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NSF/IUCRC Center for Building performance and Diagnostics at Carnegie Mellon UniversityAdvanced Building Systems Integration Consorti umGuidelines for High Performance Buildings 2004

Replace or supplement mechanical ventilation with natural ventilation or mixed-modeconditioning to achieve 47 - 79% HVAC energy savings, 0.8 - 1.3% health cost savings,

and 3 - 18% productivity gains, for an average ROI of at least 120%.

Mixed-mode conditioning systems combine natural ventilation with mechanical air conditioning and cooling.Mixed-mode systems can take many forms, but typically involve a building envelope that becomes a critical partof the HVAC system as well as an intelligent control system that allows the building to operate in both naturaland mechanical modes. Common mixed mode strategies include:

Concurrent systems, which use natural ventilation and mechanical HVAC simultaneously. Occupants arefree to open windows and the HVAC system provides supplemental ventilation, dehumidification, andcooling, while an advanced control system coordinates zone air supply rates with window positions

Changeoversystems, where the building alternates between natural and mechanical mode on a seasonal ordaily basis;

Zoned systems, in which different conditioning strategies are used simultaneously in different zones of a

building.Mixed-mode is appropriate for the design of new buildings, for the retrofit of older, naturally ventilated buildings,and for low- and medium-rise office buildings in temperate and cool climate zones. Buildings with mixed-mode systems offer four notableadvantages over air-conditioned-only buildings: reduced heating, ventilation and cooling (HVAC) energy consumption due to use of naturalventilation, reduced health symptoms due to higher outdoor air ventilation rates, higher occupant satisfaction due to improved comfort, andincreased flexibility due to the use of distributed mechanical systems and controls.

Conventional practice Improved practice

Enclosure Pressurized and sealed building Operable windows and vents

HVAC system VAV system with recirculated air Constant volume ventilation with on/off control orVAV system with supply rate linked to use of natural

ventilation or split thermal and ventilation systems

HVAC control Automati c contro l to main tain uniformconditions; ventilation air supply linkedto air temperature

Combination of automatic and user control tomaintain desired comfort conditions; ventilation airsupply independent of thermal conditioning

HVAC zoning Large zones (15-200 people); mixeddensities / functions in one zone; oftendifferent orientations in the same zone

Small, flexible zones with a maximum size of 6workstations and a shift toward individual control

Mixed-mode Conditioning Pays!Eight studies have shown that natural ventilation and mixed-mode systems can pay for themselves in less than one yeardue to energy and productivity benefits.

CMUs BIDS demonstrates that natural ventilation andmixed-mode systems yield annual energy cost savings of$110 per employee ($0.53 per square foot), health costsavings of $60 per employee, and annual productivity gainsof $3,900 per employee, for a total savings of $4,070 peremployee annually.*

With an estimated first cost premium of $1,000 per employee($5 per square foot) in new construction and a documentedfirst cost of $3,400 per employee ($17 per square foot) tomodify an existing building, the average ROI for an

investment in natural ventilation or mixed-mode conditioningis 407% for new construction and 120% for retrofits.

Costs and Benefits of Mixed-mode Conditioning and Natural Ventilation

$60

health

$3,400

retrofit

$110

energy

$1,000

new construction

$3,900

productivity

$-

$500

$1,000

$1,500

$2,000

$2,500

$3,000

$3,500

$4,000

$4,500

DollarsperEmployee

ONE-TIME first cost ANNUAL benefits

CBPD/ABSIC

BIDSTM

*Using BIDSTM baseline assumptions


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Benefits of Mixed Mode and Natural VentilationCMUs BIDS identifies three case studies that demonstrate HVAC energy savings due to mixed-mode conditioning or natural ventilation,

with average energy savings of over 59% annually. Two case studies show health cost reductions, with an average savings of 1.1%annually. Six case studies show individual productivity improvements due to mixed-mode or natural ventilation, with an averageimprovement of nearly 9% annually.

Annual HVAC Energy Savings from Mixed-mode

Conditioning and Natural Ventilation

47%52%

79%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

Bunn & Cohen 2001

(natural v ent.)

Bunn & Cohen 2001

(mixed-mode)

Rowe 2002

%S

avings

Average savings 59%

$110 per em ployee

Annual Health Cost Savings from Mixed-Mode

Conditioning and Natural Ventilation

0.80%

reduced SBSsymptoms

1.30%reduced

headache & colds

0.00%

0.20%

0.40%

0.60%

0.80%

1.00%

1.20%

1.40%

1.60%

1.80%

K roeling et al 1988 Finnegan et al 1984

%S

avin

gs

Average savings 1.1%

$60 per employee

Annual Productivity Gains from Mixed-Mode Conditioning and Natural Ventilation

3.20%71% reduced

absenteeism

5.10%40% reduced

SBS symptoms

7.50%increased

test scores

7.70%67% reduced

SBS symptoms

9.75%perceived

productivity

increase

18.0%perceived

productivity

increase

0%

4%

8%

12%

16%

20%

24%

Sterling and Sterling 1983 Skov et al 1990 Heschong Mahone 2002 Kroeling et al 1988 Leaman 2001 Rowe 2002

%I

mprovement

Average improvement 8.5%

$3900 per employee

CBPD/ABSIC

BIDSTM

CBPD/ABSIC

BIDSTM

CBPD/ABSIC

BIDSTM


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BIDSTMCase Study Examples of Mixed-mode and Natural Ventilation

39 offices

University of Sydney offices / Rowe 2002In a 2002 multiple building study of 39 offices at the University of Sydney, Australia, David Rowe identifies 79%annual HVAC energy savings and an 18% improvement in perceived productivity in offices with mixed-modeconditioning, as compared to offices with mechanical air conditioning.

Rowe, David (2002) Pilot Study Report: Wilkinson Building, The University of Sydney, Sydney, Australia.

AnnualHVAC energyuse in the PROBE buildings

0

255075

100125150175200225250275300325350375400425450475500

FRYMB

WWMCAP

UCABPOR

DMO

MBOBC

DCA

FRM

CC&G

ORC

CRS

ALDC&W

HFS

TAN

Building

kWhpersquaremeeter

NaturallyventilatedMixed mode

Air conditioned

18 buildings

PROBE / Bunn and Cohen 1994

In a 2001 multiple building study of 18 buildings in the UK, Bunn and Cohen of the PROBE team identify anaverage of 52% measured annual savings in HVAC energy in buildings with mixed-mode conditioning and anaverage of 47% measured annual savings in HVAC energy in buildings with natural ventilation, as compared to

buildings with fully mechanical ventilation and air-conditioning.

Bunn, R. and Cohen, R. (2001) Learning from PROBE. Building Services Journal, May 2001.

5 buildings

Kroeling et al 1988

In a 1988 multiple building study in Berlin and Heidelberg, Kroeling identifies a 33% reduction in reportedheadaches, a 28% reduction in reported frequency of colds and a 31% reduction in reported circulation problemsin naturally ventilated office buildings, as compared to air conditioned office buildings.

Kroeling, P. (1988). Health and well-being disorders in air conditioned buildings; comparative investigations of the building

illness syndrome. Energy and Buildings, 11(1-3): 277-282.

Vancouver office building / Sterling and Sterling 1983

In a 1983 building case study in Vancouver, B.C., Sterling and Sterling identify a 71% reduction inabsenteeismfrom 4.5% to 1.3%--when office workers moved from a building wth sealed windows andmechanical ventilation to a building with operable windows and natural ventilation.

Sterling, E. and Sterling, T. (1983) The Impact of Different Ventilation Levels and Fluorescent Lighting Types on Building

Illness: an Experimental Study. Canadian Journal of Public Health, Vol. 74, November/December 1983.


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