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T OF ENERGY

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NITED STAT S OFA ERICA

M

Research Toward Zero Energy Homes October 2006 • NREL/TP-550-39342

Projected Benefits of New Residential Evaporative 

Cooling Systems: Progress Report #2

Chuck Kutscher and Mark Eastment

National Renewable Energy Laboratory

Golden, Colorado

Ed Hancock

Mountain Energy Partnership

Boulder, Colorado

Paul Reeves

Partnership for Resource Conservation 

Golden, Colorado
Projected Benefits of New 

Residential Evaporative Cooling 

Systems: Progress Report #2 

C. Kutscher and M. Eastment 

National Renewable Energy Laboratory 

Golden, Colorado 

E. Hancock 

Mountain Energy Partnership 

Boulder, Colorado 

P. Reeves 

Partnership for Resource Conservation 

Golden, Colorado 

Prepared under Task No.BET6.8004 

Technical Report 

NREL/TP-550-39342 

October 2006 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

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recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

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Table of Contents 

1.0 Introduction and Background .................................................................................... 1 

2.0 Simulation Results..................................................................................................... 7 

3.0 Monitoring Status .................................................................................................... 17 

3.1 Borrego Springs Weather Station Specifications ................................................. 18 

3.2 Monitoring of the OASys Unit .............................................................................. 20 

3.2.1 OASys Performance Data Results from Magna ............................................ 24 

3.3 Freus Monitoring.................................................................................................. 33 

3.3.1 Monitoring Plans for Borrego Springs Freus Unit.......................................... 33 

3.3.2 Freus Performance Data Results from Magna.............................................. 45

4.0 Summary and Conclusions ..................................................................................... 47 

5.0 References.............................................................................................................. 47 

Appendix A. OASys Technical Specifications ............................................................... 48 

OASys Performance Characteristics............................................................................. 50 

iii
iv
List of Figures 

Figure 1. Schematic of OASys unit (E. Kozubal, National Renewable Energy 

Laboratory)..................................................................................................... 2 

Figure 2. Schematic of Freus Components................................................................... 3 

Figure 3. Building America prototype homes at Magna, Utah ....................................... 5 

Figure 4. OASys unit installed at Magna Unit 1............................................................. 6 

Figure 5. Freus condensing unit installed at Magna Unit 2............................................ 7 

Figure 6. Common floor plan for Borrego Springs homes ............................................. 8 

Figure 7. Distribution of hours when indoor temperature or humidity are 

too high for direct-evaporative cooler in Borrego Springs, California ........... 13 

Figure 8. Distribution of hours when indoor temperature or humidity are too 

high for OASys unit in Borrego Springs, California....................................... 14 

Figure 9. Distribution of hours when indoor temperature or humidity are 

too high for direct-evaporative cooler in Albuquerque, New Mexico ............ 14 

Figure 10. Distribution of hours when indoor temperature or humidity are 

too high for OASys system in Albuquerque, New Mexico ............................ 15 

Figure 11. Comparison of four cooling systems in various locations............................. 16

Figure 12. Comparison of fan energy use for cooling systems in various locations...... 17 

Figure 13. Supply plenum layout for Wagon OASys indirect/direct 

evaporative cooler ....................................................................................... 23

Figure 14. Outdoor conditions for Magna data-collection period................................... 25 

Figure 15. Indoor conditions for Magna data-collection period ..................................... 26 

Figure 16. Outdoor wet-bulb temperature comparison for Magna 

data-collection period................................................................................... 27

Figure 17. The EER values for OASys and OASys plus furnace AH fan ...................... 28 

Figure 18. Energy use for OASys and OASys plus furnace AH fan.............................. 29 

Figure 19. Wet-bulb effectiveness for the OASys unit at Magna................................... 30 

Figure 20. Cooling delivered by the OASys unit at Magna............................................ 31 

Figure 21. Performance during cool outdoor air temperatures...................................... 32 

Figure 22. Energy consumed by the OASys unit and furnace AH fan for early 

September .................................................................................................. 32 

Figure 23. Pitot tube traverse airflow sensors installed at the return duct 

for the DiGiorgio home................................................................................ 39 

v
Figure 24. Illustration of the non-uniform return duct and close proximity of 

Pitot tube traverse airflow sensors to outdoor air damper and 

return register assembly. ............................................................................. 40 

Figure 25. Vaisala temperature and RH sensors (left in plate shield) and custom 

forced-aspiration radiation shield with thermocouples (right, white PVC) 

installed at fence surrounding Freus condensing unit.................................. 41 

Figure 26. Thermocouple (brown wire) and Vaisala temperature and 

RH sensors (white PVC shield + gray sensor) installed at 

Freus air intake, grill removed...................................................................... 42 

Figure 27. Thermocouple (brown wire) and sampling tube (white PVC shield 

plus white tube) installed at Freus exhaust grill. Vacuum pump housing 

shown on left (White PVC pipe fitting) ......................................................... 43

Figure 28. Fan-assisted flow hood installed on the Freus unit ...................................... 44 

Figure 29. Measurements taken at Magna, Utah, showing the sensible, latent, and 

total cooling capacities of the Freus unit at two different fans speeds on 

August 10, 2005........................................................................................... 46 

List of Tables 

Table 1. Freus Parts List............................................................................................... 4 

Table 2. DOE-2 Simulation Results for the Prototype House in Borrego Springs, 

California, using Different Cooling Systems .................................................. 11 

Table 3. DOE-2 Simulation Results for a Prototype House in Albuquerque, 

New Mexico, using Different Cooling systems .............................................. 12 

Table 4. Weather Station Sensors .............................................................................. 19 

Table 5. Instrumentation for OASys Unit..................................................................... 21 

Table 6. Instrumentation for Freus Unit....................................................................... 34 

Table 7. Measurement Results for Freus Unit at Magna after 

Refrigerant was Added.................................................................................. 47 

vi
Definitions 

AH air handler 

ARI capacity American Refrigeration Institute 

BIRA Building Industry Research Alliance 

CARB Consortium for Advanced Residential Buildings 

CFL compact fluorescent lighting 

CFM cubic feet per minute 

CT Current Transducer 

db dry bulb 

DEG Davis Energy Group 

DOE U.S. Department of Energy 

DOE-2 Building energy analysis program that can predict the energy 

use and cost for all types of buildings

DX direct expansion 

ECC Evaporative Cooling Coil 

EER energy efficiency ratio 

MNPT Male, National Pipe Thread 

NPT National Pipe Thread 

NREL National Renewable Energy Laboratory 

OASys an indirect/direct evaporative cooler 

Pa Pascals 

PF power factor 

RH relative humidity 

RMS Root Mean Square 

RTD Resistance Temperature Device 

SEER seasonal energy efficiency rating 

SIP structural insulated panel 

SWA Steven Winters Associates 

wb wet bulb 

vii
viii
Projected Benefits of New Residential 

Evaporative Cooling Systems: Progress Report #2 

1.0 Introduction and Background 

The use of conventional evaporative cooling has rapidly declined in the United States despite the 

fact that it has high potential for energy savings in dry climates. Evaporative systems are very 

competitive in terms of first cost and provide significant reductions in operating energy use, as 

well as peak-load reduction benefits. Significant market barriers, such as the cost of the 

prototype evaporative cooling systems and consumer perceptions of evaporative coolers being 

unable to maintain comfort conditions, still remain and can be addressed through improved 

systems integration, including the following: 

• Innovative components 

• Better design of supply ducts and dampers 

• Identification of best climates for full cooling season comfort control and potential limits 

imposed by a rainy season 

• Development of utility partnerships to roll out evaporative cooling system design parameters 

for production builders. 

This report investigates the first of these approaches, exploring innovative components. The U.S. 

Department of Energy (DOE) Building America research teams are investigating the use of two 

promising new pieces of residential cooling equipment that employ evaporative cooling as a part 

of their system design. The OASys unit shown in Figure 1, which is a combination of direct and 

indirect evaporative cooling stages developed by Davis Energy Group (DEG) and manufactured 

by Speakman CRS, is used to ultimately provide outside air to the living space. 

1
Process Inlet Air

Exhaust Outlet Air Process Outlet Air

Some flow makes 180o turn 

and flows on wet side of 

indirect heat exchanger.

Flow exits fan and continues 

down the dry side of the indirect 

heat exchanger

The remaining cooled air is 

then cooled further through the 

evaporative pads.

Direct Evap

Pads

Indirect Evap

Pads

Air exits direct evaporative 

pads at a cooler and lower 

humidity than with traditional 

swamp coolers.

Air exhaust exits the indirect 

HX wet and warm, which 

carries away heat from the 

process inlet air.

Fan

Warm dry outdoor air enters 

the fan.

Water is distributed through 2 

distribution pipes at the top of 

the direct and indirect evap 

pads

Figure 1. Schematic of OASys unit (E. Kozubal, National Renewable Energy Laboratory) 

The outdoor air provided is indirectly and directly evaporatively cooled in two stages to a 

condition that can be below the wet-bulb (wb) temperature of the outside air, thus outperforming 

a conventional single-stage direct evaporative cooler. 

The other concept, marketed by Freus, consists of an outdoor evaporative condenser coil that is 

wetted by water spray nozzles and matched to a conventional air-conditioning system as 

described in Figure 2 and Table 1. 

2
Figure 2. Schematic of Freus components 

3
Table 1. Freus Parts List 

Part Number Description 

1 Fan grille 

2 Fiberglass cabinet 

3 Drift eliminator 

4 Distribution piping 

5 Low pressure 1⁄4-in. spray nozzles (not shown) 

6 Helical condenser coil 

7 Sump 

8 Pump 

9 Drain 

10 Flush Pump 

11 

Magnesium anode - provides sacrificial corrosion 

protection 

12 Float valve 

13 Refrigerant filter dryer 

14 Scroll compressor 

15 Additional compressor for multi-zone systems 

16 Service valves 

17 Electrical panel 

18 Louvered intake 

The wetted condenser allows the refrigerant in the condenser coil to approach the outdoor-air 

wet-bulb temperature, as opposed to the higher outdoor-air dry-bulb (db) temperature, thus 

allowing for higher energy efficiency ratios (EER). The Freus is designed to replace 

conventional air-cooled condensing units for new and retrofit applications. 

These systems are being tested in two Building America prototype home locations: three homes 

built by the Community Development Corporation of Utah in Magna, Utah, and four homes built 

by Clarum homes in Borrego Springs, California. At Magna, Unit #1 has three bedrooms and 2.5 

baths and is 1,540 ft2 plus a 770-ft2 basement and detached garage (leftmost home in Figure 3). 

4
Figure 3. Building America prototype homes at Magna, Utah 

Units #2 and #3 (rightmost and center in Figure 3) have four bedrooms and 2.5 baths. The floor 

plans of Units 2 and 3 are mirror images of each other. Unit #1 has the OASys two-stage 

indirect/direct evaporative cooler plus a “dynamic ceiling.” This system exhausts cool air though 

the attic insulation via ducts in the ceiling drywall; exhaust air then flows into a small plenum 

between the ceiling drywall and a wire mesh attached to the bottom of the trusses. Ceiling 

drywall is installed over standoffs that separate the attic insulation from the drywall. The OASys 

unit at the Magna location is shown in Figure 4. 

Unit #2 uses the Freus outdoor evaporative condenser coil in conjunction with an oversized 

Carrier indoor evaporator coil (outdoor unit pictured in Figure 5). 

Unit #3 uses a conventional split-system air conditioner with a SEER rating of 12. 

The Borrego Springs project consists of four houses, two of which use OASys systems and 

conventional split-system air-conditioner backup cooling systems, one that uses a conventional 

split-system air conditioner with a SEER rating of 21, and one that uses a Freus system. The 

house using the Freus system has an additional innovative feature. When the unit is not 

operating, such as during the evenings and swing seasons, the Freus evaporative condenser can 

operate like a cooling tower, and water can be recirculated between it and the concrete floor slab. 

One of the homes with the OASys system utilizes a similar floor-cooling feature for its 

compressor-cooling backup system, the traditional air-source outdoor condensing unit is used to 

chill water, which is circulated through the floor slab and a fan coil to provide dehumidification. 

5
The first progress report from the National Renewable Energy Laboratory (NREL; Eastment et 

al. 2005) provided an overview of benefits and barriers, detailed descriptions of the OASys and 

Freus units, and instrumentation plans. This report updates monitoring plans, provides the latest 

results of measurements of a Freus and an OASys system taken at the Magna homes, and gives 

the results of annual simulations of the two different cooling systems in various climates. 

Figure 4. OASys unit installed at Magna Unit 1 

6
Figure 5. Freus condensing unit installed at Magna Unit 2 

2.0 Simulation Results 

Annual hour-by-hour DOE-2 simulations were made for two houses: the prototype house and a 

“standard” house. Both house models are based on the same floor plan (shown in Figure 6): a 

single-story, three-bedroom house with 1,936 ft2

 of conditioned space. The prototype SIP walls 

with R-23 nominal insulation, R-42 ceiling insulation, low-e windows (SC = 0.25, U = 0.36), 

ENERGY STAR appliances, compact fluorescent lighting, whole house infiltration was modeled 

to be roughly 0.35 ACH. The prototype included 6-ft overhangs, side fins, and extensive 

landscaping. The prototype has a nominal 2-ton cooling capacity requirement. 

The “Standard” house has insulation and equipment levels more common to standard new 

construction: 2x4 wall construction with nominal R-13 insulation and exterior sheathing (R1.75), R-30 ceiling insulation, the same low-e glass, and only typical shading because of the 

soffit overhangs (no side fins, extended roof deck, or landscape shading). The standard house 

has standard lighting and appliance levels and a nominal 3-ton cooling requirement. 

7
Figure 6. Common floor plan for Borrego Springs homes 

The prototype house reduces the cooling load by about 50% compared to the standard new 

construction. Runs simulating both houses were completed for alternative cooling systems 

operating in nine climates: Borrego Springs, Phoenix, Salt Lake City, Sacramento, Denver, 

Albuquerque, Las Vegas, Los Angeles, and San Diego. 

In each climate the cooling systems for each house were changed to the following for the 

purpose of comparison: 

• Conventional SEER 10 direct expansion (DX) split-system air conditioner 

• Conventional SEER 13 

• Conventional SEER 15 

• Freus (modeled as a SEER 13 plus 100% effectiveness evaporative pre-cooling of condenser) 

• Combination of SEER 10 conventional plus OASys 

• Combination of SEER 13 conventional plus OASys 

• OASys alone with manual control 

8
• OASys alone with optimal control 

• A stand-alone direct evaporative cooler. 

All of the DX systems (direct expansion, split-system air conditioners for which Seasonal Energy 

Efficiency Ratio [SEER] values are given above) were assumed to be 3 tons (36,000 Btu/h) in 

the standard house and 2 tons (24,000 Btu/h) in the prototype house. The specifications for the 

SEER 10, 13, and 15 units were based on the “median performing units” in a Southern California 

Edison study. The two-stage OASys unit was assumed to have a heat exchange effectiveness of 

45% (based on NREL lab testing of an OASys unit) for the indirect stage and a wet-bulb 

effectiveness of 85% for the evaporative stage, independent of supply flow rate, based on 

manufacturer’s data. (Manufacturer’s data indicates that effectiveness values vary only slightly 

between low and high fan speeds.) The model used fan power values determined by 

measurements at NREL. For the stand-alone OASys systems, manual control was based on the 

assumption that the homeowner would switch speeds as needed. Optimal control saved fan 

power by minimizing full-speed operation. 

We were not able to obtain a performance map for a Freus unit. Also, we do not currently have a 

way to model the evaporative condenser of the Freus unit in DOE-2. This was, therefore, 

modeled as a stand-alone, 100% effective, evaporative pre-cooler upstream of a regular aircooled condenser. Because of the thermal resistance between the refrigerant coil and the air 

stream, this tends to underestimate heat rejection compared to a wetted coil. Thus, predicted 

energy savings for the Freus unit should be conservative. Based on manufacturer’s data, we 

assumed an electric power use of 0.0017 W per Btu of ARI capacity (American Refrigeration 

Institute). 

The DOE-2 models that combine conventional DX cooling backup and OASys evaporative 

systems assumed that the evaporative system would operate alone unless the indoor temperature 

setting could not be met, at which time the DX unit would also operate. No upper limit was set 

on indoor humidity. 

A simple, stand-alone, direct-evaporative cooler was modeled to allow comparison with the 

advanced evaporative cooling units. Its flow rate was assumed to be 1,950 CFM. 

An active data-collection program for the OASys and Freus units is being undertaken in Borrego 

Springs, California, so we will first present the simulated performance data from this site. Table 

2 shows the cooling energy required for each type of system for the prototype house. This is 

divided into the energy required by the outdoor unit (some combination of compressor, 

condenser fan, and water pump, depending on the system type). Table 2 also shows the number 

of hours any zone was under-cooled and the number of hours living-room indoor humidity and 

dry-bulb temperatures exceeded different values. Finally, the percent reductions in electric 

energy consumption compared to SEER 10 and SEER 13 units were included. 

A number of conclusions can be drawn from this analysis for a house in Borrego Springs: 

• All of the advanced concepts and the simple direct-evaporative cooler show significant 

electric energy savings compared to the reference air conditioners. 

• The OASys systems and the direct-evaporative cooler save considerably more energy than 

the Freus. The OASys provides a lower supply-air temperature and so can operate at a lower 

9
10

CFM and for a shorter period of time than a direct-evaporative cooler. However, it also has 

fan power associated with an exhaust air stream. 

• The stand-alone OASys saves more energy than the OASys units that operate in parallel with 

air conditioners. 

• An optimally controlled OASys unit has about 23% less energy consumption than the 

manually controlled one. 

• The stand-alone OASys and, especially, the simple direct-evaporative cooler result in more 

than 400 hours of operation when the indoor relative humidity (RH) is above 70% for the 

Borrego Springs climate. In addition, the direct-evaporative cooler results in more than 400 

hours when the indoor dry-bulb temperature exceeds 80°F. 
Table 2. DOE-2 Simulation Results for the Prototype House in 

Borrego Springs, California, using Different Cooling Systems 

Number of Hours 

Cooling Energy (kWh/yr) 

Any 

Zone Humidity Living R

Description 

Outdoor 

Unit Supply Fan Total 

Annual 

COP 

Annual 

Effective

SEER 

Under 

cooled > 60% > 70% > 80 F 

SEER 10 DX 3911 472 4383 2.2 7.4 0 14 0 0 

SEER 13 DX 2935 313 3248 2.9 10.0 0 14 0 0 SEER 15 DX 2845 288 3133 3.0 10.3 3 15 0 0 FREUS1

(SEER 13 + ECC) 2137 422 2559 3.6 12.4 0 14 0 0 

SEER 10 + OASYS2 2778 825 3603 2.6 8.9 113 302 88 0 

SEER 13 + OASYS2 1966 555 2521 3.8 12.8 53 507 112 0 OASYS - Manual control3 81 805 886 10.4 35.5 568 950 417 103 

OASYS - Optimal control3 61 606 667 13.8 47.2 568 950 417 103 Stand-alone Direct Evaporator 41 515 556 16.0 54.6 954 1213 474 436 

11

1 The FREUS units add an evaporative pre-cooler to the condenser with an effectiveness of 100% and an electric use of 0.0017 Watts p2 The DOE2 model of the combined DX and Evaporative system switches between DX cooling and evaporative cooling. There is no m

control. 

3 Manual control switches between high speed and low speed based on expected peak temperatures, optimal control minimizes full-spe

stand-alone OASYS systems (no DX cooling). 
12

Table 3. DOE-2 Simulation Results for a Prototype House 

in Albuquerque, New Mexico, using Different Cooling systems4

Number of Hours 

Cooling Energy (kWh/yr) Any Zone Humidity Livin

Description 

Outdoor 

Unit 

Supply 

Fan Total 

Annual 

COP 

Annual 

Effective

SEER 

Under 

cooled > 60% > 70% > 80 F 

SEER 10 DX 483 59 542 2.2 7.6 0 78 0 0 

SEER 13 DX 339 38 377 3.2 11.0 0 79 0 0 SEER 15 DX 335 36 371 3.3 11.2 0 78 0 0 FREUS1 (SEER 13 + 

ECC) 275 53 328 3.6 12.4 0 79 0 0 

SEER 10 + OASYS2 17 112 129 9.4 32.1 0 147 2 0 

SEER 13 + OASYS2 12 77 89 13.7 46.6 0 149 2 0 OASYS - Manual control3 6 60 66 17.7 60.3 0 148 7 0 

OASYS - Optimal control3 4 37 41 28.6 97.6 0 148 7 0 Stand-alone Direct Evaporator 4 55 59 20.1 68.6 0 233 12 0 

4 Notes from Table 1 apply 
Results for Phoenix are similar to those for Borrego Springs. Results for a less-extreme dry 

climate with a milder rainy season (Albuquerque) are shown in Table 3. 

Compared to Borrego Springs, the cooling season is much shorter; therefore, total annual cooling 

energy use is much less. But similar trends apply. The direct-evaporative cooler and the OASys 

unit require much less electricity than the conventional air conditioners. Because Albuquerque 

does not get as much rain as Borrego Springs, the evaporative cooling systems are not penalized 

with many hours outside the comfort zone. Both the OASys and direct evaporative coolers work 

well for this climate. 

As suggested by Tables 2 and 3, it is important not only to consider the cooling energy but also 

the number of hours the indoor conditions may be outside comfort conditions. It is also 

worthwhile to consider the distribution of these hours. Figures 7 and 8 show, respectively, the 

distribution of hours outside comfort conditions for the direct-evaporative cooler and the OASys 

for the prototype house in Borrego Springs. The blue (dark) bar indicates hours on a given date 

when the temperature, humidity, or both, were too high. The red (light) indicates the number of 

hours when only the temperature was too high. The OASys clearly does much better than the 

evaporative cooler, and it is also clear why the Borrego Springs house relies on a combination of 

the OASys and a conventional direct-expansion system. An evaporative unit alone cannot 

provide adequate comfort conditions during the monsoon season. 

Figures 9 and 10 show the rainy results for Albuquerque. Here, both the direct-evaporative 

cooler and the OASys provide sufficiently cool indoor temperatures throughout the cooling 

season. Both result in some hours when the indoor humidity is high, but the OASys is 

considerably better than the direct-evaporative cooler because of its indirect stage. 

System: Direct Evap, Climate: Borrego Springs, CA (CZ15)

0

4

8

12

16

20

24

1/1

1/15

1/29

2/12

2/26

3/11

3/25

4/8

4/22

5/6

5/20

6/3

6/17

7/1

7/15

7/29

8/12

8/26

9/9

9/23

10/7

10/21

11/4

11/18

12/2

12/16

12/30

Hours Outside Cooling Comfort Range

Outside Temperature or Humidity Range Outside Temperature Range

Figure 7. Distribution of hours when indoor temperature or humidity are too high for 

direct-evaporative cooler in Borrego Springs, California 

13
System: OASYS, Climate: Borrego Springs, CA (CZ15)

0

4

8

12

16

20

24

1/1

1/15

1/29

2/12

2/26

3/11

3/25

4/8

4/22

5/6

5/20

6/3

6/17

7/1

7/15

7/29

8/12

8/26

9/9

9/23

10/7

10/21

11/4

11/18

12/2

12/16

12/30

Hours Outside Cooling Comfort Range

Outside Temperature or Humidity Range Outside Temperature Range

Figure 8. Distribution of hours when indoor temperature or humidity are too high for 

OASys unit in Borrego Springs, California 

System: Direct Evap, Climate: Albuquerque, NM

0

4

8

12

16

20

24

1/1

1/15

1/29

2/12

2/26

3/11

3/25

4/8

4/22

5/6

5/20

6/3

6/17

7/1

7/15

7/29

8/12

8/26

9/9

9/23

10/7

10/21

11/4

11/18

12/2

12/16

12/30

Hours Outside Cooling Comfort Range

Outside Temperature or Humidity Range Outside Temperature Range

Figure 9. Distribution of hours when indoor temperature or humidity are too high for 

direct-evaporative cooler in Albuquerque, New Mexico 

14
15

System: OASYS, Climate: Albuquerque, NM

0

4

8

12

16

20

24

1/1

1/15

1/29

2/12

2/26

3/11

3/25

4/8

4/22

5/6

5/20

6/3

6/17

7/1

7/15

7/29

8/12

8/26

9/9

9/23

10/7

10/21

11/4

11/18

12/2

12/16

12/30

Hours Outside Cooling Comfort Range

Outside Temperature or Humidity Range Outside Temperature Range

Figure 10. Distribution of hours when indoor temperature or humidity are too high for 

OASys system in Albuquerque, New Mexico 
Figure 11 shows a comparison of the Freus, OASys, direct-evaporative cooling, and a reference 

SEER 13 unit in all climates; results are displayed as a fraction of SEER 13 unit energy use. In 

hot, dry climates like Phoenix, Borrego Springs, and Las Vegas, the OASys unit and a directevaporative cooler can significantly reduce cooling energy use. In cooler dry climates like 

Albuquerque and Denver, the OASys and direct-evaporative cooler cause a smaller reduction in 

the absolute amount of cooling energy because the annual cooling loads are less. However, they 

make very sizeable percent reductions in the cooling energy use. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Phoenix

Borrego Springs

Las Vegas

Albuquerque

Sacramento

Salt Lake City

Denver

San Diego

Los Angeles

Total Cooling Energy (Ratio to SEER13)

SEER 13 Evap Condenser Indirect/Direct Evap Direct Evap.

Figure 11. Comparison of four cooling systems in various locations 

16
0

0.5

1

1.5

2

2.5

3

Phoenix

Borrego Springs

Las Vegas

Albuquerque

Sacramento

Salt Lake City

Denver

San Diego

Los Angeles

Fan Energy (Ratio to SEER13)

SEER 13 Evap Condenser Indirect/Direct Evap Direct Evap.

Figure 12. Comparison of fan energy use for cooling systems in various locations 

In some climates the indirect/direct evaporative cooler uses less total energy on an annual basis, 

this is primarily a result of the fan energy use of the indirect/direct evaporative cooler. Figure 12 

shows a comparison of fan energy, results are displayed as a fraction of SEER 13 fan energy use. 

In hot climates, the OASys indirect/direct evaporative cooler spends more time running at 

maximum capacity and maintains better interior comfort than the direct evaporative cooler, but at 

the expense of added energy use. The indirect/direct evaporative cooler is able to deliver cooler 

air to the space, whereas the direct evaporative cooler is oftentimes unable to maintain 

comfortable conditions within the living space. The energy consumption of the OASys fan at 

maximum capacity is greater than the energy use of the direct evaporative cooler at maximum 

capacity. Therefore, it is not surprising that in hot climates, where both units spend more time at 

maximum capacity, the OASys will use more fan energy and total energy than the direct 

evaporative cooler. In smaller cooling load climates, the OASys indirect/direct evaporative 

cooler uses less fan energy because it spends more time in low-speed mode and can meet the 

cooling load by utilizing a more efficient part of the fan curve, compared to a single-speed fan 

used by the direct evaporative cooler. 

3.0 Monitoring Status 

Steven Winter Associates/Consortium for Advanced Residential Buildings (SWA/CARB), in 

cooperation with NREL, is collecting data on the OASys and Freus units at a Building America 

project site in Magna, Utah. In the past 6 months, NREL staff has worked closely with engineers 

17
18

from the Davis Energy Group (DEG) and Building Industry Research Alliance (BIRA) to revise 

the instrumentation and monitoring plans for the Borrego Springs houses, a second Building 

America project in a very hot-dry climate. Data collected at these sites will be used to understand 

field performance and provide better inputs to our models. The monitoring systems installed at 

each of the homes in Borrego Springs were designed to answer specific performance questions 

related to the cooling systems in addition to monitoring the performance of the home as a whole. 

The focus of the discussion here will be limited to the evaluation and comparison of the various 

cooling systems present at the Borrego Springs homes. The following list of questions were 

developed and used to direct the specification the instrumentation with the intent of providing 

answers to the cooling system performance questions: 

• Do the cooling systems perform near manufacturers’ performance specifications? 

• What are the cooling systems’ measured EER and capacity as functions of indoor and 

outdoor conditions? 

• What is the water consumption of the cooling system? 

• What is the measured wet-bulb effectiveness of the evaporative systems? 

• Do the homes maintain comfortable indoor conditions? 

• What are the actual outdoor conditions at the site? 

Some of the questions that are asked relate to the modeling effort in order to improve existing 

models or create new component models or validate simulation predictions of performance: 

• How do simulation results compare to measured cooling system performance? 

• To what extent are evaporative-cooling systems capable of meeting cooling and comfort 

criteria at various times of the year and will conventional AC backup be required? 

• In terms of annual cooling energy use how do the cooling systems or combination of systems 

compare given the same envelope, internal loads, and outdoor conditions while maintaining 

acceptable comfort conditions? 

In order to understand outdoor conditions and interpret performance data from these sites, it is 

necessary to collect weather data. Section 3.1 describes a weather station specification for the 

Borrego Springs site. Sections 3.2 and 3.3 describe the monitoring systems installed for the 

evaluation of the OASys and Freus cooling systems, respectively. 

3.1 Borrego Springs Weather Station Specifications 

To understand the performance of the homes, cooling systems, and validate simulation models it 

is important to understand the actual weather conditions at the site of interest. Horizontal 

insolation; outdoor temperature; and RH, wind speed, and wind direction are continuously 

monitored at the East Star home site at Borrego Springs. The latest specifications for the 

Borrego Springs weather station are given in Table 4. 
19

Table 4. Weather Station Sensors 

Wind speed 

Continuously monitored AC voltage 

Rotating magnet on cup-wheel shaft 03100-LXXX ±0.5m/s (1.1mph) 

Tripod on roof 1 

www

Thre

Wind direction 

Continuously monitored DC voltage Potentiometer 03100-LXXX ±5 ̊

Tripod on roof 1 www

Continuously monitored outdoorair dry-bulb temperature 

Resistance temperature device (thermistor) 

±1.1°F @ 68°F, see online specs for full accuracy 

Outdoor-air specs absolute humidity 

Continuously monitored outdoor air RH 

Variablecapacitance-type RH sensor 

Vaisala HMP-50 

0-90% RH = ±3%, 90-98% ±5%, see online specs for full accuracy specs 

Pole mounted in back yard, shielded, naturally aspirated 

1 www

Outdoor-air dry-bulb temperature 

Continuously monitored outdoorair dry-bulb temperature 

Twisted, shielded, stranded, special limits of error typeT thermocouple, Copper/CopperConstantan-Nickel 

TT-T-24STWSH-SLE-100

(1000-ft spool) 0.5 ̊C or 0.4% 

Pole mounted in back yard, shielded, forced aspiration 2 www

Data 

Required Measurement Sensor Type Sensor Part # Sensor Accuracy 

Installed 

Location Quantity 

Horizontal solar radiation 

Continuously monitored Irradiance 

Photocell-type pyranometer LI-200SZ 

Absolute Error ±5%, Typically ±3% 

Custom mounted on highest roof cupola (above dome skylight) 1 www
20

The approach for measuring most of the variables shown in Table 4 is fairly straightforward 

because the sensors are connected directly to a Datataker 800 (www.datataker.com/) datalogger, 

provided by DEG, for long-term data collection. Data is logged in 15-minute intervals and is a 

combination of 15-minute averages or totalizations depending on the measurement. Sensors were 

provided and installed by NREL. Establishing outdoor dry bulb temperature is accomplished 

with two type-T thermocouples installed in a custom-built forced-aspiration radiation shield. 

The absolute humidity of outdoor air is established by using a Vaisala HMP-50 temperature and 

relative-humidity sensor installed in a naturally aspirated plate-style radiation shield. 

Temperature and relative-humidity data from this sensor are used to determine the absolute 

humidity of air, which is logged as part of the datalogger program. 

3.2 Monitoring of the OASys Unit 

The OASys unit takes outdoor air and cools it via a two-stage indirect/direct evaporative cooling 

process. Several Building America teams are using this system in prototype homes as part of 

their whole-house research program. NREL, in cooperation with the SWA/CARB is currently 

monitoring an OASys unit at a Building America home in Magna, Utah (Unit #1) and will be 

monitoring a second OASys unit at one of the four Borrego Springs, California, homes in 

cooperation with BIRA and DEG, pending the completion of construction for these homes. At 

each of these two houses, NREL is working with the Building America team and partners to 

monitor outdoor and indoor temperature and humidity conditions, as well as the flow rate and 

temperature of the supply air. We will also monitor the amount of make-up water provided to 

and energy consumed by the OASys unit. 

To answer the questions regarding the cooling system performance and validate cooling system 

models, a long-term data-collection system was installed at the Wagon home in Borrego Springs. 

Details of the instrumentation that was selected to collect data on the OASys operation and 

performance characteristics are given in Table 5.
Table 5. Instrumentation for OASys Unit 

Data Required Measurement Sensor Type Sensor Part # Sensor Accuracy Installed Location Q

True RMS AC watt-hour transducer with pulse output 

WNA-1P-240-P 

0.45% of reading + 0.05% of full scale through 25th harmonic 

Inside OASys fan cabinet 

Evaporative cooler fan energy consumption 

Continuously monitored Watthour Totalization meter Split-core CT 

(15 amp) CTS-0750-015 Accuracy ±1% from 10% to 130% of rated current 

Installed on A- and B-phase hot leads for fan within OASys unit cabinet 

True RMS AC watt-hour transducer with pulse output 

WNA-1P-240-P 

0.45% of reading + 0.05% of full scale through 25th harmonic 

Inside OASys fan cabinet 

Evaporative system pump energy consumption 

Continuously monitored Watthour Totalization meter Split-core CT 

(15 amp) CTS-0750-015 Accuracy ±1% from 10% to 130% of rated current 

Installed on A- and Bphase hot leads for pump within OASys unit cabinet 

True RMS AC watt-hour transducer with pulse output 

WNA-1P-240-P 

0.45% of reading + 0.05% of full scale through 25th harmonic 

Inside OASys fan cabinet 

Evaporative cooler total energy consumption 

Continuously monitored Watthour Totalization meter Split-core CT 

(15 amp) CTS-0750-015 Accuracy ±1% from 10% to 130% of rated current 

Installed on main Aand B-phase hot leads within OASys unit cabinet 

21
22

Table 5 (continued). Instrumentation for OASys Unit 

Data Required Measurement Sensor Type Sensor Part # Sensor Accuracy Installed Location Quantity 

Evaporative sump-system water-inlet flow rate 

Continuously monitored water flow rate 

Paddle-wheel flowmeter with pulse output 

Model 1702 

 2% of reading for 0-0.13 GPM, ±1.5% of reading 0.13 – 13 GPM, 1⁄2-in. MNPT, 1 pulse/gal 

At water inlet for float valve on OAsys cabinet 

1 www.i

Pitot tube traverse probe 

VOLU-probe one externally mounted pitot airflow traverse probe 

2% of actual flow In supply air plenum 2 www.a

Evaporative cooler supply air volumetric flow rate 

Continuously monitored supply duct pressure differential 

Pressure transducer 

Veltron 2500 Plus 0-0.05 in. wc span 

0.25% of natural span, including nonlinearity, hysteresis, dead band, and non-repeatability. 

Mounted on side of supply plenum 

1 www.a

Evaporative cooler supply air volumetric flow rate 

Continuously monitored heat transfer from hot wire type airflow station 

Hot wire/heat transfer type airflow station 

NL 1 x 2 air flow-measuring station 

±2% of reading + 1/2% OF FS 

In supply air plenum 1 www.f

Continuously monitored Supply air dry-bulb temperature 

Resistance temperature device (thermistor) 

±0.2°F @ 68°F, see online specs for full accuracy specs Supply-air 

temperature and RH Continuously 

monitored Supply Air RH 

Variablecapacitancetype RH sensor 

Vaisala HMD60Y 0-100% RH = ±2% 

@ 68°F, see online specs for full accuracy specs 

In supply air plenum 1 www.v
The approach for measuring most of the variables shown in Table 5 is fairly straightforward 

because the sensors are connected directly to a Datataker 800 (www.datataker.com/) datalogger, 

provided by DEG, for long-term data collection. Some sensors were provided and installed by 

NREL. Data is logged in 15-minute intervals and is a combination of 15-minute averages or 

totalizations, depending on the measurement. Measuring the airflow provided by the OASys unit 

is proving to be difficult because of its ability to provide ventilation air at a flow rate of 

approximately 150 CFM. The supply plenum where the multiple airflow sensors are located has 

a square cross section of 18x24 in.; the initial supply plenum layout is shown in Figure 13. 

Figure 13. Supply plenum layout for Wagon OASys indirect/direct evaporative cooler 

23
At the lowest flow rate provided by the OASys, air velocities of roughly 50 FPM are realized 

that are nearly impossible to measure with a pressure-based sensor, such as the pitot tube traverse 

probes that were initially installed at the Wagon home. A hot-wire heat-transfer type airflow 

station was installed in order to make more accurate measurement of supply airflow rates across 

the manufacturer’s specified airflow operating range (150 to 1,500 CFM). Because of the 

minimal ducting and lack of proper airflow development length (as illustrated in Figure 12) 

associated with the air-distribution system at the Wagon home, the outputs of both airflow 

sensors (Pitot tubes plus pressure transducer and the hot-wire airflow station) will be mapped 

against a fan-assisted flow hood to establish supply-air flow rates as accurately as possible. 

Datalogger programs will be revised to reflect the sensor output to fan-assisted flow-hood 

airflow maps in order to cover the range of continuously variable airflow rates that the OASys 

unit is capable of providing. 

NREL will perform the mapping of the sensor outputs vs. know fan-flow rates during a site visit 

in the summer of 2006. Part of the difficulty in measuring the airflow for the OASys unit is that 

not all of the supply air provided by the OASys fan actually goes into the conditioned space via 

the supply air plenum. A portion of the fan airflow is exhausted to the outdoors via the indirect 

section of the cooler (Figure 1). The amount of airflow exhausted via the indirect section is 

affected by the static pressure on the supply side and changes with the continuously variable fan 

speed that the user selects. Capacity for the unit will be reported in two ways. The capacity 

relative to outdoor conditions will be determined by the change in enthalpy of the supply and 

outdoor-air conditions and the supply-air flowrate. Capacity relative to indoor conditions will be 

determined by the change in enthalpy of the supply and indoor conditions and the supply air 

flowrate. Measured values from the OASys specific sensors shown in Table 5 will be used in the 

calculations of cooling-system capacity for outdoor air or indoor air conditions (additional 

sensors within the home are part of the DEG datalogging system and are not detailed here). 

Cooling-system capacity coupled with the electrical energy measurements for the unit will allow 

use to calculate an EER for the unit. Measurements of supply air and indoor temperature and RH 

will allow us to calculate wet bulb effectiveness for the OASys. 

3.2.1 OASys Performance Data Results from Magna 

During a recent site visit, an airflow sensor was added to the long-term monitoring system 

installed at Magna Unit 1. With this additional data, the performance of the OASys unit under 

actual weather conditions can be determined. Although data collection is ongoing, only the data 

from August 18, 2005, until September 19, 2005, is analyzed here. During this time frame, 15-

minute data was collected including outdoor temperature, outdoor RH, indoor temperatures 

(basement, first floor, and second floor), indoor RH (basement, first floor, and second floor), 

OASys energy consumption (Wh), OASys supply-duct pressure (Pa, used to calculate flow rate 

in the supply duct), supply-duct temperature, supply-duct RH, and makeup water-flow rate 

(gal/min). 

During the 1-month data-analysis period, outdoor conditions varied considerably. Outdoor 

temperatures ranged from a maximum of 94oF to a minimum of 38oF and averaged 68oF during 

the data-analysis period. Relative humidity varied from a maximum of 82% to a minimum of 

26% and averaged 48% during the data-analysis period. Figure 14 plots the outdoor conditions 

at Magna, Utah, during the data-analysis period. 

24
25

Outdoor Conditions

0

10

20

30

40

50

60

70

80

90

100

8/18/05 12:00 AM8/19/05 12:00 AM8/20/05 12:00 AM8/21/05 12:00 AM8/22/05 12:00 AM8/23/05 12:00 AM8/24/05 12:00 AM8/25/05 12:00 AM8/26/05 12:00 AM8/27/05 12:00 AM8/28/05 12:00 AM8/29/05 12:00 AM8/30/05 12:00 AM8/31/05 12:00 AM

9/1/05 12:00 AM9/2/05 12:00 AM9/3/05 12:00 AM9/4/05 12:00 AM9/5/05 12:00 AM9/6/05 12:00 AM9/7/05 12:00 AM9/8/05 12:00 AM9/9/05 12:00 AM

9/10/05 12:00 AM9/11/05 12:00 AM9/12/05 12:00 AM9/13/05 12:00 AM9/14/05 12:00 AM9/15/05 12:00 AM9/16/05 12:00 AM9/17/05 12:00 AM9/18/05 12:00 AM9/19/05 12:00 AM9/20/05 12:00 AM

Day/Time

Temperature (°F)

Outdoor Temperature Outdoor Wet Bulb Temperature

Figure 14. Outdoor conditions for Magna data-collection period 

Inside the home, temperatures varied from a maximum of 79oF (second-story bedroom) to a 

minimum of 55oF (first floor) and averaged 68oF (all floors). Relative humidity varied from a 

maximum of 75% (first floor) to a minimum of 26% (first floor) and averaged 51% (all floors). 
Indoor Conditions

40

50

60

70

80

8/18/05 12:00 AM8/19/05 12:00 AM8/20/05 12:00 AM8/21/05 12:00 AM8/22/05 12:00 AM8/23/05 12:00 AM8/24/05 12:00 AM8/25/05 12:00 AM8/26/05 12:00 AM8/27/05 12:00 AM8/28/05 12:00 AM8/29/05 12:00 AM8/30/05 12:00 AM8/31/05 12:00 AM

9/1/05 12:00 AM9/2/05 12:00 AM9/3/05 12:00 AM9/4/05 12:00 AM9/5/05 12:00 AM9/6/05 12:00 AM9/7/05 12:00 AM9/8/05 12:00 AM9/9/05 12:00 AM

9/10/05 12:00 AM9/11/05 12:00 AM9/12/05 12:00 AM9/13/05 12:00 AM9/14/05 12:00 AM9/15/05 12:00 AM9/16/05 12:00 AM9/17/05 12:00 AM9/18/05 12:00 AM9/19/05 12:00 AM9/20/05 12:00 AM

Day/Time

Temperature (°F)

30%

35%

40%

45%

50%

55%

60%

65%

Relative Humidit ) (% y

Average Indoor Temperature Average Indoor RH

Figure 15. Indoor conditions for Magna data-collection period 

Figure 15 shows that the OASys unit appears to keep the house comfortable even on days with 

outdoor temperatures as high as 95oF. 

The performance of the OASys unit during the data-analysis period shows a maximum EER of 

153, wet-bulb effectiveness of 136%, cooling capacity of 29 kBtu/hr (2.4 tons), supply-air flow 

rate of 1,522 CFM, and energy consumption of 406 Wh.5

 These maximum performance values 

measured at the site do not agree with NREL laboratory measurements that place the wet-bulb 

effectiveness for the OASys are between 103% and 110%. Currently, we believe that the outdoor 

temperature and RH sensor at Magna are giving false readings of outdoor conditions. Because of 

the malfunctioning outdoor temperature and RH sensor, these maximum values are overstating 

the performance of the OASys indirect/direct evaporative cooler at Magna. Figure 16 compares 

the site wet bulb temperature to the wet bulb temperature at Salt Lake City Airport. 

 

5

 Maximum data values described here may not be coincident and are based on hourly data time steps; cooling 

capacity and EER values are relative to outdoor air conditions. 

26
Magna Site Ambient Data and Salt Lake Airport Ambient Data

35

40

45

50

55

60

65

70

8/18/058/23/058/28/05

9/2/05

Day

Temperature, °F

SLC AIRPORT WB MAGNA SITE WB

Figure 16. Outdoor wet-bulb temperature comparison for Magna data-collection period 

During the data-collection period, the site wet bulb was found to be as much as 26oF higher and 

10oF lower than wet-bulb temperatures measured at the airport. The Magna home site is roughly 

15 miles from the Salt Lake City Airport, some of the difference in wet-bulb temperatures could 

be related to differences in microclimate. Another site visit is planned for the spring of 2006 to 

replace this sensor. Monitoring of this system will continue through the summer of 2006 with 

the hope that the field performance of the unit will compare more favorably to the measured 

performance in the lab. Figure 17 shows the EER of the OASys unit relative to exhaust air 

conditions6

 with and without the furnace air-handler (AH) fan considered. The prototype home 

at Magna, Utah, directs exhaust air through up ducts in the second floor ceiling into a shallow 

plenum below the attic insulation, referred to as the Dynamic Ceiling. Attic insulation is 

suspended above the shallow plenum with a wire-mesh screen that allows the exhaust air to flow 

through the attic insulation for an additional cooling effect. SWA staff is evaluating the 

Dynamic Ceiling concept at Magna Unit 1. 

 

6

 All performance results presented relative to exhaust air conditions may not be representative of the actual 

performance of the OASys unit; the measurement of exhaust air temperature and RH is made at one up-duct location 

and may not characterize all of the exhaust air leaving the home. Performance results compared to outdoor 

conditions are not presented because of the malfunctioning outdoor sensors. 

27
OAsys Energy Efficiency Ratio

-400

-300

-200

-100

0

100

200

300

400

500

600

8/18/05 12:00 AM8/19/05 12:00 AM8/20/05 12:00 AM8/21/05 12:00 AM8/22/05 12:00 AM8/23/05 12:00 AM8/24/05 12:00 AM8/25/05 12:00 AM8/26/05 12:00 AM8/27/05 12:00 AM8/28/05 12:00 AM8/29/05 12:00 AM8/30/05 12:00 AM8/31/05 12:00 AM

9/1/05 12:00 AM9/2/05 12:00 AM9/3/05 12:00 AM9/4/05 12:00 AM9/5/05 12:00 AM9/6/05 12:00 AM9/7/05 12:00 AM9/8/05 12:00 AM9/9/05 12:00 AM

9/10/05 12:00 AM9/11/05 12:00 AM9/12/05 12:00 AM9/13/05 12:00 AM9/14/05 12:00 AM9/15/05 12:00 AM9/16/05 12:00 AM9/17/05 12:00 AM9/18/05 12:00 AM9/19/05 12:00 AM9/20/05 12:00 AM

Day/Time

EER (Btu/wh)

EER (OAsys unit only, exhaust - supply) EER (OAsys unit + AH Fan, exhaust - supply)

Figure 17. The EER values for OASys and OASys plus furnace AH fan 

The intent of comparing the exhaust-air conditions to the supply-air conditions to establish EER 

and cooling capacity is to show performance data in a way that it may be compared to the 

performance metrics for direct-expansion refrigerant-based cooling systems. However, exhaust 

air does not always leave the home through intentional openings, such as up ducts. Therefore 

measurements of exhaust air temperature and RH is not representative of all the exhaust air 

leaving the home. In addition, the monitoring system for this home does not make exhaust-air 

temperature and RH measurements at each up duct. The monitoring system is limited to a single 

measurement of the exhaust-air conditions at one up duct; this is also problematic and may not 

represent the conditions at other up ducts. 

During the site visit in early August 2005, we noted that the furnace air-handler fan was running 

continuously in fan-only mode. While this improves the uniformity of comfort conditions in the 

home, there is an energy-related penalty, as shown in Figure 18. The maximum energy 

consumed during the data-collection period by the OASys unit and the furnace air-handler fan is 

840 Wh ― double that of the OASys unit by itself. 

28
OASys Energy Use

0

100

200

300

400

500

600

700

800

900

8/18/05 12:00 AM8/19/05 12:00 AM8/20/05 12:00 AM8/21/05 12:00 AM8/22/05 12:00 AM8/23/05 12:00 AM8/24/05 12:00 AM8/25/05 12:00 AM8/26/05 12:00 AM8/27/05 12:00 AM8/28/05 12:00 AM8/29/05 12:00 AM8/30/05 12:00 AM8/31/05 12:00 AM

9/1/05 12:00 AM9/2/05 12:00 AM9/3/05 12:00 AM9/4/05 12:00 AM9/5/05 12:00 AM9/6/05 12:00 AM9/7/05 12:00 AM9/8/05 12:00 AM9/9/05 12:00 AM

9/10/05 12:00 AM9/11/05 12:00 AM9/12/05 12:00 AM9/13/05 12:00 AM9/14/05 12:00 AM9/15/05 12:00 AM9/16/05 12:00 AM9/17/05 12:00 AM9/18/05 12:00 AM9/19/05 12:00 AM9/20/05 12:00 AM

Day/Time

Energy Use (wh)

OAsys Energy Use (wh) OAsys + AH Fan Energy Use (wh)

Figure 18. Energy use for OASys and OASys plus furnace AH fan 

Wet-bulb effectiveness is commonly used to evaluate how closely an evaporative-cooling system 

approaches the outdoor (or entering) air wet-bulb temperature. Figure 19 plots wet-bulb 

effectiveness of the OASys unit for the data-collection period. Peak values are around 120%-

130% wet-bulb effectiveness, which is somewhat higher than the more accurate measurements of 

103% to 112% taken in the NREL laboratory. This is likely the result of the impaired outdoor 

temperature and RH sensor. 

Because the OASys system is an indirect/direct type evaporative system, the whole-unit wet-bulb 

effectiveness can be greater than 100%. Instrumentation was not placed such that the wet-bulb 

effectiveness of the indirect and direct sections could be evaluated separately. Instead, the 

permanent instrumentation on the OASys unit was placed such that whole-unit wet-bulb 

effectiveness could be evaluated. Note that negative effectiveness values occur. These occurred 

when the building heating system was on, and the OASys automatically provided cool outside air 

in ventilation mode. Because of the location of sensors in the supply duct, a slight heating of the 

air as a result of fan heat or heat addition to the supply duct may have raised the supply-air 

temperature slightly above the inlet-air temperature, causing the effectiveness to become 

negative when the OASys unit operates in ventilation mode. Because air is not significantly 

heated or cooled in this mode, the inlet and outlet air temperatures are very close to each other, 

so that slight errors in the temperature readings can also cause the effectiveness to be negative. 

29
OAsys Wet Bulb Effectiveness

-50%

-30%

-10%

10%

30%

50%

70%

90%

110%

130%

150%

8/18/05 12:00 AM8/19/05 12:00 AM8/20/05 12:00 AM8/21/05 12:00 AM8/22/05 12:00 AM8/23/05 12:00 AM8/24/05 12:00 AM8/25/05 12:00 AM8/26/05 12:00 AM8/27/05 12:00 AM8/28/05 12:00 AM8/29/05 12:00 AM8/30/05 12:00 AM8/31/05 12:00 AM

9/1/05 12:00 AM9/2/05 12:00 AM9/3/05 12:00 AM9/4/05 12:00 AM9/5/05 12:00 AM9/6/05 12:00 AM9/7/05 12:00 AM9/8/05 12:00 AM9/9/05 12:00 AM

9/10/05 12:00 AM9/11/05 12:00 AM

Day/Time

Wet Bulb Effectivness (%)

Figure 19. Wet-bulb effectiveness for the OASys unit at Magna

The cooling delivered by the OASys unit for the data analysis period is shown in Figure 20. The 

graph shows that the OASys unit is capable of capacities that are somewhat larger than the rated 

capacity of the unit at the manufacturer’s test condition (see Appendix A for manufacturer’s 

specifications). During the data-analysis period, the unit saw a maximum capacity of slightly 

more than 2 tons. This result may be influenced by the difficulty in accurately measuring the 

exhaust air conditions described previously. 

30
OASys Cooling Delivered

-20000

-15000

-10000

-5000

0

5000

10000

15000

20000

25000

8/18/05 12:00 AM8/19/05 12:00 AM8/20/05 12:00 AM8/21/05 12:00 AM8/22/05 12:00 AM8/23/05 12:00 AM8/24/05 12:00 AM8/25/05 12:00 AM8/26/05 12:00 AM8/27/05 12:00 AM8/28/05 12:00 AM8/29/05 12:00 AM8/30/05 12:00 AM8/31/05 12:00 AM

9/1/05 12:00 AM9/2/05 12:00 AM9/3/05 12:00 AM9/4/05 12:00 AM9/5/05 12:00 AM9/6/05 12:00 AM9/7/05 12:00 AM9/8/05 12:00 AM9/9/05 12:00 AM

9/10/05 12:00 AM9/11/05 12:00 AM9/12/05 12:00 AM9/13/05 12:00 AM9/14/05 12:00 AM9/15/05 12:00 AM9/16/05 12:00 AM9/17/05 12:00 AM9/18/05 12:00 AM9/19/05 12:00 AM9/20/05 12:00 AM

Day/Time

Cooling (Btu/hr)

Qcooling (Btu, exhaust-supply)

Figure 20. Cooling delivered by the OASys unit at Magna 

Figures 17 through 20 all show negative values for the latter part of the data-collection period 

(9/12/05 – 9/19/05, negative EER and cooling delivered). Figure 21 shows the indoor, outdoor, 

and supply-air temperatures, along with the energy and water consumed by the OASys unit from 

9/12/05 to 9/19/05. It shows the supply-air temperature tracking the outdoor air temperature 

when the unit is consuming no water during ventilation mode. 

Figure 22 shows the energy consumed by the OASys unit and the energy consumed by the 

OASys unit plus the furnace AH fan. 

From the Figure 22, it is clear that in the late evening of 9/11 the air-handler fan was taken out of 

“fan only” mode, which was helping to circulate the evaporatively cooled air in the home. At this 

point, the air-handler fan appears to cycle on periodically (this is shown wherever the two data 

series in Figure 22 deviate), which suggests that the furnace is responding to a heating load on 

the home. (The OASys and furnace have separate thermostats and can be operated 

simultaneously.) 

31
32

OASys Operation 9/12/05 - 9/19/05

0

20

40

60

80

100

9/12/05 12:00 AM9/13/05 12:00 AM9/14/05 12:00 AM9/15/05 12:00 AM9/16/05 12:00 AM9/17/05 12:00 AM9/18/05 12:00 AM9/19/05 12:00 AM9/20/05 12:00 AM

Day/Time

Temperature (°F)/Energy Use (Wh)

0

1

2

3

4

5

6

7

8

9

10

Water Consumption (GPH)

Outdoor Temperature Average Indoor Temperature Average Supply Temperature OAsys Energy Use (wh) OAsys Water Use (GPH)

Figure 21. Performance during cool outdoor air temperatures 

OASys Energy Use 9/11/05 - 9/19/05

0

100

200

300

400

500

600

700

800

900

9/11/05 12:00 AM9/12/05 12:00 AM9/13/05 12:00 AM9/14/05 12:00 AM9/15/05 12:00 AM9/16/05 12:00 AM9/17/05 12:00 AM9/18/05 12:00 AM9/19/05 12:00 AM9/20/05 12:00 AM

Day/Time

Energy Use (wh)

OAsys Energy Use (wh) OAsys + AH Fan Energy Use (wh)

Figure 22. Energy consumed by the OASys unit and furnace AH fan for 

early September 
33

It appears that the OASys unit was running in ventilation mode during this timeframe when 

outdoor temperatures were cool, in order to cool the house, and was thus fighting the heating 

system. A follow-up visit or conversation with the homeowners may be necessary to better 

understand the interaction between the OASys and the heating system. It is not entirely clear 

that this mode of operation is unintended. Fresh-air ventilation can be provided with the OASys 

unit year round; however, ventilation during the winter months will add to the heating load. 

3.3 Freus Monitoring 

There is no permanent instrumentation of the Freus unit on Unit #2 in Magna, Utah. One home 

in Borrego Springs, California, has a Freus unit that NREL intends to monitor in detail for longterm performance with the goal of answering the performance questions at the beginning of this 

section. 

3.3.1 Monitoring Plans for Borrego Springs Freus Unit 

At the DiGiorgio home, there are two separate data-collection systems. DEG has installed a 

datalogger that will be responsible for monitoring indoor temperature and RH at two locations 

within the house, attic and roof surface temperatures, floor slab circulation pump energy, PV 

energy collected, and house total energy use. When normal compressor cooling is not called for, 

the evaporative condenser will be used as a cooling tower; water from the sump of the Freus will 

be circulated to cool the concrete floor slab. The DEG datalogger will be configured to monitor 

the cooling delivered via the floor slab. NREL has intends to install permanent instrumentation 

to monitor the performance of the Freus system, including air-handler fan energy, condenser fan 

energy, compressor energy, total energy for the condensing unit, the cooling delivered to the 

conditioned space, the evaporative pump energy consumption by subtraction, and the water 

consumption of the outdoor unit. 

We will also monitor ambient temperature and humidity conditions in the vicinity of the 

condenser, condenser air-intake temperature and RH, condenser exhaust-air temperature and RH, 

outdoor-air damper status, and condenser fan status. A one-time measurement of condenser fan 

flow rate will be performed. A list of measurements for which NREL for is contained in Table 6. 

These measurements should allow us to calculate the EER, cooling capacity, water consumption, 

and electrical energy consumption for the Freus in its normal cooling mode under a variety of 

weather conditions. When combined with the DEG data, we should be able to determine EER, 

cooling capacity, water consumption, and electrical energy consumption for the Freus in its floor 

slab-cooling mode. 

In addition to performance monitoring, there are also concerns about the impact of water quality 

on outdoor coil performance. Additional monitoring points may be added to measure the 

pressure drop across the demister for a long-term look at scaling issues. Details regarding 

additional sensors are still under discussion at NREL. 

The approach for measuring some of the variables shown in Table 6 is fairly straightforward as 

the sensors are connected directly to a Campbell CR10X datalogger 

(www.campbellsci.com/index.cfm) provided by NREL, for long-term data collection. Sensors 

listed in Table 6 were provided and installed by NREL. 
Table 6. Instrumentation for Freus Unit 

Data Required Measurement Sensor Type Sensor Part # Sensor Accuracy Installed Location Quantit

True RMS AC watt-hour transducer with pulse output 

WNA-1P-240-P 

0.45% of reading + 0.05% of full scale through 25th harmonic 

Installed within to circuit-breaker panel 1 

AH fan Energy consumption 

Continuously monitored Watthour Totalization meter 

Split-core CT (15 amp) CTS-0750-015 

Accuracy ±1% from 10% to 130% of rated current 

Installed on A- and B-phase hot leads at AH fan circuit breaker 

2 

True RMS AC watt-hour transducer with pulse output 

WNA-1P-240-P 

0.45% of reading + 0.05% of full scale through 25th harmonic 

Small electrical box installed on Freus unit 

1 

Condensing unit total energy consumption 

Continuously monitored Watthour Totalization meter 

Split-core CT (30 amp) CTS-0750-030 

Accuracy ±1% from 10% to 130% of rated current 

Installed on hot leads within condensing-unit case 

2 

True RMS AC watt-hour transducer with pulse output 

WNA-1P-240-P 

0.45% of reading + 0.05% of full scale through 25th harmonic 

Small electrical box installed on Freus unit 

1 

Condenser fan energy consumption 

Continuously monitored Watthour Totalization meter 

Split-core CT (15 amp) CTS-0750-015 

Accuracy ±1% from 10% to 130% of rated current 

Installed on hot leads to condenser fan within condensing-unit case 

1 

34
Table 6 (continued). Instrumentation for Freus Unit 

Data Required Measurement Sensor Type Sensor Part # Sensor Accuracy Installed Location Qu

True RMS AC watt-hour transducer with pulse output 

WNA-1P-240-P

0.45% of reading + 0.05% of full scale through 25th harmonic 

Small electrical box installed on Freus unit 

Compressor 

energy consumption 

Continuously 

monitored Watt-hour Totalization meter 

Split-core CT (15 amp) CTS-0750-015 

Accuracy ±1% from 10% to 130% of rated current 

Installed on hot leads to compressor within condensing unit case 

Evaporative 

system pump energy consumption 

Not measured – data obtained by subtracting the compressor and condenser fan energy from the condensin

Evaporative 

sump system water inlet flow rate 

Continuously 

monitored water flow rate 

Paddle wheel flowmeter with pulse output 

FTB-4105A-P 

±1% of reading for 0.3-9LPM, 8000 pulses/L, 1⁄2-in. NPT 

At water inlet for float valve on Freus cabinet 

Calibrated fan 

 +/ 3% (using DG-700 or DG-3) 

Temporary install on site-built box above condenser fan outlet 

Condenser fan 

flow rate 

One-time pressure 

matching and flow measurement of condenser-fan operation with blowerdoor fan Pressure and Flow gauge DG-700 or |DG-3 pressure and flow gauge ± 1% of reading from –1,250 to +1,250 Pascals, or -5 to +5 inwc Handheld for one-time measurement 

Condenser fan operation 

Use milivolt output 

from DC-CT to indicate fan operation and infer fan flow over time (assume that condenser fan flow is constant at given power level) 

DC current 

transducer 

Installed on hot lead to condenser fan within condensing unit case 

35
Table 6 (continued). Instrumentation for Freus Unit 

Data 

Required Measurement Sensor Type Sensor Part # Sensor Accuracy Installed Location 

Continuously 

monitored exhaustair dry-bulb temperature 

Resistance temperature 

device (thermistor) 

±0.36°F @ 68°F, see online specs for full 

Condenser fan accuracy specs exhaust-air absolute humidity Continuously 

monitored exhaustair RH Variable-capacitance- type RH sensor 

Vaisala HMP-45A

0-90% RH = ±2%, 90%-100% ±3% @ 68°F, see online specs for full accuracy specs 

Installed in sampling tube with vacuum pump on condenserfan exhaust grill 

Condenser fan exhaust-air dry-bulb temperature 

Continuously 

monitored outdoor-air dry-bulb temperature 

Twisted, shielded, 

stranded, special limits of error type-T thermocouple, Copper/CopperConstantan-Nickel 

TT-T-24S-TWSHSLE-1000 (1,000-ft spool) 

0.5 ̊C or 0.4% 

Six thermocouples installed in parallel on condenser-fan exhaust grill adjacent to sampling tube 

Continuously 

monitored exhaustair dry-bulb temperature 

Resistance temperature 

device (thermistor) 

±0.36°F @ 68°F, see online specs for full 

Condenser fan accuracy specs intake-air absolute humidity Continuously 

monitored exhaustair RH Variable-capacitance- type RH sensor 

Vaisala HMP-45A

0-90% RH = ±2%, 90%-100% ±3% @ 68°F, see online specs for full accuracy specs 

Installed in custom shield behind condenser air intake grill 

Condenser fan Intake-air drybulb temperature 

Continuously 

monitored outdoor-air dry-bulb temperature 

Twisted, shielded, 

stranded, special limits of error type-T thermocouple, Copper/CopperConstantan-Nickel 

TT-T-24S-TWSHSLE-1000 (1,000-ft spool) 

0.5 ̊C or 0.4% 

Six thermocouples installed in parallel behind condenser air intake grill 

36
Table 6 (continued). Instrumentation for Freus Unit 

Data Required Measurement Sensor Type Sensor Part # Sensor Accuracy Installed Location 

Pitot tube traverse 

VOLU-probe 3 removable pitot airflow traverse probe 

2% of actual flow 

Directly downstream of AH filter on return/outdoor air intake 

AH fan-flow rate 

Continuously monitored return-duct pressure differential 

Pressure transducer 

Veltron 2500 Plus 0-0.05-in. wc span 

0.25% of natural span, including non-linearity, hysteresis, dead band, and nonrepeatability. 

Mounted on side of supply plenum 

Continuously monitored return-air dry-bulb temperature 

Resistance temperature device (thermistor) 

±0.2°F @ 68°F, see online specs for full 

Return-air accuracy specs temperature and RH Continuously 

monitored return-air RH 

Variablecapacitance-type RH sensor 

Vaisala 

HMD60Y 0-100% RH = ±2% @ 

68°F, see online specs for full accuracy specs 

Return/outdoor air intake duct upstream of AH filter 

Continuously monitored supply-air dry-bulb temperature 

Resistance temperature device (thermistor) 

±0.2°F @ 68°F, see online specs for full 

Supply-air accuracy specs temperature and RH Continuously 

monitored supply-Air RH 

Variablecapacitance-type RH sensor 

Vaisala 

HMD60Y 0-100% RH = ±2% @ 

68°F, see online specs for full accuracy specs 

Supply duct, placed at each supply-air main trunk off supply plenum 

37
38

Table 6 (continued). Instrumentation for Freus Unit 

Data Required Measurement Sensor Type Sensor Part # Sensor Accuracy Installed Location 

AH fan speed (fan flow rate) 

Continuously monitored fan RPM 

Pulse output from GE fan motor 

NA NA Sensor leads at GE fan motor 

Continuously monitored outdoorair dry-bulb temperature 

Resistance temperature device (thermistor) 

±1.1°F @ 68°F, see online specs for full accuracy specs Condensing-unit ambient-air 

absolute humidity Continuously 

monitored outdoorair RH 

Variablecapacitance-type RH sensor 

Vaisala HMP-50 

0-90% RH = ±3%, 90-98% ±5%, see online specs for full accuracy specs 

Mounted on fence surrounding condensing unit, shielded, naturally aspirated 

Condensing -nit ambient-air drybulb temperature 

Continuously monitored outdoor -air dry-bulb temperature 

Twisted, shielded, stranded, special limits of error type-T thermocouple, Copper/CopperConstantanNickel 

TT-T-24STWSH-SLE1000 (1000-ft spool) 

0.5 ̊C or 0.4% 

Mounted on fence surrounding condensing unit, shielded, forced aspiration 
Data is logged in 15-minute intervals and is a combination of 15-minute averages or totalizations 

depending on the measurement. Measuring the airflow provided by the Nightbreeze7 air-handler 

unit is proving to be difficult because of its ability to provide ventilation air at a flow rate of 

approximately 200 CFM. The return duct where the airflow sensors are located has a square 

cross section of 20x21 in. and is pictured in Figure 23. 

Figure 23. Pitot tube traverse airflow sensors installed at the return duct for the DiGiorgio 

home 

Unfortunately, the installed return duct dimensions were not inline with the specified duct 

dimension, and the airflow sensors were ordered according to design specification for the return 

duct system. When the time came for the installation, the sensors were too short. The addition 

of the 2x4 blocks above allowed the sensors to be installed; however, the accuracy of the sensors 

 

7

 The Nightbreeze AH is a product of DEG that incorporates an outdoor air damper for a night ventilation cooling 

ability as well as an outdoor air ventilation capability, for more on Nightbreeze see 

www.davisenergy.com/nb_page.html. 

39
is now very questionable as a result of the change in cross-sectional area for the duct. In 

addition, at the lowest flow rate provided by the Nightbreeze, air velocities of roughly 70 FPM 

are realized, which is nearly impossible to measure with a pressure-based sensor such as the pitot 

tube traverse probes that were initially installed at the DiGiorgio home. Engineers at DEG and 

NREL discussed alternatives to make the airflow measurement at DiGiorgio and considered 

ordering the correct-length Pitot type sensors. However, that option was discarded because of the 

lack of adequate airflow development length, as illustrated in Figure 24 and close proximity to 

the air handler fan as shown in Figure 23. 

Figure 24. Illustration of the non-uniform return duct and close proximity of Pitot tube 

traverse airflow sensors to outdoor air damper and return register assembly. 

Because of the duct arrangement at the DiGiorgio home, this location was identified as the only 

possible location where airflow sensors could be installed. The primary advantage to this 

location was that the AH filter was to be installed directly in front of the airflow sensors 

(installation of the filter was not complete at the time this photo was taken) and would act as an 

airflow straightener. In order to measure the airflow continuously across the manufacturer’s 

specified airflow operating range (200-2000 CFM), we decided that fan RPM will be directly 

measured from the fan motor pulse output and will be mapped against a fan-assisted flow hood 

40
to establish supply air flow rates as a function of fan RPM. Datalogger programs will be revised 

to reflect the RPM signal from the fan motor in order to cover the range of continuously variable 

airflow rates that the Nightbreeze unit is capable of providing. NREL will perform the mapping 

of the fan RPM vs. known fan flow rates during a site visit in the summer of 2006. Capacity for 

the unit running in air-conditioning mode will be determined by the change in enthalpy of the 

supply- and return-air conditions and the supply-air flow rate. Measured values from the Table 6 

will be used in the calculations of enthalpy and cooling-system capacity. Cooling-system 

capacity coupled with the electrical energy measurements for the unit will allow use to calculate 

an EER for the unit. 

Establishing outdoor dry-bulb temperatures is accomplished with two type-T thermocouples 

installed in a custom-built forced-aspiration radiation shield. The absolute humidity of outdoor 

air is established by using a Vaisala HMP-50 temperature and RH sensor installed in a naturally 

aspirated plate-style radiation shield. Temperature and RH data from this sensor are used to 

determine the absolute humidity of air, which is logged as part of the datalogger program, the 

installation is shown in Figure 25. 

Figure 25. Vaisala temperature and RH sensors (left in plate shield) and custom forcedaspiration radiation shield with thermocouples (right, white PVC) installed at fence 

surrounding Freus condensing unit 

41
The performance of the evaporative-cooled Freus condensing unit is not well known. In order to 

establish the performance of the condenser, NREL staff is attempting to measure the heat 

rejected by the condensing unit via airside measurements. For the condenser fan intake and 

exhaust air conditions, dry-bulb temperatures are measured with type-T thermocouples and 

absolute humidity is calculated from Vaisala HMP-45A temperature and RH sensors. At the 

intake and exhaust grills for the condenser support, six thermocouples are attached in parallel for 

the dry-bulb measurements. At the condenser inlet, the Vaisala temperature and RH sensor is 

installed in a custom shield (designed to prevent water from the sump from damaging the sensor) 

located behind the condenser intake grill. At the condenser exhaust grill a second Vaisala HMP45A sensor is used in conjunction with a sampling tube laid on top of the exhaust grill. Exhaust 

air is sampled through multiple holes drilled into the tube and is pulled past the Vaisala sensor by 

a vacuum pump that is mounted on the electrical box that contains the watt-hour transducers for 

the absolute humidity calculations. Figures 26 and 27 show the arrangement of the sensors on 

Freus condensing unit. 

Figure 26. Thermocouple (brown wire) and Vaisala temperature and RH sensors (white 

PVC shield + gray sensor) installed at Freus air intake, grill removed 

42
Figure 27. Thermocouple (brown wire) and sampling tube (white PVC shield plus white 

tube) installed at Freus exhaust grill. Vacuum pump housing shown on left (White PVC 

pipe fitting) 

Airflow rate for the Freus condensing unit was established by using a fan-assisted flow-hood 

arrangement that utilized a blower-door fan. Because the condenser fan is a single-speed unit, a 

one-time measurement of the condenser fan flow rate was judged to be appropriate given the 

difficulties in making a continuous measurement of the fan flow rate. Figure 28 shows the 

installation of the fan-assisted flow hood. 

43
 

Figure 28. Fan-assisted flow hood installed on the Freus unit 

44
A blower-door fan was used to determine the condenser fan flow rate by adjusting the blowerdoor fan RPM until the measured pressure difference across the foil-faced foam-board box was 

zeroed using the manometer gauge typically used with a blower door while the condenser fan 

was running. When the pressure difference inside the foil-faced foam-board box compared to 

ambient went to zero, the flow rate coincident with pressure measured at the pressure tap on the 

blower-door fan was established from manufacturer’s literature for the blower-door fan 

according to the flow ring installed on the blower-door fan. The use of the blower-door fan to 

make this measurement very closely resembles using a duct-blaster fan as a “powered capture 

hood” as described in Chapter 13 of the duct blaster manual 

(www.energyconservatory.com/download/dbmanual.pdf). 

However, a duct blaster was not used in this application because a duct blaster fan is not capable 

of achieving the flow rate necessary to zero the pressure difference across the foam-board box. 

This method showed the condenser fan flow rate to be 1490 CFM. The energy use of the 

condenser fan and compressor are sub-metered for the Freus in an effort to develop a component 

model of the Freus evaporatively cooled condenser for use with whole-buildings simulation 

tools. Currently no such model exists in typical residential annual hourly whole-building 

simulation tools. 

3.3.2 Freus Performance Data Results from Magna 

Real-time measurements of the Freus system were taken by SWA/CARB with a portable data 

logger on September 29, 2004. There are no plans to add permanent instrumentation to this 

house because it is currently occupied and adding instrumentation would be too disruptive to the 

occupants. At the time, ambient conditions were mild with an outside air temperature of about 

70°F. The measured EER of the Freus was only 6.6, which was much lower than expected. It 

was concluded in a February 4, 2005, report by SWA/CARB that there must have been an 

installation problem with the indoor evaporation coil. SWA and NREL staff returned to take 

additional test data in August 2005. 

As a result of a second site visit to the homes at Magna, Utah, in early August 2005, the Freus 

unit was found to be undercharged. A local HVAC technician added 6.5 lbs of R-22 refrigerant 

after pulling a vacuum on the system and repairing a kink in the refrigerant line. Freus engineers 

were consulted and had the expectation that a properly charged unit would see about 6oF -10oF of 

sub-cooling and 8oF -15oF of superheat. After the refrigerant was added, about 6oF -7oF of subcooling and 10oF of superheat were achieved. Line pressures were about 160 psi on the highpressure side and 78 psi on the low-pressure side. 

Field measurements of the air-handler fan flow rates were made with a Trueflow plate.8 

Temperature and RH were evaluated with Humerel temperature and RH sensors. Energyconsumption measurements were performed with a handheld GREENLEE power meter. Airhandler fan flows were adjusted by altering the jumper configuration on the furnace circuit 

board. 

 

8 www.energyconservatory.com/products/products6.htm 

45
Initial data collected on the Freus unit at Magna showed that making repairs and adding the 

refrigerant greatly improved performance. Figure 29 shows the capacity obtained at two different 

fan speeds. Table 7 shows the results of specific measurements taken at two different fan speeds 

at an outdoor condition of 90°F dry bulb/69°F wet bulb and a return-air condition of 81.3°F dry 

bulb/66.3°F wet bulb. It lists the various power levels that were measured with a GREENLEE9

CMP-200 Power Clamp (true RMS reading), the corresponding power factors (PF), the 

capacities, and the energy efficiency ratio. The EER was measured as 13.3 at the low flow rate 

(400 CFM per ton) and 13.7 at the high flow rate (500 CFM per ton). Freus has not been able to 

provide us with an EER rating with the particular 3-ton evaporator coil used at Magna. However, 

the average EER rating for a range of 2-ton coils was 14.1 from the manufacturer’s data. This 

rating was for an outdoor condition of 95°F dry bulb/75°F wet bulb and an indoor condition of 

80°F dry bulb/67°F wet bulb. 

The indoor conditions for the Magna measurement were very close to the specification 

conditions. The outdoor conditions were a little more favorable than the specification conditions. 

Because of this and because Unit #2 uses a 3-ton evaporator coil instead of a 2-ton coil, one 

would expect the measured EER to be somewhat higher than the specification, whereas it is 

actually a little lower. At Borrego Springs, we should have specification data appropriate to the 

indoor coil and will be able to make a fairer comparison between measurement and specification. 

Figure 29. Measurements taken at Magna, Utah, showing the sensible, latent, and total 

cooling capacities of the Freus unit at two different fans speeds on August 10, 2005 

 

9 www.greenlee.com

46
47

Table 7. Measurement Results for Freus Unit at Magna 

after Refrigerant was Added10

Fan Mode 

Freus 

Watts 

Freus 

PF 

Fan 

Watts 

Fan 

PF 

Total 

Watts 

Capacity 

(Btu/hr) EER

400 CFM per ton 1,850 1.00 290 0.62 2,140 28372 13.3

500 CFM per ton 1,870 1.00 450 0.63 2,320 31739 13.7

4.0 Summary and Conclusions 

Field measurements of the overall performance of the Freus and OASys cooling systems at 

Magna indicate that they are performing close to expectations. Monitoring plans have been 

developed for the forthcoming Borrego Springs installations that will allow us to take more 

detailed data on these systems. DOE-2 annual simulations for a number of cities showed that the 

Freus provides a significant improvement over a conventional DX system in dry climates and 

does not have any negative impact on indoor comfort conditions. The OASys and conventional 

evaporative cooler provide more significant reductions in cooling energy use. However, both the 

OASys and, especially, the direct evaporative cooler can compromise indoor comfort conditions 

during humid periods. For such situations they may require a parallel DX unit to maintain 

comfort and this would incur additional capital cost. The OASys will provide reasonable indoor 

comfort for a wider range of locations than the direct evaporative cooler. 

5.0 References 

Eastment, M., E. Hancock, and C. Kutscher. 2005. “Projected Costs and Benefits of New 

Residential Evaporative Cooling Systems,” Golden, CO: National Renewable Energy 

Laboratory. 

 

10 90°FDB/69°F B

WBB outdoor, 81.3°FDB/66.3°FWB return air, one-time power measurements with GREENLEE CMP200 Power Clamp (True RMS) 
Appendix A. OASys Technical Specifications

Electronic Features Thermostat multi-function electronic Control System proprietary firmware Blower Motor 3⁄4-hp variable-speed GE ECM2.3 Cooling Capacity, Btu/h11Unducted Installation 20,700 Ducted Installation 19,300 Airflow Features Unducted Ducted12Supply maximum 1650 1350 Exhaust maximum13 550 850 Electrical Characteristics Voltage 115/1 Power, max 1.2 kW Unit FLA 10.5 Amps Evaporative Media Direct cooling stage Munter’s CELdek® 5090 

Indirect cooling stage Speakman polymer parallel plate Dimensions and weight Cased Uncased W x D x H, inches 29 x 24 x 78 27 x 22 x 66 Unit weight 190 lbs 120 lbs Operating weight 240 lbs 170 lbs Water consumption Reservoir size 5 gallons Rate of evaporation 6-11 gallons per hour 

Configuration Types Uncased: Wall mount Cased: Roof mount, portable Connections Thermostat RJ45 Water fill 1⁄2-in. NPT Drain barbed hose Electrical 115-V AC terminals Photovoltaic integration 300-V DC to motor blends seamlessly with 11

11 Rated at Sacramento design conditions 12 0.2 inches external static pressure 13 airflow restrictions imposed by ducts, grilles, or inadequate venting of 

air from the house causes an increase in exhaust air and reduction in supply air 

48
49

1. 3⁄4-hp GE ECM2.3 Electronically 

Commutated Motor 

2. Venturi mounting plate 3. Morrison 11-11 squirrel-cage 

blower wheel 

4. Polyethylene rotationally molded 

cabinet 

5. Drain valve 6. Fill valve 7. Taco 003 water circulator 8. Munter’s CELdek® 5090 direct 

cooling stage 

9. Speakman indirect cooling stage 

OASys Components at a glance 
OASys Performance Characteristics 

Installation Type 

Unducted Ducted 

High 

Speed 

Medium 

Speed 

Low 

Speed 

High 

Speed 

Low 

Speed 

Fan Power, Watts 498 266 58 445 110 

Total Power, Watts 521 289 81 468 133 

Supply cfm 1551 1251 750 1250 750 

Secondary cfm (indirect 

section exhaust) 622 478 250 800 490 

Entering Air Dry Bulb, °F 104.7 103.7 104.3 103.1 106.5 

Entering Air Wet Bulb, °F 70.8 71.1 73.0 73.3 74.3 

Between stage Dry Bulb, °F 87.0 85.0 84.5 80.5 82.9 

Leaving Air Dry Bulb, °F 67.8 67.8 68.8 68.7 69.1 

Leaving Air Wet Bulb, °F 65.0 65.3 67.2 66.6 67.5 

Indirect Effectiveness, % 52.2 57.2 63.3 75.7 73.3 

Direct Effectiveness, % 87.1 87.4 91.0 84.8 89.6 

Total Effectiveness, % 108.9 110.1 113.6 115.3 116.2 

Capacity, Btu/hr* 20,660 17,155 11,128 19,257 11,770 

Capacity, tons* 1.72 1.43 0.93 1.60 0.98 

EER* 40 59 136 41 88 

* Capacity and EER are calculated using design conditions for Sacramento: 101°F db/70°F wb and an indoor 

temperature of 80°F. 

50
For more information, contact 

Covington Stanwick 

Speakman CRS 

P.O. Box 191 

Wilmington, Delaware 19899 USA 

Ph: 301.643.4115 

Fax: 302.765.0286

51
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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

October 2006 

2. REPORT TYPE

Technical Report 

3. DATES COVERED (From - To)

 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Projected Benefits of New Residential Evaporative Cooling 

Systems: Progress Report #2 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/TP-550-39342 

5e. TASK NUMBER

BET6.8004 

6. AUTHOR(S)

C. Kutscher, M. Eastment, E. Hancock, and P. Reeves 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/TP-550-39342 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

The use of conventional evaporative cooling has rapidly declined in the United States despite the fact that it has high 

potential for energy savings in dry climates. Evaporative systems are very competitive in terms of first cost and 

provide significant reductions in operating energy use, as well as peak-load reduction benefits. Significant market 

barriers still remain and can be addressed through improved systems integration. This report investigates the first of 

these approaches, exploring innovative components. The U.S. Department of Energy (DOE) Building America 

research teams are investigating the use of two promising new pieces of residential cooling equipment that employ 

evaporative cooling as a part of their system design. The OASys unit, which is a combination of direct and indirect 

evaporative cooling stages developed by Davis Energy Group (DEG) and manufactured by Speakman CRS, is used 

to ultimately provide outside air to the living space. The outdoor air provided is indirectly and directly evaporatively 

cooled in two stages to a condition that can be below the wet-bulb (wb) temperature of the outside air, thus 

outperforming a conventional single-stage direct evaporative cooler. 

15. SUBJECT TERMS

Building America; U.S. Department of Energy; evaporative cooling; energy savings; residential buildings; peak-load 

reduction; residential cooling equipment 

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energy than any other sector of the 

U.S. economy, including transportation 

and industry. Fortunately, the opportunities to reduce building energy use—

and the associated environmental 

impacts—are significant.

DOE’s Building Technologies Program 

works to improve the energy efficiency 

of our nation’s buildings through innovative new technologies and better 

building practices. The program

focuses on two key areas:

• Emerging Technologies 

Research and development of the 

next generation of energy-efficient 

components, materials, and 

equipment

• Technology Integration

Integration of new technologies 

with innovative building methods 

to optimize building performance 

and savings

For more information contact:

EERE Information Center

1-877-EERE-INF (1-877-337-3463)

www.eere.energy.gov

An electronic copy of this publication is 

available on the Building America Web 

site at www.buildingamerica.gov

U.S. Department of Energy

Energy Efficiency

and Renewable Energy

Building America Program

George S. James • New Construction • 202-586-9472 • fax: 202-586-8134 • e-mail: George.James@ee.doe.gov

Terry Logee • Existing Homes • 202-586-1689 • fax: 202-586-4617 • e-mail: terry.logee@ee.doe.gov

Lew Pratsch • Integrated Onsite Power • 202-586-1512 • fax: 202-586-8185 • e-mail: Lew.Pratsch@hq.doe.gov

Building America Program • Office of Building Technologies, EE-2J • U.S. Department of Energy • 1000 Independence Avenue, S.W. • 

Washington, D.C. 20585-0121 • www.buildingamerica.gov

Building Industry Research Alliance (BIRA)

Robert Hammon • ConSol • 7407 Tam O’Shanter Drive #200 • Stockton, CA 95210-3370 • 209-473-5000 • fax: 209-474-0817 • 

e-mail: Rob@consol.ws • www.bira.ws

Building Science Consortium (BSC)

Betsy Pettit • Building Science Consortium (BSC) • 70 Main Street • Westford, MA 01886 • 978-589-5100 • fax: 978-589-5103 • 

e-mail: Betsy@buildingscience.com • www.buildingscience.com

Consortium for Advanced Residential Buildings (CARB)

Steven Winter • Steven Winter Associates, Inc. • 50 Washington Street • Norwalk, CT 06854 • 203-857-0200 • fax: 203-852-0741 • 

e-mail: swinter@swinter.com • www.carb-swa.com

Davis Energy Group

David Springer • Davis Energy Group • 123 C Street • Davis, CA 95616 • 530-753-1100 • fax: 530-753-4125 •

e-mail: springer@davisenergy.com • deg@davisenergy.com • www.davisenergy.com/index.html

IBACOS Consortium

Brad Oberg • IBACOS Consortium • 2214 Liberty Avenue • Pittsburgh, PA 15222 • 412-765-3664 • fax: 412-765-3738 •

e-mail: boberg@ibacos.com • www.ibacos.com

Industrialized Housing Partnership (IHP)

Subrato Chandra • Florida Solar Energy Center • 1679 Clearlake Road • Cocoa, FL 32922 • 321-638-1412 • fax: 321-638-1439 • 

e-mail: subrato@fsec.ucf.edu • www.baihp.org

National Association of Home Builders (NAHB) Research Center

Tom Kenney • National Association of Home Builders (NAHB) Research Center • 400 Prince George’s Boulevard • 

Upper Marlboro, MD 20774 • 301-430-6246 • fax: 301-430-6180 • toll-free: 800-638-8556 • www.nahbrc.org/ 

National Renewable Energy Laboratory

Ren Anderson • 1617 Cole Boulevard, MS-2722 • Golden, CO 80401 • 303-384-7433 • fax: 303-384-7540 • 

e-mail: ren_anderson@nrel.gov • www.nrel.gov

Tim Merrigan • 1617 Cole Boulevard, MS-2722 • Golden, CO 80401 • 303-384-7349 • fax: 303-384-7540 • 

e-mail: tim_merrigan@nrel.gov • www.nrel.gov

Oak Ridge National Laboratory

Pat M. Love • P.O. Box 2008 • One Bethel Valley Road • Oak Ridge, TN 37831 • 865-574-4346 • fax: 865-574-9331 • 

e-mail: lovepm@ornl.gov • www.ornl.gov

Produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory, a DOE national laboratory.

October 2006 • NREL/TP-550-39342

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 10% postconsumer waste.

Visit our Web sites at:

www.buildingamerica.gov www.pathnet.org www.energystar.gov

Research Toward Zero Energy Homes