Hawaii Plumbing Contractor (C-37)
Examination Practice Exam 2026 | 100
Questions & Answers with Detailed
Rationales | Complete Exam Prep &
Study Guide
1. A plumbing contractor is sizing a building drain serving several fixture
branches. Which factor is most important when determining the required
drain size?
A. Pipe color
B. Fixture-unit loading and applicable code requirements
C. Water heater recovery rate
D. Exterior wall thickness
Answer: Fixture-unit loading and applicable code requirements
Rationale: Drainage piping is sized primarily from the anticipated fixture-unit
load, pipe configuration, slope, and governing plumbing-code requirements.
2. What is the primary purpose of a plumbing trap?
A. Increase water pressure
B. Prevent sewer gases from entering the occupied space
,C. Increase drainage velocity
D. Reduce water temperature
Answer: Prevent sewer gases from entering the occupied space
Rationale: A properly installed trap maintains a water seal that blocks sewer
gases while allowing wastewater to discharge.
3. A trap repeatedly loses its water seal when a nearby fixture is discharged.
What condition should the contractor investigate first?
A. Excessive water heater temperature
B. Improper venting or trap siphonage
C. Oversized supply piping
D. Low static water pressure
Answer: Improper venting or trap siphonage
Rationale: Improper venting can create negative pressure that siphons water
from the trap, destroying the protective seal.
4. What is the primary function of a sanitary vent system?
A. Supply drinking water
B. Increase fixture pressure
C. Protect trap seals and allow drainage-system pressure equalization
D. Heat the drainage system
Answer: Protect trap seals and allow drainage-system pressure equalization
Rationale: Venting permits air movement within the drainage system and helps
prevent positive or negative pressures from disturbing trap seals.
5. Which condition most directly indicates a possible blocked building drain?
A. High water pressure at a hose bibb
B. Multiple fixtures draining slowly or backing up simultaneously
,C. Cold water at a lavatory
D. A properly functioning water heater
Answer: Multiple fixtures draining slowly or backing up simultaneously
Rationale: When several fixtures are affected at the same time, the restriction is
often downstream in a common building drain or sewer connection.
6. Why should a cleanout generally be accessible?
A. To increase water temperature
B. To permit inspection and removal of drainage-system obstructions
C. To eliminate the need for venting
D. To increase fixture-unit values
Answer: To permit inspection and removal of drainage-system obstructions
Rationale: Cleanouts provide access for drain cleaning, inspection, and
maintenance without unnecessary demolition.
7. Which material is generally suitable for potable-water distribution when it
is approved for that application?
A. Unapproved drainage pipe
B. Approved copper tubing
C. Unlisted fuel piping
D. Sewer-only piping
Answer: Approved copper tubing
Rationale: Potable-water systems must use materials approved for drinking-
water service and installed according to the applicable requirements.
8. What is the purpose of a pressure-reducing valve on a building water
supply?
A. Increase excessive pressure
B. Control excessive downstream water pressure
, C. Remove sewer gases
D. Increase drain slope
Answer: Control excessive downstream water pressure
Rationale: A pressure-reducing valve limits excessive supply pressure to protect
fixtures, piping, and equipment.
9. A water heater has a temperature-and-pressure relief valve. What is its
primary safety function?
A. Increase water temperature
B. Relieve dangerous temperature or pressure conditions
C. Prevent all sediment accumulation
D. Replace the expansion tank
Answer: Relieve dangerous temperature or pressure conditions
Rationale: The T&P relief valve is a critical safety device designed to discharge
when unsafe temperature or pressure conditions develop.
10.Why should the discharge from a water-heater relief valve not be capped?
A. It would reduce water pressure
B. It could prevent pressure or temperature relief during a dangerous condition
C. It would increase heater efficiency
D. It would improve circulation
Answer: It could prevent pressure or temperature relief during a dangerous
condition
Rationale: Blocking the relief discharge can defeat the safety device and create
a serious explosion or scalding hazard.
11.What is the purpose of a thermal expansion control device in a closed
potable-water system?
Examination Practice Exam 2026 | 100
Questions & Answers with Detailed
Rationales | Complete Exam Prep &
Study Guide
1. A plumbing contractor is sizing a building drain serving several fixture
branches. Which factor is most important when determining the required
drain size?
A. Pipe color
B. Fixture-unit loading and applicable code requirements
C. Water heater recovery rate
D. Exterior wall thickness
Answer: Fixture-unit loading and applicable code requirements
Rationale: Drainage piping is sized primarily from the anticipated fixture-unit
load, pipe configuration, slope, and governing plumbing-code requirements.
2. What is the primary purpose of a plumbing trap?
A. Increase water pressure
B. Prevent sewer gases from entering the occupied space
,C. Increase drainage velocity
D. Reduce water temperature
Answer: Prevent sewer gases from entering the occupied space
Rationale: A properly installed trap maintains a water seal that blocks sewer
gases while allowing wastewater to discharge.
3. A trap repeatedly loses its water seal when a nearby fixture is discharged.
What condition should the contractor investigate first?
A. Excessive water heater temperature
B. Improper venting or trap siphonage
C. Oversized supply piping
D. Low static water pressure
Answer: Improper venting or trap siphonage
Rationale: Improper venting can create negative pressure that siphons water
from the trap, destroying the protective seal.
4. What is the primary function of a sanitary vent system?
A. Supply drinking water
B. Increase fixture pressure
C. Protect trap seals and allow drainage-system pressure equalization
D. Heat the drainage system
Answer: Protect trap seals and allow drainage-system pressure equalization
Rationale: Venting permits air movement within the drainage system and helps
prevent positive or negative pressures from disturbing trap seals.
5. Which condition most directly indicates a possible blocked building drain?
A. High water pressure at a hose bibb
B. Multiple fixtures draining slowly or backing up simultaneously
,C. Cold water at a lavatory
D. A properly functioning water heater
Answer: Multiple fixtures draining slowly or backing up simultaneously
Rationale: When several fixtures are affected at the same time, the restriction is
often downstream in a common building drain or sewer connection.
6. Why should a cleanout generally be accessible?
A. To increase water temperature
B. To permit inspection and removal of drainage-system obstructions
C. To eliminate the need for venting
D. To increase fixture-unit values
Answer: To permit inspection and removal of drainage-system obstructions
Rationale: Cleanouts provide access for drain cleaning, inspection, and
maintenance without unnecessary demolition.
7. Which material is generally suitable for potable-water distribution when it
is approved for that application?
A. Unapproved drainage pipe
B. Approved copper tubing
C. Unlisted fuel piping
D. Sewer-only piping
Answer: Approved copper tubing
Rationale: Potable-water systems must use materials approved for drinking-
water service and installed according to the applicable requirements.
8. What is the purpose of a pressure-reducing valve on a building water
supply?
A. Increase excessive pressure
B. Control excessive downstream water pressure
, C. Remove sewer gases
D. Increase drain slope
Answer: Control excessive downstream water pressure
Rationale: A pressure-reducing valve limits excessive supply pressure to protect
fixtures, piping, and equipment.
9. A water heater has a temperature-and-pressure relief valve. What is its
primary safety function?
A. Increase water temperature
B. Relieve dangerous temperature or pressure conditions
C. Prevent all sediment accumulation
D. Replace the expansion tank
Answer: Relieve dangerous temperature or pressure conditions
Rationale: The T&P relief valve is a critical safety device designed to discharge
when unsafe temperature or pressure conditions develop.
10.Why should the discharge from a water-heater relief valve not be capped?
A. It would reduce water pressure
B. It could prevent pressure or temperature relief during a dangerous condition
C. It would increase heater efficiency
D. It would improve circulation
Answer: It could prevent pressure or temperature relief during a dangerous
condition
Rationale: Blocking the relief discharge can defeat the safety device and create
a serious explosion or scalding hazard.
11.What is the purpose of a thermal expansion control device in a closed
potable-water system?
