HVAC System Design 3 — Questions and Answers
Question 1: Which factor does NOT directly affect the heat loss calculation through a wall assembly?
- U-factor of the wall
- Surface area of the wall
- Design temperature difference
- Color of the interior paint (Correct answer)
Correct answer: Color of the interior paint
Heat loss Q = U × A × ΔT; interior paint color has no significant effect on conductive heat loss through a wall.
Question 2: In a constant-air-volume (CAV) system, how is zone temperature control typically achieved?
- Varying the supply air volume to each zone
- Modulating the fan speed
- Using a reheat coil in the zone terminal (Correct answer)
- Adjusting outdoor air dampers
Correct answer: Using a reheat coil in the zone terminal
CAV systems maintain constant airflow and use zone reheat coils to raise supply air temperature as needed for individual zone temperature control.
Question 3: What is 'static pressure' in duct system design?
- The velocity of air moving through the duct
- The pressure acting perpendicular to airflow on duct walls (Correct answer)
- The total pressure minus dynamic pressure
- The friction loss per 100 feet of duct
Correct answer: The pressure acting perpendicular to airflow on duct walls
Static pressure is the pressure exerted outward on duct walls perpendicular to the direction of airflow, driving air through the distribution system.
Question 4: A two-pipe changeover hydronic system limitation is that it:
- Cannot serve multiple floors simultaneously
- Cannot provide heating and cooling to different zones at the same time (Correct answer)
- Requires chilled water below 32°F
- Must use gravity circulation instead of pumps
Correct answer: Cannot provide heating and cooling to different zones at the same time
Two-pipe changeover systems use the same piping for either hot or chilled water, so all zones must be in heating or cooling mode simultaneously.
Question 5: What is the purpose of a bypass duct or 'dump zone' in a zoned forced-air system?
- Provide fresh outdoor air to unconditioned spaces
- Relieve excess system pressure when zone dampers close (Correct answer)
- Connect supply and return plenums for mixing
- Increase airflow to the largest zone
Correct answer: Relieve excess system pressure when zone dampers close
A bypass duct diverts excess supply air back to the return plenum when multiple zone dampers close, preventing excessive static pressure on the blower.
Question 6: According to ASHRAE 62.2, residential ventilation rates are based primarily on:
- Number of HVAC systems installed
- Floor area and number of bedrooms (Correct answer)
- Ceiling height alone
- Occupant age and activity level
Correct answer: Floor area and number of bedrooms
ASHRAE 62.2 calculates required ventilation airflow using a formula that accounts for conditioned floor area and the number of bedrooms as a proxy for occupant count.
Question 7: In HVAC design, 'face velocity' refers to:
- The speed of the blower wheel
- Airflow velocity across the face area of a coil or filter (Correct answer)
- Duct velocity at the system outlet
- Return air speed at the grille face
Correct answer: Airflow velocity across the face area of a coil or filter
Face velocity is the air speed across the frontal area of a cooling coil, heating coil, or filter, measured in feet per minute (FPM).
HVAC System Design 3
Which factor does NOT directly affect the heat loss calculation through a wall assembly?