Mechanical & plumbing systems: NASCLA Commercial General Building Practice Questions
HVAC equipment and tonnage, ductwork, testing and balancing, drainage and venting basics, backflow, fire sprinklers and standpipes, and coordination of MEP work with the structure.
Question 1
One ton of refrigeration (air-conditioning capacity) equals how many Btu per hour?
Correct answer: 12,000 Btu/h
Trade practice (HVAC)
A ton is the heat needed to melt one ton of ice in 24 hours, 12,000 Btu/h. 3,412 Btu/h is one kilowatt.
Question 2
Calculation
A building's cooling load is 540,000 Btu/h. What is the required capacity in tons?
Correct answer: 45 tons
Trade practice (HVAC)
Divide Btu per hour by 12,000. Dividing by 3,412 (the kW conversion) gives about 158.
Worked solution
1
Formula
Tons = Btu/h ÷ 12,000
2
Values
Tons = 540,000 ÷ 12,000
3
Result
Tons = 45 tons
Question 3
In a variable air volume (VAV) system, how is the temperature of each zone controlled?
Correct answer: By varying the airflow to each zone at a terminal box
Trade practice (HVAC)
VAV boxes modulate airflow at a roughly constant supply temperature, often with reheat coils for zones that need heat.
Question 4
What is testing, adjusting and balancing (TAB) of an HVAC system?
Correct answer: Measuring and setting air and water flows to design values
Trade practice (HVAC)
An independent TAB firm adjusts dampers, valves and fan speeds and reports the results; it is usually needed before commissioning and closeout.
Question 5
Where must fire dampers be installed in ductwork?
Correct answer: Where ducts pass through rated walls, floors or shafts
Trade practice (HVAC)
Fire dampers close (usually by a fusible link) to keep fire from spreading through duct openings in rated assemblies, and need access for inspection.
Question 6
Why is condensate from cooling coils piped to an approved location?
Correct answer: The coils pull water out of the air, and it must drain safely
Trade practice (HVAC)
Condensate lines need slope, traps where required, and often secondary pans or overflow switches above finished spaces.
Question 7
What is the job of a trap under a plumbing fixture?
Correct answer: To hold a water seal against sewer gas
Trade practice (plumbing)
The water seal blocks sewer gas; vents protect the seal from being siphoned out.
Question 8
Why does a drainage system need vents?
Correct answer: To keep traps from being siphoned and let drains flow
Trade practice (plumbing)
Vents balance pressure in the drain system and carry sewer gas out above the roof.
Question 9
Calculation
A 3-inch building drain runs 80 ft at 1/4 inch per foot. How much total fall does it have?
Correct answer: 20 inches
Trade practice (plumbing)
Multiply the slope per foot by the length. 1/8 inch per foot would give 10 inches.
Worked solution
1
Formula
Fall = slope × length
2
Values
Fall = 0.25 in/ft × 80 ft
3
Result
Fall = 20 inches
Question 10
What does a reduced pressure principle (RPZ) backflow preventer protect against?
Correct answer: Backflow of contaminated water into the potable supply
Trade practice (backflow)
RPZ assemblies protect against both backpressure and backsiphonage on high-hazard connections such as fire systems with additives, boilers and irrigation; they need annual testing.
Question 11
Before concealing new drain, waste and vent piping, what is normally required?
Correct answer: A water or air test approved by the inspector
Trade practice (plumbing tests)
Rough-in piping is tested for leaks before walls and floors are closed; the code sets the test method and duration.
Question 12
What is a wet-pipe sprinkler system?
Correct answer: Pipes always full of water under pressure
Trade practice (fire sprinklers)
Wet systems are the most common; dry-pipe systems hold air and are used where pipes could freeze; deluge systems open all heads at once.
Question 13
How does a standard fusible or glass-bulb sprinkler head operate in a fire?
Correct answer: Each head opens on its own when heat reaches its rating
Trade practice (fire sprinklers)
Only the heads heated by the fire open, which is why most fires are controlled by one or two heads.
Question 14
What is the fire department connection (FDC) on a sprinklered building?
Correct answer: An outside inlet for fire department pumping
Trade practice (fire sprinklers)
The FDC lets fire engines boost pressure and flow; it must be visible, accessible and kept clear.
Question 15
Why are pipe sleeves set in concrete slabs and walls before the pour?
Correct answer: To leave openings so the concrete need not be cored
Trade practice (MEP coordination)
Sleeves must be located from coordinated drawings and approved by the structural engineer where they affect reinforcing; rated slabs then need firestopping.
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Practice questions are written in our own words and cite code sections by number. Always study from your own copy of the code and the official candidate bulletin.