Fundamentals of Heating Systems
[FrontPage Include Component]



Learning Objective: Identify types of hot-water boilers, their fittings and accessories, and their operation.

You will find both cast-iron and steel hot-water boilers as sources of heat for domestic hot-water systems in residences and other buildings. Small hot-water heaters heat the hot water for domestic and for limited industrial uses.

Hot-water boilers come in many shapes and sizes. They are constructed with a firebox for burning fuel and have provisions for passing the hot gases over the heat-absorbing surfaces of the boiler. In most cases, baffles guide the gases over the most effective route. These baffles also retard the flow of the gases from the furnace, so water can absorb as much of the heat as possible. Both ends of the boiler have openings for cleaning the boiler tubes and for washing the interior of the boiler. Since most boilers are stationary units permanently installed at the site, they have specified fittings and accessories for a specific heating job. Some boilers, however, called package boilers, are complete units, including fittings and accessories. These boilers are normally mounted on skids so they can be moved to different sites.

This accounts for the term package boiler. Package boilers usually have the same accessories and controls as the comparable stationary type of hot-water or steam boiler. Cast-iron boilers are seldom used as package boilers because of the danger of cracking the boiler sections during transportation.

Cast-iron hot-water boilers vary in size from small domestic units to moderately sized units capable of developing 31 through 98 horsepower. These boilers are usually constructed of several sections joined together by push nipples (round pieces of metal pipe tapered at both ends). Pipes, known as header connections (fig. 4-58) ordinarily connect the boiler sections.

Figure 4-58.—Cast-iron boiler castings.

Figure 4-59.—Cutaway view of a cast-iron boiler.

Cast-iron boilers normally do not have brick settings. Usually, the only bricks used with these boilers are those that are sometimes used as a base for the boilers. In most cases, the bases are made of cast iron. Square sectional cast-iron boilers are similar to the typical unit shown in figure 4-59. This boiler consists of a front and rear section and a number of intermediate sections, depending on the size of the boiler. The sections are connected on each side at the top and bottom either by push nipples or by an outside header. When nipples are used, these sections are held firmly together by rods and nuts.

The boiler has a separate base that does not contain water and, therefore, requires a floor of fireproof construction. Boilers that have water in their bases are referred to as wet-bottom boilers. These boilers are relatively small water units that may be installed on floors constructed of combustible materials. This method of installation, however, is not desirable.

The construction of square sectional boilers is ordinarily such that the sections can be taken through regular-sized doors for assembly inside the boiler room. This is a distinct advantage from the standpoint of both installing new equipment and replacing broken sections. Cast-iron boilers resist the chemical action of corrosive agents much better than steel boilers.

The disadvantage of cast-iron hot-water heating boilers is the danger of the sections cracking or breaking when improperly handled or fired.


Most steel hot-water boilers are constructed in two sections. One section consists of the water jackets, combustion chamber, and smoke passages. These components are either welded or riveted together as a unit. The other section consists of the base and either the grates or burner and is constructed according to the type of fuel used (fig. 4-60).

Figure 4-60.— Typical hot-water  boiler light oil or gas fired.

Another steel boiler is a horizontal unit of the portable type, having an internal firebox surrounded by water lanes. It rests either on a cast-iron or a brick base. The front part of the boiler rests on a pedestal. A disadvantage of this one-piece steel boiler is that it is heavy and requires special equipment to lift it.


A boiler must have a good foundation. The top surface of the foundation should be level to ensure proper alignment of the boiler sections, and thus eliminate strain on the boiler castings. The furnace foundation should be poured separately from the finished floor. It should be of sufficient width and depth to support the boiler without any settling, and it should extend 2 inches above the finished floor. Assembly procedures vary in detail for various boilers. However, manufacturers furnish detailed procedures for the assembly of their boilers. Usually, the plans for the foundations can be procured from them.


Hot-water boilers, regardless of their design and type, operate on the same basic principle. The fuel burns in the combustion chamber and produces heat. The resultant heat is radiated and conducted to the water in the water jackets surrounding the combustion chambers and passes through the boiler tubes; heat is liberated by the flue gases and absorbed by the water surrounding the tubes. The amount of heat transferred into the water depends on the rate of heat conduction through the metal in the boiler tubes and the rate of water circulation in the boiler. For this reason, boilers are designed with baffles to hold the hot gases as long as possible. They give up maximum heat before passing into the chimney.


All boilers have certain accessories for safety and ease of operation. These accessories are pressure-relief valves, pressure gauges, water-level control valves, and automatic controls.

Pressure-Relief Valve

In a closed hot-water heating system, there is always the possibility of building up a dangerous pressure. Consequently, a pressure-relief valve is installed to allow this pressure to escape. A typical pressure-relief valve is shown in figure 4-61. This valve is usually on the top of the boiler. It contains a spring-loaded valve that unseats when the pressure in the system increases to a predetermined value, thereby allowing water to escape until the pressure drops to a safe point. A valve of this type can be adjusted for different pressure.

Figure 4-61.—A typical pressure-relief valve.

Pressure-relief valves may eventually corrode and stick if they are not forced to operate occasionally. It is a good practice, once each month, to increase the pressure to a point that operates the valve. When the relief pressure on the gauge exceeds the setting of the valve, check the valve pressure with an accurate gauge and adjust it to the required amount. However, do not exceed the maximum safe pressure of the boiler.

Pressure Gauge

The operator must know the water pressure in the boiler at all times. A gauge is connected to the top of the boiler. It shows the water pressure in the boiler and in the system in pounds per square inch. This gauge is usually a combination gauge that also indicates boiler water temperature and altitude. The type shown in figure 4-62, however, indicates pressure only.

Figure 4-62.—A typical water pressure gauge.

Little maintenance is required for this unit other than to clean the glass so the gauge can be read. Some types of pressure gauges are constructed so they can be re-calibrated. However, the proper equipment to do this is not always available in the heating shop. To calibrate a pressure gauge properly, you must have either a master gauge set or a deadweight tester.

Water-Level Control Valve

Water is added to a hot-water heating system by either a manually operated water valve or an automatic valve, which is controlled by a float mechanism. Both valves are nearly identical to those used in the free-water system of a steam boiler.

Airflow Switch

The airflow switch, or "sail switch" as it is sometimes called, is in the stack, breeching, or the air inlet to the boiler. This switch shuts down the firing equipment in the event of an induced or forced draft failure. To check the operation of this switch, you restrict or shut off the draft. When you have done this, the switch should shut off the burning equipment.

Questions for Lesson 5

  1. It is a good practice to test the pressure relief valve by what method and what frequency?
  2. What is the name sometime used to refer to an airflow switch?

David L. Heiserman, Editor

Copyright ©  SweetHaven Publishing Services
All Rights Reserved

Revised: June 06, 2015