hvac air distribution systems

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”