The following article was published in ASHRAE Journal, September 2007. ©Copyright 2007 American Society of Heating, Refrigerating and Air-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 ASHRAE.

Building Simulation For LEED EQc2 Credit ®

By Tom Hudson, Associate Member ASHRAE

B

uilding owners seeking to create healthier work environments 62.1-2004, Ventilation for Acceptable Indoor Air Quality. It has been shown

or to obtain the U.S. Green Building Council’s (USGBC) Lead- that increasing ventilation rates decreases

respiratory illness and associated sick

ership and Environmental Design (LEED)® certification often want leave,1 reduces sick building syndrome (SBS) symptoms,2 and improves pro-

to know the effect of higher ventilation levels on energy use and ductivity.3 Lawrence Berkley National

Laboratory reported that almost all stud-

cost. Unfortunately, owner decisions are often based on opinions ies found that ventilation rates below 20 cfm (10 L/s–1) per person in all building rather than engineering analysis. This study is an example methodology providing energy cost of office buildings complying with the LEED credit, EQc2, Increased Ventilation, that prescribes outdoor airflow rates that exceed ASHRAE Standard 62.1-2004 minimum rates by 30%. Office buildings in Portland, Ore., Chicago, Phoenix, and Atlanta, are modeled using eQUEST®, a freeware building 58

ASHRAE Journal

energy use analysis program designed to estimate energy consumption of proposed and existing buildings using an hour-byhour simulation procedure. For LEED credit EQc2, the outdoor air ventilation rates to the breathing zone of all occupied spaces are increased by at least 30% above the minimum rates required by ANSI/ASHRAE Standard ashrae.org

types were associated with statistically significant worsening in one or more health outcomes.4 Complying with LEED credit EQc2 won’t necessarily have a negative impact

About the Author Tom Hudson is an energy engineer and commissioning agent for Green Building Services in Portland, Ore.



September 2007

on the ability to achieve credits under EAc1. In the ANSI/ ASHRAE/IESNA Standard 90.1 2004, Energy Standard for Buildings Except Low-Rise Residential Buildings, modeling protocol, Appendix G3.1.2.5 ventilation rates are considered energy neutral between the proposed and base model. You may find a net increase in energy savings over baseline when you condition more air with better equipment.5 This is a modeling study, subject to all the limitations and inadequacies inherent in using models to reflect real-world conditions that are complex and considerably more varied than can be fully represented in a single study. Nevertheless, it is hoped that this example can contribute to understanding the relationship between ventilation and energy consumption, so that engineers will be able to provide owners with a valid assessment of energy costs. Office Building

Modeling

The energy analysis of the example office buildings was performed using accepted, standard engineering calculation procedures and eQUEST, which is an interface to the DOE-2.2 energy simulation engine. DOE-2.2 is the latest privately supported extension of DOE-2. While DOE-2 is generally accepted as the most accurate energy simulation program available, the predicted energy consumption is not meant to be an absolute prediction of the actual usage. For this example, the main emphasis is on the associated costs to increase ventilation rates 30% above the Standard 62.1-2004 baseline. The wall and roof insulation, glazing properties, and HVAC efficiencies conform to the baseline prescribed by Standard 90.1-2004. The economizer is controlled via a dry-bulb setpoint in accordance with the standard, which varies from 65°F (18°C) in Atlanta to 75°F (24°C) in Portland and Phoenix. The cost per kWh and cost per therm were gathered from the “EIA 2006 Commercial Sector Average Energy Costs by State.” The cost per kWh varied by less than a penny across the four locations with an average cost of $0.074 per Zone 6 kWh. The average cost of natural gas was $1.22 per therm and varZone 4 ied by as much as $0.15.

The four-story office building selected for this discussion is a multiple-zone variable air volume (VAV) system. Each floor has a central air handler, with an outdoor air economizer; a Zone 8 variable-volume supply fan; and throttling VAV boxes with hot water reheat. Heating and reheat are provided at the zone. A hot water boiler and a centrifugal, water-cooled chiller serve the Zone Ventilation Calculations Zone 1 Zone 5 building. Buildings and spaces complyThe minimum outside airing with the LEED rating system flow at the central air handler must meet the minimum requireis determined by the designer ments of Sections 4 through 7 of Zone 3 (in accordance with the VentilaStandard 62.1-2004. To comply tion Rate Procedure in Standard with EQc2, our design calculaZone 7 Zone 2 62.1-2004) and set by the testing, tions begin by finding the baseN adjusting and balancing (TAB) line ventilation rates required contractor. Likewise, the TAB Figure 1: Typical 25,000 ft2 (2322 m2) floor plan. by Standard 62.1-2004 and then contractor verif ies that VAV adjusting those ventilation rates boxes actuate between a design maximum and minimum flow. upwards by 30%. In heating mode, the VAV box discharge temperature shall reset The Ventilation Rate Procedure in Standard 62.1-2004 has to a maximum heating setpoint at minimum design flow. Upon a specific calculations for multizone systems6 and a spreadsheet continued call for heat, the VAV box resets the zone airflow set- is available at www.ashrae.org/vrpspreadsheet. The zone and point from minimum to maximum design heating flow setpoints. system characteristics are entered into the spreadsheet and In cooling mode, zone temperature is sensed and maintained at ventilation rates are calculated at heating and cooling design the cooling setpoint by adjusting the VAV box primary airflow. conditions. The worst-case or highest required intake airflow VAV box airflow is sensed and maintained at the setpoint by may occur at the design cooling condition or design heating adjusting the position of the VAV box damper. condition and, therefore, it is necessary to check both. In a VAV The total building size is 100,000 ft2 (9290 m2) with each system, the highest required intake airflow typically occurs in 25,000 ft2 (2322 m2) floor plan consisting of identical pro- the cooling mode and this condition is reflected here. gramming (Figure 1). Each floor has eight spaces including Standard 62, Table 6.1 (not shown) prescribes minimum high-density conference rooms and low-density office spaces. people outdoor air rate (Rp) and minimum building area Each space is an HVAC zone with a thermostat controlling one outdoor-air rate (Ra). Each space consists of office area and or more VAV boxes. conference area. Both spaces require 5 cfm/person (2.4 L/s per September 2007

ASHRAE Journal

59

Ventilation Rate Procedure

Ventilation Zone

1

2

3

Ez

Voz

Rp

Pz

Ra

Az

Vbz

cfm/p

pop.

cfm/ft2

ft2

cfm

3-

4

5

Voz

Vpz

Vpz-min

cfm

cfm

cfm

cfm

adj

System Level 1

Office Area

5

70

0.06

10,498

980

1

980

1,274

7,175

2,153

2

Conference Room

5

44

0.06

1,000

282

1

282

367

1,422

569

3

Office Area

5

21

0.06

3,124

292

1

292

379

2,557

767

4

Office Area

5

21

0.06

3,124

292

1

292

379

2,348

704

5

Office Area

5

21

0.06

3,125

292

1

292

379

1,797

539

6

Office Area

5

10

0.06

1,572

147

1

147

191

982

295

7

Office Area

5

10

0.06

1,563

146

1

146

190

1,301

390

8

Conference Room

5

44

0.06

1,000

282

1

282

367

1,302

521

1. The required minimum breathing zone outdoor airflow, Vbz, is found by solving Equation 6-1 and multiplying the zone area and zone population by their respective outdoor air rates. For the south conference room (Zone 2), Vbz = 5 x 44 + 1,000 x 0.06 = 282 cfm. 2. The zone air-distribution effectiveness (Ez) is selected based on air-distribution design and is Ez = 0.8 for heating and Ez = 1.0 for cooling. 3. The zone air-distribution effectiveness modifies the breathing-zone outdoor airflow by solving Equation 6-2 to determine the minimum zone outdoor airflow, Voz. In cooling mode, the south conference room (Zone 2) remains unchanged Voz = 282 /1.0 = 282 cfm. 3-adj. To satisfy the requirements of LEED EQc2, the breathing-zone outdoor airflow rates are increased by 30%. The minimum zone outdoor airflow, Voz (Step 3), therefore, is increased 30%. In cooling mode, the new minimum zone outdoor airflow for the south conference room (Zone 2) becomes Voz = 282 x 130% = 367 cfm. 4. eQUEST was used to estimate the design cooling load and associated design flow rate or zone primary airflow (Vpz ). 5. The minimum primary airflow settings (Vpz-min) were chosen at 30% of design cooling airflow except where noted below.

Ventilation Rate Procedure

Ventilation Zone

6adj

Zd

7

8

9

10

11

D

Vou

Vps

Xs

Evz

75%

2,409

13,219

11adj

Evz

12

12adj

13

Ev

Ev

Vot

0.64

0.53

3,762

cfm System Level

13adj

Vot 5,882

0.18

1

Office Area

0.59

0.73

0.65

2

Conference Room

0.65

0.69

0.59

3

Office Area

0.49

0.80

0.74

4

Office Area

0.54

0.77

0.70

5

Office Area

0.70

0.64

0.53

6

Office Area

0.65

0.68

0.59

7

Office Area

0.49

0.81

0.75

8

Conference Room

0.70

0.64

0.53

6. The zone primary outdoor air fraction (Zp) is given by Equation 6-5 (Zp = Voz  / Vpz-min ). For the office area (Zone 1), Zp = 1,274/2,153 = 0.59. Standard 62 allows the designer to use a default or calculated value for system ventilation effectiveness (Zp). Table 6-3 (not shown) specifies the use of Appendix A for zone primary outdoor air fractions above 0.55. Appendix A is used herein. The table values for zone primary outdoor air fraction (Zp) are disregarded and the zone discharge outdoor air fraction (Zd ) is calculated for each zone. The zone discharge outdoor air fraction (Zd) is found by Zd = Voz /Vdz. For VAV systems, Vdz is the minimum expected discharge airflow for design purposes (or Vpz-min). For this example Zp = Zd. 6. (cont) When VAV systems serve spaces with varying densities, the high-density ‘critical’ zone drives the fraction of outdoor air that must be sufficient across all zones. For this example, the highest zone discharge outdoor air fraction (Zd) occurs in the north conference room (Zone 8). The conference room’s minimum damper position was increased from 30% to 40%. This decreased the maximum Zd (Zone 8) from 0.94 to 0.70. 7. An occupant diversity of 75% was estimated over the building. The sum-of-peak populations was 969, and that actual peak system population was only 727. 8. The uncorrected outdoor air intake flow, Vou, is from Equation 6-6 (Vou = D x [sum(Rp x Pz) + sum(Ra x Az)]). 9. The system primary airflow (Vps) for cooling was taken based on a 70% load diversity factor (LDF). Vps = sum Vpz x 0.70 = 18,884 x 0.7 = 13,219. 10. The average outdoor-air fraction (Xs) is Xs = Vou  / Vps = 2,409/13,219 = 0.18. 11. For each zone, the zone ventilation effectiveness (Evz) is Equation A-1 (Evz = 1 + Xs – Zd). 12. The system ventilation efficiency (Ev) is the minimum zone value found above (Ev = minimum Evz). 13. Finally, the outdoor air intake flow (Vot) for the system is Equation 6-8, Vot = Vou / Ev = 2,409 / 0.64 = 3,762. 13-adj. The new outdoor air intake flow (Vot) for the system is 5,882 cfm.

Table 1: Standard 62.1-2004, Section 6.2, The Ventilation Rate Procedure.

person) and 0.06 cfm/ft2 (0.3 L/[s·m2]). For this example, the architect has provided the intended programming and densities for the spaces. The design zone population (Pz) for the conference rooms is 44 persons/1,000 ft2 (93 m2) and the office space 60

ASHRAE Journal

is at 7 persons/1,000 ft2 (93 m2). The Table 1 zone calculations follow the Ventilation Rate Procedure for Portland. The procedural Steps 1 through 3 are common for all locations. The population density being conashrae.org



September 2007

For LEED credit EQc2, the outdoor air ventilation rates to the breathing zone of all occupied spaces are increased by at least 30% above the minimum rates required by ANSI/ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality. stant, the differences in the ventilation rates occur based on differences in design cooling load and associated design flow rate (Step 4). eQUEST was used to estimate these values. The design flow values varied from 0.58 cfm/ft2 (2.9 L/[s·m2])for the interior office (zone 5) to 1.42 cfm/ft2 (7.2 L/[s·m2]) for the south conference room (Zone 2). When VAV systems serve spaces with varying densities, the high-density critical zones drive the fraction of outdoor air that must be sufficient across all zones. For this floor plan, the high-density conference rooms returned the highest outdoor air fraction (Zd). The conference rooms drive the minimum damper position at the air-handling unit and associated outdoor air intake flow (Vot). By increasing the minimum position of the critical zone’s VAV box, the critical zone receives additional ventilation rates and the damper position at the air-handling unit is decreased. For this floor plan both conference rooms minimum damper position was increased from 30% to 40%. Table 1’s results show that the percent outside air intake at the air-handling unit can be decreased by increasing the minimum airflow of the VAV box that serves the critical zone. Other considerations include air-distribution design, acceptable diffuser air speed at the terminal unit, the first cost of increasing the terminal units’ capacity and/or duct size, and an energy balance between increased airflow versus increased ventilation. In the above case, during times the conference room is not occupied, increasing the minimum airflow may cause cycling. Increasing supply rates to an unoccupied space reSeptember 2007

quires the reheat coil to cycle on. Another exercise beyond the scope of this article is to solve, with eQUEST, the energy balance between increasing the critical zone ventilation verses increasing the overall outdoor air intake flow. A more energy-efficient technique is to use transfer air on critical zones. Appendix A and the spreadsheet both allow credit to be taken for devices such as transfer fans or fan-powered mixing boxes that transfer return air from other zones to the critical zone. Depending on the quality of the return air, this technique can substantially reduce or even eliminate the need for primary air to the zone. Demand-controlled ventilation (DCV) based on CO2 or other sensors should be considered to reduce the minimum VAV box flow when applicable.

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Operations Summary Results

Table 2 lists the ventilation summary and simulated runtimes of various modes of operation. eQUEST’s estimates for design flow rates showed increasing design flow in locations with higher cooling degree days. Phoenix returned the largest design flow. Adjusting ventilation rates upwards by 30%, increased the ratio of outside air to design flow by ~10% at each location. As expected, locations with more heating degree days experienced more hours at minimum outside air introduction. Portland and Chicago experienced more hours at minimum outside air during heating mode than either Atlanta or Phoenix. In Phoenix, 97.4% of the hours that operated at minimum ventilation were due to the outside air dry-bulb limit being exceeded (see Note 2, Table 2). ASHRAE Journal

61

Simulation Results

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The energy use to comply with the increased ventilation rates correlated to the amount of hours the HVAC system was operating at minimum outside air ventilation and in heating mode. This varied widely by location. Portland showed the highest proportion of hours (78%, Table 2), operating in heating mode and at minimum outside air ventilation. eQuest returned the highest proportional increase in energy consumption for this location. When the minimum OSA rate was increased for the Portland location, the building’s natural gas consumption increased 50% (Table 3). The natural gas consumption for all buildings is for the hot water boiler serving VAV box reheat coils at the zone level. In heating mode, as outside air is introduced to the space, the reheat coils maintain discharge air temperature. In Phoenix, the HVAC system operated at the minimum outside air rate for more than 60% of the operating hours. Of those hours, only 2.7% (Table 2) were in heating mode. When the minimum outside air was increased, eQuest returned a negligible increase in energy consumption. The system was in cooling mode for the majority of hours the system operated at minimum outside air. While operating in cooling mode, increased introduction of OSA incurs a cooling penalty. The majority of the supply air is recirculated and conditioned to meet internal heat gains and solar heat gain. The energy required to meet the internal heat gains and solar heat gain overshadow the energy associated with outside air conditioning. When multiple zones require cooling, the quantity of airflow introduced to the mechanical equipment is small. The simulated cost to comply with LEED EQc2 during all occupied hours ranged from $2.94 per occupant per year in Phoenix to $25.50 in Portland. The largest impact was on the heating system. The heating energy consumption increased close to 45% and 48% in Atlanta and Portland respectively.

used to simulate the energy cost of LEED credit, EQc2, Increased Ventilation on office buildings with VAV multiple-zone systems. The credit requires minimum outdoor airflow rates increased 30% above Standard 62-2004 minimums. The primary impact on energy consumption was increased natural gas consumption during the heating season. In cooling dominated climates, the energy required to meet the internal heat gains and solar heat gain overshadow the energy associated with OSA conditioning. The increase in heating energy to comply with LEED EQc2 ranged from 4% in Phoenix to 50% in Portland. On a perperson basis, the additional cost ranged from $2.94 per occupant in Phoenix to $25.50 per occupant in Portland. The cost to improve indoor air quality may be insignificant based on documented increases in productivity and decreased sick days. It is important for engineers to provide a valid cost estimate so that owners can make an informed decision on whether to seek the LEED credit. The increase in energy consumption may not be small and will increase site energy use intensity (EUI) and associated green house gas emissions. Using Appendix A in Standard 62 allowed a more precise calculation of the required outside air intake. Increasing the minimum design flow or transfer return air to the critical zone are options worthy of further investigation and energy study. Adjusting the distribution of air to the critical zone has a dramatic effect on the percent outside air required at the system level. Using heat recovery ventilators to decouple the ventilation system from the heating and cooling system is also worth considering. The first cost to comply with this credit may be negligible. In heating-dominated climates, equipment may need to be upsized or life-cycle assessments may justify heat recovery equipment. In cooling-dominated climates, the loads may be sufficiently diversified not to affect the total system sizing.

Summary

References

The simulation program eQUEST was 62

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ashrae.org

1. Milton, K., et al. 2000. “Risk of sick leave



September 2007

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Location

Design Flow

Minimum 62.1-2004

Minimum LEED EQc2

HDD

CDD

cfm

cfm

Base 65

Base 65

62.1

EQc2

cfm

% Hours at Minimum cfm1

Heating Mode2

Portland

18,884

3,762

20%

5,882

31%

4,400

398

10.4%

35.2%

78.0%

Chicago

19,323

3,690

19%

5,707

30%

6,498

835

47.5%

67.6%

57.2%

Atlanta

22,400

3,803

17%

5,982

27%

2,827

1,810

53.1%

60.1%

15.4%

Phoenix

26,078

3,937

15%

6,322

24%

1,027

4,355

56.5%

58.0%

2.6%

1. As expected, the percent of hours the system runs at minimum OSA increases when the minimum volume of OSA is increased. This correlates to more hours at minimum OSA rates in lieu of economizer operation. 2. This percentage is the ratio of hours the system is operating in heating mode and minimum OSA to the number of hours the system is operating at minimum OSA. The remaining hours at minimum occur when the drybulb limit is surpassed; i.e., cooling mode. 3. Flow rates are for a typical 25,000 ft2 (2322 m2) floor plan.

Table 2: Ventilation summary results and simulated results of modes of operation. Space Cool (kWh)

Heat Reject. (kWh)

Space Heat (kBtu)

Vent. Fans (kWh)

Pumps (kWh)

All Energy (kWh)

Electricity EUI1 (kWh)

Natural Gas EUI1 (kBtu)

Energy Cost

Base Ventilation

167,900

1,700

1,603,068

79,800

Plus 30%

169,100

1,700

3,086,646

82,500

62,600

781,832

3.1

16.0

$41,527

62,900

1,220,844

3.2

30.9

$60,068

Difference

0.7%

0.0%

48.1%

3.3%

0.5%

36.0%

1.3%

48.1%

$18,541

Base Ventilation

207,100

4,600

Plus 30%

216,600

4,800

2,792,085

85,300

68,900

1,184,213

3.7

27.9

$58,263

4,099,622

86,300

71,000

1,580,230

3.8

41.0

$73,562

Difference

4.4%

4.2%

31.9%

1.2%

3.0%

25.1%

3.4%

31.9%

$15,299

Base Ventilation

307,800

Plus 30%

325,800

7,800

795,304

102,700

82,000

733,390

12.3

8.0

$50,193

8,300

1,440,041

104,700

85,000

945,952

14.7

14.4

$60,898

Difference

5.5%

6.0%

44.8%

1.9%

3.5%

22.5%

16.1%

44.8%

$10,705

Base Ventilation

376,600

8,500

1,530,476

135,100

86,600

1,055,357

16.6

15.3

$61,859

Plus 30%

391,800

9,000

1,589,509

136,900

89,000

1,092,558

17.2

15.9

$63,994

Difference

3.9%

5.6%

3.7%

1.3%

2.7%

3.4%

Cost/ person

Portland

$25.50

Chicago

$21.04

Atlanta

$14.72

Phoenix

3.3%

3.7%

$2,135

$2.94

1. Energy Use Intensity (EUI) in terms of kWh/ft2 for electric EUI and kBtu/ft2 for natural gas EUI. The initial eQUEST run returned low values for natural gas consumption at all office locations. To provide realistic values for this study, the energy consumption was normalized against energy use from metered office buildings in similar climate zones. The Commercial Buildings Energy Consumption Survey (CBECS) provided natural gas EUIs for office buildings in similar climate zones. The percentage increase in energy consumption between the minimum OSA rates and increased OSA rates were not altered. (2003 “Commercial Buildings Energy Consumption Survey [CBECS]” Table C20. Electricity Consumption and Conditional Energy Intensity by Climate Zone. Energy Information Association, U.S. Department of Energy. www.eia.doe.gov/emeu/cbecs/.)

Table 3: Annual energy costs for the simulated office buildings. associated with outdoor air supply rate, humidification, and occupant complaints.” Indoor Air 10:212–221. 2. Apte, M.G., et al. 2002. “Indoor Carbon Dioxide Concentrations, VOCs, Environmental Sensitivity Association with Mucous Membrane and Lower Respiratory Sick Building Syndrome Symptoms in the BASE Study: Analyses of the 100 Building Dataset.” Indoor Environment Department, Lawrence Berkeley National Laboratory, Berkeley, Calif., LBNL-51570. http://eetd. lbl.gov/ie/viaq/pubs/LBNL-51570.pdf. 3. Fisk, W.J. 2000. “Health and Productivity Gains from Better Indoor Environments and Their Relationship with Building Energy Efficiency.” Indoor Environment Department, Environmental Energy 64

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Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, Calif. http://eetd.lbl.gov/ie/viaq/pubs/FiskAnnualReviewEE2000.pdf. 4. “Sick Building Syndrome, Commercial Building Ventilation and Indoor Environmental Quality.” Indoor Environmental Department, Environmental Energy Technologies Division. Lawrence Berkeley National Laboratory, Berkeley, Calif. Mar. 30, 2007. http://eetd.lbl.gov/ie/viaq/v_syndrome_2.html. 5. Taylor, S.T. 2005. “LEED® and Standard 62.1.” ASHRAE Journal 47(9):S4–S8. 6. Stanke, D. 2005. “Standard 62-2001 Addendum 62n: Single-path multiple-zone system design.” ASHRAE Journal 47(1):28–35. ashrae.org



September 2007