HVAC Air Distribution Systems

HVAC Air Distribution Systems Eli P. Howard, III Executive 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 Handbook  ASHRAE Duct Fitting Database  SMACNA HVAC Duct Construction Standards-Metal & Flexible

HVAC DUCT PRESSURE-VELOCITY CLASSIFICATION Static Pressure Class

Operating Pressure

Units in. w.g.

Units in. w.g.

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

Type of Pressure

Seal Class

Maximum Velocity Units fpm

Duct Design>>>Construction DESIGN ENGINEER Design Considerations: CFM Static Pressure Duct Size Fitting Type Construction Pressure Class Seal Class (if more stringent than “standard”)

DUCT CONTRACTOR Construction Considerations: Panel Thickness (Gage) Pressure Class (as specified ) 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  Space

Pressure/Duct Const.

Allocation

Duct Construction Aspects  SMACNA/ANSI

HVAC-DCS 3rd

Edition  G60 ASTM A924/A653 (0.60oz/sf)  Tie Rods  Duct 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.

Duct Leakage (Duct vs. Equipment) 

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) 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 by The Artist

“Fini”