100+ Free CSLB C-55 Practice Questions

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2026 Statistics

Key Facts: CSLB C-55 Exam

31%

Troubleshooting & Repair

CSLB Study Guide

28%

Planning & Estimating

CSLB Study Guide

3.5

Hours allowed

PSI Exams

$450

Application Fee

CSLB

4 Years

Experience Req

CSLB

The CSLB C-55 Water Conditioning examination is a 3.5-hour closed-book, multiple-choice test administered by PSI. It requires 4 years of journey-level experience and a $450 application fee. The exam covers planning, component installation and startup, troubleshooting and repair, and safety.

Sample CSLB C-55 Practice Questions

Try these sample questions to test your CSLB C-55 exam readiness. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.

1A water analysis report shows hardness of 10 grains per gallon (gpg) as CaCO3. According to WQA classification, this water is:

A.Hard water

B.Soft water

C.Moderately hard water

D.Very hard water

Explanation: WQA classifies 7.0–10.5 gpg as hard water; 10 gpg falls in that range. Soft is 0–1 gpg, moderately hard is 3.5–7 gpg, and very hard is above 10.5 gpg.

2What pH range is generally considered acceptable for drinking water per EPA secondary standards?

A.6.5 – 8.5

B.5.5 – 7.0

C.7.0 – 9.5

D.6.0 – 7.5

Explanation: EPA secondary drinking water regulations recommend a pH range of 6.5 to 8.5 for aesthetic quality. Water outside this range can taste metallic or feel slippery, though pH is not directly regulated as a health standard.

3Total Dissolved Solids (TDS) is best measured with which instrument?

A.A conductivity meter

B.A turbidity tube

C.A pH meter

D.A hardness test kit

Explanation: TDS is measured indirectly using a conductivity meter, which measures the electrical conductivity of dissolved ions. Turbidity measures cloudiness, pH measures acidity, and hardness kits measure calcium and magnesium specifically.

4Which form of iron is soluble in water at the source but oxidizes upon exposure to air?

A.Ferrous iron (Fe2+)

B.Ferric iron (Fe3+)

C.Bacterial iron

D.Organic iron

Explanation: Ferrous iron (Fe2+) is dissolved and clear when drawn, but oxidizes to ferric iron (Fe3+) upon exposure to oxygen, causing red/brown staining. This is the most common form in well water.

5A water sample has total hardness of 8 gpg and total alkalinity of 5 gpg (both as CaCO3). What is the estimated carbonate hardness?

A.5 gpg

B.8 gpg

C.13 gpg

D.3 gpg

Explanation: Carbonate (temporary) hardness equals the lesser of total hardness or total alkalinity, so it is 5 gpg. The remaining 3 gpg is non-carbonate (permanent) hardness.

6A water softener has a resin capacity of 32,000 grains. The water hardness is 10 gpg and daily usage is 400 gallons. How many days between regenerations?

A.8 days

B.4 days

C.12 days

D.16 days

Explanation: Daily grain load = 10 gpg × 400 gal = 4,000 grains/day. Days between regenerations = 32,000 ÷ 4,000 = 8 days.

7When evaluating a job site for a water softener installation, what is the minimum recommended clearance around the unit for service access?

A.12 inches on all sides

B.6 inches on all sides

C.24 inches on all sides

D.3 inches on all sides

Explanation: At least 12 inches of clearance on all sides is recommended to allow access for maintenance, brine tank refilling, and valve operation. Less clearance can make service impossible without removing the unit.

8Before installing a water treatment system, what should be checked first regarding the home's water supply?

A.Static water pressure and flow rate

B.Water temperature

C.Pipe color

D.Drain line distance only

Explanation: Static water pressure and flow rate must be checked first because treatment equipment has operating pressure ranges and flow requirements. Low pressure or flow can prevent the system from functioning properly.

9Why is it important to determine whether a home is on a private well versus a municipal water supply during site evaluation?

A.Well water may have different contaminants and no disinfection residual

B.Well water always has higher pressure

C.Municipal water requires no treatment

D.Well water is always safer

Explanation: Well water often contains iron, hardness, and bacteria without any disinfection residual, while municipal water is treated but may have chlorine and chloramines that affect equipment. The source determines which treatment approach is appropriate.

10What is the purpose of a pressure test during site evaluation before installing an RO system?

A.To verify the feed water pressure meets the RO membrane's minimum requirement

B.To check for pipe leaks in the home

C.To measure water temperature

D.To determine the TDS level

Explanation: RO membranes require a minimum feed pressure (typically 40–60 psi) to produce rated output. A pressure test confirms the system will function properly and identifies whether a booster pump is needed.

About the CSLB C-55 Exam

The C-55 Water Conditioning exam assesses the knowledge and skills necessary to plan, install, start up, service, and repair water conditioning and treatment systems, including softeners and reverse osmosis equipment.

Assessment

Question count not published by the exam provider

Time Limit

3.5 hours

Passing Score

Disclosed at test site

Exam Fee

$450 application fee (California Contractors State License Board (CSLB))

CSLB C-55 Exam Content Outline

28%

Planning and Estimating

Water analysis, job site evaluation, system selection, and planning and estimating.

24%

Water Systems Component Installation and Startup

Piping and valves, equipment installation, drain connections/backflow, electrical connections, startup and calibration.

31%

Troubleshooting, Maintenance, Repair, and/or Replacement of Water Systems and Components

Service and maintenance, troubleshooting, repair and replacement, disinfection.

17%

Safety

Personnel safety, handling hazardous materials, environmental safety, and tool and equipment safety per Cal/OSHA Title 8.

How to Pass the CSLB C-55 Exam

What You Need to Know

Passing score: Disclosed at test site
Assessment: Question count not published by the exam provider
Time limit: 3.5 hours
Exam fee: $450 application fee

Keys to Passing

Work through all 100 available questions
Review every answer and explanation
Track weak areas and revisit them
Use our AI tutor for tough concepts

CSLB C-55 Study Tips from Top Performers

1Review the 2025 California Plumbing Code (Title 24) and the California Health & Safety Code Safe Drinking Water Act provisions, which are the editions the CSLB study guide lists as references.

2Practice energy and operating-cost calculations for reverse osmosis systems using kWh and run-time factors.

3Know backflow prevention requirements and the correct procedure for non-polarized receptacle electrical connections.

4Study the Water Quality Association knowledge base for water analysis, softening, and disinfection procedures.

5Familiarize yourself with Cal/OSHA Title 8 personnel, hazardous-materials, and tool safety provisions.

Frequently Asked Questions

Are these practice questions the real CSLB C-55 exam questions?

No. These are independently written study questions built from the topic areas and weights in CSLB's official C-55 examination study guide. They are a study aid for the knowledge the exam covers and do not reproduce CSLB's item bank, replace the required journey-level experience, or substitute for the official examination.

What is the format of the C-55 exam?

The exam is a closed-book, multiple-choice test administered by PSI, taking up to 3.5 hours. A calculator is provided and some questions require computation.

How much experience is required?

You need at least four years of journey-level experience within the last ten years.

Are code books allowed in the exam?

No, it is a closed-book examination. No outside reference materials or code books are permitted.

Is the exam offered in Spanish?

CSLB publishes a Spanish-language study guide for this classification, but the examination itself is administered in English only. Spanish exam versions are available for only 10 commonly taken classifications; for other exams CSLB may approve a translator to read the exam to you.

100+ Free CSLB C-55 Practice Questions

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