Boilers
Boilers transfer heat from the combustion of a fuel or electric resistance to a fluid. They are used in residential, commercial, institutional, industrial, and transportation applications. The most common fuel for buildings is natural gas, and the fluid is typically water or steam. Boilers are a well-established technology, and from a modeling perspective, the most important aspects are usually related to the boiler's efficiency at design and off-design conditions and the boiler’s use of electricity for parasitics like the draft fan.
How It Works
Boilers come in many different configurations, and it is important to be conversant with the various classifications when interacting with the mechanical design team. The classification of boilers is related to the following:
- Working pressure and temperature
- Fuel used
- Construction materials
- Type of draft
- Condensing or non-condensing
Low-pressure steam boilers operate at pressures no more than 15 psig, and low-pressure hot water boilers operate at pressures no more than 160 psig and 250 F supply water temperature. This is the most common pressure and temperature range for boilers in commercial and residential applications. Medium-pressure steam boilers operate from 15 to 160 psig, and high-pressure steam boilers operate above that. The medium temperature supplies water from 250F to 350F for hot water boilers, and the high temperature is above that.
Boilers are typically designed for specific fuels, but the types of fuels can vary widely. Natural gas is the most common for residential and commercial applications, but electricity, propane, fuel oil, waste, oil, wood, and coal can also be fuels. Dual-fuel boilers can operate on two different fuels, most typically natural gas and fuel oil. Combustion boilers are closed vessels containing a burner and a heat exchanger, as well as all the necessary connections to supply the fuel and exhaust combustion products. Electric boilers include resistance heating elements, usually within the fluid flow. Both contain the necessary controls and sensors for operation and safety. Air to water heat pumps, water to water heat pumps, and heat recovery chillers can sometimes substitute for boilers for projects looking to avoid onsite fuel consumption.
Depending on the manufacturer and application, boilers can be made of cast iron, steel, copper, copper over steel, stainless steel, or aluminum. Condensing boilers require corrosion-resistant materials. The type of materials also influences the configuration of whether water flows through the tubes of the heat exchanger, called “water tube,” or if flue gases flow through the tubes, called “fire tube.” The location of the combustion chamber also has corresponding terminology. If the combustion chamber is below the heat exchanger, that is called “dry-base,” and if the heat exchanger surrounds the combustion chamber, it is called “wet-base” or “wet-leg.” Boilers are often made from identical sections attached next to each other so that a range of capacities can be met with the same overall design but a different number of sections.
A natural draft boiler has air flowing through the combustion chamber by buoyancy alone. In contrast, a forced draft boiler has a fan before the combustion chamber, and an induced draft boiler has a fan after the combustion chamber.
Non-condensing boilers are designed not to allow the flue gas to condense within the boiler. This means the inlet water temperature must stay hot, usually above 140F. This is the traditional boiler approach and does not require corrosion-resistant materials, but it also limits the boiler's efficiency since lowering the water temperature will increase the efficiency of the process. Dropping the inlet water temperature can often substantially increase the boiler's overall efficiency but requires using materials that can withstand corrosion. Condensing boilers, since they are designed for lower return water temperatures, also often can be operated at lower part loads.
Additional details can be found in Chapter 32, Boilers of the ASHRAE Handbook on Systems and Equipment 2024.
Information Needed for the Model
The modeler needs to understand the boiler's performance for design conditions, but since boilers are most often operated at part load, the performance at off-design conditions is even more critical. Unfortunately, that information is sometimes very difficult to access and, when available, may be challenging to interpret. The efficiency of a boiler can be expressed using the following terms:
- Annual Fuel Utilization Efficiency (AFUE)
- Combustion Efficiency (Ec)
- Thermal Efficiency (Et)
AFUE is used primarily by residential-sized boilers and attempts to represent the ratio of annual heat produced by the boiler divided by fuel consumption. Combustion efficiency is a ratio of heat output that does not include flue losses from combustion divided by fuel consumption. Thermal efficiency removes both the flue and jacket losses from the boiler in its estimate of heat output and is intended to portray the heat added to the water. Thermal efficiency is sometimes also called overall efficiency. Combustion and thermal efficiency are at rated conditions, and most boiler manufacturers only provide these values at full load. Most BEM software needs the full load thermal efficiency for the boiler, so boiler specifications that only include the combustion efficiency need to be adjusted, or a request should be made to the manufacturer for the estimated thermal efficiency.
It is crucial to adjust the efficiency for operating conditions experienced during the simulation. Most simulation programs use adjustment curves based on the part-load ratio and the entering or leaving temperature of the hot water. This temperature-dependent curve is especially important for condensing boilers. Some software includes default curves that can be used for these inputs, but the best option is to have curves for the specific boiler. They may be requested from the manufacturer, but since it is rare for boilers to be tested other than at full load conditions, the data may not be available.
Other inputs that are commonly needed to simulate boilers include
- Boiler capacity
- Fuel
- Rated and designed supply and return temperatures and limits
- Rated and designed water flow rate
- Minimum, maximum, and optimum part load ratios
- Start-up and standby time
- Electrical parasitics for sensors and controls
- How the boiler is attached to the loop for the supply and return water
Most of these data are available from the boiler specifications or the mechanical design.
In many climates, the building load on a boiler is minimal for much of the year. At those times, the jacket losses from the boiler and the losses from the hot water distribution system can be larger than the load from the building itself. Care must be taken to model this behavior of hot water systems properly under minimal loads and predict fuel consumption correctly. This is especially important when comparing options that may not have these types of standby losses when no or little heating is required.
Common Measures
Some common measures that can be applied to central heating plants that include boilers include:
- Higher efficiency boilers
- Switching to condensing boilers
- Reducing the hot-water return temperature
- Sequencing smaller boilers
- Variable flow hot water distribution systems
- Preheating return water using heat recovery
- Using a dedicated service hot water system
- Electric boilers
- Heat recovery chillers supplying hot water
- Use of heat recovery for providing reheat during cooling
- Different system configurations to reduce the need for reheat during cooling
- Additional insulation on the hydronic piping system
Common Control Options
The boiler control options modeled in BEM software are limited and include when they are scheduled to be available. Most controls within BEM software are related to the hot water system operation[3] [4] , such as
- changing the hot-water supply and return temperatures
- resetting the supply temperature based on load, time of year, or time of day
- staging or sequencing of multiple boilers
- bypassing heating coils in air handling units, terminals, or perimeter baseboards based on the time of year.
Common Applications
Boilers are used in many different building applications across almost all building types. Hot water systems using boilers are commonly used for many heating applications. While very small units have been used in residential applications, most of the boiler market is medium to large buildings where centralized control and maintenance are desired. Almost any type of commercial building can use hot water systems, but they are more common in medium to large multi-floor buildings. Smaller buildings typically use decentralized packaged HVAC[5] [6] systems.
According to the 2020 ASHRAE Handbook on HVAC Systems and Equipment, central plants are commonly used in:
- Campus environments with distribution to several buildings
- High-rise facilities
- Large office buildings (typically over 150,000 ft2)
- Large public assembly facilities, entertainment complexes, stadiums, arenas, and convention and exhibition centers
- Urban centers (e.g., city centers/districts)
- Shopping malls
- Large condominiums, hotels, and apartment complexes
- Educational facilities
- Hospitals and other healthcare facilities
- Industrial facilities (e.g., pharmaceutical, manufacturing)
- Large museums and similar institutions
- Locations where waste heat is readily available (result of power generation or industrial processes)
- Larger systems where higher efficiency offsets the potentially higher first cost of a chilled-water system
Model Output Checks
The first check is to compute the operation of the boiler performance model as close as possible to full load. Following that, check the boiler performance at low part loads and make sure the effects of distribution losses and jacket losses are properly accounted for. Unless special steps are taken, the low part load performance should be significantly worse than the full load performance.
The chiller page describes other modeling checks that can be adapted for hot water systems.
Related Energy Code Requirements
ASHRAE Standard 90.1-2019 and 90.1-2022 have the same tabular requirements for gas-fired boilers for those put in place after March 2, 2022:
Gas-Fired Boiler Requirements from 90.1-2019 and 90.1-2022
| Type | Size (kBtu/h) | Efficiency | Test |
| Hot-water | <300 | 82% AFUE | 10 CFR 430 App N |
| 300 to 2,500 | 80% Et | 10 CFR 431.86 | |
| >2,500 | 82% Ec | 10 CFR 430 App N | |
| Steam | <300 | 80% AFUE | 10 CFR 430 App N |
| >300 to 2,500 | 79% Et | 10 CFR 431.86 |
These test requirements from the Code of Federal Regulations generally reference AHRI Standard 1500-2015. Other testing methods include CSA Standard Z21.13 and UL Standard 795. The main drawback of many of these test standards is that they rate the boiler only at full load, but for most buildings, it rarely operates at full load. ASHRAE has developed a new Standard 155, including testing at part load conditions. It will provide much more information for modelers for boilers that are rated by the new standard, but so far, as it is a new standard, it is not clear yet if the industry will start rating their boilers according to this until an energy code or standard references it.
Section 6.5.4.8 of 90.1-2022 requires an additional gas boiler efficiency of Et of 90% for new buildings with 1 to 10 million Btu/h of total heating system capacity, with some exceptions for site-recovered energy and specific heating systems. This requirement also requires that the design return temperature be 120F or less.
In addition to these efficiency requirements, 90.1-2022 also includes some other requirements for boilers, including:
- Section 6.5.4.1 Boiler Turndown requires that boilers have a minimum turndown of 3 to 1 for those between 1 and 5 million Btu/h, 4 to 1 for 5 to 10 million Btu/h, and 5 to 1 for over 5 million Btu/h.
- Section 6.5.4.3.2 Boiler Isolation requires that flow through a boiler in a multiple boiler system stop when the boiler is shut off.
The standard also has requirements for the operation of heating systems.
Similar or Related Systems
Furnaces, heat pumps, and heat recovery chillers can sometimes substitute for boilers.
Additional Resources
Peterson, Kent. Design Tips to Avoid Boiler Short Cycling. ASHRAE Journal. July 2018.
Condensing Boiler Wikipedia article.
Boiler Retrofits and Decarbonization in Existing Buildings: HVAC Designer Interviews - Lamon, Emily. Raftery, Paul. Schiavon, Stefano. UC Berkeley. August 2022.
Hot Water Heating - Design and Retrofit Guide. Version 1.0. March 2024. Hwakong Cheng, Pat Wendler, Paul Raftery. Taylor Engineers. University of California at Berkeley Center for the Built Environment.
Guidance Document on Space Heating Electrification for Large Commercial Buildings with Boilers. April 2024 U.S. Department of Energy. William Goetzler, Jim Young, Mark Butrico, Ryan Murphy.
Phase-out of Fossil Fuel Boilers Wikipedia article.
Boiler Research Project Final Report in Support of ASHRAE Standard 155P - Jeff Stein, Jingjing (Sabrina) Wang, Gypsy Achong, Peter Mustacich, Kevin Choi, Meg Waltner. PG&E. December 2022.
References
Code of Federal Regulations 431.86
Code of Federal Regulations 430 App N
AHRI Standard 1500-2015. Performance Rating of Commercial Space Heating Boilers. Air-Conditioning, Heating and Refrigeration Institute. Arlington VA.
CSA Standard Z21.13-2010 Gas-fired Low-Pressure Steam and Hot Water Boilers. CSA International. Mississauga, Ontario.
UL Standard 795. Commercial-industrial gas heating equipment. Underwriters Laboratories. Northbrook
Standard 155-2024. Method of Testing for Rating Commercial Space Heating Boiler Systems. ASHRAE. Atlanta, Georgia.
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