Energy and indoor air quality implications of alternative minimum ventilation rates in California offices

Energy and indoor air quality implications of alternative minimum ventilation rates in California offices

Building and Environment 82 (2014) 121e127 Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/loc...

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Building and Environment 82 (2014) 121e127

Contents lists available at ScienceDirect

Building and Environment journal homepage: www.elsevier.com/locate/buildenv

Energy and indoor air quality implications of alternative minimum ventilation rates in California offices Spencer M. Dutton*, William J. Fisk Lawrence Berkeley National Laboratory, Berkeley, CA, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 May 2014 Received in revised form 2 August 2014 Accepted 8 August 2014 Available online 19 August 2014

The energy and IAQ implications of varying prescribed minimum outdoor air ventilation rates (VRs) in California office buildings were estimated using the EnergyPlus building simulation software tool. Weighting factors were used to scale these model predictions to state wide estimates. Energy use predictions were then verified using surveyed California building energy end use data. Models predicted state wide office electricity use that was within 15% of reported electricity consumption from power utilities. The HVAC energy penalty of providing the current Title-24 VRs was approximately 6%, of the total HVAC energy use. Having economizers installed reduced average indoor formaldehyde exposure by 38% and lowered HVAC EUI by 20%. For California offices with economizers, 50% and 100% increases in Title-24 prescribed minimum VRs increased heating, ventilating, and air conditioning (HVAC) modeled energy use by 7.6% and 21.6%, respectively, while decreasing the annual average workplace formaldehyde exposure by 8.6% and 14.4%, respectively. Economizers increased VRs above the minimum 79% of the time lowering annual average concentrations of formaldehyde. Decreasing minimum VRs below the Title-24 rate would have smaller predicted effects on energy use and comparatively larger effects on formaldehyde concentrations. In buildings without economizers in many climate zones, increasing VRs up to 150% of the current Title-24 minimum would save HVAC energy and significantly reduce formaldehyde. A key conclusion is that raising future minimum VRs in California offices is unlikely to significantly improve time-averaged IAQ in buildings with economizers. Lowering future minimum VRs would be unlikely to deliver substantive energy savings. © 2014 Published by Elsevier Ltd.

Keywords: Office ventilation energy use Indoor air quality EnergyPlus Contaminant modeling Prescribed minimum ventilation Title-24

1. Introduction California office buildings consume approximately 16000 GWh of electricity and 5400 GWh of gas per year. Of this, 38% of the electricity use and 80% of the gas use are used for HVAC [1]. A proportion of this HVAC energy is used to condition ventilation air [2]. The ventilation is needed to flush out indoor-generated air contaminants that would otherwise build up to unhealthy concentrations.

Abbreviations: CEUS, California End Use Survey; EUI, energy use intensity; GWh, gigawatt hours; HVAC, heating, ventilation, and air conditioning; IAQ, indoor air quality; kWh, kilowatt-hour; L/s, liters per second; mg/h/m2, micrograms per hour per square meter; m2, square meter; ppb, parts per billion; VR, ventilation rate; W, Watt; U.S. DOE, United States Department of Energy; U.S. EPA, United States Environmental Protection Agency. * Corresponding author. 1 Cyclotron Road, Berkeley, CA 94720, USA. Tel.: þ1 (510) 486 7179; fax: þ1 (510) 486 6658. E-mail addresses: [email protected], [email protected] (S.M. Dutton). http://dx.doi.org/10.1016/j.buildenv.2014.08.009 0360-1323/© 2014 Published by Elsevier Ltd.

Increases in VRs generally increase building energy use but also reduce indoor air contaminant concentrations. Minimum VRs specified in standards such as California's Title-24 building energy efficiency standards [3] aim to strike a balance between the energy use associated with providing ventilation and the indoor air quality (IAQ) improvements that ventilation provides. A building's VR impacts energy use because outdoor air often requires heating and possibly humidification, or cooling and possibly dehumidification, before it is supplied to indoor spaces. Several previous studies have shown that modifying the minimum indoor VR in commercial buildings significantly affects cooling energy in warmer climate zones and heating energy in cooler zones. Based on simulations of the full U.S. commercial building stock [2], an estimated 6.6% of total building site energy, corresponding to about 16.5% of HVAC energy, is used to condition ventilation air that is supplied mechanically. Conditioning of the additional ventilation provided by infiltration uses additional energy. Analyses by the U.S. Environmental Protection Agency [4] found that raising minimum VRs from 2.5 to 10 (L/s) per person

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in U.S. offices increased HVAC energy costs by 2%e10%. This percentage increased significantly when occupant densities were high or economizers were present [4]. The U.S. EPA report also identified a significant impact of increased minimum VRs on peak energy loads: raising minimum VRs from 2.5 L/s to 10 L/s per person increased peak electricity demand by 25%e35% in educational buildings and by 35%e40% in auditoriums. This peak load effect could limit HVAC system downsizing, which is often part of an energy-efficiency strategy. Haves et al. [5] used a model of a big-box store to assess the effectiveness of reducing the minimum VR. This study analyzed minimum VRs that were reduced by 50% for each of seven store models. On average, gas heating energy usage decreased by 60%, and electricity usage decreased by 1.3%, resulting in an overall 7% reduction in average total building energy use. However, no prior studies were identified that estimated the energy use associated with different minimum VRs specifically in commercial buildings in California. This study estimates the statewide HVAC energy use and IAQ implications of the minimum Title24 VR for offices and six alternative minimum VRs. The alternative minimum VRs analyzed in this study are expected to directly affect indoor contaminant concentrations and therefore to affect indoor air quality (IAQ). Prior analyses by Parthasarathy et al. [6] provide the foundation for the current assessment of the IAQ implications of minimum VRs in commercial buildings. Through mass balance modeling and risk analyses, prior studies [6,7] found that ventilation primarily helps limit health risks from indoor-generated gaseous contaminants, particularly volatile organic compounds because many commercial buildings have minimal indoor sources of inorganic gaseous contaminants. Increased VRs often increase the indoor concentrations of some outdoor air contaminants. Modeling in prior studies [8,9] found that indoor particle concentrations are not highly affected by minimum VRs because particles are present in outdoor air as well as being emitted from indoor sources. In addition, the high removal rates of particles by filters and substantial loss of particles by deposition on indoor surfaces lessen the influence of VRs on indoor concentrations of particles. Consequently, in the current work, the research team modeled an indoor-generated gaseous contaminant, formaldehyde, as an indicator of the impacts of the VRs on IAQ. Formaldehyde is a leading health concern among VOCs [8] and is also ubiquitous in offices [10]. Additionally it has a source that is independent of occupancy, a low (but not zero) outdoor concentration level, and limited loss from deposition on surfaces, chemical reaction, or filtration. Thus, the results for formaldehyde are generally applicable to other gaseous contaminants commonly found in indoor environments that also meet these criteria. Contaminant concentrations were modeled using EnergyPlus's multi-zone contaminant modeling capabilities. This relatively recent feature (added in EnergyPlus version 7.2) was first validated in a currently unpublished study. In this study, using a comparable model input settings, indoor contaminant concentrations predicted by EnergyPlus were found to agree with predictions using accepted closed form analytical mass balance models with a Pearson product moment correlation coefficient of 0.9999 (to at least 4 significant figures). Based on modeling, this paper evaluates the energy and IAQ implications of various fixed minimum VRs, with results presented first from models for buildings that include economizers and then from models for buildings without economizers. An economizer is a hardware and control system that increases the VR above a minimum value when outdoor air can be used to eliminate or reduce the need for mechanical cooling. The results of this analysis provide data to inform the development of new ventilation standards for California. Though based in California, the results of this study are

broadly applicable, and the problem of balancing energy use while providing adequate IAQ is universal. 2. Methods 2.1. Modeled scenarios Simulations encompassed seven scenarios; the California Title24 prescribed minimum VR and six alternative minimum VRs that are 100%, 50%, and 30% above and below the Title-24prescribed minimum. The minimum VRs employed in the modeling are mechanical VR, therefore100% below Title-24 (0% Title-24), means there is zero mechanical ventilation; however, the building still has unintentional air infiltration. Building performance was simulated under each ventilation scenario in each of the 16 Title-24 climate zones, shown in Appendix Fig. A1. Economizers are widely used in offices; a 2003 survey of U.S. offices [11] found that 50% of the floor stock employed economizers, specifically in California offices, as of 2003e2004, 55% of the floor stock used economizers [12]. This percentage will likely be significantly higher in new California construction because of both current Title-24 regulations and increased awareness of economizers' energy-saving potential. New office buildings built in California will need to comply with Title-24 requirement that each cooling fan system that has a design total mechanical cooling capacity exceeding 15.6 kW (54 kilo-British thermal units per hour) must include an economizer [3]. A survey of small- and mediumsize California commercial buildings revealed that 38% did not provide any mechanical ventilation [13]. Despite these two factors, a proportion of new office space will likely not have economizers. To determine the potential impact that this would have on the relationship between VRs, energy use, and IAQ, this analysis first assumed 100% economizer penetration and then assumed no economizers. 2.2. Model selection The building and IAQ models used in this study were defined in the EnergyPlus building energy simulation program. EnergyPlus has been, and continues to be validated through both analytical tests and comparative tests with other tools [14,15]. 2.3. Building models Small, medium, and large office buildings were defined for the simulations. The reference small office is a 372-square-meter (m2), single-zone building with an aspect ratio of 2.5 and an HVAC system consisting of a packaged terminal air conditioner with a DX cooling coil and a heating coil (gas). A prior study [16] used this model, which was heavily informed by the U.S. Department of Energy (U.S. DOE) reference small office model originally developed by the National Renewable Energy Laboratory [17]. The medium office is a three-story, 4982-m2 building with three multi-zone, variable air volume, packaged HVAC units. This system has three gas boilers to provide heat but also includes electric reheat coils. The large office is a 12-story, 42,757-m2 building with five thermal zones in each story. The HVAC system for the large office has two water-cooled chillers and a variable air volume HVAC system. The reference medium and large offices are based on U.S. DOE reference building models. In all cases, HVAC systems include economizer controls that increase VRs above the minimum when increases in VR save energy by reducing the need for mechanical cooling. Additional simulations assumed no economizers. This suite of reference models was designed to be representative of 70% of current U.S. commercial building stock. The original reference

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models were modified to conform to California Title-24 2008 building codes. Changes to the reference models included updates to envelope performance, weather files [18], and building operation. Brunswick et al. [16] describes in detail the California building reference models and the Title-24-specific model changes. The reference building models developed by Griffith et al. [17] represent infiltration as a simple constant air change rate. In the current study, the airflow network model was used to estimate unintentional infiltration through the building façade over time. The modeling assumed an effective leakage area of 4 square centimeters per m2 evenly distributed over the building envelope, based on measured infiltration rates reviewed and analyzed by Persily and Ivy [19]. Economizer operation was an important consideration in defining the building models; the proportion of the time that the models operate under minimum VR conditions is largely a function of economizer control settings and outdoor weather conditions. Referencing work by Taylor [20], economizer control was modeled based on a differential air temperature (dry-bulb), with the economizer activated when outdoor air temperature is less than returnair temperature, and de-activated at a maximum outdoor air temperature threshold of 28  C during these periods. The research team expected that different outdoor air VRs would result in different annual energy use and peak loads. EnergyPlus's system auto-sizing feature sized HVAC components differently in each model to meet the different load requirements. Models with higher minimum VRs require larger HVAC systems, resulting in increased energy use per unit of delivered ventilation air. Prior studies of the energy cost of ventilation air [20] avoided these secondary energy use impacts by standardizing HVAC system size. However, the current study examines the energy use of new buildings built to comply with ventilation standards; therefore, in this case it is more appropriate to size the HVAC systems to meet the projected loads of each model. Thus, HVAC system sizes vary for the different VRs studied and for different climates. Weighting factors were used to scale modeled energy consumption values to produce estimates for all offices in California. The scaling accounted for the mix of office buildings by size and by the estimated floor area of offices in each climate zone. Weighting factors were calculated using 2011 census population data [21] and 2006 electricity use data from the California End Use Survey (CEUS) Consultant Report [1]. A detailed explanation of the weighting factor calculation method is available in the Appendix [37,38]. The final energy use index or intensity (EUI) weighting factors in Table 1 were calculated by dividing the electricity use in each Title24 climate zone (from Appendix Fig. A3) by the total California electricity consumption. The figures in Table 1 represent the percentage weighting assigned to each building size and climate zone, with the sum of the figures equaling 100%. Statewide average EUI were calculated by multiplying the EUI weighting factors for each climate by the predicted EUIs from the simulation model for each building size and corresponding climate. Calculating California energy use entailed multiplying office building floor areas for each climate zone by the corresponding EUIs predicted using simulation models. Total statewide energy use was the sum of energy use in each Title-24 climate zone. Office floor

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areas were estimated using the electricity use by Title-24 climate zone given in Fig. A3 and the surveyed electricity EUIs from CEUS. Potential future ventilation standards would likely not apply to existing building stock; however, over time, existing stock will eventually be renovated or replaced. Energy impact estimates did not consider projected growth in stock or any regional differences in construction rates. Ventilation energy costs were based on the predicted gas and electricity use and energy prices. The average 2012 gas price paid by U.S. commercial companies [22], was the basis for the unit energy costs of gas: 0.028 $/kilowatt-hour (kWh) ($8.13 per thousand cubic feet). Unit electricity prices of 12.09 cents per kWh are based on the February 2013 year-to-date average price paid by commercial customers in California [23]. 2.4. Building model validation To validate the reference office building models the research team compared the projected baseline energy use to data from comparable building categories identified in the CEUS Consultant Report [1]. A detailed explanation of the building model validation method in the Appendix [39]. Given the inherent differences between our building models and the buildings surveyed in CEUS the research team considered the observed agreement between the measured and simulated end use data to be an acceptable validation of our models. Though given these limitations, this validation exercise could only realistically identify large modeling errors. 2.4.1. Uncertainty Building energy simulations are by their nature approximations of reality; all of our modeling assumptions (such as occupancy schedules and HVAC system performance) will have varying degrees of uncertainty associated with them which impact their accuracy when used to make real world building predictions. The research team identified three main sources of uncertainty in the model predictions. Sources of uncertainty include the accuracy of the EnergyPlus tool, how representative the modeled buildings are of real buildings, and uncertainty in the model input parameters. The research team attempted where possible to limit these uncertainties by; using a modeling tool, EnergyPlus, that has been extensively validated; using reference models that were developed based on surveyed data [17]; ensuring that all changes to these original model inputs were defensible and based on referenced sources; and by comparing predictions of HVAC energy by end use, and predictions of state wide electricity use, to surveyed energy use data. Without performing a more detailed uncertainty analysis EnergyPlus does not currently give estimates of uncertainty in simulation results given uncertainty in model inputs though this feature is under consideration. 2.5. Indoor air quality models EnergyPlus modeled continuous emission of formaldehyde generated by the building's interior, using the program's ZoneAirContaminantBalance generic contaminant object. Prior studies [24,25] have shown that emission rates of formaldehyde from manufactured wood products are significantly influenced by both

Table 1 EUI percentage weights for small, medium, and large offices by climate zone.

Small Medium Large

CZ01

CZ02

CZ03

CZ04

CZ05

CZ06

CZ07

CZ08

CZ09

CZ10

CZ11

CZ12

CZ13

CZ14

CZ15

CZ16

0.16% 0.16% 0.00%

0.38% 0.89% 1.13%

2.24% 7.25% 10.96%

0.82% 1.89% 2.45%

0.11% 0.25% 0.32%

1.44% 4.19% 3.54%

1.91% 3.14% 2.92%

2.42% 6.80% 5.42%

1.66% 3.69% 2.48%

2.92% 4.89% 3.06%

0.35% 0.49% 0.19%

2.80% 5.73% 5.60%

1.08% 1.42% 0.51%

0.49% 0.85% 0.40%

0.15% 0.24% 0.07%

0.05% 0.07% 0.03%

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humidity and temperature. Other studies [26,27] have found that formaldehyde emission rates from manufactured wood products are significantly impacted by indoor formaldehyde concentrations, with higher indoor concentrations resulting in reduced emission rates. Models [28,29] of formaldehyde emission rates from particle board have been validated experimentally. However, the sources of formaldehyde in offices are varied and manufactured wood products, which are likely a key source, have changed as a consequence of new standards. Thus, insufficient data are available to determine how formaldehyde emission rates in current-generation commercial buildings are affected by indoor formaldehyde concentrations, temperature and humidity. In this study, indoor temperature and humidity were not expected to vary between our modeled scenarios; however, indoor formaldehyde concentrations will vary as the ventilation rates are changed. By not accounting for any coupling between formaldehyde emission rates and ventilation rates, the modeling may over predict actual changes in indoor formaldehyde concentrations. The indoor emission rate used was a continuous 25.4 mg per hour per m2 (mg/h/m2), based on measured formaldehyde emission rates in commercial buildings [10]. Outdoor concentrations of formaldehyde were a constant 4.8 mg/m3 (4 parts per billion [ppb]), approximately based on mean observed outdoor concentrations of formaldehyde of 3.01 mg/m3 plus one standard deviation of 1.78 mg/ m3, totaling 4.8 mg/m3 [30]. A conservative estimate for the outdoor concentration of formaldehyde was selected, higher than published data, to account for the limited number of buildings surveyed in the reference. 3. Results The following results represent the outcomes of the EnergyPlus simulations over the range of climates and ventilation scenarios. Presented statewide model predictions assume hypothetical scenario where all California office conform to California Title-24 [3]. In order to use these model predictions to understand the potential impact changes in prescribed minimum ventilation rates would theoretically have on real buildings it is essential to consider the inherent uncertainty in our models and their limitations discussed in the limitations section below. In buildings with economizers, the proportion of occupied time during which economizers operate varies significantly by climate, with minimum prescribed VRs provided when the economizer was inactive. For the Title-24 minimum VR scenario, economizers were deactivated on average 21% of the time on average. Economizers were deactivated up to 57% of the time in climate zone 15, and as little as 10% of the time in climate zones 1 and 5. Fig. A8 shows that the proportion of the occupied time when economizers were inactive increased with increasing minimum VR. Minimum VRs were provided during hot periods, when the outdoor air temperature was above the return air temperature, and during cold periods when heating was required. Fig. 1 gives the combined gas and electricity, HVAC EUI that includes all heating, cooling, fan and pump energy, for offices with economizers in each climate zone, under all seven ventilation scenarios. The scenario identified as “0% Title-24” indicates that the applied minimum mechanical VR was zero with only unintentional infiltration remaining. In Fig. 1, the HVAC energy use is the weighted average energy use of small, medium, and large offices. Appendix Figs. A9eA11 present the total HVAC energy use for each ventilation scenario, by climate, for the small, medium, and large offices, respectively, assuming 100% economizer penetration. Total HVAC energy increased with increased VRs. Unintentional infiltration accounted for approximately 20%, 10%, and 2% of whole

Fig. 1. Statewide office HVAC EUI, by climate zone and minimum VR, assuming 100% economizer.

building VR, for the small medium and large office respectively when no economizer was modeled. The relationship between HVAC energy use and VR was non linear with regard to the ventilation scenario although excluding the highest VR resulted in an approximately linear relationship in some climate zones. One factor driving this non-linearity was variable magnitude change in minimum VR (ranging from 20% to 50% of the Title-24 minimum VR) among adjacent ventilation scenarios. The increased VRs resulted in a non-linear increase in heating energy use in climate zone 16. Fig. 2 shows the impact of the seven VRs on office building HVAC EUI, assuming no economizers. Appendix Figs. A12eA14 give the equivalent data for small, medium, and large offices individually. Results from models without economizers showed an overall increase in HVAC energy use, compared to models with economizers. For the 100% Title-24 VR scenario, the weighted average HVAC EUI increased when no economizer was used by on average 26%. The impact of VRs on HVAC energy also depended strongly on whether economizers are present. In several climates, higher VRs provided additional free ventilation cooling, lowering total HVAC energy use up to and including 150% Title-24 rates. Fig. 3 gives the predicted state wide HVAC energy use by end use, after applying the weighting factors to the individual predictions and assuming that all buildings use economizers. Increasing the minimum VR from the current Title-24 prescribed minimum rate to double this rate increased air conditioning electricity use by 5.5% and heating energy use by 160%. Increasing the minimum VR to 130%, 150%, and 200% of Title-24 VRs increased the modeled total HVAC energy use by 3.8%, 7.6%, and 21.6%, respectively. Given an electricity cost of $0.121/kWh and a gas cost of

Fig. 2. Statewide office HVAC EUI, by climate zone and minimum VR, assuming no economizer use.

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6000 5000

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area. Appendix Figs. A15eA17 give the modeled statewide HVAC EUI by end use for the small, medium, and large offices. The small office model does not include an economizer, but the medium and large offices do.

4000 3000 2000 1000 0

Fig. 3. HVAC energy end use with economizers.

$0.028/kWh, these percentage increases translate to increased energy costs of $15.6 million, $32.5 million, and $96.5 million, respectively. Decreasing VRs to 70%, 50%, and 0% of the Title-24 rate reduced total HVAC energy use by 2.4%, 3.5%, and 5.6% respectively, saving $8.5 million, $12.2 million, and $17.6 million in energy costs compared to the costs associated with the Title-24 rate. Fig. 4 shows the predicted statewide HVAC energy use for all office buildings assuming no economizers. The impact of VRs on buildings without economizers shows a trend of decreasing cooling energy use with increased VRs. Total HVAC energy use is higher without economizers and falls as VRs increase up to 150% of Title24 rates. The heating electricity use originated from the medium building model, which includes both gas boilers and electric reheat. The gas boilers only provided heat when all of the five zones (one core, four perimeter) required heating. For the majority of the time that heating is required, only the perimeter zones required heat, provided using the electric reheat coils. Modeled total electricity use (excluding exterior lighting) for all offices under the Title-24 prescribed VR was 15,629 gigawatt hours (GWh). This compares well with the CEUS reported total electricity use of 18,012 GWh. The disparity relates to the difference between the measured and modeled EUIs and uncertainties in the floor stock

6000 5000

3.1. Indoor air quality results Fig. 5 shows the predicted average indoor formaldehyde concentration relative to the 0% Title-24 reference case in small, medium, and large offices with economizers, weighted by occupancy, for periods when minimum outdoor air is being provided, i.e., excluding periods when economizers operate. Fig. 6 shows the relative weighted average indoor contaminant concentration for all occupied periods in offices with economizers. Results showed that for periods when economizers are de-activated, formaldehyde concentrations fell significantly with increasing minimum VR. For example, in the medium and large offices, doubling the current Title-24 minimum VR reduced the average formaldehyde concentration during periods of minimum outdoor air supply by approximately 40%. This translates to a substantially smaller but still significant decrease in average contaminant concentrations for all occupied periods. For the three scenarios with increased minimum VRs e i.e., 130%, 150%, and 200% of Title-24 epredicted average formaldehyde concentrations in all offices decreased by 5.5%, 8.6%, and 14.4% respectively. For the three scenarios that decrease minimum VRs, predicted indoor contaminant concentrations for all offices increased by 6.9%, 13.0%, and 41.8%, respectively. The effect of minimum VRs on contaminant concentration was moderated by air infiltration and economizer use. The average whole-building VR, including mechanical ventilation and infiltration, was notably higher in the small office than the medium or large offices, primarily because of increased infiltration. The small-office infiltration rates were higher with an average of 0.23 ACH compared to the medium and large, where infiltration rates were approximately 0.05 ACH. The research team identified two reasons for this discrepancy: first, the small office comprises a single zone with multiple openings on each of the four walls, allowing for infiltration-based ventilation from all wind directions, where as the medium and large offices include a large core with no infiltration from the outdoors and second, the ratio of envelope area to volume is greatest in the small office. Fig. 7 shows the relative weighted average indoor contaminant concentration in small, medium, and large offices without economizers. Results showed that under the 100% Title-24 VR scenario, average indoor contaminant concentrations were 62% higher with no economizer compared to reference models with economizers. In the large- and medium-size offices, doubling the current Title-24

4000 3000 2000 1000 0

Fig. 4. HVAC energy end use without economizers.

Fig. 5. Relative contaminant concentration, occupied times at minimum VR, with economizers.

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Fig. 6. Relative contaminant concentration, all occupied times, with economizers.

minimum VR decreased the average indoor contaminant concentration during occupied periods by approximately 40%. 4. Discussion and analysis The predicted energy impacts of minimum VRs varied substantially with both climate and building size. Raising the minimum VR had the largest effects on energy use in climates with higher heating demand and in smaller offices. Climate affects how minimum VRs influence energy use more than climate affects how minimum VRs affect IAQ. Consequently, the benefit-to-cost ratio of increased minimum VRs will also vary with climate and building size. For California offices with economizers a 50% increase in minimum VR, from the current Title-24 VR, increased predicted HVAC energy use 7.6%, costing $32.5 million per year for energy, and decreased the predicted average contaminant concentration during occupancy by 8.6%, with a substantially larger decrease in contaminant concentration during periods of minimum outdoor air supply. Reducing minimum VR's by 50% reduced predicted HVAC energy use by 3.5%, saving $12.2 million in annual energy costs while increasing predicted office work time exposure to formaldehyde by 13%. The economic cost of the negative health outcomes that might result from this increased contaminant exposure are unknown; however, prior studies [31e33] have found significant economic costs of health outcomes associated with indoor contaminant exposures and different VRs. Under typical operating conditions in California, economizers provide additional ventilation air above the minimum VR on average for 79% of occupied time. The predicted impacts of changes in minimum VRs on the time-averaged indoor concentrations of

formaldehyde were moderate because of the extensive periods of economizer use and the presence of significant infiltration. Actual changes in formaldehyde may be smaller due to coupling of ventilation rates with indoor formaldehyde emission rates. Broader use of economizers and assurance that buildings provide minimum ventilation in practice may have a greater potential to reduce occupant exposure to indoor-generated contaminants than changes in minimum VRs. When simulations were repeated assuming no installed economizers, the results indicate that there would be both overall energy and IAQ benefits to higher minimum VRs for buildings without economizers. In theory, one potential mechanism for realizing these dual benefits would be prescriptions for different minimum VRs depending on whether or not a building has an economizer. Despite the modest impacts of minimum VRs on predicted indoor formaldehyde concentrations, experimental data [31] indicate that VRs in offices have small but economically significant effects on work performance and substantially affect rates of sick building syndrome symptoms experienced at work. One study in offices and two studies in schools [34-36], found that VRs affected rates of sick leave. Analyses of these effects in the entire stock of U.S. offices indicates that the economic benefits of improved health and performance when minimum VRs are increased far outweigh the increases in energy costs [31]. The predicted effects of VRs on both energy and IAQ were non linear. Reducing VRs below the current Title-24 VRs saved little energy with substantial increases in predicted formaldehyde concentrations during periods of minimum VR although the magnitude of increase depended strongly on the infiltration rate. The modeled infiltration rates were based on average estimates of envelope leakage rates from a limited survey of buildings [19]. In the survey, envelope leakage rates varied significantly among buildings and no attempt was made to account for this variation in this current study. If future building envelopes are more air tight, lowering of mechanical VRs will increase formaldehyde concentrations more than predicted in this paper. The minimum VR has a much larger impact on heating energy use than on cooling energy use, a finding also reported by Benne et al. [2]. As internal heat generation rates are reduced to save energy, the effects of minimum VRs on heating energy consumption would likely increase. Our results do not support arguments that lowering minimum VR based will save significant energy. Additionally, this study shows that raising minimum VRs for offices would not substantially reduce state-wide time-average indoor concentrations of airborne contaminants in buildings with economizers because of the large fraction of time when economizers provide more than the minimum VR in California. 5. Conclusions

Fig. 7. Average contaminant concentration, without economizers.

 Providing Title-24-prescribed minimum VRs increases predicted HVAC energy use by approximately 6% when compared to a condition with no outdoor air ventilation.  Minimum VRs impacted HVAC heating energy significantly more than HVAC cooling energy use.  Economizer use depended significantly on climate; on average in the modeled California offices, economizers provided free cooling and improved IAQ during 79% of the time that occupants were at work.  Having installed economizers reduced the predicted average indoor formaldehyde exposure by 38% and lowered HVAC EUI by 20% compared to reference models without economizers.  In offices without economizers, minimum VRs up to 150% of the Title-24 rates lowered total statewide HVAC energy use and

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improved IAQ. However, energy use is expect to increase in all small offices, and by a substantial amount for those that are located in more severe climate zones.  Annual average indoor formaldehyde concentrations in offices with economizers were only moderately affected by minimum VRs, primarily because of extensive economizer use.  The predicted effects of VRs on both energy and IAQ were non linear. Reducing VRs below the current Title-24 VRs substantially increased formaldehyde concentrations during periods of minimum VRs and produced little energy savings. Increasing the minimum VR above 150% of the current Title-24 minimum VR reduced indoor formaldehyde concentrations during periods of minimum VR only slightly while increasing energy consumption significantly. Acknowledgments The research reported here was supported by the California Energy Commission Public Interest Energy Research Program, Energy-Related Environmental Research Program, award number 500-09-049 via Contract DE-AC02-05CH11231 between the University of California and the U.S. Department of Energy. The authors thank Marla Mueller for program management. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.buildenv.2014.08.009. References [1] California Energy Commission. California commercial end-use survey (CEUS): consultant report; 2006. Table 7e2, http://www.energy.ca.gov/ 2006publications/CEC-400-2006-005/CEC-400-2006-005.pdf. [2] Benne K, Griffith B, Long N, Torcellini P, Crawley D, Logee T. Assessment of the energy impacts of outside air in the commercial sector. National Renewable Energy Laboratory; 2009. NREL/TP-550e41955. [3] California Energy Commission. Building energy efficiency standards for residential and nonresidential buildings. Sacramento CA: California Energy Commission; 2013. [4] United States Environmental Protection Agency. Energy cost and IAQ performance of ventilation systems and controls. EPA-4-2-S-01-00; 2000. [5] Haves P, Coffey B. Benchmarking and equipment and controls assessment for a ‘Big Box’ retail chain. In: ACEEE 2008 summer study proceedings. Pacific Grove CA: The American Council for an Energy-Efficient Economy; 2008. p. 661. [6] Parthasarathy S, Chan WR, Fisk WJ, McKone T. Modeling indoor exposures to VOCs and SVOCs as ventilation rates vary. Berkeley, CA: Lawrence Berkeley National Laboratory; 2012. report LBNL-5548E. [7] Dutton SM, Mendell MJ, Chan WR, Barrios M, Sidheswaran MA, Sullivan DP, et al. Evaluation of the indoor air quality minimum ventilation rate procedure for use in California retail buildings. Indoor Air 2014. http://dx.doi.org/ 10.1111/ina.12125. [8] Parthasarathy S, Fisk WJ, McKone TE. Effect of ventilation on chronic health risks in schools and offices. Berkeley, CA: Lawrence Berkeley National Laboratory; 2012. report LBNL-6121E. [9] Dutton SM, Chan WR, Mendell MJ, Barrios M, Parthasarathy S, Sidheswaran M, et al. Evaluation of the indoor air quality procedure for use in retail buildings. Berkeley CA: Lawrence Berkeley National Laboratory; 2013. report LBNL6079E. [10] Wu X, Apte MG, Maddalena R, Bennett DH. Volatile organic compounds in small-and medium-sized commercial buildings in California. Environ Sci Technol 2011;45:9075e83. [11] United States Energy Information Administration. Commercial buildings energy consumption survey. p. Table B14. Selected principal activity: part 2, floorspace for non-mall buildings; 2003. http://www.eia.gov/consumption/ commercial/data/archive/cbecs/cbecs2003/detailed_tables_2003/2003set4/ 2003html/b14.html [accessed: October 2013]. [12] California Energy Commission. California commercial end-use survey (CEUS): database; 2006. http://www.energy.ca.gov/ceus/ [accessed: May 2014].

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[13] Bennett DH, Fisk WJ, Apte MG, Wu X, Trout A, Faulkner D, et al. Ventilation, temperature, and HVAC characteristics in small and medium commercial buildings in California. Indoor Air 2012;22:309e20. [14] Henninger RH, Witte MJ. EnergyPlus testing with global energy balance test; 2013. [15] Henninger RH, Witte MJ. EnergyPlus testing with HERS BESTEST tests from ANSI/ASHRAE standard 140e2011; 2013. [16] Brunswick S, Dutton SM, Banks D, Adams K, Haves P. A framework for estimating the potential energy savings of natural ventilation retrofits for California commercial buildings. Madison, Wisconsin: IBPSA-USA SimBuild; 2012. [17] Griffith BT, Long N, Torcellini P, Judkoff R, Crawley D, Ryan J. Methodology for modeling building energy performance across the commercial sector. National Renewable Energy Laboratory; 2008. [18] United States Department of Energy. Weather data. Available at: http://apps1. eere.energy.gov/buildings/energyplus/weatherdata_about.cfm accessed: October 2013. [19] Persily AK, Ivy EM. Input data for multizone airflow and IAQ analysis. US Department of Commerce, Technology Administration, National Institute of Standards and Technology; 2001. [20] Taylor ST, Cheng C. Why enthalpy economizers don't work. ASHRAE J 2010;52. [21] California Department of Finance. E-1 population estimates for cities, counties and the state with annual percent change e January 1, 2011 and 2012.2012. http://www.dof.ca.gov/research/demographic/reports/estimates/e-1/view. php. [22] United States Energy Information Administration. U.S. price of natural gas sold to commercial customers; 2013. http://www.eia.gov/dnav/ng/hist/ n3020us3A.htm [acessed: October 2013]. [23] United States Energy Information Administration. Average retail price of electricity to ultimate customers by end-use sector, by state, year-to-date through February 2013 and 2012. Table 5.6.B; 2013., http://www.eia.gov/ electricity/monthly/epm_table_grapher.cfm?t¼epmt_5_06_b [accessed: October 2013]. [24] Lee Y-K, Kim H-J. The effect of temperature on VOCs and carbonyl compounds emission from wooden flooring by thermal extractor test method. Build Environ 2012;53:95e9. [25] Haghighat F, De Bellis L. Material emission rates: literature review, and the impact of indoor air temperature and relative humidity. Build Environ 1998;33:261e77. [26] Brown S. Chamber assessment of formaldehyde and VOC emissions from wood based panels. Indoor Air 1999;9:209e15. [27] Myers GE, Nagaoka M. Emission of formaldehyde by particleboard: effect of ventilation rate and loading on air contamination levels. For Prod J 1981;31: 39e44. [28] Hoetjer JJ. Introduction to a theoretical model for the splitting of formaldehyde from composition board. DelfzijilP: Methanol chemie nederland; 1978. p. 391e8. [29] Han K, Zhang JS, Wargocki P, Knudsen HN, Varshney PK, Guo B. Model-based approach to account for the variation of primary VOC emissions over time in the identification of indoor VOC sources. Build Environ 2012;57:403e16. [30] Bennett D, Apte M, Wu X, Trout A, Faulkner D, Maddalena R, et al. Indoor environmental quality and HVAC survey of small and medium size commercial buildings. California Energy Commission; 2011. CEC-500-2011-043, Table 45 SMCB: Distribution of Outdoor Concentrations of VOCs Across All Buildings. [31] Fisk WJ, Black D, Brunner G. Changing ventilation rates in US offices: implications for health, work performance, energy, and associated economics. Build Environ 2012;47:368e72. [32] Logue JM, Price PN, Sherman MH, Singer BC. A method to estimate the chronic health impact of air pollutants in US residences. Environ health perspect 2012;120:216. [33] Dutton SM, Banks D, Brunswick SL, Fisk WJ. Health and economic implications of natural ventilation in California offices. Build Environ 2013;67:34e45. [34] Myatt TA, Staudenmayer J, Adams K, Walters M, Wand M, Rudnick S, et al. An intervention study of outdoor air supply rates and sick leave among office workers. ln: Indoor air O2: proceedings of the 9th international conference on indoor air quality and climate. Monterey, CA, vol.1; 2002. p. 778e83. [35] Shendell DG, Prill R, Fisk WJ, Apte MG, Blake D, Faulkner D. Associations between classroom CO2 concentrations and student attendance in Washington and Idaho. Indoor Air 2004;14:333e41. [36] Mendell MJ, Eliseeva EA, Davies MM, Spears M, Lobscheid A, Fisk WJ, et al. Association of classroom ventilation with reduced illness absence: a prospective study in California elementary schools. Indoor Air 2013;23:515e28. [37] California Energy Commission. Building standards reference appendices (Appendix JA2); 2008. http://www.energy.ca.gov/maps/renewable/building_ climate_zones.html [accessed: May 2014]. [38] ENVIRON and California Air Districts. California emissions estimator model appendix F; 2013. http://www.aqmd.gov/caleemod/doc/AppendixF.pdf. [39] California Energy Commission. Nonresidential alternative calculation method reference manual; 2013.