HVAC Sheet Metal and Fabrication

Question 1

What is the most common gauge of galvanized sheet metal used for residential ductwork?

Accepted Answer

26 gauge (0.018")

Answer

30 gauge (0.012")

Answer

24 gauge (0.024")

Answer

20 gauge (0.036")

Question 2

What is the purpose of 'seam' construction in sheet metal ductwork?

Accepted Answer

To join the longitudinal edges of sheet metal to form a duct shape, providing structural strength and airtight construction

Answer

To allow thermal expansion of the ductwork as it heats and cools

Answer

To create mounting points for hanging the ductwork from the structure

Answer

To allow sections to be separated for cleaning access

Question 3

What is 'Pittsburgh lock' seam and when is it used in ductwork fabrication?

Accepted Answer

A longitudinal seam type using a rolled mechanical lock; standard for rectangular ductwork fabrication

Answer

A high-security locking mechanism for equipment access panels

Answer

A transverse connection between duct sections using slip flanges

Answer

A corner seam used only at duct elbows

Question 4

What is the difference between 'transverse' and 'longitudinal' joints in ductwork?

Accepted Answer

Transverse joints run across the duct (connecting duct sections end-to-end); longitudinal joints run along the length of a duct section (forming the duct shape from flat sheet)

Answer

Transverse joints connect duct to equipment; longitudinal joints connect duct sections together

Answer

Transverse joints use flanges; longitudinal joints use slips

Answer

Transverse joints are for rectangular duct; longitudinal joints are for round duct

Question 5

What is 'TDF' (Transverse Duct Flange) connection?

Accepted Answer

A rolled flange formed on the duct end that accepts a flange connector (clip), providing airtight, rigid connections faster than slip-and-drive for commercial ductwork

Answer

Thermal Duct Fitting; used only for insulated ductwork connections

Answer

A type of flexible duct connection for residential systems

Answer

A self-sealing tape used for commercial ductwork connections

Question 6

What is the difference between 'galvanized steel' and 'stainless steel' ductwork?

Accepted Answer

Galvanized is zinc-coated mild steel for standard applications; stainless steel is chromium-alloyed for corrosion-resistant applications like kitchen exhaust and chemical environments

Answer

Galvanized is for outdoor use; stainless is for indoor use only

Answer

Galvanized is standard for commercial; stainless is standard for residential

Answer

There is no functional difference; stainless is just cosmetically preferred

Question 7

What is a 'radius elbow' versus a 'square throat elbow'?

Accepted Answer

A radius elbow has a curved inner turning surface; a square throat elbow has a sharp 90° inner corner; radius elbows have significantly lower pressure drop and turbulence

Answer

A radius elbow is used in circular ducts; square throat in rectangular ducts

Answer

A radius elbow has a larger radius but same pressure loss; square throat is preferred for space constraints only

Answer

The terms are interchangeable—both have the same pressure loss characteristics

Question 8

What is 'duct leakage' and how does SMACNA classify acceptable leakage?

Accepted Answer

Airflow escaping through unsealed seams and joints; SMACNA specifies leakage classes (LC) of 3, 6, 12, 24, 48 CFM/100 sq ft at 1 inch WG, with lower LC numbers indicating tighter construction

Answer

Air leaking from ductwork is called duct loss; SMACNA allows 100% leakage at low pressure

Answer

Condensate leaking from cooling coils; SMACNA classifies by volume per hour

Answer

Refrigerant leaking from split systems connected to ducts; SMACNA specifies ppm limits

Question 9

What type of sealant is used for duct seams and joints to achieve airtight construction?

Accepted Answer

Water-based mastic duct sealant applied to all seams and joints, or pressure-sensitive foil tape meeting UL 181A or 181B

Answer

Standard silicone caulk from a hardware store

Answer

Duct tape (cloth-backed adhesive tape) is the industry standard

Answer

Fiberglass mesh tape embedded in joint compound

Question 10

What is a 'duct liner' and how does it differ from 'duct wrap'?

Accepted Answer

Duct liner is insulation installed inside the duct (thermal insulation + sound absorption); duct wrap is insulation applied to the outside of the duct (primarily thermal insulation for ducts in unconditioned spaces)

Answer

Duct liner is fiberglass on the outside; duct wrap is fiberglass on the inside—they have opposite effects

Answer

They are identical products with different names used in different regions

Answer

Duct liner is rigid foam board; duct wrap is flexible fiberglass