The Essential Guide to Industrial Hot Water Boilers

There are many types of industrial hot water boiler systems, including high temperature hot water boilers, medium temperature hot water boilers and low temperature hot water boilers. As you can imagine, the primary difference between these types of hot water boiler systems is the maximum temperature. The pressure also plays a significant role in the classification of these boiler systems. For instance, a high temperature hot water boiler includes any boiler with a maximum temperature exceeding 250°F and/or maximum pressures exceeding 160 pound per square inch, or PSIG.

We will explore the various types of hot water boiler systems in this article. However, we want to first take a step back and explain the role of the American Society of Mechanical Engineers (ASME) in classifying boilers as well as clarify the difference between industrial steam boilers and hot water boiler systems.

The Importance of ASME and Boiler Pressure Vessel Code

The American Society of Mechanical Engineers (ASME) provides specific guidelines for the different classifications of industrial boilers. Every two years, ASME releases a set of books called the Boiler and Pressure Vessel Code (BPVC). The BPVC sets the standard for the classification, design, manufacturing and certification of boilers and related equipment. Essentially, the ASME is the governing body over industrial boilers and the BPVC is the material that is used by professionals in the boiler industry to make sure you are up to code.

The Difference Between Steam Boilers and Hot Water Boiler Systems

Most people in the boiler industry are familiar with steam boilers, yet don’t know much about hot water boilers. We’re here to clear any confusion. Steam boilers are classified as either Section 1 (> 15 PSIG) or Section IV (≤ 15 PSIG) depending upon their Maximum Allowable Working Pressure (MAWP) design. Meanwhile, an ASME Section I Power Boiler may also be designed as a hot water boiler where the maximum temperature is above 250°F and/or the MAWP is above 160 PSIG. Additionally, an ASME Section IV Heating Boiler may also be designed as a hot water boiler where the maximum temperature does not exceed 250°F and the MAWP does not exceed 160 PSIG.

Understanding the classification of steam and hot water boilers must be accounted for when you are looking to purchase a boiler. In some cases, a boiler may be built for service as a both a hot water or steam boiler, in which case, the overall classification would be determined by the desired operation of either configuration. For instance, if one configuration requires the design to be in accordance with ASME Section 1, then the overall design would need to be classified as an ASME Section 1.

Lastly, steam boilers and hot water boiler systems have different applications. A steam boiler is more suitable for sterilization, humidification, direct injection and heat exchangers. Meanwhile, a HTHW boiler is more suitable for district and campus heating.

Industrial Hot Water Boilers

Types of Hot Water Boiler Systems

Now that we know the difference between industrial steam boilers and hot water boiler systems, let’s take a deeper dive into the types of hot water boiler systems. As mentioned, the main difference between the hot water boiler systems is the temperature. Hence, the names provide some insight into the temperature associated with the system. In this section, we’ll explain three types of hot water boiler systems: high temperature hot water, medium temperature hot water and low temperature hot water boilers. We’ll provide the definition of these boiler systems from ASME as well as what it typically looks like in application.

High Temperature Hot Water Boilers (HTHW)

According to ASME, a High Temperature Hot Water Boiler is an ASME Section I Power Boiler and includes any boiler with a maximum temperature exceeding 250°F and/or maximum pressures exceeding 160 PSIG. In application, HTHW systems refer to those designs where the temperatures exceed 350°F. Typically, a HTHW system operates with a maximum operating pressure of less than 300 PSIG. These systems are ideal for larger systems, like district and campus heating due to the large heat loads, extensive networks of piping, and the overall size of the facilities. Large process applications are also an ideal fit for high temperature hot water boiler systems due to the high temperature requirement which cannot be achieved by the low and medium temperature systems.

Medium Temperature Hot Water Boilers (MTHW)

A Medium Temperature Hot Water Boiler refers to a boiler with a temperature, ranging from 250°F to 350°F, with a maximum operating pressure of 150 PSIG. This means that a MTHW system may require an ASME Section I boiler for some designs and an ASME Section IV boiler for others. Each particular system must be compared to the ASME BPVC to ensure the boiler for that system is designed in accordance with the applicable section. Systems that use a medium temperature hot water boiler are district and campus energy loops, apartment and hotel complexes, and small processes that require the medium range temperatures.

Low Temperature Hot Water Boilers (LTHW)

According to ASME Section IV, a heating boiler includes any boiler with a maximum temperature below 250°F and a maximum pressure below 160 PSIG. In application, LTHW systems refer to those designs where the temperatures are below 250°F. Typically, a LTHW system operates with a maximum operating pressure of less than 30 PSIG. This means that a LTHW system is normally going to use a boiler built to ASME Section IV code. These boilers are typically used in smaller buildings and even residential homes. Most homes utilize a forced air system for heating and cooling, but coils may be placed in the air handler, utilizing the hot water to provide the heating during the colder months. Other homes and buildings that incorporate a low temperature hot water boiler utilize radiators to distribute the heat from the water to condition the various occupancy and living spaces.

Advantages of High Temperature Hot Water Boilers

There are several advantages of hot water boiler systems, particularly HTHW boilers, compared to steam boilers. Some examples include low costs and higher efficiency, which can be a huge benefit to your operation. After all, we’re all looking to increase boiler efficiency while simultaneously minimizing costs.

Lower Initial Equipment Costs

A HTHW boiler typically comes with fewer boiler room components. Since there are less boiler parts associated with this boiler system, your initial investment of the equipment is lower.

Lower Operating Costs

We know that ongoing operating costs can really add up, and HTHW boiler eliminate some of the expenses that steam boilers generate. For example, HTHW eliminates typical losses associated with steam boilers such as steam traps, steam leaks, vents, blowdowns, etc and is less affected by transmission losses as a result of temperature and pressure drops.

Higher System Efficiency

High load spikes are often absorbed by the large heat storage capacity of a HTHW system. As a result, the burner is operated more evenly and efficiently. Overall, the system is able to accommodate small load swings without increasing the burner firing rate which lowers the total energy consumption of the system.

Elevation Changes Don't Affect System

Elevation is a common design consideration for steam systems. However, HTHW boilers use circulation pumps, which overcome total system losses, meaning elevation doesn’t really matter because what goes up must come down.

High Temperature Hot Water Boilers Considerations

There are a few considerations that you should factor into your decision-making process for a high temperature hot water (HTHW) boiler system. We’ve provided a few criteria for both existing and new systems.

  • Existing Systems: Steam systems and HTHW operate completely differently. It is not a simple task to convert a facility that has been running a steam loop to then run on a HTHW loop. A complete redesign would be required, so it is necessary to have critical information detailing your current boiler efficiency and finances.
  • New Systems: If you are moving towards a HTHW boiler system, you should ask think about the design, distribution type and usage. We’ve highlighted a few questions for you.
    • Design: What temperature and pressure are necessary? What is the cost optimization between higher temperature and pressure in relation to the cost of the equipment?
    • Distribution Type: How will circulation be controlled through the boiler and distribution system to reach the end use components?
    • Usage: What will be the primary use?
Powerhouse Hot Water Boiler System

Powerhouse and Our High Temperature Hot Water Boilers

Most new boilers being purchased will be built for a specific operation at the customer’s facility. As we design a new boiler system for your operations, we will make sure to make the best decision for your specific needs.

However, for our boiler rentals, we design for every scenario because we never know where our boilers will be rented. That’s why our rental boilers are designed with both steam and HTHW capabilities. Our RL-50 units utilize an ASME Section IV boiler that has been designed to run at ≤ 15 PSIG steam and ≤ 100 PSIG hot water. Our RH units utilize an ASME Section I Power Boiler, with a MAWP of 150 - 250 PSIG depending on the unit, and are capable of running both high pressure steam and high temperature hot water. Then, we have WT units which are built to ASME Section I, with a MAWP of 399 PSIG, and are capable of running both high pressure steam and high temperature hot water.

Have a question?

Contact Us

Related Boiler Education Pages

Trailer Mounted Boiler

Need a boiler fast? The trailer mounted boiler systems offer the most economical option to get a rental boiler at your facility in the fastest way possible.

READ MORE →

Portable Boiler Room

When a shutdown (emergency or planned) of your plant includes all the components to operate your boiler the only solution is a completely furnished Portable Boiler Room.

READ MORE →

Still have questions about boilers? We'd love to help!