Home
   Dedicated Outdoor Air
   Systems (DOAS)

   Environmental Safety
   Radiant Ceiling Panels
   Economic
   Considerations

   Proof of Concept
   Technical Papers
   PPT Presentations @
    ASHRAE
Meetings
    Links                                 
   LEED Green Building
              Email:    Dr. Stanley A. Mumma,
   Ph.D., P.E

   

      

   Since February 15, 2001


Economic Considerations: DOAS/Radiant vs. VAV

The DOAS/Radiant cooling approach provides superior indoor air quality (IAQ), humidity control and thermal comfort.  How significant is this superior IAQ
, humidity control and thermal comfort?  It is estimated by Lawrence Berkeley National Laboratory that US companies currently loose as much as $48 billion annually in medical expenses due to biological contaminants that lead to sick building illnesses and $160 billion annually in lost productivity due to inadequate ventilation. The DOAS/Radiant cooling approach, by virtually eliminating these problems, offers a huge and largely overlooked economic benefit.  Financial benefits of that magnitude alone should be sufficient incentive for every engineer/architect/owner to use the DOAS/Radiant cooling system. However, it is a well-recognized fact that these issues are not always sufficiently compelling to motivate prospective investors. They generally expect to realize at least a first cost benefit. Operating cost savings are just an added benefit, but rarely a major factor in the decision making process. Therefore, a careful first cost analysis is necessary to justify this design approach.

The major factors that impact the first cost reduction of the DOAS/radiant system compared to a conventional all air VAV system are:
  • Chiller size reduction due to need for less OA and total heat recovery in the DOAS.
  • Pump size reduction due to chiller size reduction.
  • Ductwork reduction due to the small (only about 15 to 20%) DOAS air flow rate when
        compared to an all air VAV system.
  • Plenum depth reduction, and the associated savings in building enclosure/interior
        partition/ structural/plumbing/ fire protection/and vertical transportation materials due
        to the smaller ducts and elimination of terminal VAV boxes.
  • Air handling unit size reduction.
  • Electrical services reduction for the mechanical equipment due to smaller chiller, fans
        and pumps.
  • Piping reduction due to functional integration of the thermal and fire suppression
        transport systems.
  • Acoustical ceiling panels reduction where replaced with the radiant system.
  • Mechanical shaft reduction due to the much lower air volumes.

  • The major factor contributing to the first cost of the DOAS/radiant system is:
  • The radiant panels.
  • A first cost analysis (using representative pricing data extracted from Means Cost Data Manuals) for a 6 story, 31,000 sq ft per floor, office building (in compliance with the ASHRAE energy conservation std. 90.1-2004) located in Philadelphia, PA is presented in the table below.

     

    Cost item

    Unit cost

    Units VAV

    Units DOAS/radiant

    Cost savings

    Chiller

    $1,000/ton

    506 ton

    306 ton

    $200,000

    Chilled water pump

    $25/gpm

    1215 gpm

    737 gpm

    $11,950

    Ductwork

    $1/sq-ft DOAS, $4/sq-ft VAV

    186,000 ft2

    186,000 ft2

    $558,000

    AHU

    $2/cfm VAV

    $4/cfm DOAS

    135,000 cfm

    25,000 cfm, 100% Ventilation air

    $170,000

    Electrical Serv.

    $50/kW

    630 kW

    372 kW

    $12,400

    Facade/partitions

    $35/ft2 of facade

    No depth reduction

    1 ft plenum depth/floor or 4308 ft2

    $150,780

    Integrated thermal and fire suppression piping.

    $0.65/ft2 savings

    NA

    186,000 ft2

    $120,900

    Drop ceiling

    $1.50/ ft2

    NA

    79,200 ft2

    $118,800

    Mechanical shaft impact on lost rentable space

    $125/ ft2

    NA

    500 ft2
     saved

    $62,500

    Savings
    Subtotal

     

     

     

    $1,405,300

    Radiant Panel

    $13/ ft2 of panel

    NA

    79,200

    $(1,029,600)

    Net savings

     

     

     

    $375,700
    or
    $2/
    ft2

     

    The operating cost figures also favor the DOAS/Radiant system. An hourly energy analysis for 12 hours a day 5 days per week provides the following op cost data. The utility rate for this project was:

    Energy Charges:     Demand block 1: 200 kWh/kW, $0.065/kWh
                                   Demand block 2: 200 kWh/kW, $0.052/kWh
                                   Demand block 3: remaining kWh, $0.05/kWh

    Demand Charge:     $6.94/kW



    System

    Annual Mechanical

    Annual Total Mech, Illum, and Equip

    PW annual Total, 3yr, 8% interest

    Total present worth =
    PW annual - Net savings

    VAV

    $77,350

    $299,914

    $772,900

    $772,900

    DOAS/radiant

    $59,730

    $273,565

    $705,000

    $329,300

     

    These cost considerations are extremely encouraging. However the structure of professional fees may discourage crediting items outside ones own discipline (i.e. Mechanical in this case). For example, reductions in the first cost of the building enclosure, mechanical shaft floor area reduction, and dual use of fire suppression piping could be perceived as having an adverse impact on the Architects fees. If actual practice supports these estimates, there can be no impediment to rapid implementation, short of defending the status quo!


    Please visit the Papers section of this site for more information on
          Dedicated Outdoor Air Systems and Radiant Ceiling Panels.



    Home l  Dedicated Outdoor Air Systems (DOAS) l  Environmental Safety l  Radiant Panels l  Economic Considerations l
    Proof of Concept l  Technical Papers l  ASHRAE Presentations l  Links l  LEED Green Building l  Email l


    Dedicated Outdoor Air Systems & Radiant Ceiling Panels
    This site maintained by S. A. Mumma ©2001
    All Rights Reserved. All material on this site are trademarks or registered trademarks of their respective companies or mark holders and remains their property.
    Copyright 2001 Bob Hedman.