335 Washington DC Sixth Class Stationary Boilers Exam: Complete Guide and Preparation Tips

The 335 Washington DC Sixth Class Stationary Boilers exam evaluates knowledge of low-pressure boiler operation, safety, steam and hydronic systems, fuel and draft, water treatment, controls, fittings, and basic engineering calculations. The current PSI bulletin lists the examination as 70 multiple-choice questions, closed book, with a 2½-hour time limit.

What Is the 335 Sixth Class Stationary Boilers Exam?

The 335 examination is part of the District of Columbia’s stationary boiler licensing examination program. It is intended for professionals who need to demonstrate knowledge of operating and maintaining stationary boiler systems within the applicable D.C. licensing framework.

The District of Columbia Department of Licensing and Consumer Protection (DLCP) administers occupational and professional licensing programs and provides information about stationary steam engineer licensing and examinations.

The Sixth Class category covers steam and hot-water heating boilers within specified capacity and pressure limits. D.C. licensing instructions state that Class 6 applies to steam and hot-water heating boilers where steam pressure does not exceed 15 psi and the applicable boiler capacity does not exceed 200 horsepower.

How Is the 335 Exam Structured?

The current PSI examination bulletin identifies 70 multiple-choice questions and a 2½-hour testing period. The examination is listed as closed book.

The content is divided evenly across ten areas:

• Boiler Operation and Safety — 10%
• Fuel and Draft System — 10%
• Steam Systems — 10%
• Boiler Water Treatment — 10%
• Engineering Calculations — 10%
• Boiler Fittings and Accessories — 10%
• Hydronic Systems — 10%
• Feedwater Systems — 10%
• Controls — 10%
• Boiler Design and Principles — 10%

Because every section carries the same weighting, candidates should avoid focusing exclusively on one technical area.

What Should You Know About Boiler Operation and Safety?

Safe operation is fundamental to stationary boiler work. Candidates should understand normal operating procedures, monitoring, startup and shutdown principles, and the purpose of safety devices.

Important concepts include:

• Maintaining appropriate water levels
• Monitoring pressure and temperature
• Recognizing abnormal operating conditions
• Following safe startup procedures
• Understanding shutdown procedures
• Responding to unsafe conditions
• Maintaining operating records

D.C. regulations also establish requirements concerning boilers and pressure vessels, including permitting and appropriate licensed-engineer responsibilities. The regulations require daily logs documenting testing of boiler safeties and controls for each tour of duty.

How Do Fuel and Draft Systems Work?

Fuel and draft systems influence combustion efficiency and safe boiler operation.

Candidates should review:

• Fuel characteristics
• Burner operation
• Combustion fundamentals
• Draft
• Air supply
• Flue gases
• Combustion safety
• Basic troubleshooting

Understanding the relationship between fuel, air, combustion, and draft can help when evaluating operating conditions and identifying potential problems.

What Are Steam Systems?

Steam-system knowledge is another core part of the examination.

Candidates should understand how steam is generated, distributed, and returned through a heating system. Important concepts include pressure, temperature, condensation, steam distribution, and condensate return.

It is useful to understand why proper steam-system operation matters for both efficiency and safety.

Why Is Boiler Water Treatment Important?

Water quality directly affects boiler performance and equipment life.

Poor water conditions can contribute to:

• Scale
• Corrosion
• Deposits
• Reduced heat transfer
• Equipment damage
• Operational problems

Candidates should understand the purpose of water treatment and why maintaining appropriate water conditions is important for boiler operation.

What Engineering Calculations Should You Review?

The examination includes a dedicated Engineering Calculations section worth 10% of the test.

Preparation should include basic calculations relevant to boiler operation and heating systems.

Depending on the question, candidates may need to work with concepts involving:

• Pressure
• Temperature
• Capacity
• Flow
• Energy
• Horsepower
• Heating requirements

Practice calculations carefully rather than relying only on memorized formulas.

What Boiler Fittings and Accessories Should You Know?

Boilers rely on fittings and accessories to operate safely and efficiently.

Candidates should understand the purpose and general operation of equipment such as:

• Safety valves
• Pressure gauges
• Water-level controls
• Low-water cutoff devices
• Relief devices
• Valves
• Expansion-related components
• Other boiler-room accessories

The important point is to understand what each component does and why it is required.

How Do Hydronic and Feedwater Systems Work?

Hydronic systems circulate heated water through a building or process. Candidates should understand circulation, pumps, expansion, temperature, pressure, and system components.

Feedwater systems are associated with supplying appropriate water to a boiler. Preparation should cover the basic relationship between water supply, feedwater equipment, boiler operation, and water treatment.

Understanding these systems as connected processes is more useful than studying individual components in isolation.

Why Are Boiler Controls Important?

Controls help regulate and protect boiler operation.

Candidates should become familiar with the basic purpose of:

• Temperature controls
• Pressure controls
• Water-level controls
• Burner controls
• Safety interlocks
• Low-water protection
• Automatic shutdown functions

A key preparation goal is understanding how controls respond when operating conditions move outside safe limits.

What Should You Know About Boiler Design and Principles?

The final examination category covers Boiler Design and Principles and represents 10% of the test.

Candidates should review fundamental boiler concepts, including:

• Heat transfer
• Steam generation
• Boiler components
• Pressure and temperature relationships
• Combustion
• Heating surfaces
• Boiler efficiency
• Basic system design

A good understanding of these fundamentals can make the other examination sections easier to understand.

How Can You Prepare for the 335 Exam?

1. Follow the Official Content Outline

The PSI outline provides an effective starting point because the ten subject areas are equally weighted.

2. Study Low-Pressure Boiler Fundamentals

Focus on how boilers generate and distribute heat and how operating conditions affect system performance.

3. Review Safety Devices

Understand the purpose and operation of safety valves, low-water cutoffs, pressure controls, and other protective equipment.

4. Practice Calculations

Set aside dedicated study time for engineering calculations rather than leaving them until the end of your preparation.

5. Connect Related Systems

Study fuel, combustion, draft, water treatment, steam, hydronic systems, and controls as parts of one operating system.

6. Use Practice Questions

Practice questions can help identify weak areas and improve familiarity with technical terminology. They should reinforce your understanding rather than replace official references and technical study.

7. Verify Current Requirements

Before scheduling the examination, confirm current licensing, application, scheduling, and examination requirements with the District of Columbia DLCP. The District states that exams may be scheduled online or by phone through its examination provider.

How Can CertsArea Support Preparation?

CertsArea provides certification-focused preparation resources that can be incorporated into a broader study routine. Practice material can help candidates review boiler terminology, identify knowledge gaps, and become more comfortable with question-based assessment.

For the 335 examination, preparation should be combined with the current PSI content outline, appropriate boiler references, and the latest D.C. licensing information.

Frequently Asked Questions

What is the 335 Washington DC Sixth Class Stationary Boilers exam?

It is a District of Columbia stationary boiler examination covering boiler operation, safety, fuel and draft, steam systems, water treatment, calculations, fittings, hydronic systems, feedwater, controls, and boiler principles.

How many questions are on the 335 exam?

The current PSI bulletin lists 70 multiple-choice questions.

How much time is allowed for the exam?

The listed time limit is 2½ hours. The exam is identified as closed book.

What are the main topics on the 335 exam?

There are ten equally weighted content areas, ranging from boiler operation and safety to fuel systems, steam systems, water treatment, calculations, fittings, hydronic systems, feedwater, controls, and boiler design.

What does a D.C. Class 6 stationary boiler license cover?

D.C. licensing instructions describe Class 6 as covering steam and hot-water heating boilers with steam pressure not exceeding 15 psi and applicable boiler capacity not exceeding 200 horsepower.

Is the 335 exam open book?

No. The current PSI bulletin identifies the 335 examination as closed book.

What reference is listed for the 335 exam?

The PSI bulletin lists Low Pressure Boilers, ATP as the approved reference and states that any edition is acceptable.

What is the best way to prepare for the 335 exam?

Study all ten content areas, give particular attention to boiler safety and operating principles, practice engineering calculations, understand boiler controls and fittings, and use practice questions to reinforce your technical knowledge.

Final Thoughts

The 335 Washington DC Sixth Class Stationary Boilers exam covers a broad but clearly defined set of boiler-operation fundamentals. Since all ten content areas carry equal weight, a balanced preparation strategy is important.

Candidates should develop a solid understanding of safe boiler operation, combustion and draft, steam and hydronic systems, water treatment, controls, fittings, calculations, and boiler design principles. Combining technical study with practice questions and current D.C. licensing information can provide a stronger foundation for the examination.