This presentation discusses underfloor air distribution (UFAD) systems. It outlines some disadvantages of traditional HVAC systems, such as temperature variations and energy wasted moving air to unoccupied spaces. UFAD systems use an access floor plenum to distribute air through floor diffusers, exploiting thermal stratification for improved comfort and efficiency. Case studies of UFAD installations demonstrate strategies for primary air systems, perimeter systems, leakage control, and construction sequencing.

1. UNDERFLOOR AIR DISTRIBUTION
DANIEL H. NALL, PE, FAIA, FASHRAE, LEED FELLOW, BEMP, HPDP
SYSKA HENNESSY GROUP
MAY 2017

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3. • Temperature, Humidity, Ventilation all controlled by
a single Sensor
• Much Energy Expended in Moving Air Around
• Energy Expended for Conditioning Areas where
People Aren’t
• Poor Comfort Maintenance due to Space
Temperature Variation
DISADVANTAGES OF COMMON HVAC SYSTEMS

4. WHAT IS UNDERFLOOR AIR DISTRIBUTION?
• Use of an Access Floor Plenum as Supply Air
Path
• Use of Floor Diffusers to Supply Air to Space
• Exploitation of Space Temperature Stratification
• High Return

5. • Plenum Facilitates IT and Power Cabling
• Air Distribution Simplified by Floor Plenum
• Economical Accommodation of Space
Reconfiguration (Churn)
• Economical Provision of Personal HVAC Control
• Improved Indoor Air Quality because of
Stratification of Contaminants
WHY UNDERFLOOR AIR DISTRIBUTION?

6. COMPONENT TRADITIONAL UNDERFLOOR HVAC SYSTEM
Amounts $/SF $ $/SF $
3. Floors and Roof
Beam Penetrations 100,000 SF 0.25 $25,000 0 EA 0.00 $0
Premium for Structural details - SF 0.00 $0 100,000 SF 0.50 $50,000
Raised Floor - SF 0.00 $0 100,000 SF 8.50 $850,000
0.25 $25,000 9.00 $900,000
4. Interior Partitions
Electrified furn/partitions premlum 400 STN 1000.00 $400,000 $0
Drywall 4,000 LF 65.00 $260,000 4,000 LF 70.00 $280,000
Cores, conference, etc. 6.60 $660,000 2.80 $280,000
7. Finishes
Paint Ceiling & services 100,000 SF 1.00 $100,000 100,000 SF 1.00 $100,000
Carpet 11,112 SY 30.00 $333,360 11,112 SY 30.00 $333,360
4.33 $433,360 4.33 $433,360
11. HVAC
Main Loop 66,000 LB 3.50 $231,000 20000 LB 3.50 $70,000
Branch ducts 50,000 LB 3.50 $175,000 8,000 LB 4.00 $32,000
Plenum Divers - FT 0.00 $0 20,000 FT 8.00 $160,000
Diffusers/registers 500 EA 68.00 $34,000 40 EA 68.00 $2,720
Insulation 60,000 SF 1.50 $90,000 0 SF 0.00 $0
Hot water reheat 5,000 LF 22.00 $110,000 5,000 LF 18.00 $90,000
Terminals 80 EA 1600.00 $128,000 280 EA 500.00 $140,000
7.68 $768,000 4.95 $494,720
12. Electrical
Power Distribution 100,000 SF 2.00 $200,000 100,000 SF 1.50 $150,000
Receptacles 926 EA 76.00 $70,376 300 EA 70.00 $21,000
Duct (in floor) outlets & raceway 626 EA 50.00 $31,300 0 EA 0.00 $0
Cable management 4,000 LF 26.00 $104,000 2,000 LF 3.00 $6,000
Light fixtures 2,000 EA 220.00 $440,000 2,000 EA 220.00 $440,000
Low voltage signal and alarm systems 100,000 SF 1.50 $150,000 100,000 SF 1.50 $150,000
9.96 $995,676 7.67 $767,000
Subtotal 28.82 $2,882,036 28.75 $2,875,080
Design Contingency 10% 2.88 $288,204 2.88 $287,508
Total 31.70$ 3,170,240$ 31.63$ 3,162,588$
% Savings Net 1st Cost Savings 0.2% 7,652$
FIRSTCOSTCOMPARISONS:
UNDERFLOORVS.CONVENTIONAL

7. DESIGN PROCESS
• Rethink design issues and typical solutions
• Consider expanded opportunities
• Integrated design - trade-offs between systems
• Address code issues & local approvals

8. TECHNICAL ISSUES
• Avoiding Cold Feet
• Exploiting Stratification
• Uniform Supply Temperatures
• Controlling Humidity
• Modulating Heating and Cooling Capacity
• Leakage
• Construction Sequence/ Housekeeping

9. ACCESS FLOOR AND WIRING

10. FLOOR DISPLACEMENT DIFFUSER

11. FLOOR VARIABLE VOLUME TERMINAL AND DIFFUSER

12. SUPPLY AIR JET PERSISTENCE

13. STRATIFICATION
• Artifact of Buoyancy and Non-Mixing Air
Delivery
• Potentially Increases Comfort
• Improves Occupant Air Quality
• Reduces total Air Delivery Volume
• Maximize temperature rise between chest
level and ceiling
• Minimize temperature rise from foot to chest

14. STRATIFICATION FACTORS
• Supply Air Jet Persistence
• Ratio of Air Volume to Cooling Load
• Concentration of Heat Gains

15. RATIO OF SUPPLY AIR FLOW TO LOAD

16. SUPPLY TEMPERATURE BEHAVIOR -
TEMPERATURE
• Heat Gain from Structural Slab and Access Floor Tile
• Flow Distance from Plenum Injection to Diffuser -
Vortices

17. Supply
Plenum
Ceiling-floor radiation
Floor-room convection
Return Plenum
Return
Plenum
Slab
Slab
Treturn
Treturn
Treturn
Tceiling
Tcarpet
Tplenum
Troom, near floor = 72°F
Slab-supply plenum conduction/convection
Slab-supply plenum conduction/convection
Floor-supply plenum conduction/convection
Return-ceiling convection
Return-slab convection
Room
Raised Floor
Panels
Ceiling-slab radiation
0.6 cfm/ft2, 65°F
78°F
PLENUM HEAT BALANCE

18. PREDICTED DISTRIBUTION OF ROOM COOLING LOAD
Heat gain
into space
100%
Extraction
57%
Through floor
14%
Through slab
29%
Total into plenum
43%
Baseline results – hung ceiling

19. CORPORATE HEADQUARTERS BUILDING:
SUPPLY PLENUM TEMPERATURE VARIATION

20. PLENUM DESIGN – NY TIMES HQ

21. Typical Tower
Floor Plan
Third Floor
Plan
Ground
Floor Plan
Typical Floor
Plan

22. CFD MODEL

23. NY TIMES – INITIAL PLENUM DESIGN

24. NY TIMES – NO STUB DUCTS – FULL
FLOW

25. NY TIMES – NO STUB DUCTS – FULL
FLOW

26. PLENUM DESIGN - VERIFICATION
Images Provided by the
Center for the Built
Environment, University
of California Berkeley

27. PRIMARY AIR SYSTEMS – ISSUES
• Required Supply Air Dry Bulb Temperature
• Required Supply Air Dew Point
Temperature
• Required Plenum Pressure
• Adequate Supply Air Distribution

28. PRIMARY AIR SYSTEMS –
TECHNOLOGIES
• Bypass Central Station Units
Underfloor Air Highways
Pressure Control Dampers
• Air Towers (Column Fans)
Direct Pressure Fan VFD Control
Distributed Fan Locations

29. CENTRAL STATION BYPASS AIR HANDLING UNITS

30. NY TIMES PLAN WITH UNDERFLOOR AIR HIGHWAYS

31. AIR TOWERS

32. AIR TOWERS

33. PERIMETER SYSTEMS - ISSUES
• Cooling
Thermostatic Control for Envelope Load Variations
Maintenance of Stratification
• Heating
Thermostatic Control of Heat/Cool Changeover
Control of Window Wall Downdrafts
Energy Efficiency

34. PERIMETER SYSTEMS - TECHNOLOGY
• Underfloor Fan Coils
Updraft Flow Enhances Stratification
Maintenance/Furniture Conflict
Continuous Updraft vs. Incremental
• Overhead Fan Coils
Downdraft Flow Conflicts with Interior Zone Flow
Overhead Maintenance May not Require Furniture
Disassembly

35. NEW YORK TIMES HEADQUARTERS
UNDERFLOOR FAN COILS WITH CONTINUOUS DIFFUSERS

36. UNDERFLOOR FAN COILS

37. PERIMETER FAN COILS WITH INCREMENTAL DIFFUSERS
Images and Equipment by York International

38. UPFLOW PERIMETER SYSTEM STRATIFICATION
NY Times – Transverse Section

39. UNDERFLOOR HEATING AND COOLING FAN
COILS

40. CHEUNG KONG CENTER

41. OVERHEAD FAN COILS
• Accessibility Advantages
• Heating Issues
• Cooling Issues

42. OVERHEAD FAN COILS – COOLING AT
DIFFUSER

43. OVERHEAD FAN COILS – HEATING

44. OVERHEAD FAN COILS: STRATIFICATION
Return
Grills
Lights
Supply
Plenum
Return
Plenum
Curtain
Wall
Linear
Ceiling
Diffusers
People
Sun
Spot

45. Discovery Communications Headquarters
Silver Spring, Maryland
CASE STUDY – SAP US HEADQUARTERS

46. East/West
 Thermostatic Underfloor VAV Fan Terminals
 Two pipe changeover coils for heating/peak cooling
46
North Perimeter
 Perimeter Upflow Slot Diffusers, non-fan forced
 Thermostatic Dampers for Diffusers
 Overhead Electric Radiant Panels interlocked with
cooling dampers
SAP HEADQUARTERS: PERIMETER
STRATEGIES

47. SAP HQ INTERIOR HVAC STRATEGY
 Manually Adjustable Swirl Diffuser for Open- Office
 Thermostatic Diffusers for Private Offices
 Fan Coils with Electric Reheat and CO2 Sensors for
Large Conference Rooms
47

48. SAP: UNDER FLOOR AIR SYSTEM
48

49. CASE STUDY – BASF US HEADQUARTERS

50. UFAD SYSTEM FLOOR LAYOUT
50

51. 51
CFD STUDY OF FLOOR SUPPLY PLENUM

52. BASF EVAPORATIVE CONDENSER DX
CUSTOM PENTHOUSE
 Refrigerant Temperature Range Comparable to Water Cooled
Water Chiller
 Condenser Recirculation Pump Much Smaller than Condenser
Water Pump
 No Chilled Water Pump
 Reduced Fouling of Refrigerant Heat Transfer Surfaces
 Small Centrifugal Compressors Slightly Higher EIR Compared
with Large Centrifugal Water Chillers
 Variable Speed Drive Improves part Load Performance
 Overall Energy Savings Compared with Water Cooled Chiller
Plant
52

53. 53
CUSTOM PENTHOUSE LAYOUT

54. 54
HUDSON YARDS, NYC FINANCIAL HQ

55. 55
HUDSON YARDS, NYC FINANCIAL HQ

56. HUDSON YARDS, NYC FINANCIAL HQ
 Over 28% energy savings for chilled water service with
dual temperature plant
o 38% reduction in IPLV for 55°F (12.8°C) chiller compared with
standard
o Waterside economizer full cooling almost 50% of hours
compared with 34% of hours for standard system
 Significantly reduced interior zone overcooling due to
high temperature SAT (66°F, 18.9°C)
 Significantly reduced thermal degradation due to high
temperature of supply air in plenum.
 Reduced airflow due to low temperature (60°F, 15.6°FC)
recooled air at perimeter.
 ASHRAE Journal “Engineers Notebook”, June, 2017
56

57. • Overwhelming Cause of Project Failure
• Mutli-Disciplinary, Multi-Trade Issue
• Leakage Rates of 40% have Been Identified
• Three Types of Leakage:
–Into the Occupied Space
–Into the Return Path, Bypassing Occupied Space
–Into the Exterior Envelope
LEAKAGE

58. • Allowable up to 0.25 cfm per square foot
• Gasketed Floor Boxes
• Leakage Rated Access Tiles
• Offset Grid of Access Floor Tile and Carpet
Tile
• Trim Detail for Access Floor at Walls and
Columns
LEAKAGE INTO OCCUPIED SPACE

59. • Completely Undesirable
• Tape and Seal all Walls at Floor Slab
• Firestopping Detail at Slab/Exterior Wall Join
• Firestop all Piping, Cabling Floor Penetrations
• Isolate and Seal All Core Wall Penetrations for
Cabling
• Make Underfloor Supply Plenum a Bathtub
LEAKAGE INTO RETURN PLENUM

60. • Can Cause Water Damage to Exterior Wall
• Provide Solid Interior Finish Surface to Spandrel
Panel in Plenum
• Firestopping Details
• Curtain Wall Details
• Make Underfloor Supply Plenum a Bathtub
LEAKAGE INTO EXTERIOR WALL

61. PLENUM LEAKAGE CONTROL

62. • Supply Plenum Must be Completely Clean for
Operation
• Concrete Floor Slab is Sealed
• Sequence of Cleaning and Installation
–Slab Sealing
–Drywall Installation and Finishing
–Pedestal Installation
–Final Floor Installation
• Designer/Contractor/Owner Summit to Create
Installation Sequence
CONSTRUCTION
SEQUENCE/HOUSEKEEPING

63. Acknowledgements:
Alcoa Corporate Headquarters
Architect – The Design Alliance, Pittsburgh, PA
Building Services Engineers – Roger Preston +Partners
Atlanta, GA, Dodson Engineering, Pittsburgh PA
New York Times Headquarters
Architect – FX Fowle, New York, NY
Renzo Piano Building Workshop
Building Services Engineers – Flack + Kurtz, NYC

64. Questions
THANK YOU
Underfloor Air Distribution Systems
Daniel H. Nall, FAIA, PE, FASHRAE, LEED, Fellow, BEMP HBDP
dnall@Syska.com