HVAC Ductwork 5

Question 1

When retrofitting insulation on existing sheet metal ducts in an unconditioned attic, what minimum R-value does IECC 2021 typically require for climate zones 4 and above?

Accepted Answer

R-8

Answer

IECC 2021 requires a minimum of R-8 duct insulation for ducts in unconditioned spaces in climate zones 4 and above to limit thermal losses.

Question 2

What is 'duct conduction loss' and how does it affect system efficiency?

Accepted Answer

Heat transfer through duct walls that warms or cools supply air before it reaches registers

Answer

Duct conduction loss occurs when heat flows through duct walls between conditioned supply air and surrounding unconditioned space, reducing the effective capacity delivered to the room.

Question 3

A return air grille is significantly undersized. What is the most likely symptom in the living space?

Accepted Answer

Negative pressure in the room causing doors to slam shut or stick

Answer

An undersized return grille restricts airflow back to the air handler, creating negative pressure in the served space and causing pressure-related door and draft problems.

Question 4

What is the purpose of a turning vane installed inside an HVAC elbow?

Accepted Answer

Guide airflow around the bend to reduce turbulence and pressure drop

Answer

Turning vanes inside elbows redirect airflow smoothly around the bend, reducing turbulence and cutting pressure drop compared to a plain 90° elbow.

Question 5

In a commercial VAV system, when zone cooling demand drops to minimum, the VAV box may switch to reheat mode. What is the energy concern with this operation?

Accepted Answer

Reheating previously cooled air wastes both cooling and heating energy simultaneously

Answer

Reheat systems cool air centrally and then heat it locally at the VAV box, consuming both cooling energy (chiller) and heating energy (boiler or electric coil) at the same time — an inherently wasteful process.

HVAC Practice Test

HVAC Practice Test

HVAC Ductwork 5