Building and Environment 82 (2014) 121e127
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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 . A proportion of this HVAC energy is used to condition ventilation air . 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  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 , 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  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 . 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.  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.  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  and is also ubiquitous in ofﬁces . 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  found that 50% of the ﬂoor stock employed economizers, speciﬁcally in California ofﬁces, as of 2003e2004, 55% of the ﬂoor stock used economizers . 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 . A survey of small- and mediumsize California commercial buildings revealed that 38% did not provide any mechanical ventilation . 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  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 . 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 , and building operation. Brunswick et al.  describes in detail the California building reference models and the Title-24-speciﬁc model changes. The reference building models developed by Grifﬁth et al.  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 . 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 , 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  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  and 2006 electricity use data from the California End Use Survey (CEUS) Consultant Report . 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
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 , 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 . 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 . A detailed explanation of the building model validation method in the Appendix . 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 ; 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
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 . 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 . 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 . 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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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
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  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 . 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 . 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. . 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  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.  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.  California Energy Commission. Building energy efﬁciency standards for residential and nonresidential buildings. Sacramento CA: California Energy Commission; 2013.  United States Environmental Protection Agency. Energy cost and IAQ performance of ventilation systems and controls. EPA-4-2-S-01-00; 2000.  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.  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.  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.  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.  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.  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.  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].  California Energy Commission. California commercial end-use survey (CEUS): database; 2006. http://www.energy.ca.gov/ceus/ [accessed: May 2014].
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