Development and Evaluation of Applicable Optimal Terminal Box Control Algorithms for Energy Management Control Systems

Transcription

1 sustainability Article Development and Evaluation of Applicable Optimal Terminal Box Control Algorithms for Energy Management Control Systems Yoon-Bok Seong 1 and Young-Hum Cho 2, * 1 Senior Research Engineer, Construction & Energy Business Division, Korea Conformity Laboratories, 199, Gasandigital 1-ro, Geumcheon-gu, Seoul 853, Korea; nike21@snu.ac.kr 2 School of Architecture, Yeungnam University, 2 Daehak-Ro, Gyeongsan, Gyeongbuk 38541, Korea * Correspondence: yhcho@ynu.ac.kr; Tel./Fax: Academic Editor: Marc A. Rosen Received: 16 August 16; Accepted: 25 October 16; Published: 9 November 16 Abstract: Common energy management control systems (EMCS) in building HVAC systems could be made much more energy efficient without sacrificing comfort. Most researchers have focused on implementing optimal control algorithms in Variable Air Volume (VAV) systems with EMC functions. Previous studies have paid little attention to using terminal box EMC functions integrated with main AHU (Air Handling Unit) systems. Terminal boxes with EMCS may cause occupant discomfort and waste excessive energy if y do not have proper operation control functions. objective of this study is to evaluate impact of energy consumption and estimate building energy savings on optimal minimum of single duct VAV terminal boxes and develop applicable optimal terminal box control algorithms for EMCS. This paper presents a dynamic model of a VAV terminal box with hydronic reheat, develops optimal terminal box control algorithms and applies developed EMCS algorithms to an actual building. results of this study show that optimal terminal box control algorithms can stably maintain set room and reduce energy consumption for varying heating loads compared to conventional control algorithms. Keywords: VAV terminal box; control algorithms; energy savings; rmal comfort 1. Introduction HVAC systems consume more than 3% of building energy. Methods of saving building HVAC energy include installation or replacement of HVAC system and upgrading energy management control system (EMCS). Even with improved HVAC equipment, a building cannot achieve energy savings if system has an unsuitable operation control system. Considerable energy savings can be attained by properly operating HVAC system. A typical EMCS in a building HVAC system could be made much more energy efficient without sacrificing comfort. Most researchers have focused on implementing optimal control algorithms in VAV systems with EMC functions. Elovitz et al. [1] and Krakow et al. [2] discussed minimum outside control method and economizer for VAV systems. Optimal supply control in VAV systems was suggested by Ke et al. [3]. Zaheer-uddin [4] suggested optimal operating strategies. Engdahl and Johansson [5] investigated energy savings potential of a controlled supply of a VAV based system by a comparison with a constant supply. Wei [6] examined major factors that impact supply reset schedule which minimizes overall heating energy, cooling energy and fan power consumption. Nassif and Moujaes [7] proposed a new damper control strategy for outdoor, discharge and recirculation dampers of economizer in a VAV system. Taylor et al. [8] discussed minimum set point of terminal units. Cho and Liu Sustainability 16, 8, 1151; doi:1.339/su

2 Sustainability 16, 8, of 22 identified relationship between supply and minimum rate [9] and correlation between minimum and discharge that will maintain room rmal comfort using a CFD simulation [1]. Kang et al. [11] determined relationship between supply, rate and energy by height and presented a terminal unit rate control method considering stratification and IAQ. Kim et al. [12] presented a method for controlling supply and rate of a terminal unit to control stratification. Taylor et al. [13] conduct an energy study on systems that use conventional box and dual maximum box control methods. Zhang et al. [14] investigated how VAV terminal box minimum setting influences system energy use as well as building IAQ. However, previous studies have paid little attention to using terminal box EMC functions that are integrated with main AHU system. Terminal boxes are one of major building HVAC components and directly impact building room comfort and energy costs. minimum of terminal boxes is a key factor for comfort, indoor quality (IAQ) and energy cost. Current terminal boxes with EMCS may cause occupant discomfort and waste energy because y have inappropriate operation control functions of minimum. Existing terminal boxes in EMC algorithms use mostly empirical methods by using a fixed minimum. This control sequence can cause occupant discomfort or use excessive energy. In addition, advanced optimization and control techniques are too complex between maximum and minimum set point and hard to implement on existing commercially EMCS. refore, optimized control strategy and operation schedules of terminal boxes with proper minimum should be studied to improve indoor quality, rmal comfort and energy savings. objective of this study is to evaluate impact of energy consumption and estimate building energy savings on optimal minimum of single duct VAV terminal boxes and develop applicable optimal terminal box control algorithms for EMCS. This paper presents a dynamic model of a VAV terminal box with hydronic reheat, develops optimal terminal box control algorithms and evaluates developed EMCS algorithms to an actual building. terminal box energy consumption and rmal comfort are compared between conventional and improved control using measured data. 2. VAV Terminal Box Modeling single-duct pressure independent VAV terminal box with hydronic reheat system consists of a modulation damper and sensor and a reheat coil installed in discharge for spaces requiring heating. 1 presents schematic diagram of single-duct pressure independent VAV terminal box with hydronic reheat. To analysis energy consumption by terminal box, zone load of simple zone sensible load modeling should be calculated as shown in Equation (1); discharge and pressure of reheating coil modeling should be calculated as shown in Equations (2) (11); and rate of damper and valve modeling should be calculated as shown in Equations (12) (16) Zone Model Using energy balance principle by neglecting heat gains from ducts, a simple zone sensible load diagram is shown in 2.

3 Sustainability 16, 8, of 22 Sustainability 16, 8, of of Schematic diagram of single-duct pressure independent VAV terminal box with hydronic 1. Schematic diagram diagram of a of single-duct a single-duct pressure pressure independent independent VAV terminal VAV terminal box with box hydronic with reheat. hydronic reheat. reheat. w w p, w s, w r, w Q w m a w cp, wt s, w Tr, w a p, a d, s Ts Q m c T T a a p, a d, s s Q Q exterior exterior Ui Ai C p, exterior Ui Ai C p, exterior T T set set T T out out T Q int erior U j Aj set Tad, j Q int erior interioru j Aj Tset Tad, j int erior 2. terms in a zone load calculation. 2. terms in zone load calculation. 2. terms in a zone load calculation. fully mixed mass is given by Equation (1). fully mixed mass is given by Equation (1). fully mixed mass dtr 1 q i is given by Equation (1). Ui Ai mv C a Tr Tout U j Aj Tr Tad, j dt dt V1 Ca exterior (1) { ( R ) int erior q d T i Ui Ai mv C R 1 a Tr Tout U j Aj Tr Tad, j (1) = dt qv Ca exterior int erior dt ρ V C i + a U i A i + ) m v C a (T r T out ) + U j A j (T } r T ad,j (1) exterior interior 2.2. Reheating Coil Model Reheating Reheating Coil Coil Model Model reheating coil is most important connection between AHUs and zone distribution reheating reheating system. coil coil is is reheating most most coil important important model uses connection connection an approximate between between equation AHUs AHUs for effectiveness and and zone zone as a distribution distribution function of NTU system. system. (number reheating reheating of transfer coil coil units) model model for uses a uses typical an an approximate cross approximate heat equation equation exchanger for for effectiveness with effectiveness both fluids as as a function unmixed of type. NTU Total (number heat transfer of transfer rate, units) water for and a typical side cross heat flux heat can be exchanger calculated with by Equations both fluids unmixed type. Total heat transfer rate, water and side heat flux can be calculated by Equations

4 Sustainability 16, 8, of 22 a function of NTU (number of transfer units) for a typical cross heat exchanger with both fluids unmixed type. Total heat transfer rate, water and side heat flux can be calculated by Equations (2) (4). When room can maintain a constant, total heat transfer, and water side heat flux have a heat balance as in Equation (5). Q total = ε C min (T s,w T s ) (2) Q w = C w (T s,w T r,w ) (3) Q a = C a (T d,s T s ) (4) Q a = Q w = Q total (5) steady state and water outlet are found using definition of heat exchanger effectiveness by Equations (6) and (7). T d,s = T s + ε Cmin C a (T s,w T s ) (6) T r,w = T s,w C a C w (T d,s T s ) (7) Air and water dynamic outlet are calculated using coil time constant by Equations (8) and (9). dt D,d,s dt dt D,r,w dt = T d,s T D,d,s t coil (8) = T r,w T D,r,w t coil (9) Air and water inlet pressure are calculated from outlet pressure and rates by Equations (1) and (11). 2 P a,i = P a,o + k a m a (1) 2.3. Damper and Valve Model P w,i = P w,o + k w m w 2 A damper or valve is essentially a variable fluid resistance. method in which rate varies with damper or valve position is known as damper or valve characteristic. model calculated inlet pressure as a function of mass rate and relative damper or valve position. pressure loss coefficient for steady-state is derived as Equations (12) and (13). se equations can be related to volume rate entering terminal box as in following Equations (14) and (15). rate related to resistance can be calculated by Equation (16). (11) P box = k box P v (12) P box = k box ρ v2 2 P box = k box ρ m b 2 2 A b 2 (13) (14) P box = S box m 2 (15) P m = box (16) S box

5 box 3. Optimal Minimum Air Flow 3.1. Minimum Air Flow Based on Heating Load Sustainability 16, 8, of 22 minimum for a heating load is required by room design heating load. 3. Optimal minimum Minimum Air Flow needed to satisfy building heating load requirement can be calculated 3.1. Minimum with Equation Air Flow Based (17): on Heating Load minimum for a heating load is required by room design heating load. minimum needed to satisfy, = (17) building heating load requirement can be calculated with (, ) Equation (17): where Q V min,h = h (17), volumetric rate for heating ρ Cload, p (T d,s ft 3 /min T r ) where Room heating load, Btu/hr T r Room Vmin,h volumetric, F rate for heating load, ft 3 /min Q h Room heating load, Btu/hr, Discharge, T r Room, F F ρ standard density, lbm/ft T d,s Discharge, 3 F specific ρ standard heat of density,, Btu/lbm lbm/ft 3 F In order C p specific to calculate heat of, required Btu/lbm F for heating load, discharge should first In order be to determined. calculate This required for can be heating calculated load, based discharge on Cho and Liu s [1] determined should first optimal be determined. discharge This. can be It calculated is used to based find on Cho proper and Liu s room [1] determined distribution optimal discharge and minimum. rate of energy It is used usage. to find proper 3 compares room optimal distribution and s and using minimum differing ratefixed of energy discharge usage. s 3 compares at various optimal occupied conditions. and s using required differing in unoccupied fixed discharge conditions s was higher at various than occupied that in occupied conditions. conditions required because of in a high unoccupied conditions was higher than that in occupied conditions because of a high room heating room heating load requirement. During high room heating load condition (at low outside load requirement. During high room heating load condition (at low outside condition), discharge condition), discharge is higher than 9 is higher than 9 F based on F based on calculated optimal value. calculated However, optimal discharge value. However, is discharge lower than 9 F during low is room lower heating than load 9 F condition. during low refore, room heating discharge load condition. refore, cannot be fixed discharge at a specific ; cannot instead, be it varies fixed at a specific according ; to room instead, heating it load varies and according. to room heating load and. Air (CFM) 2 1 T=optimal Ts=1F Ts=F Ts=9F Air (CFM) 2 1 T=optimal Ts=1F Ts=F Ts=9F Outside (F) (a) Outside (F) (b) 3. Cont.

6 Sustainability 16, 8, of 22 Sustainability 16, 8, of 22 Sustainability 16, 8, of 22 2 Air (CFM) Air (CFM) T=optimal Ts=1F T=optimal Ts=F Ts=1F Ts=F Ts=9F Ts=9F Outside Outside (F) (F) (c) (c) Comparison of calculated with with different different discharge discharge for different 3. Comparison of calculated with different discharge for different for occupied conditions: (a) unoccupied condition; (b) 5% occupied condition; and (c) fully occupied occupied different conditions: occupied (a) conditions: unoccupied (a) unoccupied condition; condition; (b) 5% occupied (b) 5% occupied condition; condition; and (c) and fully (c) occupied fully condition. condition. occupied condition. 4 compares reheating energy consumption of optimal discharge and fixed 4 49 compares F discharge reheating. energy consumption optimal of of discharge optimal discharge discharge reduces and energy and fixed consumption fixed9 9 F F discharge by 33% to 35%.. During unoccupied optimal optimal conditions, discharge discharge even more reheating reduces energy energy reduces energy can consumption be saved. by 33% by to33% 35%. to During 35%. unoccupied During unoccupied conditions, even conditions, more reheating even more energyreheating can be saved. energy can be saved. Energy consumption(btu) Energy consumption(btu) 45, 45,,, 35, 35, 3, 3, 25,, 25, 15,, 1, 15, 5, 1, 5, Supply temp. = 9 F Optimal Supply supply temp. = 9 F Optimal supply temp. Unoccupied condition 5% occupied condition Fully occupied condition 4. Comparison Unoccupied of reheating conditionenergy consumption 5% occupied between condition fixed supply Fully occupied condition 4. Comparison of reheating energy consumption between fixed supply and and optimal optimal supply supply Comparison compares of minimum reheating energy rates consumption of actual between fixed in an supply existing system with and optimal 5 supply compares. minimum rates of actual in an existing system with potentially improved (simulated). se are based on heating load requirement under potentially improved (simulated). se are based on heating load requirement under differing differing occupied conditions. discharge used calculated optimal value. occupied 5 conditions. compares discharge minimum rates used of actual calculated optimal in an value. room heating load was 297 Btu/hr at an outside of 3 F in existing fully occupied system with potentially roomimproved heating load (simulated). was 297 Btu/hr se at are an outside based on heating of 3 conditions, with a calculated minimum for room heating load requirement load F in requirement fully occupied of CFM, under conditions, with a calculated minimum for room heating load requirement of CFM, differing which is occupied actual minimum conditions. in discharge existing system. calculated used minimum calculated rate optimal of value. which is actual minimum in existing system. calculated minimum rate of for for room heating load requirement was 297 Btu/hr was at an CFM. outside minimum of ratio 3 F for in fully room occupied room heating load requirement was CFM. minimum ratio for room heating conditions, heating load with requirement a calculated was minimum % 3% for a design for maximum room heating of load CFM. requirement Lowering of CFM, which rate load of is occupancy requirement actual minimum increases was % 3% required for a design in minimum maximum existing system.. of CFM. Lowering rate of occupancy increases required minimum. calculated minimum rate of for room heating load requirement was CFM. minimum ratio for room heating load requirement was % 3% for a design maximum of CFM. Lowering rate of occupancy increases required minimum.

7 Sustainability 16, 8, of 22 Sustainability 16, 8, of 22 Air rate (CFM) Air rate (CFM) Air rate (CFM) Outside (F) (a) Outside (F) (b) Outside (F) (c) 5. Comparison of between actual in existing system and calculated 5. Comparison of between actual in existing system and calculated based on heating load requirement for different occupied conditions: (a) fully occupied condition; based on heating load requirement for different occupied conditions: (a) fully occupied condition; (b) 5% occupied condition; and (c) unoccupied condition. (b) 5% occupied condition; and (c) unoccupied condition Minimum Air Flow Based on Ventilation Requirement 3.2. Minimum Air Flow Based on Ventilation Requirement minimum rate of that satisfies ventilation requirements can be calculated using Equations minimum(18) (). rate of that satisfies ventilation requirements can be calculated using Equations (18) (). (( α oa = Min(Max( T R T s,,min),1) minoa), (18) (18) T R T OA where α oa AHU outside intake ratio, % T R Return, F F T OA Outside, F F T F S Supply, F V f ( R p Pz a AZ ) / E (19) z V f = (R p P z + R a A Z )/E z (19)

8 Sustainability 16, 8, of 22 Sustainability 16, 8, of 22 where volumetric rate for fresh requirement, ft where 3 /min, V volumetric rate for ventilation, ft f volumetric rate for fresh requirement, 3 /min ft 3 /min outdoor rate required per person as determined from ASHRAE Standard 62, 4 V min,v volumetric rate for ventilation, ft 3 zone population, person /min R p outdoor rate required per per person unit area as determined as determined from from ASHRAE ASHRAE Standard Standard 62, 4 62, P z zone 4 population, person R a zone outdoor floor area, ftrate 2 required per unit area as determined from ASHRAE Standard 62, 4 A z zone floor area, ft 2 V f V min,v =, = () () α oa 6 compares between between actual actual fresh fresh with different with different minimum minimum outside outside intake ratios intake andratios fresh and requirements fresh requirements at different occupied at different conditions. occupied During conditions. occupied During and 1% occupied minimum and 1% outside minimum intake outside conditions, intake conditions, actual fresh actual fresh was lower than was lower required than minimum required minimum. This may. cause This anmay IAQcause issue. an IAQ issue. Air rate (CFM) Actual fresh (min oa = 3%) Actual fresh (min oa = 1%) Actual fresh (min oa = %) Fresh requirement Air requirement for room load Outside (F) (a) Air rate (CFM) Actual fresh (min oa = 3%) Actual fresh (min oa = 1%) Actual fresh (min oa = %) CFMfresh_require Air requirement for room load Outside (F) (b) 6. Cont.

9 Sustainability 16, 8, of 22 Sustainability 16, 8, 8, of of Air rate (CFM) Actual Actual fresh fresh (min (min oa oa = 3%) 3%) Actual Actual fresh fresh (min (min oa oa = 1%) 1%) Actual Actual fresh fresh (min (min oa oa = %) %) Fresh Fresh requirement requirement Air Air requirement requirement for for room room load load Outside Outside (F) (F) (c) Comparison of between actual fresh with with different minimum outside outside intake intake ratio ratio and fresh and fresh requirement requirement for different for different occupied occupied conditions: conditions: (a) fully occupied (a) fully condition; occupied condition; (b) 5% occupied (b) 5% condition; occupied condition; and (c) unoccupied and (c) unoccupied condition. condition. 7 shows calculated minimum based on fresh requirement at different occupied conditions. percentage of ofoutside in inan anahu is is in in range of 3% % of full outside intake according to outside ranges. determined minimum for for fresh fresh requirement requirement was was CFM, CFM, and and minimum minimum ratio for ratio ventilation for ventilation requirement requirement was 9% 3% was during 9% 3% fullyduring occupied fully conditions. occupied conditions. Air rate (CFM) Air rate (CFM) Air based on fresh requirement Air based on fresh requirement Outside (F) Outside (F) (a) 1 Air based on fresh requirement 1 Air based on fresh requirement Outside (F) Outside (F) (b) Calculated Calculated minimum minimum based based on fresh on fresh requirement requirement for different for occupied different conditions: occupied conditions: (a) fully occupied condition; and (b) 5% occupied condition. (a) fully occupied condition; and (b) 5% occupied condition.

10 Sustainability 16, 8, of 22 Sustainability 16, 8, of Minimum Air Flow Requirement minimum set set point point can can be set be so set that so it that equals it equals eir eir highest highest rate required rate required by room by design room heating design load heating or load minimum or minimum rate required rate for required ventilation. for ventilation. min = V max( min,h, min V h V min,v v )) (21) (21) 8 shows shows calculated calculated minimum minimum for for room room load load and fresh and fresh requirements requirements under under differing differing occupied occupied conditions. conditions. During During fully occupied fully occupied conditions, conditions, fresh requirement fresh requirement served as served a more as critical a more factor; critical however, factor; during however, unoccupied during unoccupied conditions, minimum conditions, minimum can generally can be generally determined be by determined load requirement. by load requirement. Air rate (CFM) Outside (F) 1 (a) Air rate (CFM) Outside (F) (b) 8. Determined minimum for room load requirement and fresh requirement for 8. Determined minimum for room load requirement and fresh requirement for different occupied conditions: (a) fully occupied condition; and (b) 5% occupied condition. different occupied conditions: (a) fully occupied condition; and (b) 5% occupied condition. 9 compares existing fixed minimum and determined, which is selected maximum 9 compares value between existing fixed ventilation minimum requirement and and heating determined load requirement,, which at is different selected maximum occupied conditions. value between ventilation calculated requirement minimum and heating under load unoccupied requirement, conditions at different was lower occupied than conditions. in occupied conditions. calculated minimum under unoccupied conditions was lower than in occupied conditions.

11 Sustainability 16, 8, of 22 Sustainability 16, 8, of 22 Air rate (CFM) Air rate (CFM) Air rate (CFM) Outside (F) (a) Outside (F) (b) Outside (F) (c) Comparison of of between between existing existing fixed fixed minimum minimum and and determined determined for different for different occupied occupied conditions: conditions: (a) fully (a) fully occupied occupied condition; condition; (b) 5% (b) 5% occupied occupied condition; condition; and (c) and unoccupied (c) unoccupied condition. condition. range with fixed minimum and optimal in differing occupied range with fixed minimum and optimal in differing occupied conditions. range of optimal was 6% to 35% of design. shaded area conditions. range of optimal was 6% to 35% of design. shaded area between fixed minimum and optimal represents potential amount of between fixed minimum and optimal represents potential amount of energy energy saved. This graph reveals that energy consumption of optimal minimum can saved. This graph reveals that energy consumption of optimal minimum can be less be less than that of conventional fixed minimum. than that of conventional fixed minimum. Table 1 displays calculated minimum and its ratio under differing occupied Table 1 displays calculated minimum and its ratio under differing occupied conditions. conditions. fixed conventional minimum set point was determined in unoccupied fixed conventional minimum set point was determined in unoccupied conditions. conditions. range of minimum ratio in unoccupied conditions was greater than in range of minimum ratio in unoccupied conditions was greater than in occupied occupied conditions. graph furr indicates that potentially greater energy savings can be conditions. graph furr indicates that potentially greater energy savings can be expected in expected in unoccupied conditions. unoccupied conditions.

12 Sustainability 16, 8, of 22 Table 1. Determined minimum for different occupied conditions and its ratio. Design Air Flow (CFM) Minimum Air Flow (CFM) Minimum Air Flow Ratio (%) Base Case Improved Case Base Case Improved Case Fully occupied condition % 14% 35% 5% occupied condition % 11% 35% Unoccupied condition % 6% 35% 4. Optimal Terminal Box Control Algorithms Generally, EMCS achieve energy efficiency by improved scheduling and operation without sacrificing rmal comfort. EMCS functions can be activated based on time of year, month and daily operation of building system. In this study, applicable optimal terminal box control algorithms for EMCS functions integrated with AHUs are developed General Operation Function for an EMCS Objective: To improve rmal comfort and reduce energy consumption Definitions: Seasonal cycle: summer, winter and spring/fall Weekend cycle: day of year is a weekend or holiday Occupied cycle: day of year is a weekday Nighttime cycle: terminal box is disabled Start/stop time cycle: before and after working hours Process: EMCS can automatically classify what season it is currently, depending on month. re are three seasonal operation modes: summer, winter and spring/fall. For example, if month of year is August, terminal box goes to summer cooling operation mode. EMCS considers wher it is a normal cycle or weekend cycle by day of year. If day of year is a weekend or holiday, terminal box goes to weekend cycle. During weekend cycle, terminal box is usually disabled based on room and outside conditions. During week, system goes to daily normal operation cycle. In daily normal operation cycle, a day is divided into four time periods: nighttime, start time, stop time and occupied time cycle. During nighttime, terminal box is disabled independent of AHU operation. If room and outside condition is extremely cold or hot, terminal box will be enabled during nighttime cycle. decision when terminal box needs to be enabled from disabled period is important for energy consumption because most rooms are not continuously occupied. If occupied time is regular, e.g., an office area, system can easily figure out occupied or unoccupied time. Before occupied operation time, terminal box goes to start time cycle. system decides enabled time based on room conditions needed to pre-cool or warm to attain desired room set point before occupied time. During start/stop cycle, if room can maintain room set point, terminal box is disabled. system will also need to add special equipment like a motion sensor if occupied time is too variable. 1 shows general terminal box operation cycle.

13 Sustainability 16, 8, of 22 Sustainability 16, 8, of General General terminal terminal box box operation cycle. cycle Occupied Occupied Time Time Operation Operation for for EMCS EMCS most most complex complex operation operation strategy strategy is is occupied occupiedtime time cycle, cycle, when when occupants occupants stay stay in in room. room. Occupied Occupiedperiod period control control sequences sequences are are most most important important and require and require complex complex EMC functions EMC functions because because of effect of on occupant effect on rmal occupant comfort. rmal Most comfort. of all, Most HVAC of control all, HVAC through control EMCS through should EMCS give rmal should give comfort rmal to occupants. comfort to occupants. system can also system save energy can also with save anenergy optimal with control an optimal sequence. control five sequence. modes during five modes occupied during time cycle occupied are astime follows: cycle are as follows: Ventilation mode: When room is equal to its set point deviation (d), Ventilation mode: When room is equal to its set point deviation (d), reheat reheat valve is closed; set point is maintained by calculated required fresh valve is closed; set point is maintained by calculated required fresh intake. intake. Cooling Cooling mode: mode: When When room room is is above above its its set set point point deviation deviation (d), (d), reheat reheat valve valve is is closed; closed; room room set set point point is maintained is maintained by modulating by modulating between between maximum maximum and minimum and minimum values. values. Maximum Maximumcooling coolingmode: When When room room is is above its its set set point point deviation deviation (2d), (2d), reheat reheat valve valve is closed; is closed; maximum maximum of this of mode this mode is higher is higher than that than ofthat cooling of cooling mode. mode. Heating mode: When room is below its set point deviation (d), discharge Heating mode: is When maintained room by modulating reheat is valve below between its set its point maximum deviation and minimum (d), discharge set point; setis point maintained is calculated by modulating required ventilation reheat valve between. its maximum and Maximum minimum heating set point; mode: When set room point is calculated is below required its set ventilation point deviation. (2d), Maximum isheating maintained mode: between When its maximum room and minimum is below values. its set point deviation (2d), is maintained between its maximum and minimum values. 11 shows control algorithms for operation during occupied time. 11 shows control algorithms for operation during occupied time.

14 Sustainability 16, 8, of 22 Sustainability 16, 8, of Control algorithms for occupied time operation. When terminal box is enabled, terminal box is set at ventilation mode. When terminal box is enabled, terminal box is set at ventilation mode. rate that satisfies outside ventilation requirements can be calculated by Equations (18) and rate that satisfies outside ventilation requirements can be calculated by Equations (18) and (19). (19). outside intake ratio can be derived from AHU s control variable. outside intake ratio can be derived from AHU s control variable. Under ventilation mode, controller checks two conditions: (1) wher room Under ventilation mode, controller checks two conditions: (1) wher room is higher than its set point deviation (d); and (2) wher room is lower is higher than its set point deviation (d); and (2) wher room is than its set point deviation (d). lower than its set point deviation (d). When first condition is satisfied, terminal box is switched to cooling mode. In When first condition is satisfied, terminal box is switched to cooling mode. In cooling mode, set point varies between design cooling and required cooling mode, set point varies between design cooling and required ventilation ventilation to maintain room. During cooling mode, room to maintain room. During cooling mode, room is higher is higher than its set point deviation (2d), and terminal box is switched to than its set point deviation (2d), and terminal box is switched to maximum cooling mode. maximum cooling mode. Set maximum cooling to α times current design cooling Set maximum cooling to α times current design cooling set point. If room set point. If room is lowered, terminal box is switched from maximum is lowered, terminal box is switched from maximum cooling to cooling mode. cooling to cooling mode. If room satisfies within its set point deviation (d), If room satisfies within its set point deviation (d), terminal box is switched from terminal box is switched from cooling mode to ventilation mode. If second condition is satisfied, cooling mode to ventilation mode. If second condition is satisfied, terminal box is switched terminal box is switched from ventilation mode to heating mode. heating is from ventilation mode to heating mode. heating is set between required set between required ventilation and design heating to maintain room ventilation and design heating to maintain room. discharge. discharge has a maximum set point which cannot be more than has a maximum set point which cannot be more than F above room, as recommended by ASHRAE Standard F above room, as [15]. If room recommended by ASHRAE Standard [15]. If room is lower than its set point is lower than its set point deviation (2d), terminal box is switched to maximum deviation (2d), terminal box is switched to maximum heating mode. Set maximum heating heating mode. Set maximum heating to α times current design heating set to α times current design heating set point. If room increases, point. If room increases, terminal box is switched from maximum heating to terminal box is switched from maximum heating to heating mode. If room heating mode. If room is satisfied within its set point deviation (d), terminal is satisfied within its set point deviation (d), terminal box is switched from heating mode to box is switched from heating mode to ventilation mode. Table 2 shows control variables for ventilation mode. Table 2 shows control variables for each operation mode. each operation mode.

15 Sustainability 16, 8, of 22 Sustainability 16, 8, of 22 Sustainability 16, 8, of 22 Table Summary of of operation control variables. Table 2. Summary of operation control variables. Discharge Temperature Air Air Set Point Set Point Discharge Temperature Set Point Mode Mode Mode Mode Period Period Discharge Set Temperature Air Set Point Point Mode Mode Period Maximum Minimum Maximum Set Point Maximum Minimum Maximum Minimum Maximum Minimum Maximum Minimum Ventilation Ventilation Tset Tset d < Tr d < Tr Tset < Tset + d + d CFM_ventilation CFM_ventilation CFM_ventilation CFM_ventilation Ts Ts Ts Ts Ventilation Cooling Cooling Tset Tr > Tset Tr d < > Tr + Tset < Tset d + d + d CFM_ventilation CFM_cooling CFM_cooling CFM_ventilation CFM_ventilation CFM_ventilation Ts Ts Ts Ts Ts Ts Occupied Maximum Cooling cooling cooling Tr > Tr Tset Tr > Tset + Tset 2d + d 2d α CFM_cooling α CFM_cooling CFM_cooling CFM_ventilation CFM_ventilation CFM_ventilation Ts Ts Ts Ts Ts Ts Occupied Maximum Heating Heating cooling Tr < Tr Tset Tr > < Tset Tset d+ 2d d CFM_heating α CFM_heating CFM_cooling CFM_ventilation CFM_ventilation CFM_ventilation Tr + 15Tr Ts + 15 Ts Ts Ts Maximum Maximum Heating heating heating Tr < Tset Tr Tr < Tset 2d d 2d α CFM_heating α CFM_heating CFM_heating CFM_ventilation CFM_ventilation CFM_ventilation Tr + 15 Tr Tr Ts Ts Ts Maximum heating Tr < Tset 2d α CFM_heating CFM_ventilation Tr + 15 Ts Cooling Cooling Tr > Tset Tr > + Tset 4d + 4d CFM_cooling CFM_cooling Zero Zero Ts Ts Ts Ts Unoccupied Cooling Heating Heating Tr Tr < Tset Tr > < Tset Tset + 4d 4d 4d CFM_cooling CFM_heating CFM_heating Zero Unoccupied Zero Zero Tr + 15Tr Ts + 15 Ts Ts Ts Heating Tr < Tset 4d CFM_heating Zero Tr + 15 Ts In current practice, a proportional-only controller can result in as fast a response as an offset In current practice, a proportional-only controller can result in as fast a response as an offset controller. If gain is higher, response of control system will also increase, but higher controller. If gain is higher, response of control system will also increase, but higher gain also causes overshoot and hunting. To solve this problem, proportional and integral control is gain also causes overshoot and hunting. To solve this problem, proportional and integral control is most common and widely used used control control method method in HVAC in HVAC systems, systems, although although it is difficult it is difficult to select to most common and widely used control method in HVAC systems, although it is difficult to appropriate select appropriate proportional proportional and integral and integral gains for gains multiple for control multiple loops. control loops. 12 shows a 12 schematic shows a select appropriate proportional and integral gains for multiple control loops. 12 shows a diagram schematic for diagram an for an control loop. control To maintain loop. To maintain room room set point, set controller point, schematic diagram for an control loop. To maintain room set point, calculates controller calculates desired desired set point. set point. damper modulates damper modulates to maintain to maintain set point. controller calculates desired set point. damper modulates to maintain set point. 13 shows a 13 schematic shows a diagram schematic for diagram discharge for discharge control loop. control To maintain loop. To set point. 13 shows a schematic diagram for discharge control loop. To room maintain room set point, controller set point, calculates controller desired calculates discharge desired discharge set point. maintain room set point, controller calculates desired discharge reheating set valve point. modulates reheating to maintain valve its modulates set point. to maintain its set point. set point. reheating valve modulates to maintain its set point. fan Q S fan Q 2 2 Q P S box box 12. Schematic diagram for control loop. 12. Schematic diagram for control loop. 13. Schematic diagram for discharge control loop. 13. Schematic diagram for discharge control loop. 5. Application Application Application applied building is a medical center located in Omaha, Nebraska. re are 232 terminal boxes in applied applied building. building building is Most is a of medical medical terminal center center located located boxes are in in Omaha, Omaha, a single-duct Nebraska. Nebraska. re pressure re are independent are terminal terminal VAV boxes boxes terminal in box building. building. Most of terminal boxes are a single-duct pressure independent VAV with hydronic Most ofreheat terminal and serve boxes both are interior a single-duct and exterior pressurezones. independent One typical VAV terminal terminal box with hydronic reheat and serve both interior and exterior zones. One typical terminal box, located at south exterior office room, was chosen in this study to apply optimal terminal box, located at south exterior office room, was chosen in this study to apply optimal terminal

16 Sustainability 16, 8, of 22 box with hydronic reheat and serve both interior and exterior zones. One typical terminal box, located at south exterior office room, was chosen in this study to apply optimal terminal box control sequence. re is normally one person occupying this office; occupied hours on weekdays are 8: a.m. to 6: p.m. HVAC system operates 24 h a day, 7 days a week. room set point is 73 F and AHU supply temp set point is 55 F. HVAC system EMCS software (Andover, Omaha, NE, USA) is Andover with Plain English programming. Table 3 shows applied building system information. Table 3. Building information. Item EMCS Software Zone People Operation condition EMCS control point Terminal box information Information Andover Continuum with Plain English Programming Location Purpose Size Number Schedule HVAC system AHU supply Supply hot water Room set point AHU Input Terminal box Input Terminal box Output Type Omaha, Nebraska Office room Floor area: 215 ft 2, Volume: 155 ft 3 Normally one Person during occupied hour Occupied hours (8: a.m. 6: p.m.) 24 h, 7 days working 55 F 1 F 73 F Outside Mixed Supply Return Space room Discharge Air rate Damper position Reheating valve position Single-duct pressure independent terminal box with hydronic reheat coil 5.1. Conventional Terminal Box Control type of terminal box in this building is a single-duct pressure independent VAV terminal box with hydronic reheat coil. existing control sequence is following. On a call for cooling, will increase as damper opens towards fixed maximum set point (25 CFM) to satisfy room set point. On a call for less cooling, will decrease as damper closes towards fixed minimum set point to satisfy room set point. With a furr call for heat, drops to minimum set point and reheat coil valve is modulated to maintain room heating set point. fixed minimum set point (125 CFM) is based on design heating load. terminal box operates 24/7 to maintain room set point. 14 shows conventional terminal box control sequence. To analyze room control in an actual system, room conditions were measured. range of room was 71.5 F 74.6 F, discharge was 55.4 F F, and supply was CFM as shown in s 15 and 16. With a conventional control sequence, room is not able to maintain set point (73 F) within control deviation. re are two reasons that room cannot stably maintain set room. One reason is high minimum set point, while anor is high discharge. It often causes significant simultaneous heating and cooling and excessive fan power in mild wear. Although a high minimum set point causes excess energy consumption, it produces a large amount of fresh to occupants during operations

17 Sustainability 16, 8, of 22 Sustainability 16, 8, of 22 as shown Sustainability 16, 8, refore, terminal boxes need to optimize control sequence17 operations as shown in 16. refore, terminal boxes need to optimize according of 22 control Sustainability 16, 8, of 22 to building sequence operation according to conditions. operations as shown building in operation 16. refore, conditions. terminal boxes need to optimize control sequence operations according as shown to building in operation 16. refore, conditions. terminal boxes need to optimize control sequence according to building operation conditions. Temperature (F) Temperature (F) (F) Temperature (F) Temperature (F) (F) Conventional terminal box control sequence. 14. Conventional terminal box control sequence. 14. Conventional terminal box control sequence. 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/9/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/9/7 4/8/7 3:: 3:: 4/8/7 AM 6:: AM 6:: 4/8/7 AM 9:: AM 9:: 4/8/7 AM 12:: PM 3:: PM AM 12:: 4/8/7 6:: PM 9:: PM Time PM 3:: 4/8/7 PM 6:: 4/8/7PM 4/8/7 9:: PM 4/9/7 3:: AM 6:: AM 9:: AM 12:: Time PM 3:: PM 6:: PM 9:: PM Time 15. Temperature trending data for conventional control Temperature trending data for conventional control. 15. Temperature trending data for conventional control. 16. Air trending Time data for conventional control. 16. Air trending data for conventional control Air Air trending trending data data for for conventional conventional control. control. Disch Temp Disch Supply Temp Temp Supply Room Temp Disch Temp Temp Room Oa Temp Supply Temp Oa Temp Room Temp Oa Temp 3 Room Temp 3 Oa Room Temp Temp 3 Room Temp 1 Sup Oa Oa Temp Temp Fresh 1 Sup 1 Sup 25 Fresh Fresh /8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/8/7 4/9/7 4/8/7 3:: 4/8/7 AM 6:: 4/8/7 AM 9:: 4/8/7 AM 12:: 4/8/7 PM 3:: 4/8/7 PM 6:: 4/8/7 PM 9:: 4/8/7 PM 4/9/7 4/8/7 4/8/7 3:: AM 4/8/7 6:: AM 9:: 4/8/7 AM 12:: 4/8/7 Time PM 3:: 4/8/7 PM 6:: 4/8/7 PM 9:: 4/8/7 PM 4/9/7 3:: AM 6:: AM 9:: AM 12:: Time PM 3:: PM 6:: PM 9:: PM Air Air (CFM) (CFM) Air (CFM)

18 Sustainability 16, 8, of 22 Sustainability 16, 8, of Improved Terminal Box Control With new terminal box control sequence, EMCS can automatically check operation cycle based on previous optimal control algorithm such as seasonal operation cycles, normal cycle or weekend cycle and daily occupied time operation cycle. outside fresh intake ratio for minimum ventilation mode can be calculated by Equations (17) and (18) from AHU control input. To maintain room, variable and and discharge discharge set point set point can be can calculated be calculated by by previous previous control control loop. loop. Finally, Finally, damper damper and and valve valve modulate modulate to maintain to maintain room room set point. set point Stability of Room Air Temperature Control An analysis of stability of room control in an actual system shows that range of room is is 7.9 F 73.6 F, discharge is is 55.6 F 86.9 F F and supply is CFM during occupied time as shown in s 17 and 18. room is able to maintain set point (73 F) within control deviation. average room is lower than its set point. During occupied period, AHU s outside intake is is at at minimum condition and and supply set point is higher than economizer season set point to maintain fresh requirement as as shown in in 18. During unoccupied period, lowest room is 68.7 F; however, system is is maintained in a disabled condition because room cannot reach unoccupied operation heating set point (Tr < Tset 4d). If outside condition is extremely cold or hot during winter or summer season, system will respond quickly based on room conditions. During start time, system tries to to catch room set set point point before before occupancy. refore, refore, it is assumed it is assumed that re that re is proper is proper and stable and stable room room control. control. 1 Disch Temp SupplyAir Temp Room Temp Oa Temp 1 Temperature (F) 9/27/7 12: AM 9/27/7 3: AM 9/27/7 6: AM 9/27/7 9: AM 9/27/7 12: PM Time 9/27/7 3: PM 9/27/7 6: PM 9/27/7 9: PM 9/28/7 12: AM 17. Temperature trending data of improved control. 17. Temperature trending data of improved control rmal Comfort rmal Comfort If minimum is less than required, re may be rmal comfort issues such as If minimum is less than required, re may be rmal comfort issues such as indoor indoor quality problem and lack of circulation. Measurement of CO2 in occupied spaces has quality problem and lack of circulation. Measurement of CO 2 in occupied spaces has been been widely used to evaluate amount of outdoor supplied to indoor spaces. To verify indoor widely used to evaluate amount of outdoor supplied to indoor spaces. To verify indoor quality due to reductions in minimum, measurements were performed on CO2 levels quality due to reductions in minimum, measurements were performed on CO 2 levels using indoor quality meters. average CO2 level was in range of ppm. average using indoor quality meters. average CO 2 level was in range of ppm. average outdoor concentration was 3 ppm. According to ASHRAE Standard 62, comfort criteria are outdoor concentration was 3 ppm. According to ASHRAE Standard 62, comfort criteria are likely to be satisfied if ventilation results in indoor CO2 concentrations less than 7 ppm above outdoor concentration, which is representative of delivery rates of outside. refore, it is assumed that any IAQ problems due to reduction of minimum set point will not happen.

19 Sustainability 16, 8, of 22 likely to be satisfied if ventilation results in indoor CO 2 concentrations less than 7 ppm above outdoor concentration, which is representative of delivery rates of outside. refore, it is Sustainability assumed that 16, any 8, 1151 IAQ problems due to reduction of minimum set point will not happen. 19 of Room Temp Oa Temp Supply Fresh 3 25 Temperature (F) 15 Air (CFM) 5 9/27/7 12: AM 9/27/7 3: AM 9/27/7 6: AM 9/27/7 9: AM 9/27/7 12: PM Time 9/27/7 3: PM 9/27/7 6: PM 9/27/7 9: PM 9/28/7 12: AM Air Air trending trending data data of of improved improved control. control Potential Potential Energy Energy Savings Savings reheat reheat energy energy savings savings can can be be considered considered as as reheat reheat energy energy consumption consumption required required to to heat heat reduced reduced from from supply supply to to room room.. reheating reheating coil coil energy energy consumption consumption can can be be estimated estimated based based on on Equation Equation (22). (22). cooling cooling energy energy savings savings equals equals cooling cooling energy energy consumption consumption to to cool cool reduced reduced from room conditions to supply condition. When room moisture production is from room conditions to supply condition. When room moisture production is neglected, neglected, potential cooling energy consumption equals potential reheat energy potential cooling energy consumption equals potential reheat energy consumption. cooling consumption. cooling energy consumption can be estimated based on Equation (23) when energy consumption can be estimated based on Equation (23) when outside is outside is higher than supply. When outside higher than supply. When outside is lower than supply is lower than supply, mechanical cooling can be eliminated by using an, mechanical cooling can be eliminated by using an economizer. refore, potential economizer. refore, potential cooling energy savings is zero when an economizer is used. If cooling energy savings is zero when an economizer is used. If an economizer is not used, reduced an economizer is not used, reduced can decrease cooling energy consumption even can decrease cooling energy consumption even when outside is lower when outside is lower than supply. than supply. When impact of fan efficiency decrease is neglected, potential fan power savings can When impact of fan efficiency decrease is neglected, potential fan power savings can be estimated based on Equation (24). When supply fan is reduced, supply fan speed be estimated based on Equation (24). When supply fan is reduced, supply fan speed should be reduced and total fan power will be dropped. refore, significant fan power should should be reduced and total fan power will be dropped. refore, significant fan power should be saved. With proper static pressure control from handling unit, rooms receive adequate be saved. With proper static pressure control from handling unit, rooms receive adequate cooling and do not overwhelm fan. cooling and do not overwhelm fan. Erh m p ( min min ) ( Tr Ts ) (22) E rh = m C p (α min α min ) (T r T s ) (22) Ec m p ( min min )( hr hs ) (23) E c = m C p (α min α min )(h r h s ) (23) 2 E f m Ps, d ( 1s Ps, d )/ f, d E f = m P s,d (1 α 2 s P s,d )/η f,d (24) (24) In order to evaluate rmal energy consumption in actual system, reheating energy In order to evaluate rmal energy consumption in actual system, reheating energy consumption can be calculated with Equation (4). energy consumption is 119,721 Btu/hr day when consumption can be calculated with Equation (4). energy consumption is 119,721 Btu/hr day re is a conventional control sequence. On or hand, energy consumption is 6758 Btu/hr when re is a conventional control sequence. On or hand, energy consumption is 6758 Btu/hr with an improved control sequence. rmal energy consumption with improved minimum is less than that with conventional minimum. Based on trending data, reheating energy cannot be compared between conventional and improved control sequences because outside conditions are not same. However, reduction of supply and

20 Sustainability 16, 8, 1151 of 22 with an improved control sequence. rmal energy consumption with improved minimum is less than that with conventional minimum. Based on trending data, reheating energy cannot be compared between conventional and improved control sequences because outside conditions are not same. However, reduction of supply and discharge should save fan power, cooling and heating energy consumption by optimal terminal box control algorithms. 6. Conclusions In this study, applicable optimal terminal box control algorithms for EMCS are suggested. To improve conventional terminal box control sequence, dynamic models were analyzed, optimal control algorithms were developed, and developed EMCS algorithms were n applied to an actual building. terminal box energy consumption and rmal performance were compared between conventional and improved control. results are as follows. (1) In conventional control, room could not maintain its set point because minimum supplies an inadequate for a conditioned space without considering building operation conditions. minimum is higher than required, which often leads to significant simultaneous heating and cooling, in addition to excessive fan power. (2) Improved control can stably maintain set room and reduce energy consumption, compared to conventional control. Measurements of CO 2 levels show re is no indoor quality problem when minimum set point is reduced. (3) In energy measured results, rmal energy consumption with improved control is less than that with conventional control. refore, if developed EMCS control algorithms are applied, a terminal box can reduce rmal energy. Acknowledgments: This research was supported by a grant (16AUDP-B7914-3) from Architecture & Urban Development Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government. Author Contributions: All authors contributed to this work. Yoon-Bok Seong performed result analysis and wrote major part of this article. Young-Hum Cho was responsible for this article and gave conceptual advice. Conflicts of Interest: authors declare no conflict of interest. Nomenclature T r T out T ad,j q i m v Room, F outside, F interior, F internal heat gains, Btu/hr mass rate of ventilation, lbm/h ρ standard density, lbm/ft 3 V volume of room mass, ft 3 C a U i U j A i area of exterior wall, ft 2 A j area of interior wall, ft 2 T s T d,s T s,w T r,w m a m w specific heat of, Btu/lbm F heat transfer coefficient of exterior wall, Btu/lbm ft 2 F heat transfer coefficient of interior wall, Btu/lbm ft 2 F steady state supply, F steady state discharge, F steady state supply water, F steady state return water, F mass rate of, lbm/h mass rate of water, lbm/h

21 Sustainability 16, 8, of 22 C w = m w c p,w C a = m a c p,a C min = Minimum(C a, C w ) C max = Maximum(C a, C w ) C r = C min C max NTU = C UA min [ ε = 1 exp NTU.22 C {exp r specific heat of water, Btu/lbm F specific heat of, Btu/lbm F equal to C a or C w, whichever is smaller. equal to C a or C w, whichever is bigger. heat capacity ratio [ number of transfer units C r (NTU).78] }] 1 heat exchanger effectiveness with both fluids unmixed T D,d,s dynamic discharge, F T D,r,w dynamic return water, F t coil coil time constant P a,i Inlet pressure P a,o outlet pressure k a resistance coefficient m a rate P w,i Inlet water pressure P w,o outlet water pressure k w water resistance coefficient m w water rate volumetric rate for fresh requirement, V f ft 3 /min α oa AHU outside intake ratio, % S box terminal box resistance E f fan power, kw h r room enthalpy, Btu/lbm h s supply enthalpy, Btu/lbm α min minimum ratio in conventional conditions α min minimum ratio in improved conditions α s supply ratio η f,d fan efficiency static pressure set point, in.wg P s,d References 1. Elovitz, D.M. Minimum outside control method for VAV systems. ASHRAE Trans. 1995, 11, Krakow, K.I. Economizer control. ASHRAE Trans., 16, Ke, Y.P. Optimized supply (SAT) in variable--volume (VAV) systems. Energy Int. J. 1997, 22, [CrossRef] 4. Zaheer-uddin, M. VAV system model to simulate energy management control functions: Off-normal operation and duty-cycling. Energy Convers. Manag. 1994, 35, [CrossRef] 5. Engdahl, F.; Johansson, D. Optimal supply with respect to energy use in a variable volume system. Energy Build. 4, 36, [CrossRef] 6. Wei, G.; Claridge, D.E.; Liu, M. Optimize Supply Air Temperature Reset Schedule for a Single Duct VAV system. In Proceedings of Twelfth Symposium on Improving Building Systems in Hot and Humid Climates, San Antonio, TX, USA, May ; pp Nassif, N.; Moujaes, S. A new operating strategy for economizer dampers of VAV system. Energy Build. 8,, [CrossRef] 8. Taylor, S.T.; Stein, J. Sizing VAV Boxes. ASHRAE J. 4, 46, Cho, Y.; Liu, M. Minimum reset of single duct VAV terminal boxes. Build. Environ. 9, 44, [CrossRef] 1. Cho, Y.; Liu, M. Correlation between minimum and discharge. Build. Environ. 1, 45, [CrossRef] 11. Kang, S.; Kim, H.; Cho, Y. A Study on control method of single duct VAV terminal unit through determination of proper minimum. Energy Build. 14, 69, [CrossRef]

22 Sustainability 16, 8, of Kim, H.; Kang, S.; Cho, Y. A study on control method without stratification of single duct VAV terminal units. J. Asian Arch. Build. Eng. 15, 14, [CrossRef] 13. Taylor, S.; Stein, J.; Paliago, G.; Cheng, H. Dual maximum VAV box control logic. ASHRAE J. 12, 54, Zhang, B.; Li, Y.; Lau, J.; Liu, M. Demand control ventilation: Influence of terminal box minimum setting on system energy use. Energy Build. 14, 79, [CrossRef] 15. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). Ventilation for Acceptable Indoor Air Quality (IAQ); ASHRAE Standard ; ASHRAE: Atlanta, GA, USA, by authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under terms and conditions of Creative Commons Attribution (CC-BY) license (