HVAC Air Distribution Systems
Eli P. Howard, IIIExecutive Director Technical Services
SMACNA
SMACNA SMACNA is an international trade
association and ANSI standards developer. SMACNA represents sheet metal contractors in the arenas of commercial, industrial, architectural, residential and custom fabricating. SMACNA produces over 30 standards that are widely used by the industry and adopted by the model building codes.
Today's Program
Duct Pressure Classification
Duct Construction Aspects
Duct Fitting Selection
Duct Leakage (Duct vs. Equipment)
Basis of Program
SMACNA HVAC Systems Duct Design Manual
ASHRAE Fundamentals HandbookASHRAE Duct Fitting Database SMACNA HVAC Duct Construction
Standards-Metal & Flexible
Static Pressure
Class
Operating Pressure Type of
Pressure Seal Class
Maximum Velocity
Units in. w.g. Units in. w.g. Units fpm
1/2 Up to 1/2 Pos/Neg C 2000
1 Over ½ to 1 Pos/Neg C 2500
2 Over 1 to 2 Pos/Neg C 2500
3 Over 2 to 3 Pos/Neg B 4000
4 Over 3 to 4 Pos A 4000
6 Over 4 to 6 Pos A As Specified
10 Over 6 to 10 Pos A As Specified
HVAC DUCT PRESSURE-VELOCITY CLASSIFICATION
Duct Design>>>ConstructionDESIGN ENGINEER
Design Considerations:CFM
Static Pressure
Duct Size
Fitting Type
Construction Pressure Class <or 1 & 2 w/ VAV>
Seal Class (if more stringent than “standard”)
DUCT CONTRACTORConstruction Considerations:
Panel Thickness (Gage)
Pressure Class (as specified <or 1 & 2 w/VAV>)
Panel Width/Height
Joint Type/Spacing
Intermediate Reinforcement External/Internal (tie rods)
Duct Construction Aspects
SMACNA/ANSI HVAC-DCS 3rd
Edition
Proper Duct Fitting Ratios
Operating Pressure/Duct Const.
Space Allocation
Duct Construction Aspects
SMACNA/ANSI HVAC-DCS 3rd
EditionG60 ASTM A924/A653 (0.60oz/sf)Tie RodsDuct Cleanliness Seismic Restraint (OSHPD)
Proper Duct Fitting Ratios
ASHRAE Loss Coefficient Database
SMACNA HVAC Systems Duct Design Manual
20% Duct Fitting Ratios – Energy Efficient Air Distribution System
Factors Affecting Duct Pressure Losses
Air Velocity Pressure Class Number of Fittings Type of Fittings Shape-Aspect
Ratio/Rect/Round
Type of Filters Type of Outlets System Effect
Duct Leakage (Duct vs. Equipment)
ASHRAE 90.1 – 2010– 25% of duct operating above 3 in. w.g.– All duct located outside the building
envelope– Seal class A– Leakage class 4
IECC– 25% of duct operating above 3 in. w.g.
IGCC V2– 25% of duct operating above 3 in. w.g.– All duct located outside the building envelope– Seal class A
LEED®– References ASHRAE 90.1
ASHRAE 189.1– Does not require any testing– Requires Seal Class “A”
Duct Leakage (Duct vs. Equipment)
All methods use a leakage class not an arbitrary percent
LEED®, 189, IGCC are all “reach” documents
The allowable leakage has been reduced by 1/3 in SMACNA’s latest duct leakage standard
Duct Leakage (Duct vs. Equipment)
Scandinavian Standards-10%/20%/100% testing of round ductwork-20%/40%/100% testing of rectangular ductwork-Based on duct surface area (SMACNA)
HVCA DW 143 (UK & Central Europe)-Adheres to ASHRAE 90.1-Based on duct surface area (SMACNA)
Duct Leakage (Duct vs. Equipment)
Duct Leakage (Duct vs. Equipment)
ASHRAE Research RP 1292 Data clearly shows “no” VAV can attain
2% (average was 10-20% leakage) 5% for the system?
– Just testing VAV boxes not systems– (3) 8” units 12%, 10%, 5%– The best leaked 5% only at a low pressure
high flow test actually hit 15% at low flow
Where is the Industry Going?
SMACNA and ASHRAE are developing a System Air Leakage Standard.
It will provide a method of test intended for field application
It will provide a method to develop a REAL pass/fail criteria – not a random value independent of the system.
Proposed Industry Consensus Duct Leakage Testing – “Ductwork shall be leak tested
according to SMACNA HVAC Air Duct Leakage Test Manual. Representative sections totaling 25% of the total installed duct surface area for each pressure class shall be tested. If the initial representative section fails the test, an additional 25% of that pressure class shall be tested. If the second representative section fails 100% of the duct constructed for that pressure class must be tested. The maximum permitted leakage class (CL) is 4 cfm/100 sq ft @ 1” WG (SMACNA Leakage Class 4) (0.05 L/s/meter sq). Test pressures shall not exceed the construction pressure class. Positive Pressure tests are permitted for negative pressure systems.”
As not spoken byThe Artist
“Fini”