APPENDIX 7-A. MODIFICATIONS TO SIMULATIONS FOR ENERGY USE 

CALCULATIONS

TABLE OF CONTENTS

7-A.1 STATISTICAL PROCESS FOR ASSIGNING VENTILATION RATES

EMISSIONS .......................................................................................................... 7-A-1

7-A.2 ECONOMIZER USAGE AND PERFORMANCE ............................................... 7-A-4

7-A.3 BLAST PACKAGED DX COOLING EQUIPMENT MODEL ........................... 7-A-5

7-A.3.1 DX Condensing Unit.............................................................................................. 7-A-5

7-A.3.1.1 Available Capacity ............................................................................... 7-A-6

7-A.3.1.2 Full Load Power Consumption ............................................................ 7-A-7

7-A.3.1.3 System Power Consumption ................................................................ 7-A-8

7-A.4 DEVELOPMENT OF PART-LOAD PERFORMANCE CURVE ....................... 7-A-9

7-A.4.1 Degradation of Compressor Performance .............................................................. 7-A-9

7-A.4.2 Minimum Saturated Condensing Temperature .................................................... 7-A-10

LIST OF TABLES

Table 7-A.1.1 Statistical Summary of Minimum Outdoor Air in BASE Study Buildings .. 7-A-3

Table 7-A.1.2 Minimum Outdoor Ventilation Rate for 1,033 Buildings ............................. 7-A-3

Table 7-A.4.1 Coefficients for BLAST Fraction of Full Load Curve.................................. 7-A-9

LIST OF FIGURES 

Figure 7-A.1.1 Annual Consumption Loss for Lower Temperature Changes in DICE, 

FUND, and PAGEFrequency Distribution of Minimum Outdoor Air 

Flow for the BASE Study Office Buildings and a Log Normal 

Distribution (n = 5,000) ................................................................................ 7-A-2

Figure 7-A.1.2 Frequency Distribution of Outdoor Ventilation Rates for Each Building 

Type .............................................................................................................. 7-A-4

Figure 7-A.4.1 Calculated Condenser Efficiency Degradation Factor Used in Current 

Analysis and Comparison with other Data Sources .................................... 7-A-10

7-A-i
APPENDIX 7-A. MODIFICATIONS TO SIMULATIONS FOR ENERGY USE 

CALCULATIONS

 As described in chapter 7, the U.S. Department of Energy (DOE) performed simulations 

of cooling loads for 1,033 commercial buildings in order to calculate the energy use of 

commercial unitary air conditioners (CUAC) operating under a range of ambient conditions. The 

simulation runs provided hourly building cooling loads and fan operating hours. The Department 

used the Building Loads and System Thermodynamics (BLAST) program to calculate the 

building air-conditioning loads and fan operating hours.1

 The methods and parameters for the 

simulations were based on those described in the Technical Support Document (TSD) for the 

Advance Notice of Proposed Rulemaking (ANOPR)2

 for CUAC that DOE published in 2004. 

Some of the methods and parameters were modified and updated to better capture the on-site 

performance of CUAC when operating. Listed below are the parameters and methods that were 

modified for the current simulation of CUAC operation. 

1) Statistical process for assigning ventilation rates

2) Economizer usage and performance

3) BLAST Packaged DX cooling equipment model 

a. DX condensing unit 

i. Available capacity

ii. Full load power consumption 

iii. System power consumption 

4) Development of part-load performance curve 

a. Degradation of compressor performance 

b. Minimum saturated condensing temperature 

 Each area of modification is discussed further below. 

7-A.1 STATISTICAL PROCESS FOR ASSIGNING VENTILATION RATES

EMISSIONS

 In the present analysis DOE assigned minimum outdoor ventilation flow rates for each of 

the 1,033 buildings in the set of buildings simulated based partly on the U.S. Environmental 

Protection Agency's Building Assessment Survey and Evaluation (BASE) study performed by 

the National Institute for Standards and Testing (NIST).3 The field data for BASE were collected 

from many randomly selected General Services Administration (GSA) office buildings 

throughout the United States, where a standardized protocol was used to measure ventilation 

rates. NIST excluded from the data set buildings known to have indoor air quality problems. The 

Department found the average ventilation rates in those office buildings to be approximately 

two-thirds of the ventilation rates derived from ASHRAE Standard 62-1999, but with significant 

variation among the buildings NIST studied. For this analysis, the ventilation rate was modified 

for some building types, and a Monte Carlo distribution of multipliers was applied based on data 

from the NIST BASE study to establish a minimum outdoor air ventilation rate for each of the 

1,033 buildings analyzed. 

7-A-1
 For this analysis, the variable of interest is the volume of outdoor air, reported in cubic 

feet per minute per square foot (cfm/ft2

), under minimum ventilation conditions. Those data were 

available for 148 of the BASE buildings. The data are summarized in Table 7-A.1.1 and shown 

graphically in Figure 7-A.1.1. Also shown in Figure 7-A.1.1 is a log normal frequency 

distribution having the same mean and standard deviation as the BASE buildings, but using a 

sample size of 5,000 buildings. Based on preliminary examination of the data, the Department 

assumes that the minimum outdoor air volume follows a log normal distribution. 

Figure 7-A.1.1 Annual Consumption Loss for Lower Temperature Changes in DICE, 

FUND, and PAGEFrequency Distribution of Minimum Outdoor Air Flow for the BASE 

Study Office Buildings and a Log Normal Distribution (n = 5,000) 

7-A-2
Table 7-A.1.1Statistical Summary of Minimum Outdoor Air in BASE Study Buildings

Statistical Parameter Outdoor Air (cfm/ft2)

Mean 0.097

Standard Deviation 0.078

Minimum 0

10th Percentile 0.036

25th Percentile 0.046

Median 0.09

75th Percentile 0.125

90th Percentile 0.17

Maximum 0.608

 For each building type in the 1,033-building set, DOE used a log normal random number 

generator to generate a minimum outdoor ventilation rate. The random number generator 

required as input two variables—the population mean and the standard deviation. For each 

building type, the mean ventilation rate from the ANOPR TSD was adjusted by a factor of 0.643 

to provide the statistical mean used in the log normal random number generator. This adjustment 

represents the ratio of the median ventilation rate for office buildings in the BASE study (0.09) 

to the ventilation rate for office buildings used in the ANOPR TSD (0.14). For all but the retail 

building type, the mean given in the ANOPR TSD is the value used in the ANOPR analysis. The 

mean retail ventilation rate varied depending on the number of stories in the building. For this 

analysis the mean value for retail buildings from the ANOPR TSD (0.283 cfm/ft2

) was adjusted 

by a factor of 0.643 to provide the statistical mean input to the ventilation rate. 

 The Department used the standard deviation for the minimum ventilation rate in the 

BASE study (0.078) for all building types in the 1,033-building set as the standard deviation in 

the log normal distribution. Table 7-A.1.2 summarizes the minimum ventilation rates calculated 

based on this statistical approach and then used in the analysis. Figure 7-A.1.2 shows the 

statistical distribution of ventilation rates for each building type in the 1,033-building set. 

Table 7-A.1.2 Minimum Outdoor Ventilation Rate for 1,033 Buildings 

Building 

Type

Number of 

Buildings

Outdoor Ventilation Rate (cfm/ft2)

Average Maximum Minimum

Assembly 75 0.59 0.82 0.41

Education 99 0.27 0.44 0.15

Food 

Service 57 0.38 0.58 0.26 

Office 357 0.09 0.51 0.01

Retail 408 0.19 0.55 0.04

Warehouse 37 0.04 0.29 0

7-A-3
Figure 7-A.1.2 Frequency Distribution of Outdoor Ventilation Rates for Each Building 

Type 

7-A.2 ECONOMIZER USAGE AND PERFORMANCE

The data set from DOE’s 1995 Commercial Building Energy Consumption Survey 

(CBECS95), described in detail in chapter 7, has a single variable (ECN6) that indicates whether 

the building in question has an economizer to provide “free cooling.” The survey provides no 

indication of the control or operation status of the economizer, or whether it is used on all the 

building's CUAC systems. The Department modified the assumption regarding economizer 

usage. Field studies indicate that economizers experience a wide range of failure modes and 

deviations from proper operation, which are difficult to quantify.4, 5, 6, 7 The studies, however, 

indicate that at least a third of installed economizers do not function properly and that 

economizer controls often are disconnected from the HVAC system. Given the available data, 

the Department changed the modeling by assigning to each building in CBECS95 that reported 

economizer use a 30-percent probability of having a non-operational economizer. Nonoperational means that DOE assumed a fixed ventilation rate for all operating hours of the year. 

7-A-4
7-A.3 BLAST PACKAGED DX COOLING EQUIPMENT MODEL 

The simulations for this analysis utilized BLAST’s Packaged DX cooling system model. 

The BLAST model, which is a mathematical representation of the performance of a direct 

expansion (DX) cooling system (e.g., a packaged unitary air conditioner), is used to calculate the 

sensible and latent cooling capacity, condenser energy consumption, and fan power energy 

consumption of a CUAC system, hour-by-hour, in response to the cooling requirements of a 

building. The model comprises three components: the DX evaporator coil model, to deal with 

calculating the cooling coil loads on the system; the DX condensing unit model to determine the 

energy consumption of the system in response to coil loads; and the supply fan model, to 

calculate the necessary supply air temperature and fan energy consumption. Detailed information 

on the DX evaporator and condensing unit models and the calculation of fan energy is found in 

the BLAST Technical Reference Manual. 

 The following sections describe changes DOE made to BLAST model parameters used in 

the ANOPR TSD. Models for the evaporator coil model, DX condensing unit, and available 

capacity were either changed or expanded.

7-A.3.1 DX Condensing Unit 

 For the ANOPR analysis of building energy and end use loads, the simulations were 

based on default performance curves incorporated in the BLAST software package.

DOE used BLAST’s Packaged DX condensing unit model to determine the required hourly 

energy input to the condensing unit (the compressor and condenser fans are treated as a single 

condensing unit) in response to the total cooling load from the DX cooling coil model. The 

condensing unit model adjusts the condenser's efficiency in response to outdoor air conditions 

and cooling coil load. 

 The BLAST formulation for the DX condensing system performance is based on a curve 

fit of the actual system performance of a particular piece of equipment to a thermodynamic 

model of the ideal performance of the system. In addition, the DX condensing unit model has a 

default value for the overall heat-transfer coefficient of the condensing coil (referred to as the 

condenser UA value, CUA) that is consistent with the BLAST default system capacity. To 

accurately represent performance of cooling systems at capacities other than the BLAST default 

capacity, the default value is replaced by a user-specified input value for CUA. This substitution

enables the model to accurately calculate system efficiency under design temperature and load 

conditions that match the user-input nominal cooling efficiency (representative of the 

performance of the condensing unit under nominal full load design cooling conditions). 

Three separate curves are provided in BLAST to simulate the performance of the 

condensing system. These curves are used to calculate the (1) available capacity (AVAILCAP) 

for a given hour; (2) the full load power ratio (FPLR), the ratio of the electrical input power for 

the condenser section to the cooling provided (in consistent units); and (3) the fraction of full 

load power (FFL), which provides the ratio of the actual condenser power requirements to the 

7-A-5
full load power requirements for a given hour’s cooling load. The formulation of the 

performance curves is described below.

7-A.3.1.1 Available Capacity 

 In BLAST the formulation of the curve for available cooling capacity (AVAILCAP) is 

based on a curve fit of the actual system performance to a theoretical model of the condenser 

system (Equation 7-A.3.1). 

AVAILCAP = NOMCAP × (RCAVCD1 + RCAVCD2 × ∆T + RCAVCD3 × ∆T2)

Where:

NOMCAP = nominal capacity of the cooling system, and default values for the coefficients are:

RCAVCD1 = 0.98772 

RCAVCD2 = -0.02288 

RCAVCD3 = 0.00027 

ΔT = “equivalent temperature difference” defined by:

∆T = SCT − DSCT

SCTTR − (SST − DSST)

Where:

SCT = saturated condensing temperature (°F), 

DSCT = design saturated condensing temperature (°F),

SCTTR = SCT temperature rise coefficient (°F),

SST = saturated suction temperature (°F), and

DSST = design saturated suction temperature (°F).

DSST, DSCT, and SCTTR are given as parameter inputs to the model. The default values 

for those parameters are:

DSST = 40 (°F) 

DSCT = 122 (°F) 

SCTTR = 2.63 

In order to calculate ΔT, first the SST is estimated by:

SST = DSST + (1 − CPLR) × UTR

Where:

SST = actual saturated suction temperature,

CPLR = cooling part-load ratio, calculated by coil modeling,

DSST = design saturated suction temperature, and 

7-A-6
UTR = unloader throttling range.a

The default value for UTR, which is a parameter input to the model, is 4 °F.

Next the saturated condensing temperature is estimated as:

SCT = Max � MSCT

LOAD ×

1+DFLPR

CUA

 

Where:

SCT = actual saturated condenser temperature,

OADB = outdoor dry-bulb temperature, 

LOAD = total load on the condensing unit calculated by coil modeling,

MSCT = minimum saturated condenser level,

DFLPR = design full load power ratio, and 

CUA = condenser U-factor-area product.

 The DX condensing unit model has a default value for the overall heat-transfer 

coefficient of the condensing coil (the CUA). BLAST also provides default values for system 

capacity. The default system capacity for the BLAST model is 563 kBtu/h; the default CUA is 

27.43 kBtu/h-°F. To represent performance of cooling systems accurately at capacities other than

the BLAST default capacity, the user specifies an input value to replace the default CUA value.

The user-specified value enables the model to accurately calculate system efficiency under 

design temperature and load conditions that match the user-input nominal cooling efficiency 

(representative of the performance of the condensing unit under nominal full load design cooling 

conditions). When analyzing equipment correctly sized for a simulation, the CUA must scale 

with the nominal equipment capacity in order to maintain the same system performance when 

modeling equipment of different sizes. DOE maintained the ratio between the two values to 

preserve the form of the BLAST capacity and full load performance curves for the CUAC 

analysis.

7-A.3.1.2 Full Load Power Consumption 

 Full load power consumption is a function purely of the SST and SCT. BLAST first 

estimates the Carnot efficiency ηc of a thermodynamic heat pump operating between the SST and 

SCT. 

a This equation reflects modifications to the BLAST model to account for the condensing unit capacity under design 

conditions. The equation was confirmed at Pacific Northwest National Laboratory and verified by the BLAST 

support office at the University of Illinois on 10/4/2002 and by the algorithm’s author (Prof. Doug Hittle, Colorado 

State University) on 10/16/2002. The modification made to the BLAST model also allow the use of the nominal 

value of 563 kBtu/h identified in the air conditioner performance data described in BLAST's documentation.

7-A-7

 
ηc = SCT − SST

SCT + 460°F

 Next, the actual full load power ratio (FLPR) (the instantaneous full load system 

efficiency) is calculated based on the DFLPR and an adjustment curve fit that uses the theoretical 

Carnot efficiency as an input: 

FLPR = DFLPR × (ADJECD1 + ADJECD1ηc + ADJECD1 × ηc

2)

Where:

FLPR = full load power ratio 

ADJECD1 = 0.2984 

ADJECD2 = 0.1334 

ADJECD3 = 34.603 

The values for ADJECD1, ADJECD2, and ADJECD3, which are provided as system defaults, 

allow for adjusting the nominal full load power ratio to different environmental temperatures.

7-A.3.1.3 System Power Consumption 

 The system power consumption for a given hour is determined through a three-step 

process. First, the part load ratio (PLR) on the equipment is calculated as:

PLR = LOAD

AVAILCAP

Where LOAD is the cooling coil load for that hour. 

 Next, the fraction of full load power (FFL) is calculated based on a curve fit of the PLR

defined above. A default curve fit is provided by BLAST: 

FFL = RPWRCD1 + RPWRCD2 × PLR + RPWRCD3 × PLR2

Where: 

RPWRCD1 = 0.1456 

RPWRCD2 = 0.9554 

RPWRCD3 = -0.10476. 

The basis of the default FFL curve fit, as defined in the BLAST Technical Reference and Help 

Manual1

, derives from the original performance data under part load conditions. Finally, the 

hour’s condensing unit power consumption (Power) is calculated as:

Power = AVAILCAP × FLPR × FFL

7-A-8
7-A.4 DEVELOPMENT OF PART-LOAD PERFORMANCE CURVE

As the BLAST DX model operates at part load, less heat moves to the condenser, which 

results in a cooler average condenser temperature for the hour, further modifying system 

performance through the other system curves. The Department developed a new part-load 

performance curve for the BLAST DX condenser unit model, whereby overall performance 

would better represent a multi-compressor system. DOE's use of a two-compressor system was 

based on the modeling methodology used in the Federal Energy Management Program's 

(FEMP's) high efficiency rooftop procurement project.8

The Department used several parameters to define the performance of the BLAST DX 

unit at other than full-load design conditions. These parameters included a system performance 

curve to determine the variation in cooling capacity when deviating off of design-temperature 

conditions, a system performance curve to determine the variation in hourly full-load power 

consumption (e.g., maximum instantaneous power) at off-design cooling conditions, and a 

system performance curve to determine actual power consumption at part load.

7-A.4.1 Degradation of Compressor Performance 

The Department used a degradation factor for compressor performance to represent 

losses during cycling. The factor provides for a 25-percent degradation of the compressor's 

coefficient of performance (COP) at a compressor load of 0 percent of capacity, decreasing to a 

0-percent degradation at 100-percent capacity. The Department based this factor on the 

degradation factors used in the FEMP cost estimator for unitary equipment.10

 DOE developed new coefficients using a successive approximation method for the 

quadratic fraction of the full load (FFL) curve in BLAST. This method provides a performance 

degradation that approximates the trends shown for the idealized multi-compressor model for a 

range of outdoor air temperatures. Table 7-A.4.1 shows the original values for the BLAST FFL 

curve and the modified values DOE developed for the current analysis. 

Table 7-A.4.1 Coefficients for BLAST Fraction of Full Load Curve 

Source RPWRCD(1) RPWRCD(2) RPWRCD(3)

BLAST Default FFL

Coefficients 0.1456 0.9554 -0.10476 

UAC FFL Coefficients

Used for Current 

Analysis

0.0925 1.2396 -0.3432 

Figure 7-A.4-1 shows the idealized two-compressor curve used in BLAST, in which the 

efficiency degradation factor, which is the ratio of the degraded system COP to the full load 

COP, is shown as a function of the system part load ratio (PLR) for an idealized, two-compressor 

model. The idealized cycling behavior in this model overlooked some factors, such as minimum 

compressor run times. Those other factors would result in a somewhat higher efficiency of 

system operation at system load points of about 0 percent and 50 percent. The idealized curve in 

7-A-9
Figure 7-A.4-1 represents a system operating with an average condenser efficiency degradation 

factor of greater than 97 percent when the system load exceeds 40 percent.

Figure 7-A.4-1 shows the new BLAST performance degradation curve assuming a 

minimum saturated condensing temperature (MSCT) of 80°F and a load profile that varies 

linearly from 0 percent system load at 70 °F, to 100 percent system load at 95 °F. The modified 

FFL curve provides a curve for condenser efficiency degradation having somewhat higher 

performance than the idealized model, especially at PLR values above 20 percent, although 

performance is slightly lower at PLR values below 20 percent. 

Figure 7-A.4.1 Calculated Condenser Efficiency Degradation Factor Used in Current 

Analysis and Comparison with other Data Sources 

7-A.4.2 Minimum Saturated Condensing Temperature

The BLAST default value for the minimum saturated condensing temperature (MSCT) 

allowed the refrigerant in a CUAC to reach 100 °F. DOE lowered the user-input parameter 

representing the allowed MSCT to the minimum condensing temperature of 80 °F reported in the 

compressor performance literature.

7-A-10
REFERENCES

1. BLAST Support Office, Volume 1: Building Loads and System Thermodynamics 

(BLAST) User Manual, 1991. University of Illinois at Urbana-Champaign. 

2. U.S. Department of Energy, Technical Support Document: Energy Efficiency Program 

for Commercial and Industrial Equipment: Commercial Unitary Air Conditioners and 

Heat Pump, Office of Energy Efficiency and Renewable Energy, Editor. 2004: 

Washington, D.C. http://www.regulations.gov/#!documentDetail;D=EERE-2006-STD0103-0078

3. Persily, A. and J. Gorfain, Analysis of Ventilation Data from the U.S. Environmental 

Protection Agency Building Assessment Survey and Evaluation (BASE) Study, 2004. 

National Institute for Standards and Testing. 

4. Ecotope, Enhanced Operations & Maintenance Procedures for Small Packaged Rooftop 

HVAC Systems–Protocol Development, Field Review, and Measure Assessment—Final 

Report, 2002. Eugene Water and Electric Board. 

5. Cowan, A., A Review of Recent Commercial Roof Top Unit Field Studies in The Pacific 

Northwest and California, 2004. Prepared by New Buildings Institute for Northwest 

Power and Conservation Council and Regional Technical Forum. 

<http://rtf.nwcouncil.org//NWPCC_SmallHVAC_Report%28R3%29Final.pdf> 

6. California Energy Commission, Integrated Energy Systems: Productivity & Building 

Science, 2003. Prepared by New Buildings Institute for the Public Interest Energy 

Research Program. <http://www.energy.ca.gov/pier/project_reports/500-03-082.html> 

7. Portland Energy Conservation Inc, Retrocommissioning Report–Facility A, Clearlake, 

CA—Final Report, 2001. Prepared by PECI, Portland, OR, in conjunction with the 

Institute for Market Transformation (IMT) and Pacific Gas and Electric 

<http://www.imt.org/resources/detail/retrocommissioning-report-long-term-care-facilitya> 

8. Federal Energy Management Program, FEMP High-efficiency Rooftop Air-conditioner 

Procurement Project. <http://www.pnl.gov/uac/costestimator/main.stm > 

7-A-11