Determining which boiler system to design
- Understand the difference between condensing and noncondensing hot water boilers.
- Learn about electrical alternatives for producing hot water.
- Recognize the code sections applicable to boiler system designs.
There are many advantages to using boilers as a primary component of a central energy plant. Boilers are pressure vessels in which fluids, most often water, are heated. The term boiler also includes vessels in which the fluid does not necessarily boil or vaporize.
Boilers are widely accepted for many applications and are an established heating source for generating heating hot water, domestic hot water and steam, particularly in colder climates or large facilities. When designing central heating plants, there are several types of boilers and applicable codes to consider.
While there are many different configurations of boilers, three primary types widely used for environmental and process systems include steam, noncondensing hot water and condensing hot water. In the past, steam boilers were very common, but are routinely being minimized or eliminated in current design practice due to their limited efficiency, specialized operation and maintenance requirements and high–pressure steam systems add safety concerns.
The primary advantage of steam is the ability to distribute a greater quantity of heat in a smaller volume, via the latent heat of vaporization and superheat. One of the downsides is the piping is pressurized and requires condensate return as typically enough heat is lost in distribution to cause a phase change from vapor to liquid. Most steam systems currently exist on large campuses or in health care facilities. However, even health care facilities have started to move to point of use steam generators for their sterilizers, cooking and humidification, while using hot water boilers for the heating and domestic hot water (also known as service water heating, or SWH).
Hot water boilers don’t sufficiently heat the water to vaporize it at their operating pressure, so they don’t have the advantage of storing the additional latent heat of vaporization in the fluid like steam boilers. Noncondensing boilers are typically designed with hotter return water temperatures to avoid condensing in their heat exchangers because they aren’t rust resistant.
Condensing boilers have nearly become the standard practice boiler for most designs because of their increased efficiency. They use cold return water to condense water out of the combustion products to recover some of the lost heat from the exhaust flue. Because they are designed for condensing, their heat exchangers are stainless steel. The key is to ensure the return heating hot water temperature is below the dewpoint of the combustion products, which is about 125 F for natural gas.
As the return heating hot water temperature gets further below the combustion dewpoint temperature(125 F), more water is condensed and the efficiency approaches 100%. The typical operational efficiency for condensing boilers is about 95%. When compared to the typical thermal efficiency of noncondensing hot water boilers at 80%, the increase in performance is substantial. Heating coils in buildings do have to be sized for a higher delta T in order to facilitate the colder return water temperature, which can increase the cost of the coils but decreases pumping energy.
While not a boiler in the historic context of steam, the use of heat recovery or heat pump chillers to create heating hot water is also becoming more common, especially when looking toward a carbon–free future. Heat recovery chillers can produce 140 F heating hot water while reducing the load on the chilled water loop, so they typically operate with a heating performance of three to four coefficient of performance.
If the benefit of both the heating hot water and chilled water is accounted for, the coefficient of performance can increase to as high as seven. They also have the advantage of not requiring on-site combustion as they use electricity for operation. This is an advantage for projects looking to reduce Scope 1 carbon emissions or pursuing certain third-party certifications, such as Living Building Challenge.
Codes and standards
There are several codes governing the design of boiler systems. In the building industry the most relevant are the International Mechanical Code, International Energy Conservation Code and ASHRAE Standard 90.1: Energy Efficiency for Buildings Except Low-Rise Residential Buildings.
Chapter 10 of the 2018 IMC describes the design requirements for boilers, water heaters, expansion tanks and other pressure vessels. Section 1004 states that “boilers must be designed, constructed and certified in accordance with the American Society of Mechanical Engineers Boiler and Pressure Vessel Code, Section I or IV.” There also are requirements for clearances and valves based on the pressure and capacity of the boiler. The requirements of this code are to ensure the design of safe boiler systems.
The IECC and ASHRAE 90.1 both govern the energy–efficiency requirements of heating hot water and domestic hot water boiler systems. The standards have several similar requirements, but some that are different, so it’s important to know which you are designing around.
The 2018 IECC and ASHRAE 90.1-2016 have similar efficiency requirements with respect to boilers. The pertinent sections and their equivalent in the other standard are shown in Table 1. Except for some slight changes in wording, the requirements are the same in both standards, including the specific minimum efficiencies and turn down requirements. It is important to understand that the service water heating section in both ASHRAE 90.1-2016 (section 7) and IECC-2018 (C404) are mandatory.
There is a change underway for how we design heating hot water systems. The historic default has been to use combustion to produce steam or hot water that can be circulated through buildings. Several utilities have recently put moratoriums on new gas services, such as Consolidated Edison in Westchester County, New York, and National Grid in Long Island, New York, which means new facilities will have to produce their heating hot water and domestic hot water by other means.
Also, the New York city council recently based the Climate Mobilization Act, which sets carbon emissions limits for covered buildings. There are multiple reasons for this change. Some areas are planning for a fully electrified, carbon–free future. Others are just responding to supply and demand. Other countries are making the change as well. The United Kingdom and Netherlands are reducing their gas production and consumption in response to aggressive climate goals. As engineers, we need to be ready with innovative solutions to provide efficiently generated heating hot water and domestic hot water for our clients’ facilities.