HVAC Sheet Metal and Fabrication 5 — Questions and Answers
Question 1: What is the primary reason galvanized steel is the most common material for HVAC ductwork?
- It provides corrosion resistance without requiring additional coatings (Correct answer)
- It has the highest thermal conductivity of any duct metal
- It is the lightest duct material available
- It is the only metal approved by SMACNA for duct fabrication
Correct answer: It provides corrosion resistance without requiring additional coatings
The zinc coating on galvanized steel provides inherent corrosion resistance, making it durable and cost-effective for most HVAC duct applications.
Question 2: Which fitting is used to offset duct around an obstruction while maintaining the same duct size?
- Offset fitting (Correct answer)
- Tee fitting
- Wye fitting
- Boot fitting
Correct answer: Offset fitting
An offset fitting shifts the duct laterally or vertically to route around a beam, pipe, or other obstruction while keeping the same duct dimensions.
Question 3: When a wye fitting splits a round duct into two branches, what should the combined cross-sectional area of the branches equal for proper air distribution?
- Approximately equal to or greater than the main trunk area (Correct answer)
- Exactly half the main trunk area
- Twice the main trunk area
- The branch areas do not need to relate to the trunk area
Correct answer: Approximately equal to or greater than the main trunk area
The combined area of the two branches should at least equal the main trunk area to avoid excessive velocity increase and pressure loss at the split.
Question 4: What type of mastic sealant is recommended for sealing HVAC duct joints in unconditioned spaces?
- UL 181-listed water-based duct mastic applied with mesh tape (Correct answer)
- Standard latex caulk
- Duct tape (cloth-backed adhesive tape)
- Silicone sealant
Correct answer: UL 181-listed water-based duct mastic applied with mesh tape
UL 181-listed duct mastic reinforced with fiberglass mesh tape provides a durable, airtight seal that remains flexible across temperature cycles in unconditioned spaces.
Question 5: What is 'parallel pattern development' used for in HVAC sheet metal layout?
- Developing the flat pattern of cylinders and prisms that have parallel lateral edges (Correct answer)
- Laying out fittings that taper from a large to a small opening
- Calculating the surface area of a cone-shaped fitting
- Marking hole locations for duct hangers on a flat sheet
Correct answer: Developing the flat pattern of cylinders and prisms that have parallel lateral edges
Parallel pattern development (parallel line development) is used to unfold fittings such as cylinders, rectangular ducts, and elbows whose lateral edges are parallel.
Question 6: A duct hanger rod for a rectangular duct should be sized based on which primary factor?
- The weight of the duct and any insulation it must support (Correct answer)
- The duct air velocity
- The duct static pressure class
- The distance from the air handler
Correct answer: The weight of the duct and any insulation it must support
Hanger rods are sized by the load they must support, which is determined by the weight of the duct section (and insulation) between hanger points.
Question 7: When fabricating a square-to-round transition, which development method is most appropriate?
- Triangulation method (Correct answer)
- Parallel line method
- Radial line method
- Direct projection method
Correct answer: Triangulation method
The triangulation method is used for square-to-round transitions because the surfaces are warped (non-developable) and must be divided into triangles to approximate the flat pattern.
HVAC Sheet Metal and Fabrication 5
What is the primary reason galvanized steel is the most common material for HVAC ductwork?