 ADM APN017 EN November 2005
21. we produce Appendix C of ASHRAE 62 1 2004 provides the following mass balance equation to predict the difference between indoor Cs and outdoor Co concentrations of carbon dioxide at steady state conditions given a S TRANE constant perperson ventilation rate Vo and a constant CO2 generation rate N where Vo outdoor airflow rate cfm person N CO2 generation rate cfm person Cs CO2 concentration in the space ppm Co CO concentration in the outdoor air ppm Implementing CO2 based DCV is a matter of estimating the CO2 generation rate of the occupants N measuring the concentration difference in the space versus outdoors Cs Co and then using this difference to determine the rate at which ventilation air Vo on a perperson basis is delivered to the space In most locations the outdoor concentration Co of carbon dioxide seldom varies by more than 100 ppm from the nominal value Because of this and in lieu of installing an outdoor CO2 sensor most designers use either a one time reading of the outdoor CO2 concentration at the building site or a conservative value from historical readings This simplifies control M Schell S Turner and R O Shim Application of CO2 based demand controlled ventilation using ASHRAE Standard 62 Optimizing energy use and ventilation ASHRAE Transactions 1998 lowers the installed cost and usually increases accuracy because it avoids the pote
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