Introduction to boilers for the entry-level engineer

Learning objectives

• Provide a primer for young engineers to understand the basics of condensing hydronic boilers. 
• Know the difference between the common types of commercial hydronic boilers. 
• Learn about the specifications used when selecting commercial hydronic boilers. 

When beginning a career in consulting mechanical engineering, there is a lot to learn. Specifications can seem like a foreign language and the details of mechanical equipment can overwhelm the young engineer. With regards to hydronic heating systems, the beginning of this navigation starts with the commercial hydronic boiler.  

The hydronic boiler is the heart of the hydronic heating system. The heating system consists of many parts including the boiler itself, the piping distribution, pumps, central and terminal devices that deliver the hot water to where it’s needed and building automation systems to control how much heat is being delivered. The boiler is a pressurized vessel that burns combustible fuel to heat water that is used to heat a commercial building. 

What defines a hydronic boiler? 

A hydronic boiler can be either a condensing or noncondensing boiler. Both types of boilers can be either a fire– or a water-tube boiler. When selecting a fire-tube boiler, it can be categorized either as a wetback or a dryback boiler. 

Since we will use the term Btu/hour frequently throughout this article, a definition is in order. The Btu is the heat required to raise the temperature of 1 pound of water by 1 F. The heating capacity of boilers is rated in Btu/hour; 1,000 Btu/hour is referred to as MBH. 

A condensing boiler typically ranges from 400 MBH up to 3 million Btu/hour and a noncondensing boiler typically ranges from 400 MBH up to 6 million Btu. A condensing boiler has two heat exchangers and a lower temperature of combustion products (around 130 F).  

Efficiencies of condensing boilers reach up to 98%. This is accomplished by condensing water vapor and other components in the exhaust gases to recover latent heat of vaporization while preheating the entering water stream. The condensate is acidic, with a pH between 3 and 4. The majority of condensing boiler combustion chambers are stainless steel construction to withstand the acidic condensate.  

In comparison, a noncondensing boiler has a single combustion chamber and a single heat exchanger with higher temperature products of combustion (around 350 F). Their combustion chambers are not required to be acid corrosion resistant because the flue gases don’t condense and acidify. Also, the heat of the flue gases is wasted when the products of combustion are discharged straight out the exhaust flue.  

In short, a condensing boiler has a higher initial cost due to the corrosion–resistant construction and multiple heat exchangers and it is more cost–efficient to operate. Where budget is a concern, engineers should choose noncondensing boilers. 

A commercial hydronic boiler can be either a fire- or water-tube boiler. Water-tube boilers consist of water flowing through tubes that are encased by hot combustion gases. Conversely, fire-tube boilers consist of hot combustion gases passing through tubes surrounded by water. Fire-tube boilers are further classified by the type of reversal chamber between passes (see Figure 1) through which flue gases travel through the furnace. If the reversal chamber is surrounded by water, it is defined as a wetback boiler and if the reversal chamber contains a lined rear wall, it is a dryback boiler.  

Fire-tube boilers also have much longer fire-up times and require longer adaptation periods to altering demands due to their high thermal mass (high volume of water in the boiler). These types of boilers also require regular and difficult maintenance periods. Water-tube boilers have relatively fast fire-up times and respond easily to frequently changing demands due to their small thermal mass (low overall water volume) compared to equivalent fire-tube boilers. 

Boiler definitions and classifications 

After navigating which type of commercial hydronic boiler will best serves the needs of a particular project, one must then dive into the specifications that will define and describe the boiler for the project.  

A specification for a boiler can have confusing verbiage that is difficult to navigate. Key components include its burner, combustion chamber, heat exchanger, controls and exhaust stack. The burner of a boiler provides the flames that heat the water in the boiler, while the combustion chamber is the area within the boiler where fuel is burned to heat the water. This chamber holds the burner and is usually made of cast iron or steel. The heat exchanger of a boiler allows the transfer of the heat produced by the burners to the water in the boiler. To set the water temperature, ignition, air and fuel supply mixtures and internal pressure, every boiler will have systems controls.  

These systems controls also contain a safety control to ensure that the internal pressures in the boiler don’t get too high. These safety controls ensure that the water temperature stays within a safe range and the system is running as designed.  

The final component of a boiler is the exhaust stack. The exhaust stack contains all of the pipes used to carry exhaust gases from the boiler to the outside of the building. This component is crucial to the safety of the system because of the toxicity of carbon monoxide in any building.  

When choosing a hydronic boiler, another key term to look at is its turndown ratio. The turndown ratio of a boiler is the ratio between full boiler output and the boiler output when operating at low fire. Typical boiler turndown is 4:1 meaning a 400-horespower boiler with a turndown of 4:1 will modulate down to 100 horsepower before cycling off.  

When an engineer needs to develop a boiler specification for a specific project, they typically look in their company’s custom master specification library or one of the model master specifications like the American Institute of Architect’s MasterSpec library to look for the applicable master specification under Division 23. Specifically, they would look for section 23 52 16 condensing boilers, section 23 52 33 water-tube boilers or section 23 52 39 fire-tube boilers, depending on the specific type of boiler the project engineer for the project has selected in the basis of design. One of these specifications would be selected and edited for the project.  

To demonstrate how a master specification is edited for a particular project, we will use specification section 23 52 16 condensing boilers for a boiler being implemented in a local public school district. 

Part 1 of the condensing boiler specification will the general description what type of condensing boiler is being described in the section of this article titled “What defines a hot water boiler?”

Part 1: General 

1.1 Related documents 

Retain or delete this article in all Sections of Project Manual. 

A. Drawings and general provisions of the contract, including general and supplementary conditions and Division 01 specification sections, apply to this section. 

1.2 Summary 

A. Section includes gas-fired, [pulse-combustion] [fire-tube] [water-tube] [water-jacketed] condensing boilers, trim and accessories for generating [hot water] [and] [steam]. 

To follow the type of boiler illustrated in Figure 1, we would edit the following raw specification sections as shown below while confirming the features with the basis of design condensing boiler: 

Part 2: Products 

2.1 Forced-draft, fire-tube, condensing boilers  

B. Manufacturer: Design note: Select manufacturers from owner’s approved list, if applicable or based on engineer preference and local engineering and maintenance support. 

C. Description: Factory-fabricated, -assembled and -tested, fire-tube condensing boiler with heat exchanger sealed pressure tight, built on a steel base, including insulated jacket; flue-gas vent; combustion-air intake connections; water supply, return and condensate drain connections; and controls. Water-heating service only. 

D. Heat exchanger: Nonferrous, corrosion-resistant combustion chamber. 

E. Pressure vessel: Carbon steel with welded heads and tube connections. 

F. Burner: [Natural] [Propane] gas, forced draft. Design note: Pick whichever is common for the region. 

G. Blower: Centrifugal fan to operate during each burner firing sequence and to prepurge and postpurge the combustion chamber. 

Default motor characteristics are specified in Section 230513 “Common Motor Requirements for HVAC Equipment.” 

1. Motors: Comply with National Electrical Manufacturers Association designation, temperature rating, service factor and efficiency requirements for motors specified in Section 230513 “Common Motor Requirements for HVAC Equipment.”

a. Motor sizes: Minimum size as indicated; if not indicated, large enough so driven load will not require motor to operate in service factor range above 1.0. 

H. Gas train: Combination gas valve with manual shut-off and pressure regulator. 

I. Ignition: Spark ignition with 100% main-valve shut-off with electronic flame supervision. 

J. Casing: 

1. 1. Jacket: [Sheet metal] [Plastic], with snap-in or interlocking closures.
   2. Control compartment enclosures: NEMA 250, Type 1A. 

If retaining second option in “Jacket” Subparagraph above, delete “Finish” Subparagraph below. 

3. Finish: [Baked-enamel] [Powder-coated] protective finish. 

4. Insulation: Minimum 2-inch–thick, [mineral-fiber] [polyurethane-foam] insulation surrounding the heat exchanger. 

5. Combustion-air connections: Inlet and vent duct collars. 

If Project has more than one type or configuration of boiler, delete “Capacities and Characteristics” Paragraph below and schedule boilers on Drawings. 

K. Capacities and characteristics: 

1. 1. Heating medium: Hot water. 
   2. Design water–pressure rating: [160 pounds/square inch gage] Design note: This is the standard working pressure for American Society of Mechanical Engineers Boiler and Pressure Vessel Code, Section IV Heating Boilers Class. 
   3. Safety relief valve setting: Design note: Value ranges from 30 psig up to the design water-pressure rating and is selected by the consulting engineer based on the highest pressure point in the system, in psig. 
   4. Entering-water temperature: Design note: Value is based on the project design requirement, in degrees F. 
   5. Leaving-water temperature: Design note: Value is based on the project design requirement, in degrees F. 
   6. Design water flow rate: Design note: Value is based on the project design requirement, in gallons/minute. 
   7. Minimum water flow rate: Design note: This value is based on the basis of design boiler’s stated minimum flow requirement, in gpm. 
   8. Design pressure drop: Design note: Value is based on the basis of design boiler listed pressure drop, in psig. 

Retain “Minimum Efficiency AFUE,” “Minimum Thermal Efficiency,” or “Minimum Combustion Efficiency” Subparagraph below. Specify standing or intermittent pilot with minimum AFUE. Sustainable design systems require compliance with ASHRAE/IES 90.1 and may require efficiency in excess of minimum efficiency required by ASHRAE/IES 90.1. 

9. Minimum efficiency annual fuel utilization efficiency: Design note: Value is based on the basis of design boiler listed efficiency in percentage. 

10. Minimum thermal efficiency: Design note: Value is based on the basis of design boiler listed thermal efficiency in percentage. 

11. Minimum combustion efficiency: Design note: Value is based on the basis of design boiler listed combustion efficiency in percentage. 

Retain “AGA Input” or “Gas Input” Subparagraph below. 

12. American Gas Association input: Design note: Value is based on the basis of design boiler listed data, in MBH. 

Consider actual Btu content of fuel source if retaining “Gas Input” Subparagraph below. Contact fuel supplier and boiler manufacturers to determine impact. Add text indicating Btu content of fuel if applicable. 

13. Gas input: Design note: Value is based on the basis of design boiler listed data, in cubic feet per hour. 

Retain “AGA Output Capacity,” “DOE Output Capacity,” or “Equivalent Direct Radiation” Subparagraph below for rating methods. 

14. AGA output capacity: Design note: Value is based on the basis of design boiler listed data, in MBH. 

15. Department of Energy output capacity: Design note: Optional value is based on the basis of design boiler listed data, in MBH. 

16. Equivalent direct radiation: Design note: Value is based on the basis of design boiler, listed data as EDR. 

Consider impact of site altitude on fan and motor. 

17. Blower: 

a. Motor horsepower: Design note: Value is based on the basis of design boiler listed data. 

b. Revolutions/minute: Design note: Value is based on the basis of design boiler listed data. 

18. Electrical characteristics: 

a. Volts: [115] [208] [230] [460] Design note: Value is based on the basis of design boiler listed data and project conditions. 

b. Phase: [Single] [Three] Design note: Value is based on the basis of design boiler listed data and project conditions. 

c. Hertz: [50] [60] Design note: 60 hertz is the standard in the U.S. 

d. Full-load amperes: Design note: Value is based on the basis of design boiler listed data. 

e. Minimum circuit ampacity: Design note: Value is based on the basis of design boiler listed data, for wire sizing. 

f. Maximum overcurrent protection: Design note: Value is based on the basis of design boiler listed data, for power circuit breaker sizing. 

Note that this specification’s edits are for a standard condensing boiler project in the U.S. For a given project, the young engineer should request more guidance from the project engineer and customize the options and construction to match owner requirements, local code requirements and project budget. Once the verbiage and classification are defined for a commercial hydronic boiler, the specifications can then reflect the boiler being used for the project. To properly specify the boiler, the consulting engineer needs to understand the application considerations of the boiler chosen.  

Commercial application considerations 

Commercial boilers are used typically in hospitals, offices, hotels and schools. These boilers typically work with heat output from 6,000 MBH to as low as 400 MBH. Commercial condensing boilers are available in fire– and water–tube designs. They typically use propane or natural gas to provide hot water. These systems in a condensing mode operation can have fuel efficiency as high as 98%, depending on the extent of condensing to capture the latent heat of the flue gas by the return water. 

Most architectural projects with large amount of glazing will favor the hydronic boiler because it is an efficient and cost-effective boiler to be used for heating schools, offices and other commercial buildings. Overhead air heating doesn’t work well with tall glazing higher than 5 feet, so some type of local perimeter heating, whether baseboard or radiant ceiling panels, provides an effective solution. For most applications and regions in the U.S., hydronic heating generally provides a more cost-effective solution than electric heating. 

Although gas boilers are typical 10% to 25% higher in price than their oil-fired counterparts — the quick payback due to lower natural gas prices per gallons/minute of heated water compared to oil boilers makes it worth the investment. Natural gas-fired boilers also boast approximately 10% higher AFUE than oil-fired boilers, a measure for a boiler’s combustion efficiency.  

Fuel oil also frequently lacks the pipeline infrastructure, which means certain means of resupply and storage have to be taken into consideration. Oil has its advantages in terms of providing maximum heat; the boilers are capable of supplying the amount of heat required to reach a certain setpoint two times faster than natural gas boilers due to the higher temperatures oil burns at. The higher burning temperatures also ensure lower amounts of condensation, which leads to a longer life span of the boiler if the boiler construction isn’t corrosion resistant. 

Condensing boilers are the most popular commercial water boiler due to the high combustion energy efficiency levels they are able to achieve. However, multiple important factors should be taken into consideration when deciding whether the initial price increase, typically in the 30% cost premium range, is worth it for the system, whether it be a new or retrofit project. Beyond first cost, system operation also plays a significant role in their selection.  

Generally, condensing boilers operate at lower supply and return water temperatures, typically 160 F supply and 130 F return, to achieve their optimal energy savings. Systems in northern climates that require a higher supply temperature during peak heating season tend to operate outside the condensing return water temperature for much of the heating season. They do provide high energy efficiency during the shoulder months when only lower–quality heat is adequate. 

Condensing boilers achieve their highest efficiencies around 98% when the return water temperature is as low as possible, typically around 80 F. However, typical return water temperatures for conventional hydronic systems is around 130 F, which yields overall boiler efficiency around 90%. The lower hot water return temperature aids in condensing the flue gases produced from the combustion of natural gas, which in turn causes a release of energy that heats the return water before entering the combustion chamber of the boiler and thus raising the overall efficiency of the boiler. 

They are available in fire– and water–tube configurations. If the system is expected to be running at high supply and return temperatures for most of its operating hours, a condensing boiler system will not be the optimal system selection because the hot water return temperature will always be too high for condensing operation. The equipment that is being served by the boilers also should be one of the deciding factors for the boiler type.  

Some terminal heating equipment require higher temperature heating fluid to ensure proper operation. Radiant ceiling panels are a good example of such equipment that perform most optimally at supply temperatures around 180 F. For systems with a large number of these devices, a condensing boiler may not the optimal choice. Conversely, applications such as underfloor heating and variable air volume box reheats and installations in milder winter climates (Department of Energy and International Code Council climate zones 2, 3 and 4) are excellent for taking advantage of the benefits of condensing boilers with lower supply water temperatures around 150 F to 160 F. 

Hybrid systems 

Combining condensing and noncondensing boilers in the same system, also called a hybrid system, can help improve overall system efficiency. As discussed above, the return water to the boiler has to be a low enough temperature for the flue gases to condense. Usually, this happens when the weather conditions do not require the boiler system to be firing at high combustion, which usually happens around the shoulder seasons of fall and spring. This is where hybrid systems are optimal by selecting just enough condensing boilers to pick up the load for the shoulder months along with lower water temperature loops such as reheats and underfloor heating, if they are part of the project. By doing so, the system minimizes capital investment in condensing boilers and at the same time takes advantage of the higher efficiencies condensing boilers are capable of for a significant portion of the year. 

After selecting the suitable condensing boiler for the application, special precautions have to be taken into consideration when designing the flue exhaust stack and condensate drainage. Both the gaseous exhaust and condensate from a condensing boiler are moderately acidic. Therefore, the flue for condensing boilers is typically fabricated with AL-29-4C stainless steel to resist corrosion.  

A neutralization kit containing calcium carbonate (limestone chips) on the boiler’s drain line is required to neutralize the acid before releasing into the sanitary drain system. The condensate that drains to the neutralizing basin has to be corrosion–resistant while complying with local plumbing code–acceptable materials. Sizing and routing of the exhaust and drainage is usually specified per the boiler manufacturer’s recommendations.  

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