Back to basics: Boilers and hot water systems

Learn about hot water systems, boilers and their applications in commercial buildings

By Paulina Diaz September 22, 2022
Courtesy: WSP USA Buildings


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 combustion process that introduces outside air into the boiler burner energy is generated and released from the fuel to the fluid. Boilers are designed and built to meet the standards of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code.

The first boiler and pressure vessel code was issued in 1914 and published in 1915. It is now considered the most comprehensive standard and guideline regarding the construction and operation of boilers. The BPVC applies to boilers not only for the use in heating applications for human comfort, but also for power generation and nuclear energy, among others uses.

The BPVC is comprehensive in nature with more than 10 sections. Section I provides guidelines regarding the construction of power boilers, while Section VII provides details for the care and maintenance of these pieces of equipment. Section VIII is dedicated to pressure vessels operating at pressures higher than 15 pounds per square inch (psig).

Heating boilers fall under Section IV and include boilers for steam generation, potable water heaters and hot water system applications. This section provides requirements for installation and inspection, particularly for low-pressure applications. Heating boilers for steam generation fall under two pressure ranges. Low-pressure applications for steam boilers are considered for those less than 15 psig and for hot water boilers less than 160 psig. These boilers can be fired by natural gas, fuel oil, electricity or coal. Furthermore, Section VI provides guidelines for care and operation to include controls for heating water boilers.

In addition to the BPVC, there are other codes and standards that design engineers use when specifying boilers for comfort heating. Under the International Code Council several codes are provided such as the International Building Code, the International Mechanical Code, the International Plumbing Code and International Fuel Gas Code. These codes give the design engineer the minimum criteria to implement in the design of a functional and safe system.

Boilers are commonly used in heating, ventilation and air conditioning systems, typically for use in comfort heating. Water heaters are referred to equipment serving domestic systems for potable water; specific guidelines for water heaters are indicated in the IPC.

Figure 1: Detail indicates a heating water boiler for small type application. The goal is to show what is discussed in the article as safety valve and low-water cutoff. Courtesy: WSP USA Buildings

The 2018 edition of the IMC is comprehensive and includes 15 chapters and two appendices. Chapter 7 provides guidelines for combustion air. The most common options are providing two openings, one high and one low or one single large opening. The opening size is calculated by adding all the fuel burning equipment inside the mechanical room using a minimum of 1 square inch per 1,000 British thermal units per hour or 3,000 Btu/hour, depending on the option selected for the design. These specific methods are detailed under the IFGC.

Chapter 10 of the IMC provides guidelines for boilers, water heaters and pressure vessels. There are a few key items worth noting within this chapter:

Section 1004 – Boilers:

  • As discussed earlier, boilers must comply with the ASME BPVC.
  • Safety and controls required to design fuel fired rated appliances must follow ASME-CSD-1, which is the ASME standard for Controls and Safety Devices for Automatically Fired Boilers. Under the IMC, boiler governing standard fall into two ranges, the first range is less than 12.5 million Btu/hour input rating, which follows the ASME BPVC under Section I or IV. A boiler exceeding that value falls under NFPA 85: Boiler and Combustion Systems Hazards Code. This includes safeties to prevent explosions in boilers with this large input.
  • Installation of the boiler must comply with the manufacturer’s recommendations.
  • Clearances are important. The design engineer must always keep in mind the available working space around the boilers; in addition to the available space to remove and replace it. Under this section, the code provides the minimum dimensions for accessibility and equipment maintenance. Clearances are indicated to be a minimum of 18 inches around the equipment; however, this may exceed 18 inches, depending on the boiler type, size and application. A few items to keep in mind when coordinating and designing boiler rooms.
    • The engineer should validate the real estate required for such large pieces of equipment. Due to cost optimization, these rooms are designed to the necessary space to service & remove the equipment.
    • Engineers should account for changes to the equipment specified as basis of design versus awarded as dimensions vary from vendor to vendor.
    • Today, three-dimensional modeling is a vital tool used by design engineers to ensure the appropriate clearances for maintenance of equipment are allocated.
  • Steam boilers require additional overhead clearance, in particular this must be taken into consideration in retrofit spaces as the existing floor-to-floor structure space may be limited.
    • Under subsection 1004.3.1, the IMC provides a table 1004.3.1 indicating boiler top clearances; the reader should review when designing a new boiler room.
  • Heating water boilers must be in their own room enclosure and the walls are to be fire rated per the IMC, IBC and NFPA. These code sections were reviewed in my previous article for heating hot water systems.

Section 1005 – Boiler connections:

  • All boilers must be provided with a makeup water supply and a shut-off valve at both the return and the supply pipe serving it. In addition, a low water cutoff must also be provided as these are responsible for turning off the combustion process in the event of low water levels. These cutoffs are automatic. This is indicated under section 1007 (see Figure 1).

Section 1006 – Safety and pressure relief valves and controls:

  • Specific criteria for both hot water boiler and steam boilers is provided. Boilers are to be provided with safety valves and safety relief valves, with a rating capacity for the system it serves. Safety valves are often spring-loaded. This is to allow the valve to relieve in the event the system experiences a pressure over setpoint.

Boiler water distribution

Taking a step back in history, piping systems can be one-pipe, two-pipe or four-pipe. First, single-pipe systems are implemented only for heating systems on a dedicated loop, which ties back the return line downstream to the equipment. There are a few disadvantages, given that the equipment is tied in series, which causes an increase in head pressure and therefore makes the pumping system larger. Additionally, temperature loss throughout the run. This directly impacts overall operational costs.

An example of single-pipe heating system is perimeter radiation, mostly specified in the colder weather areas, these systems are known as monoflow. Second, two-pipe systems are designed to provide cooling and heating using supply and return piping alternately for heating and cooling. This forces the system to have a changeover depending on the season.

Last is a commonly specified four-pipe system. This system has a dedicated loop for chilled water and another for hot water. The operational cost may vary and it is assessed on a design basis to suit the facility needs.

Pipe sizing is key for the distribution loop. Water moving inside pipes must overcome friction losses; therefore, the velocity at which the flow is traveling is important. One key concept the engineer must understand is the Reynolds number  This number helps us identify if the flow is laminar or turbulent inside the pipe, its definition includes the density of the fluid (, velocity of the fluid (, characteristic linear dimension ( and dynamic viscosity if the fluid (.

The formula is expresses as . Typically, lower Reynolds numbers indicate laminar flow while higher indicate turbulent. The engineer uses charts such as the Moody Diagram, with the known Reynolds number and pipe roughness the flow type may be determined. Furthermore, standards and code provide minimum guidelines for recommended velocities for specific systems. One example is found in ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings. Chapter 6, Table provides the recommended flow rates and maximum velocities.

Figure 2: Heating water system expansion tank detail with safeties. Detail is general to provide guideline as supporting document to the hot water heating system article. Courtesy: WSP USA Buildings

Pumping options

There are two common pumping options for designing a hot water central plant. First, a system designed with primary pumps serving the building, commonly referred as variable primary. Common advantages are lower operating cost and centralized maintenance.

The second is a primary/secondary system, often used when the central plant is located farther away from the building it serves or serving multiple buildings with different requirements. The design engineer must consider and evaluate the return on investment to provide the most cost-effective design for the owner. Typically, owners prefer to see less than five- or seven-year return on investment for decision-making purposes.

Hot water system accessories

Closed loop systems, in this case heating water systems, are provided with accessories that assist the system performance such as expansion tanks, air separators, pressure gauges and temperature sensors. When heated, fluid increases its volume depending on the system temperature and pressure and these accessories provide safeties.

There are several expansion tank types, the most common are bladder and diaphragm. Both tanks are closed type. Bladder type expansion tanks have a bladder inside to hold the water, this bladder expands as the system temperature increases, both air and water are self-contained inside the tank.

Diaphragm type tanks have a diaphragm inside located about midrange. The lower portion holds the water and the upper portion the compressed air. The diaphragm oscillates like a sine wave when the water expands. The expansion tank selection will depend on the specific design. These tanks are recommended to be installed on the return as to avoid pumping through these (see Figure 2).

Air separators are also a common accessory in closed systems. An air separator ensures the system does not accumulate air. Typically, at system startup, air may be entrapped within the system, when the system is first filled the circulation of the flow will ensure air leaves the system. This is key to protect the major pieces of equipment and the system performance (see Figure 3).

Other important accessories are the pressure gauges, Pete’s plugs and thermometers, which are always specified at each piece of equipment within the system.

Temperature sensor and pressure gauges are required at connections to equipment and cooling or heating coils.

Figure 3: Heating water system air separator detail. Detail is general to provide a guideline as supporting document to the hot water heating system article. Courtesy: WSP USA Buildings

Boiler load

Two types of boilers commonly specified in comfort heating water systems: condensing type and noncondensing. Condensing type boilers have become more popular due to their higher efficiency at lower return water temperatures. These boilers allow condensation of the flue gases releasing latent heat of vaporization. The condensate is then channeled to the drain passing through a neutralization kit  This is required to protect floor drains as this water is very corrosive if not pretreated.

Noncondensing type boilers do not allow condensation within the flue gases for this reason design engineers need to specify design temperatures that align with the boiler type specified. Otherwise, the cast iron steel and copper components will damage with time reducing the life expectancy of the boiler. Noncondensing boilers work at higher temperature ranges such as 180°F to 150°F degrees and are much larger in footprint than condensing type.

Boilers are specified to provide hot water for comfort heating, domestic heating or process heating. For comfort heating, a design engineer’s first step is to perform a building load calculation to size the equipment. A building load is dependent on many factors such as building function, location, orientation, envelope, percent and type of glazing and space layout among others.

Loads can be calculated using software. The No. 1 goal is to obtain total peak load expressed in British thermal units per hour and gallons per minute (gpm) required to maintain occupant comfort. Using the peak conditions, the engineer designs a system with a specified temperature difference.

The temperature difference is important as it directly impacts the system heating hot water flow rate as seen in the following theoretical example: A 50,000-square-foot medical office building with a calculated total peak load of 1.5 million Btu/hour and a design of 30°F temperature difference will yield 100 gpm flow rate. The latter number is obtained from solving for gallons per minute in the formula below with the specified temperature difference.

Keep in mind that if the design specifies a larger temperature difference, the resulting gpm is lower. This translates in savings on pump energy, as the horsepower of the pump is smaller. However, the engineer must weigh the advantages and disadvantages on a design basis.

Understanding the formula and the units, assuming water as the fluid in question. For further detail, see ANSI/ASHRAE Handbook — Fundamentals.

Another important concept in heating systems is the sensible load, which is calculated using the following equation:

The constant is a result of air density (pounds mass/cubic feet) its specific volume (cubic feet/pounds mass) at standard conditions. In the psychrometric chart a sensible load indicates moving in a straight line horizontally from point A to point B. In heating system design, two stages are key: preheating, which is related to pretreating the outside air and reheating, which is heating at the local level.

A good example is at the variable air volume box. These design parameters are important as these are considered to size the boiler plant. Therefore, not only the building envelope load but the process as well. Furthermore, due to condensing boilers benefiting from lower return water temperature, the VAV reheat coil design is key. Lower temperature heat has a lower energy; therefore, more surface area is needed to achieve the same heat transfer. VAV systems with design temperatures lower than 130°F should consider three-row coils to achieve the leaving air temperature.

In summary, boilers and their applications are vast, key reason for the engineer to have a thorough understanding of the applicable codes and standards. Moreover, it is known in our industry that condensing boilers benefit from lower return water temperatures. However, design parameters need to suit the goal and purpose of the building system or process as lower temperatures have other impacts throughout the system such as three-row coils for VAV, as an example. It is essential to take the time to brainstorm the design parameters before jumping into the drawing board.

Author Bio: Paulina Diaz is a senior associate at WSP USA Buildings. She is a mechanical engineer with more than 10 years of experience and is focused on health care design.