Getting boiler design right

Understand factors to help decide which types of boilers are available and what is appropriate to heat a building

By Ian Marchant and Alexandria Stuart October 7, 2021
Courtesy: CDM Smith


Learning Objectives

  • Learn the selection process for determining the appropriate boiler system.
  • Understand the different components of a boiler system.
  • Review the codes and standards that govern boiler design and specification.

Boilers transfer heat to a fluid, typically through combustion. The most widely used boilers burn fuel to heat water. This heated water is piped to building occupants who then use the thermal energy. Hot water is used to heat buildings or processes.  

Boilers also can be used to create steam. Steam can be used in similar ways to hot water and can be used as an energy source to power equipment such as electric generators, pumps and chillers. Steam is also used for building humidification.  

Heat from both steam and hot water can be used in specialized processes like absorption chillers, sterilizers and distillers.  

To properly design a boiler system, it’s necessary to have an understanding of proper selection of equipment and materials as well as system function. 

Boiler selection process

To size the system, the maximum load needs to be considered. This includes the total load of all connected systems, losses, diversity, etc. Additionally, the boiler system is unlikely to be used to its full potential all the time, so the turndown of the boiler must be appropriate so that the boiler does not short cycle. 

Beyond boiler characteristics, other parameters are necessary to specify the best system. Space constraints, space purpose, redundancy requirements and other factors affect the design process.  

The best boiler system may incorporate multiple smaller boilers to handle the load instead of a larger one. Multiple smaller boilers offer several advantages over a single larger unit. The turndown can be greater because the turndown of the smallest boiler in the system dictates the turndown of the total installed capacity. Multiple smaller boilers offer better reliability over a single large unit; a failure of one smaller unit will not leave the system without heat. In systems with large seasonal changes in load, several smaller boilers can be kept offline in summer, thereby reducing standby losses.  

An important distinction in boiler type is whether it is considered commercial or residential. This leads to different code and standard requirements. According to the ASHRAE HVAC Systems and Equipment Handbook, systems can be classified as residential or commercial. A commercial boiler requires an input of 300,000 Btu/hour or greater. A residential boiler requires an input of less than 300,000 Btu/hour.  

Energy is lost in the process of the boiler transferring heat to the water. This energy loss dictates the efficiency of a boiler. Boiler efficiency can be defined in a few ways. The most common include combustion efficiency, thermal efficiency and seasonal efficiency.  

Combustion efficiency has a typical rating of 76% to 86% for noncondensing boilers and 88% to 95% for condensing boilers. This rating is determined by subtracting the stack loss from the input and dividing that value by the input as shown: 

Combustion efficiency=Energy input−Stack loss energy/Energy input


Thermal efficiency must be determined in a laboratory setting for a precise result. This value is calculated by measuring the gross energy output (by the steam or water exiting the boiler) divided by the energy input as shown: 

Thermal efficiency=Gross energy output/Energy input


The seasonal efficiency details the actual efficiency of a boiler at a variable load as shown: 

Seasonal efficiency=Seasonal energy output/Seasonal energy input


This efficiency is important because it is unlikely a boiler system would run at its maximum capacity. This rating provides a more accurate picture as to the boiler performance at a more realistic demand.  

These efficiencies are not necessarily provided by boiler manufacturers. Standard and code requirements detail which efficiency type a piece of equipment is approved or listed under by the appropriate listing agency such as the Air-Conditioning, Heating and Refrigeration Institute, or AHRI 

Figure 1: To properly design a boiler system, an understanding of proper selection of equipment and materials as well as system function is necessary. Courtesy: CDM Smith

Figure 1: To properly design a boiler system, an understanding of proper selection of equipment and materials as well as system function is necessary. Courtesy: CDM Smith

Boiler types

There are two common types of boilers: hot water and steam. Both systems need an energy source to heat water. The key difference between these two is the temperature to which they heat water.  

Hot water boilers heat the water to a setpoint below boiling temperature. This water flows through piping to the end users of the energy being supplied from the boilers.  

Steam boilers heat water to its boiling point to produce steam. The steam then flows through piping to the heat users. The steam condenses and, similar to the hot water boilers, the cooler water travels back to the boilers to begin the process over again.  

Steam has an advantage over hot water boilers because the latent heat of evaporation requires a much smaller mass flow than water to deliver an equal amount of heat. Therefore, the pumps and piping required are much smaller than a hot water system of equal heating capacity. Conversely, a leak in a steam system has much greater energy losses than from a hot water system. 

Boiler system components

A boiler system is made up of the boiler, the boiler controls, a burner and the ancillary equipment that allow the hot water to circulate and release heat into spaces.  

Boiler: Boilers are usually a cast-iron, aluminum or carbon or stainless-steel pressure vessel with heat exchangers to allow heat transfer to a fluid, which is typically water.  

Controls: To ensure efficient and safe operation, boilers have automatic control of water flow, fuel flow and air flow. With the progression of technology, traditional relays and switches have been replaced with microprocessor controls. The water control adjusts the rate at which water flows through the boiler. If a boiler’s water level were to drop below a predefined limit, overheating can occur, damaging the system or — more seriously — causing a boiler explosion. Fuel and air are controlled in tandem as they contribute to the combustion process. Safety relief valves are also an important component that act as a last resort should the other controls fail to limit combustion rate in the boiler.  

Burner: The boiler’s heating and combustion control is provided by the burner. The burner mixes air with the fuel to create an ideal mixture for combustion. This mixture is then ignited to create heat. The important components of the boiler are the fuel and the ignition source. Boilers typically derive the energy needed to provide heat with fossil fuels. The most common sources are natural gas, No. 2 fuel oil and biogas. It is also important to note that electric boilers do exist, as such, they do not have a burner and rely on electric heat to increase water temperature.  

The flame from the burner is directed into the boiler’s combustion chamber. This is where the combustion reaction occurs, creating heat. The main heat transfer process occurs in this section. In this chamber the flame heats the water through radiation directly from the flame through the heat transfer surface and into the circulating water. 

Post-combustion, the byproducts of this process then travel to the heat exchanger. Heat transfer occurs through convection and conduction in heat exchange channels as the hot combustion products, and the remainder of air that is unused in the combustion process passes through. The heat exchanger area is often configured with multiple passes to maximize the efficiency of the heat transfer and to minimize the footprint of the boiler. 

Ancillary equipment: For a boiler to provide heat to spaces, more equipment is required than the boiler itself. The full system might include expansion tanks, piping, circulating pumps and the heat -using equipment such as heating coils, radiators or heat exchangers. 

Expansion tanks are required as part of a hot water boiler system to prevent excess water pressure due to thermal expansion from the increase in temperature. These expansion tanks are selected based on temperature, pressure and volume of the heating system. The two main types are compression and bladder tanks. Compression tanks typically are mounted above the boiler while bladder tanks often sit on the floor. 

Piping material and size are in accordance with local building codes and manufacturer recommendations. Appropriate sizing is paramount to provide an efficient system. Piping systems are insulated to prevent heat loss.  

Circulating pumps allow the water to flow to and from the boiler. These pumps are selected based on temperature of the fluid as well as head loss of the system. These pumps must be selected appropriately to allow for the most efficient system. 

For space heating, radiators allow the heat to transfer from the boiler system into a space. Radiator size and configuration are determined based on the space needs and heat load calculations. The radiator capacities are the driving factor in the sizing of the boiler systems. Radiators come in different forms including baseboard, ceiling, wall and others.  

Historically, steam radiators were used because the heavy cast iron could store a large amount of heat and allow cycling of the steam system to control space temperature. These units typically caused high temperature swings in the space. Newer and more modern high-efficiency radiators are low mass and use low-temperature water and electronic controls to operate at a temperature required to keep a constant temperature in the space. The lower temperatures required allow for high-efficiency boilers to be used. Nonboiler heat sources such as heat pumps or waste energy sources also can be considered. 

Valves, pressure gauges, temperature gauges and other devices allow for the control, monitoring and troubleshooting required to properly operate the boiler system. Water to supply and provide makeup water to the boiler system is necessary. The water source is most often required to be potable per the local plumbing codes. 

Figure 2: To ensure efficient and safe operation, boilers have automatic control of water flow, fuel flow and air flow. Courtesy: CDM Smith

Figure 2: To ensure efficient and safe operation, boilers have automatic control of water flow, fuel flow and air flow. Courtesy: CDM Smith

Boiler codes, standards

Boiler codes and standards can be broken into multiple subcategories including jurisdictional codes, safety codes, performance codes and standards and other important considerations. 

Requirements for clearances around boilers allow for proper operation, service and inspection. These can be outlined in local building codes. When they are not, defer to the boiler manufacturer’s requirements. Frequently these conditions are not defined in building codes but instead governed by other agencies. State labor departments are often the agency that publishes and enforces these additional rules.  

The America Society of Mechanical Engineers maintains a code that addresses boiler safety concerns. ASME’s Boiler and Pressure Vessel Code is divided into sections and divisions to form comprehensive guidelines. There are multiple sections that cover construction of power boilers, material specifications, welding specifications and recommended rules for the care and operation of heating boilers. It also has multiple sections that provide alternative rules for construction and other information. ASME’s BPVC certification program confirms that a company conforms to the rules governing the design, fabrication, assembly and inspection of boiler and pressure vessel components during construction.  

NFPA 85: Boiler and Combustion Systems Hazard Code is considered the governing code for boiler systems. This code is only applicable to boilers that are larger than 12.5 MMBtu/hour and provides guidance for safe operation and preventing explosions and implosions. 

ASME CSD-1: Controls and Safety Devices for Automatically Fired Boilers covers requirements of automatically operated boilers that are directly fired with gas, oil, gas-oil hybrid or electricity, having fuel input ratings less than 12,500 MMBtu/hour and includes the assembly, installation, maintenance and operation of controls and safety devices. 

UL 834: Standard for Heating, Water Supply and Power Boilers – Electric for packaged electric boilers details electric heating, water supply and power boilers rated at 15,000 volts or less. It is intended for commercial, industrial or residential use space heating applications using hot water or steam.  

UL 2523: Standard for Solid Fuel-Fired Hydronic Heating Appliances, Water Heaters and Boilers provides requirements that apply to factory-built manually and/or automatically fueled solid fuel-fired hydronic heating appliances, water heaters and boilers. The appliances are intended to burn solid fuels, such as wood, coal or any other biomass fuel.  

Performance codes, standards

Commercial boiler performance is tested according to standards developed by three different agencies. These agencies include the Air-Conditioning, Heating and Refrigeration InstituteUL and the American Gas Association. These tests provide the full-load steady-state efficiency of commercial boilers. This means comparing the heat output of the boiler from combustion when it is operating under a full load to the energy input. AHRI 1500: Performance Rating of Commercial Space Heating Boilers provides testing requirements for the thermal efficiency and combustion efficiency of gas and oil-fired steam and hot water heating boilers with inputs ranging from 300 to 2,500 MBtu/hour. 

Residential boiler performance is rated according to standards by the U.S. Department of Energy. The laboratory tests on- and off-cycle losses, which are then applied to a computer simulation that will predict an annual fuel utilization efficiency. The AFUE is a percentage that predicts the portion of energy from the boiler’s fuel source that is converted to useful heat. As of the 2021 regulations, gas-fired water boilers must achieve 84% AFUE. Gas-fired steam boilers must achieve 82% AFUE. Oil-fired water boilers must achieve 86% AFUE. Oil-fired steam boilers must achieve 85% AFUE. 

ASME Fired Steam Generators Performance Test Code 4 provides information on commercial-industrial and packaged fire-tube boilers test procedures. It provides significant performance parameters and efficiency calculating and testing via the energy balance method. 

Other factors

The requirements for wiring external devices that not provided with the manufacturer-provided controls are often overlooked. One is the requirement for interlocking the boilers with the source of combustion air. The intake dampers or combustion air fans are not integral components of the controls often supplied by the burner manufacturer. Another external control element is the remote shutdown switch. These elements external to the burner/boiler controls require the specifying engineer to include these interface requirements in the design documents. 

Determining the correct method of providing heat to the system depends on a variety of factors. Further, determining which boiler type is most advantageous for its application is critical to providing a solid engineered design. Boiler controllers and ancillary equipment factor into the heating system as a whole. Numerous codes and standards govern a boiler system and following the applicable ones are essential for the heating system designer or engineer. 

Author Bio: Ian Marchant is a senior mechanical engineer at CDM Smith. Alexandria Stuart is a mechanical engineer at CDM Smith.