Back to basics: Boilers and hot water systems

Courtesy: WSP USA Buildings

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Learning Objectives

• Know and understand the applicable codes and standards for boilers.
• Learn about water distribution, pumping options and hot water system accessories.
• Understand boiler applications for comfort heating and improving boiler efficiency.

Boiler insights

• Boilers, frequently used in commercial buildings, can heat both air and fluids.
• The International Mechanical Code identifies design and specification requirements for boilers and their use in hot water systems.

Boilers are robust pieces of equipment with integral heat exchangers capable of increasing the temperature of a fluid to meet a building load or a process. A common temperature difference specified in comfort heating system design is 30°F or 20°F between the entering water temperature and leaving water temperature serving the building. Boilers are energized by a fuel, which is either natural gas, diesel or propane.

Through a combustion process, outside air is introduced into the boiler burner, generating energy released from the fuel to the fluid. These machines are designed to meet the standards set forth by the American Society of Mechanical Engineers Boiler and Pressure Vessel Code.

The first boiler and pressure vessel code was published in XXXXXX and recognized internationally for its comprehensive nature. It governs not just heating applications but also power generation and nuclear energy among others.

The BPVC is extensive with more than 10 sections. Section I gives guidelines for the construction of power boilers, while Section VII outlines the care and maintenance required for the equipment. Section VIII is specifically for pressure vessels operating at pressures above 15 psig.

Heating boilers fall under Section IV and encompass steam generation boilers, potable water heaters, and applications associated with hot water systems. This section establishes requirements for installation and inspection, especially for low-pressure applications. Heating boilers for steam generation are referenced in low-pressure contexts for those under 15 psig and for hot water boilers under 160 psig, and can be fired by natural gas, fuel oil, electricity, or coal. Furthermore, Section VI provides operational guidelines, including controls for heating water boilers.

Beyond the BPVC, engineers also rely on other codes and standards when specifying boilers for comfort heating. The International Code Council publishes several codes, including the International Building Code, International Mechanical Code, International Plumbing Code, and International Fuel Gas Code, which serve as minimum criteria for safe and functional designs.

Boilers are commonly found in heating, ventilation, and air conditioning systems, primarily used for comfort heating. Water heaters serve domestic systems for potable water, with specific guidelines delineated in the IPC.

The IMC edition is thorough, comprising 15 chapters and two appendices. Chapter 7 outlines combustion air guidelines, recommending options such as two openings—one high and one low or a single large opening. The size of the opening is calculated by the total fuel-burning equipment within the mechanical room, adhering to a minimum of 1 square inch per 1,000 Btu/hour or 3,000 Btu/hour depending on the selected design.

Chapter 10 provides essential guidelines for boilers, water heaters, and pressure vessels. Here are some highlights:

• Boilers must comply with the ASME BPVC.
• Safety controls must follow ASME-CSD-1, ensuring standards for fuel-fired rated appliances. Under the IMC, there are two ranges; they span less than 12.5 million Btu/hour input rating culminating in ASME BPVC, while those exceeding this threshold fall under NFPA 85: Boiler and Combustion Systems Hazards Code.
• Installation must align with the manufacturer’s recommendations.
• Clearances are critical. Designers must ensure adequate working space is available around the equipment for maintenance and removal. Minimum clearance is set at 18 inches, though this may increase based on the boiler type and size.

Steam boilers necessitate greater overhead clearance, especially in retrofit scenarios where existing floor-to-floor space may be limited.

Heating water boilers must reside within their own fire-rated room enclosures as mandated by the IMC, IBC, and NFPA.

Boiler connections:

• Each boiler must have a makeup water supply and shut-off valve at both the return and supply pipe. A low water cutoff is vital for turning off the combustion process during low water levels.

Safety and pressure relief valves and controls:

• Boilers must include safety valves and relief valves rated for the system it serves, typically spring-loaded to relieve pressure at setpoints.

Boiler water distribution

Historically, piping systems can be classified as one-pipe, two-pipe, or four-pipe systems. One-pipe systems are typically used in dedicated loops for heating, which can increase head pressure leading to higher pumping costs.

A single-pipe heating system example is perimeter radiation, commonly utilized in colder climates, referred to as monoflow. The two-pipe system alternates supply and return piping for heating and cooling, while a four-pipe system has individual loops for both chilled and hot water.

Choosing the correct pipe size is essential as water flow must overcome friction losses; thus, flow velocity is crucial. The Reynolds number helps identify whether the flow inside the pipe is laminar or turbulent.

The engineer must consider maintaining design velocity recommendations from standards like ASHRAE Standard 90.1, which offers guidelines on flow rates and maximum velocities.

Pumping options

Typical hot water central plants employ two primary pumping designs. A variable primary system provides lower operating costs and centralized maintenance.

The secondary design is commonly used when the plant's distance from the building increases, requiring consideration for cost-effectiveness and a return on investment.

Hot water system accessories

Closed-loop heating water systems incorporate accessories such as expansion tanks, air separators, pressure gauges, and temperature sensors, which enhance performance and ensure safety as the heated fluid expands.

Diaphragm and bladder-type expansion tanks serve to accommodate this volume increase. Air separators prevent air accumulation within the system, which is essential for protecting the major equipment and optimizing performance.

Boiler load

There are two primary boiler types in comfort heating: condensing and non-condensing. Condensing boilers are favored for their higher efficiency at lower return water temperatures, allowing condensation of flue gases.

Non-condensing boilers operate at higher temperature ranges, necessitating careful temperature design to avoid damage to components. Load calculations are crucial in sizing equipment, based on various building factors.

Understanding these calculations is vital for boiler plant design, ensuring comfort and operational efficiency.

To summarize, knowledge of the codes and standards governing boiler operations is essential. Condensing boilers with lower return temperatures offer multiple benefits, but their design must suit the building's operational goals and physical layout.

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