Retrofitting Air Conditioning 

and Duct Systems in Hot, 

Dry Climates 

C. Shapiro, R. Aldrich, and L. Arena

Consortium for Advanced Residential Buildings 

(CARB)

July 2012 
NOTICE

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iii

Retrofitting Air Conditioning and Duct Systems in Hot, Dry Climates

Prepared for: 

Building America

Building Technologies Program

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by:

Carl Shapiro, Robb Aldrich, and Lois Arena

Steven Winter Associates, Inc.

of the Consortium for Advanced Residential Buildings (CARB)

61 Washington Street

Norwalk, CT 06854

NREL Technical Monitor: Cheryn Engebrecht 

Prepared under Subcontract No. KNDJ-0-40342-02

July 2012
iv

[This page left blank]
v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

Executive Summary................................................................................................................................. viii

1 Introduction........................................................................................................................................... 1

1.1 Prior Research......................................................................................................................1

1.2 Research Focus ....................................................................................................................3

2 Technical Approach ............................................................................................................................. 4

2.1 Portable EER Measurement System....................................................................................4

3 CCCRD and CARB Standard Performance Tests.............................................................................. 7

3.1 Duct System Modifications and Performance .....................................................................8

3.2 System Airflows and Static Pressures..................................................................................9

4 Portable EER Measurement System Results................................................................................... 10

5 Predicted Cost Savings ..................................................................................................................... 15

6 Conclusion .......................................................................................................................................... 16

Appendix: Program Evaluation Report for CCCRD............................................................................... 17

References................................................................................................................................................. 25
vi

List of Figures

Figure 1. Example effects of flow rate on AC sensible capacity, latent capacity, and power 

consumption at constant indoor and outdoor conditions and static pressure differential. 

Numbers above data points refer to sensible EER (Sensible Btu/Wh)........................................... 4

Figure 2. EER portable test system suitcase without sensors .............................................................. 5

Figure 3. Manufacturer listed and measured sensible capacity. CCCRD measured flow rate used 

for House 2.......................................................................................................................................... 13

Figure 4. Manufacturer listed sensible EER........................................................................................... 14

Unless otherwise noted, all figures were created by CARB.

List of Tables

Table 1. Heating and Cooling System Type, Capacity, and Efficiency.................................................. 8

Table 2. Space Conditioning Equipment Model Numbers...................................................................... 8

Table 3. Duct System Airflow and Leakage (CCCRD Measurements)................................................... 9

Table 4. System Airflows and Static Pressures..................................................................................... 10

Table 5. Coil Sizing and Impact on Slope of Sensible EER Improvement .......................................... 12

Table 6. Cost of Increased Sensible Cooling Efficiency Measures ..................................................... 15

Table 7. Cost Analysis Assumptions and Results ................................................................................ 15

Unless otherwise noted, all tables were created by CARB.
vii

Definitions

AC air conditioning

ACCA Air Conditioning Contractors of America

AFUE annual fuel utilization efficiency

AHU air handling unit

BPI Building Performance Institute

CARB Consortium for Advanced Residential Buildings

CCCRD Clark County Community Resources Division

DHW domestic hot water

CFM cubic feet per minute

EER energy efficiency ratio

HERS Home Energy Rating System

HUD U.S. Department of Housing and Urban Development

HVAC heating, ventilation, and air conditioning

iwc inch water column

NSOP normal system operating pressure

NSP Neighborhood Stabilization Program

RESNET Residential Energy Services Network

SEER seasonal energy efficiency ratio

SHR sensible heat ratio

SNRHA Southern Nevada Regional Housing Authority

SWA Steven Winter Associates, Inc.
viii

Executive Summary

Since 2009, Clark County, Nevada, has received significant funding through the Neighborhood 

Stabilization Program (NSP), which is administered by the Department of Housing and Urban 

Development (HUD). The purpose of this funding was to stabilize communities that have 

suffered from foreclosures and abandonment. The Clark County Community Resources Division 

(CCCRD) has used these funds to purchase foreclosed properties, perform audits on the homes, 

and implement health, safety, and energy improvements to the properties. The renovated homes 

are rented or sold to qualifying buyers.

The Consortium for Advanced Residential Buildings (CARB) has worked with CCCRD over the 

past two years to help optimize renovations with respect to health, safety, energy performance, 

and cost. CARB has also assessed the training needs of contractors and auditors and has helped 

CCCRD implement appropriate trainings. Because air conditioning is the dominant load in Las 

Vegas, CARB has worked with CCCRD to develop procedures for upgrading cooling systems.

This report focuses on CCCRD’s procedures for increasing sensible efficiencies through 

increased flow rates across the air handling unit and increased indoor coil sizes. Although higher 

flow rates often would not be possible with an existing duct system, CCCRD and CARB 

recognized that, when substantial energy improvements result in a significant reduction in design 

cooling loads, an older and constrictive duct system may allow higher flow rates (CFM/ton) 

when combined with replacing an older existing condensing unit with a smaller unit. In practice, 

however, auditors and contractors determined that existing duct systems needed replacement in 

most cases due to factors such as poor layout, damage, and improper design. 

Detailed monitoring of five air conditioning systems provided valuable information about the 

success of CCCRD guidelines to increase sensible cooling efficiency. Three of the five homes 

met or exceeded CCCRD requirements for flow rate. Of the two homes that failed to meet the 

flow targets, one home had a packaged rooftop unit connected to ductwork that was never 

designed for flow rates greater than 400 CFM/ton. Although issues with sensors used with the 

portable energy efficiency ratio (EER) measurement system precluded direct measurement of 

total system efficiency, the measured sensible capacity of the system met manufacturer ratings at 

the measured operating conditions. An examination of the manufacturer ratings shows that when 

indoor coils are oversized, the improvement in the sensible efficiency is more pronounced as 

airflows increase.

Since duct systems needed replacement in most cases for reasons unrelated with the increased 

flow rates, the cost associated with increasing flow rates across the air handling unit was

minimal. The resulting utility bill savings of this measure, although small, are cost effective. The

annualized rate of return of this measure was 6% to 32%.
1 

1 Introduction

For more than two years, Nevada has suffered from the highest foreclosure rate in the nation, an 

unemployment rate that has reached almost 14%, and a dramatic bursting of a housing bubble 

that has left most homes in the state worth less than half the purchase price (Medina 2011). To 

combat these economic woes, the Department of Housing and Urban Development (HUD) 

granted the state of Nevada almost $40 million dollars in 2009 through the Neighborhood 

Stabilization Program (NSP), which aims to stabilize communities that have suffered from 

foreclosures and abandonment. In turn, Clark County, Nevada, which is the home of Las Vegas,

received just over $25 million in state and federal funding under the NSP. 

Through the Clark County Community Resources Division (CCCRD), Clark County has 

developed a renovation program that uses NSP funding to: (1) purchase foreclosed properties; 

(2) perform audits to assess code compliance, energy efficiency, and health issues; (3) implement 

repairs recommended by the audits; and (4) resell the properties to qualifying buyers or transfer 

the properties to the Southern Nevada Regional Housing Authority (SNRHA) for rental. The 

most recent round of purchases and renovations, which are the focus of this report, were 

performed between 2010 and 2011. During this period, CCCRD performed renovations—

including energy efficiency upgrades, health and safety repairs, cosmetic repairs, and 

improvements to bring homes into compliance with local codes—on 184 homes constructed 

between 1979 and 2007. 

This report focuses on CCCRD’s procedures for increasing sensible efficiencies through 

increased flow rates across the air handling unit and increased indoor coil sizes. Although higher 

flow rates often would not be possible with an existing duct system, CCCRD and CARB 

recognized that when substantial energy improvements result in a significant reduction in design 

cooling loads, an older and constrictive duct system may allow higher flow rates (CFM/ton) 

when combined with replacing an existing condensing unit with a smaller unit. In practice, 

however, auditors and contractors determined that existing duct systems needed replacement in 

most cases due to factors such as poor layout, damage, and improper design. 

1.1 Prior Research

Since January 2010, CARB has partnered with CCCRD to provide technical support and 

assistance toward meeting the energy efficiency goals of their NSP program, which include 

reducing the Home Energy Rating System (HERS) index of any home purchased under this 

program to less than 50 (lower values signify less energy usage). CARB has provided Clark 

County with: 

1. Modeling assistance

2. Systems research with respect to attic insulation and mechanical ventilation 

3. Support with performance testing 

4. Ongoing technical assistance with heating, ventilation, and air conditioning (HVAC) 

design and quality installation of air conditioning (AC) and heat pump systems 
2 

5. Practical guidelines in 2010 for Clark County remodelers to comply with Building 

Performance Institute (BPI) and Air Conditioning Contractors of America (ACCA) 

standards and other best practices (Arena 2010). 

In 2011, CARB’s support and evaluation of CCCRD’s NSP program had two main aspects. First, 

CARB focused on evaluating the overall effectiveness of CCCRD’s NSP program with respect 

to energy efficiency. Second, CARB evaluated the effectiveness of CCCRD’s AC design 

standards, particularly with respect to air flow and duct reconfigurations. The former research 

was published as a report to CCCRD (see Appendix) and is summarized below, whereas the 

latter research is the primary focus of this report. 

Through analysis of program documentation and data from 40 of the program’s homes

(representing approximately 22% of the program total), CARB evaluated the overall 

effectiveness of the energy efficiency measures implemented in the most recent round of 

renovations. This analysis determined the degree of energy savings, the difference between 

actual and estimated costs, and the energy savings potential of the program by house 

construction date. 

In most cases, the final (test-out) average energy efficiency of each component met or exceeded 

the value recommended by the auditors. Based on the test-in and test-out results, the average 

utility bill savings for this sample is anticipated to be approximately $806 per year, and the 

average HERS index improved from 121 to 69. Although CCCRD did not meet their goal of an 

average HERS index of 50, they succeeded in significantly reducing the energy usage of the 

purchased properties.

Costs were first analyzed broadly by major improvement category (i.e. energy efficiency, health 

and safety, and code/cosmetic) and then explored in more detail at the energy efficiency level. 

While actual costs were higher than estimated costs in each major category, the biggest 

differences were associated with code and cosmetic improvements. When comparing actual and 

estimated costs for the energy efficiency upgrades, the most significant differences were in the 

mechanical systems category. On average, the actual costs for mechanical system improvements 

(HVAC, domestic hot water (DHW), and mechanical ventilation combined) were 27% higher 

than the initial bids. Underbidding was the most apparent reason for this discrepancy, although 

many other factors may have contributed. 

Mechanical contractors were unfamiliar with the amount of testing associated with installing 

HVAC systems to the standards required by Clark County, which come from ACCA’s standards 

for quality installation of air conditioning equipment.1

 While these standards should be standard 

practice for the industry, issues with compliance are rampant across the country. Performing 

design calculations and commissioning the systems once installed were tasks with which most 

HVAC contractors in the Las Vegas area were unfamiliar. Third party verification by BPI 

auditors and/or Residential Energy Services Network (RESNET) certified energy raters was 

 

1

 See Arena (2010) for more details. Various technical manuals and standards give information 

about quality installation of air conditioning equipment (ACCA 2007, ACCA 2008, ACCA 

2009a, ACCA 2009b, ASHRAE 2004, BPI 2003, CARB 2009, Hohman 2009, Hohman 2010, 

Krigger and Dorsi 2006, Krigger and Dorsi 2008, Moravek 2009, Rutkowski 1995, Rutkowski 

2006). 
3 

required on all projects, and initial inspections confirmed that commissioning and Manual J 

(Rutkowski 2006) calculations were not being properly performed on the vast majority of the 

houses. As a result, the auditors found themselves training the contractors on the proper methods 

and had to perform repeated inspections before the HVAC systems complied with BPI standards. 

Not understanding the associated requirements, contractors typically underbid the work and then 

regularly increased their prices during renovation. 

1.2 Research Focus

Based on the program evaluation performed during the first half of 2011, CARB and CCCRD 

deemed CCCRD’s efforts to optimize air conditioning and duct system retrofits to be the most 

pressing subject requiring further research. Air conditioning is typically the largest load in hot, 

dry climates like Las Vegas, and CCCRD has appropriately placed a large emphasis on AC 

efficiency and performance. The large discrepancy between bid and actual costs for mechanical 

system upgrades in CCCRD’s NSP program, however, is a cause for concern, and given these 

higher costs, greater research is needed to ensure that CCCRD’s exacting standards are 

producing results. 

An important requirement CCCRD employs to produce higher AC efficiencies is a specification 

for larger indoor cooling coils and higher air handling unit (AHU) flow rates (400-480 

CFM/ton). While many other factors, such as flow rate, liquid line restrictions, coil obstructions, 

refrigerate charge, and non-condensable matter in the refrigerant lines can affect performance, air 

flow rates and indoor coil size were the primary target of this research. Design manuals indicate 

that increasing airflow and indoor coil size will increase sensible capacity and decrease latent 

capacity (Rutkowski 1995, Rudd 2006). In hot, dry climates where large latent cooling loads 

generally do not exist, trading latent capacity for sensible capacity is often desirable. 

Furthermore, the increased sensible cooling capacity is associated with a much smaller increase 

in power consumption, which is mostly caused by increased fan energy usage. As a result, 

effective sensible EER ratings are increased.

For example, the 3-ton split system shown in Figure 1 experiences an improvement in sensible 

efficiency of 9% when flow rate is increased from 363 CFM/ton to 450 CFM/ton. A review of 

various split system air conditioners indicates that increasing coil size by one unit size increases 

sensible cooling capacity and rated EER by 1%-2%.The increase in sensible capacity for both 

measures is accomplished by essentially trading latent capacity for sensible capacity. In hot, dry 

climates, latent capacities are generally small and increased sensible capacities are desired. In 

hot, humid climates, the opposite is generally true, and latent capacities can be increased by 

reducing air flow rates per ton and reducing indoor coil size compared to the outdoor unit. 

When retrofitting existing homes, however, achieving higher flow rates can be quite challenging. 

In many older homes, duct systems are very leaky and quite constrictive, meaning that the ducts 

are not large enough to handle larger AC flow rates. Higher flow rates may require larger ducts, 

but the modest efficiency improvements may not be worth the cost of major duct system 

upgrades. When substantial energy improvements are part of a home retrofit effort, however, the 

resulting design cooling loads can be significantly lower, and an older, constrictive duct system 

may be adequate for the target flow rates of a smaller-capacity system. 
4

8.2 8.9 9.2 9.7

0

1

2

3

4

5

6

7

6

9

12

15

18

21

24

27

30

800 900 1000 1100 1200 1300 1400

Power [kW]

Cooling Capacity [kBtu/h]

Flow Rate [CFM]

Example Effects of Flow Rate

Sensible Capacity

Latent Capacity

Power

Figure 1. Example effects of flow rate on AC sensible capacity, latent capacity, and power 

consumption at constant indoor and outdoor conditions and static pressure differential. Numbers 

above data points refer to sensible EER (Sensible Btu/Wh).

2 Technical Approach

To determine the effectiveness of CCCRD’s AC standards, short-term performance monitoring 

of the cooling systems in five unoccupied homes was conducted in late September 2011. Testing 

was conducted using CARB’s portable EER measurement system, which measures system

operating conditions and power consumption. These measurements are used to determine the 

efficiency and capacity of the cooling system. In addition to the EER testing, measurements of 

system air flows and static pressures were taken. Normal system operating pressure (NSOP) and 

return static pressure measurements were taken using an Energy Conservatory DG-700 Pressure 

and Flow Gauge. Total system airflow was measured using an Energy Conservatory TrueFlow 

flow measurement plate and a DG-700. Air flow rates through individual registers were 

measured using an Alnor LoFlo Balometer.

2.1 Portable EER Measurement System

The portable EER measurement system consists of a Campbell Scientific CR-10X datalogger 

coupled with a Campbell Scientific AM416 relay multiplexer and other supporting sensors. The 

datalogger, multiplexer and other support equipment are installed in a Pelican high-impact, hardsided suitcase.
5

Figure 2. EER portable test system suitcase without sensors

Sensors in the EER measurement system are sampled at 10-second intervals with data output at 

1-minute intervals in the form of averages, minimums, maximums and/or totals over the 1-

minute time period. Several parameters, listed below, are directly measured at ten-second 

intervals.

• Ambient (outdoor) air temperature and relative humidity

• Three AHU supply air temperature relative humidity measurements (to be averaged)

• AHU return air temperature and relative humidity

• AHU velocity pressure

• Condensing unit energy consumption (Wh)

• AHU energy consumption (Wh).

Air temperature and relative humidity measurements are made using Humirel HTM2500 

temperature and relative humidity probes. For ambient measurements, the probe is located to 

minimize heat transfer from radiation and surrounding equipment. Electrical energy consumption 

measurements are made using Continental Control Systems WattNodes coupled with 

appropriately-sized current transformers (typically 30A at the AHU and 50A at the condensing 

unit). The WattNode is a true-RMS watt‐hour transducer equipped with a pulse-output. The 

pressure measurement used in calculating airflow is made using a Setra Systems Model 264 

differential pressure transducer with a range of 0-1.0 inches of water column and equipped with a 

4-20mA output. Velocity pressure at the air handler is measured with an Energy Conservatory 

TrueFlow flow measurement plate.
6

Based on the measured parameters, the following values were calculated. Most calculations are 

performed using equations from the 2009 ASHRAE Fundamentals Handbook (ASHRAE 2009).

• Saturation vapor pressure

• Water vapor partial pressure

• Supply humidity ratio

• Return humidity ratio

• Supply enthalpy

• Return enthalpy

• Enthalpy differences

• Air density

• Cooling capacities (sensible and latent)

• Sensible heat ratio (SHR)

• EER (Btu/Wh).

The saturation pressure over liquid water pws (psia) is found as a function of dry bulb temperature

T (°R)

ln pws C /T C C T C T C T C13 lnT 3

12

2 = 8 + 9 + 10 + 11 + + ,

where

C8 = -1.0440397 × 104,

C9 = -1.1294650 × 101

,

C10 = -2.7022355 × 10-2,

C11 = 1.2890360 × 10-5

,

C12 = -2.4780681 × 10-9

, and

C13 = 6.5459673 × 100

.

The partial pressure of water vapor pw (psia) is found as a function of the saturation pressure over 

liquid water and relative humidity φ (fraction)

pw = pwsφ .

The humidity ratio W is calculated as a function of the partial pressure of water vapor and 

atmospheric pressure p (14.696 psia)

w

w

p p

p W − = 0.621945 .

The specific enthalpy of dry air hda (Btu/lbda) can be approximated as a function of dry bulb 

temperature t (°F)
7

h t da = 0.240 .

The specific enthalpy of saturated water vapor hg (Btu/lbw) is similarly approximated as a 

function of dry bulb temperature

h t g = 1061+ 0.44 .

The specific enthalpy of moist air h (Btu/lbda) is calculated as a function of the specific enthalpy 

of dry air, humidity ratio, and the specific enthalpy of water vapor

da Whg h = h + .

The total cooling capacity Qtotal

 (Btu/hr) is calculated as a function of flow rate V (CFM), supply 

enthalpy supply h (Btu/lbda), return enthalpy hreturn (Btu/lbda), and the density of dry air ρ da

(0.076474 lbda/ft3

)

( ) hour

minutes Qtotal = Vρ da hsupply − hreturn × 60   .

The sensible cooling capacity Qsensible

 (Btu/hr) is calculated similarly to the total cooling 

capacity, but uses the supply and return enthalpies of dry air at the measured dry bulb 

temperature

( ) hour

minutes = , − da,return × 60 supply sensible da da Q Vρ h h  

.

The sensible heat ratio SHR is the dimensionless ratio of the sensible cooling capacity to the total 

cooling capacity

total

sensible

Q

Q SHR 

 = .

The energy efficiency ratio EER (Btu/kWh) is ratio of total cooling capacity to the electricity 

consumption of the cooling system E (kW)

E

Q EER total



 = .

3 CCCRD and CARB Standard Performance Tests

Each house in this study has a natural gas furnace for heating and an air conditioning system for 

cooling. Four units have split system air conditioners, and the remaining house has a rooftop 

packaged unit. With SEER ratings of 15-16, cooling efficiencies for all systems are moderately 

high. The heating systems installed with the split system air conditioners are condensing natural 
8

gas furnaces with efficiencies rated at 95% AFUE, but the packaged rooftop unit is rated at a 

moderate efficiency of 80% AFUE. System capacity, type, and efficiency for each house are 

shown in Table 1.

Table 1. Heating and Cooling System Type, Capacity, and Efficiency 

System Type SEER

AC 

Capacity 

(tons)

AFUE

Heating 

Capacity 

(kBtu/h)

House 1 Split System AC 16 2 95.5% 69

House 2 Split System AC 15 2 97.5% 60

House 3 Rooftop Package Unit 15 4 80.0% 115

House 4 Split System AC 16 3 95.5% 69

House 5 Split System AC 16 4 95.7% 92

The space conditioning equipment model numbers for each house are shown in Table 2.

Table 2. Space Conditioning Equipment Model Numbers

Indoor Coil Condensing Unit AHU/Furnace

House 1 Aspen

CE30D44175L004

Amana

ASX160241CA

Amana

AMVC950704CXAA

House 2 York

ASLB2424A286TV+S

York

CZE02411A

York

YP9C060B12MP11A

House 3 N/A N/A Goodman

GPG154911541AA

House 4 Aspen

CQ60A34210T000

Amana

ASXC160361BB

Amana

AMVC950704CXAA

House 5 Goodman

CHPF486D6DA

Amana

ASXC160481AA

Amana

AMVC950905DXAB

3.1 Duct System Modifications and Performance

At the program planning stages, CARB and CCCRD recognized that when substantial energy 

improvements are part of a home retrofit effort, the resulting design cooling loads can be 

significantly lower. As a result, an older, constrictive duct system may provide higher flow rates 

(CFM/ton) when existing condensing units are replaced with smaller units that are sized for the 

reduced cooling loads. The NSP program provided a unique opportunity to realize increased 

sensible efficiencies through higher flow rates per ton without replacing the entire duct system. 

CCCRD program administrators, auditors, and contractors expected to keep, scrap, or modify the 

existing duct system, depending on the predicted energy savings and the conditions of the 

existing system. Auditors evaluated the duct systems based on static pressure measurements, 

kinks and compression of existing ducts, flows at the registers, and the overall condition of the 

ducts. Recommendations were made for repair, replacement or addition of ductwork based on 

the initial audit. The contractors would further assess the adequacy of the existing system by 

taking into account the size of the new HVAC system to be installed and the practicality of 

repairing or replacing the systems. 
9

CARB initially intended to evaluate the duct system screening criteria for replacement or 

modification, but the practical aspects of the NSP program made this investigation unnecessary. 

During program implementation, CCCRD auditors, contractors, and program administrators 

discovered that existing duct systems were commonly not adequately or properly installed. With 

high static pressures, excessively long runs, and inadequate return ducting, existing duct systems 

did not meet quality standards. As a result, auditors and contractors recommended new ductwork

when practical, and in practice this meant different approaches for one-story and two-story 

homes. In one-story homes, all ductwork was replaced because the entirety of the system was 

easily accessible in the attic. Since two-story homes were not gutted, first floor ducting was left 

in place and only second floor ductwork was replaced.

Duct systems in these five homes were greatly improved during renovation. The associated duct 

leakages to the outdoors are below 6% of measured system flow rate in all but one case. System 

duct leakages and air flow, as measured by CCCRD auditors, are shown in Table 3.

Table 3. Duct System Airflow and Leakage (CCCRD Measurements)

Number of 

Floors

Duct Leakage 

(CFM@25Pa)

AHU Flow Rate 

(CFM)

Duct 

Leakage (%)

House 1 1 77 1081 7.1%

House 2 1 42 1225 3.4%

House 3 1 74 1406 5.3%

House 4 2 65 1460 4.4%

House 5 2 94 1970 4.8%

3.2 System Airflows and Static Pressures

Two methods were typically used to measure system airflow during field testing. The first 

method used a flow plate and a digital manometer to measure air flow at the return plenum of the 

air handling unit. The second method used a balometer to measure flows through the supply 

registers. The flow plate method is considerably more accurate than the balometer method, 

which is known to have very high errors on the order of 20% (Wray et al. 2002). 

Clark County auditors measured system airflow using the balometer method. System airflow was 

measured by CARB using a flow plate, but balometer measurements were also taken to compare 

individual register airflows to the mechanical design. CCCRD and CARB measured airflows and 

total external static pressures are compared in Table 4. Calculated total flow measurements using 

CARB’s balometer measurements are listed to provide a more appropriate comparison against 

CCCRD measurements.

The measurements listed for House 4 were taken at different system stages. CARB measured the 

system at low stage operation because the system could not be forced into high speed. CCCRD 

measurements, on the other hand were taken at high stage operation. 

CARB measurements for airflow at House 2 are approximate because the configuration of the 

duct system precluded the use of typical measurement techniques. Several registers were larger 

than the balometer hood and were measured in parts. Airflow could not be measured at the air 

handling unit, and instead was measured using two flow plates at the return grilles, which is not 

as accurate a method.
10

Table 4. System Airflows and Static Pressures

CCCRD Measurements CARB Measurements

AHU Flow 

Rate –

Balometer 

(CFM/ton)

Total 

External 

Static 

Pressure 

(iwc)

AHU Flow Rate 

– Manometer 

with Flow Plate 

(CFM/ton)

AHU Flow 

Rate –

Balometer 

(CFM/ton)

Total 

External 

Static 

Pressure 

(iwc)

House 1 541 0.62 465 434 0.37

House 2 613 0.32 525* 441* 0.46

House 3 352 0.59/0.70 333 308 0.61

House 4 487 N/A‡ 320† 296† 0.37†

House 5 493 0.46 358 342 0.46

* CARB could not accurately measure the flow rates using the balometer or flow plate.

† CARB measured in low speed.

‡

External static pressure not reported by CCCRD.

Table 4 shows large discrepancies between the measurements taken by CCCRD and CARB. The 

airflow measurement discrepancies could be attributed to errors associated with flow hoods 

(Wray et al. 2002). It is also possible that different brands of flow hoods were used. While total 

external static pressure measurements can vary somewhat with the exact location of pressure 

probes, the exact reasons for the disparity between some of CARB’s and CCCRD’s 

measurements are presently unknown.

The airflow measurements show that three of the five houses achieved high flow rates per ton 

and met CCCRD guidelines (400-480 CFM/ton). The packaged rooftop unit (House 3) failed to 

meet airflow requirements. The duct system was initially sized to provide 392 CFM/ton, and the 

high measured static pressures suggest that higher flow rates could not be achieved using the 

installed ductwork. Although House 5 met CCCRD requirements according to the auditor 

measurements, CARB’s measurements show the system failing to meet those requirements.

4 Portable EER Measurement System Results

Although the portable EER system is meant to measure sensible cooling capacity, total cooling 

capacity, and mechanical system power consumption, issues with some of the sensors calls into 

question the accuracy of the power consumption and total cooling capacity measurements. 

Electric consumption measurement inaccuracy stemmed from a severe inaccuracy in the 

Continental Control Systems CTS Series split core current transformers used in the portable EER 

measurement system. Total system cooling capacity inaccuracy stemmed from a faulty return 

RH sensor and error propagation from the Humirel HTM2500 temperature and relative humidity 

probes.

During a field EER test, CARB noticed discrepancies in power consumption measurements, and 

contacted Continental Control Systems to resolve the problem. As explained by the application 

engineer, these current transformers experience rather alarming phase angle inaccuracies when 

used to monitor currents below 70A, information that was not previously published. Since the 

current transformers used in this application were rated at 30 and 50 amps, significantly below 
11

the 70A phase angle error threshold, monitored power consumption is inaccurate. The 

inaccuracies are even more severe when the monitored load has a low power factor. The CTS 

Series datasheet now states that the phase angle is rated to be <2° at 50% of rated current, but 

this rating is valid only for 70A or higher. Below 70A, the error is unknown and could be 

considerably worse, particularly if the load being monitored also has a low power factor

(Continental Control Systems 2011).

When analyzing the EER system data, the measured total cooling capacities were considerably 

larger than the rated capacities under the measured conditions. After extensive analysis, CARB 

determined that this discrepancy was caused by an unfortunate combination of a faulty return 

duct RH sensor and surprising error propagation in dry climates. CARB tested all humidity 

sensors in the lab under standard room conditions and observed a 4.2% humidity difference 

between the return RH sensor and the three supply RH sensors, which were within 1.5% RH of 

each other.

The measured 4.2% humidity difference between the sensors was amplified through error 

propagation in the cooling capacity calculations. Extensive analysis of the effect of the humidity 

sensor error at various conditions showed that the operating conditions of the monitored Las 

Vegas air conditioning systems provided a worst case scenario for error propagation. The latent 

capacity calculations are dependent on the humidity ratio, which is more sensitive to relative 

humidity at higher dry bulb temperatures. As a result, a combination of a large temperature 

difference across the coil and high return temperatures will produce the largest propagation of 

error from the relative humidity measurements to the latent capacity. In these homes, thermostats 

in these unoccupied residences were set to around 80°F before the testing commenced, and the 

operating conditions created a typical temperature difference of 20°F across the coil, which is at 

the high end of the manufacturer specifications. CARB is currently investigating more robust 

sensors for use in calculating latent cooling capacity.

Despite the issues with the relative humidity sensors and current transformers, the portable EER 

measurement system provided valuable information about the performance of the cooling 

systems through measurement of the sensible cooling capacities. These measured capacities can 

be compared to manufacturer-listed capacities to ensure that these systems are performing as 

expected.

System cooling performance is typically listed by the manufacturer in tabular form as a function 

of indoor dry bulb temperature, indoor wet bulb temperature, outdoor dry bulb temperature, and 

airflow. Listed values include total system capacity, sensible system capacity, and power 

consumption. These values are listed for a specific combination of air handling unit, outdoor 

unit, and indoor coil. If a specific combination was not listed, or if all of the equipment was not 

manufactured by the same company, CARB selected a comparable combination based on 

professional judgment.

Since the listed power consumption and capacities are given in tabular form with relatively large 

differences between the listed values of the dependent variables, an interpolation method was 

used to match the listed values to the measured system variables. A quadratic interpolation 

method using the equation below was used to predict the performance of the system under the 

measured conditions
12

k l

out db

j

in wb

i

in db

i j k l

p b aijkl t t t V , , ,

2

0

2

0

2

0

2

0

∑∑∑∑ = = = =

= + ,

where

aijkl = interpolation coefficients,

b = constant interpolation coefficient,

p = predicted value of power, total capacity, or sensible capacity,

tin,db = indoor dry bulb temperature (°F),

tin,wb = indoor wet bulb temperature (°F),

tout,db = outdoor dry bulb temperature (°F), and

V = flow rate (CFM).

Interpolations with independent coefficients, aijkl and b, were generated for power, total capacity,

and sensible capacity for each cooling system monitored. Each interpolation has its own set of 

interpolation coefficients. All interpolations had R2 values greater than 0.96.

Figure 3 shows measured system sensible cooling capacities against the interpolations of 

manufacturer’s listed capacities at the measured system operating conditions. The interpolated 

manufacturer data is shown as a function of system airflow to display the impact of airflow rates 

on system capacity. In four of the five cases, the measured system capacity was close to the 

manufacturer rated values. This shows that by increasing the flow rate across the coils, CCCRD 

was able to increase system capacity. System capacity at House 3 was considerably lower than 

the manufacturer’s listed specifications, which indicates a potential issue with the operation of 

the unit.

Although the total system efficiency of the units could not be measured, Figure 4 shows the 

predicted impact on the sensible efficiency of the unit based on the manufacturer’s rated 

specifications. Efficiency is listed as sensible EER. The packaged rooftop unit was excluded 

from these graphs because the system does not operate near the manufacturer’s rating.

These graphs show that sensible EER improvements vary significantly between systems. For 

example, the House 2 system has a slight slope of improvement, while the House 1 system has a 

steeper slope of improvement. The sensible EER improvement correlates strongly with the 

degree of oversizing of the indoor coil against the outdoor unit. A qualitative analysis is shown 

in Table 5, where coil sizes are compared to a qualitative observation of the slope of the sensible 

EER improvement shown in Figure 4.

Table 5. Coil Sizing and Impact on Slope of Sensible EER Improvement

Indoor Coil 

Tonnage

Condensing 

Unit Tonnage

Slope of 

Sensible EER 

Improvement

House 1 3 2 Steep

House 2 2 2 Slight

House 4 5 3 Steep

House 5 4-5 4 Moderate
13

180 230 280 330 380 430 480

12

18

24

30

36

42

48

Airflow (CFM/ton)

System Capacity (MBh)

House 4

Operating in Low Stage

300 350 400 450 500

24

30

36

42

48

54

60

Airflow (CFM/ton)

System Capacity (MBh)

House 5

230 280 330 380 430

24

30

36

42

48

54

60

Airflow (CFM/ton)

System Capacity (MBh)

House 3

Packaged Rooftop Unit

300 350 400 450 500 550 600

0

6

12

18

24

30

36

Airflow (CFM/ton)

System Capacity (MBh)

House 2

Flow Measured at Return Grilles

290 340 390 440 490

0

6

12

18

24

30

36

Airflow (CFM/ton)

System Capacity (MBh)

House 1

Listed Sensible Capacity - Low

Listed Sensible Capacity - High

Measured Sensible Capacity

Figure 3. Manufacturer-listed and measured sensible capacity. CCCRD measured flow rate used for House 2.
14

Sensible EER (BTU/Wh)

Airflow (CFM/ton)

House 4

200 250 300 350 400 450 500 550 600

10

11

12

13

14

15

Sensible EER (BTU/Wh)

Airflow (CFM/ton)

House 5

200 250 300 350 400 450 500 550 600

10

11

12

13

14

15

Sensible EER (BTU/Wh)

Airflow (CFM/ton)

House 2

200 250 300 350 400 450 500 550 600

10

11

12

13

14

15

Sensible EER (BTU/Wh)

Airflow (CFM/ton)

House 1

200 250 300 350 400 450 500 550 600

10

11

12

13

14

15

Manufacturer Listed Sensible EER

At Measured Flow

At Nominal Flow At Nominal Flow

At Nominal Flow

At Measured Flow

At Nominal Flow

At Measured Flow

At Measured Flow

Figure 4. Manufacturer-listed sensible EER
15

5 Predicted Cost Savings

The cost savings of this measure were calculated by comparing the energy savings associated 

with increasing the airflow of the system from 360 CFM/ton to 450 CFM/ton for a 1,700-ft2

representative home in the Clark County NSP project. The representative home was modeled in 

BEopt, and the energy efficiency improvement associated with the increased airflow was 

assumed to be a 9% increase in the EER rating of the air conditioning unit. If this home were 

able to achieve higher flow rates (450 CFM/ton), the annual cooling costs would be reduced by 

$33.27 per year (assuming $0.114 per kWh).

Achieving higher flow rates in cooling systems is probably only practical and cost effective 

when the duct system is in need of major repairs or replacement, or the home energy 

improvements reduce the design load such that the existing ducts can carry a higher flow rate 

(per ton) with the new, smaller cooling equipment. In the case of CCCRD, the auditors 

determined that the duct systems needed to be replaced for reasons independent of the system 

flow rate. As a result, the costs associated with this measure are labor and material costs of 

installing a larger indoor coil and larger ductwork. CARB assumed that the increased airflows 

would require contractors to install coils and ductwork one size larger than the typical sizes for 

the unit capacity. For this home, the increased duct sizes are assumed to cost $0.30 per linear 

foot and the larger coil costs $100. The costs assumptions are shown in Table 6.

Table 6. Cost of Increased Sensible Cooling Efficiency Measures

Larger Indoor Coil $100

Larger Duct Sizes $100

Labor and Miscellaneous Charges $0-$200

Total $200-$400

The cost analysis was performed as described by Polly et al. (2011). The assumptions and results 

are listed in Table 7, and the annualized rate of return was found to be 6% to 32%. The costs 

were assumed to be wrapped into a 5-year loan at a 7% interest rate.

Table 7. Cost Analysis Assumptions and Results

Analysis Period 30 years

Inflation Rate 3%

Loan Rate 7%

Loan Period 5 years

Measure Cost $200 – $400

Measure Lifetime 20 years

Annualized Rate of Return 6% - 32%

These savings may be modest when compared to home envelope energy improvements, but they 

may be comparable to strategies such as duct sealing. According to the National Residential 

Efficiency Measures Database (NREL 2011), the cost of duct sealing in this example home to 

reduce leakage from 30% to 15% might range from $250 - $1,250, resulting in an annual cooling 

savings of $20-$50.
16

6 Conclusion

In hot, dry climates, sensible cooling efficiencies can be improved by increasing the flow rate 

across the air handling unit and increasing the size of the indoor coil. Design manuals indicate 

that increasing airflow and indoor coil size will increase sensible capacity and decrease latent 

capacity (ACCA 1995, Rudd 2006). In hot, dry climates where large latent cooling loads 

generally do not exist, trading latent capacity for sensible capacity is often desirable. 

Furthermore, the increased sensible cooling capacity is associated with a much smaller increase 

in power consumption, which is mostly caused by increased fan energy usage. As a result, 

effective sensible EER ratings are increased. Manual S (Rutkowski 1995) considers mismatched 

coils acceptable as long as performance criteria are met. Although larger indoor coils are 

preferable for hot, dry climates, furnace-condenser-indoor coil matches should have available 

performance ratings to avoid potential performance issues and conform to energy program 

documentation requirements. 

Although higher flow rates may not be possible with existing duct systems, CCCRD and CARB 

recognized that when substantial energy improvements result in a significant reduction in design 

cooling loads, an older and constrictive duct system may adequately provide higher flow rates

(CFM/ton) when a smaller condensing unit is installed. The CCCRD NSP program provided an 

opportunity to test the feasibility of using older ductwork to provide higher flow rates per ton as 

part of a comprehensive energy efficiency upgrade. In practice, however, CCCRD auditors and 

contractors determined that most existing duct systems needed to be replaced for reasons such as 

improper design, damage, or poor workmanship.

Detailed monitoring of five air conditioning systems in CCCRD-retrofitted homes provided 

valuable information about the success of CCCRD in achieving air flow targets set from their 

guidelines and the potential to increase the sensible efficiencies of their mechanical systems. 

CARB determined that three of the five homes met or exceeded CCCRD targets for flow rate. Of 

the two homes that failed the requirements, one contained ductwork that was never designed for 

flow rates greater than 400 CFM/ton. The second home met the requirements under the auditor 

ratings, but failed to meet the flow rate requirements under CARB testing.

Although issues with the portable EER measurement system precluded direct measurement of 

efficiency, measured sensible capacities showed that in most cases the sensible capacity of the 

system met manufacturer’s ratings. The only case that did not meet the manufacturer’s ratings 

was a packaged rooftop unit. A qualitative analysis of the manufacturer ratings showed that 

when indoor coils are oversized, the improvement in the sensible efficiency is greater as airflows 

improve.

Since duct systems were typically replaced during renovation for reasons beyond total system

flow rate, the overall cost associated with increasing flow rates across the air handling unit were 

minimal. The resulting utility bill savings of this measure, although small, are cost effective. The 

annualized rate of return of this measure was 6% to 32%.
17

Appendix: Program Evaluation Report for CCCRD
18
19
20
21
22
23
24
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DOE/GO-102012-3691 ▪ July 2012

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