Fundamentals of Heating Systems
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Learning Objective: Identify the different types of warm-air heating units and equipment and basic installation and maintenance guidelines.

Advances in the field of warm-air heating have made it one of the most popular and widespread forms of heating in use today. It has the advantage of adaptability with various fuels and can be used in a variety of buildings, including barracks, hangars, personnel housing, schools, and theaters. It is likely, therefore, that at one time or another you will be responsible for performing technical maintenance and repair and installation of warm-air heating equipment and systems.

The different types of heating equipment that will be discussed include unit heaters, electric and gas- and oil-fired space heaters, and gas-fired and oil-fired furnaces.


In this course the term unit heater is defined as an installed equipment item and a component of a system consisting of an extended finned heat transfer surface (coil) and a propeller or blower fan to create airflow through it. Unit heaters are indirect units that differ from space heaters because they generate heat indirectly from a medium of steam or hot water piped through a central distribution system. Space heaters are direct-fired units that generate heat directly by the use of an electrical coil or by a combustible fuel.

Unit heaters can be used for many heating requirements, the major limiting factor being the availability of a steam or hot-water system. They are commonly used with heating systems in shops, offices, dining halls, and warehouses. There are three basic types: (1) the suspended horizontal discharge, (2) the suspended vertical discharge, and (3) the floor-mounted or horizontal type of blower unit (fig.4-2).

Figure 4-2.—Unit heaters:
A. Suspended vertical discharge; B. Suspended horizontal disc harge.

The units are rated in Btu or equivalent direct radiation heat output and cubic feet per minute air discharge capacity at a given fan or motor speed. These ratings are important in the application of unit heaters.

Manufacturers furnish information regarding the area effectively heated by units to enable proper planning and location of the units. Generally, units under 50,000 Btu per hour are designated to operate on low-pressure steam or high-temperature hot water.

Space Heaters

Space heaters are used for heating rooms and similarly enclosed spaces, either in addition to, or in place of, a central heating system. They are desirable as a means of providing heat to a small space because of their simplicity of construction, low initial cost, and reasonable fuel consumption. They may be placed directly in the space or at such a location where heat can be delivered through a single register into the space.

Space heaters are sometimes classified by the manner in which they transfer heat to the space to be heated; for example, by radiation and/or convection. The terms direct-fired and indirect-fired are also used to identify such heaters. In this manual, space heaters are identified as direct-fired units and by their heat source or fuel. This discussion will include electric, gas-fired, coal-fired, and oil-fired units.

Electric Heaters and Installation

Space heaters with electrically powered heating elements are used in spaces where it is desired to eliminate cold spots and maintain uniform temperatures, where other fuels are useful as portable units on the floor to overcome floor drafts, and as fixed units mounted in, or to walls or ceilings. They are generally rated in kilowatts (kW). One kW (1,000 watts) is equal to 3,415 Btu per hour.

Electric space heaters are available in two general types—the radiant and natural convection type and the forced warm-air (fan) type. In the radiant and natural convection type, heat from electric elements rises and strikes parabolic (bowl-shaped) reflectors. The reflectors are highly polished curved metal surfaces, which deflect the heat outward into the place where heat is desired (fig. 4-3). Some radiant heat units have no deflectors but provide a combination of radiant and natural convection heat, which rises from the coils into a chamber open on the side where heat is required. The electric baseboard convection heater is an example of this type. The forced warm-air type uses a motorized fan to circulate heat from the heating element outward into the space (fig. 4-4). The electric units are operated manually with an ON-OFF switch or automatically with a thermostat.

Figure 4-3.—Radiant electric space heater.


Figure 4-4.—Forced warm-air electric space heater.

In the selection and installation of electrical space heaters, safety must be assured. Units that are to be installed should bear the label of the Underwriter’s Laboratories (UL). They should also conform to the safety standards outlined in space heating equipment UL-573. All electrical work required for an installation should be done according to the manufacturer’s instructions and by a qualified electrician.

Gas Heaters and Installation

Gas-fired space heaters are clean in operation; they are easily operated and require no fuel handling. They are adaptable for use with natural gas, manufactured gas, or liquefied petroleum gas. Their construction features are similar regardless of the type of gas used. Basically, there are two types—the vented and the unvented.

VENTED UNITS are enclosed metal cabinets with either top and bottom or front and rear grilles for warm-air circulation. The flame burns in a closed combustion chamber, and the heater vent carries away the gases (fig. 4-5). The flow of heat is maintained by a motor-driven fan and is controlled by vanes, fins, louvers, or diffusers. This type of unit is more satisfactory than the unvented type because there is less danger of carbon monoxide poisoning. A panel unit is one type of vented unit (fig. 4-6). It may be recessed or surface-mounted in either an interior or exterior wall with a vent properly insulated and run up through the wall. This type of unit has the advantage of requiring less floor or ceiling space.

Figure 4-5.—Rear view of a vented gas-fired space heater.

Figure 4-6.—Gas-fired panel space heaters.

UNVENTED UNITS are usually the open-flame type where the gas burns in an open combustion chamber. These heaters should be used in a well-ventilated area. Ventilation ensures that the carbon monoxide produced by the gas flame is removed.

Gas-fired space heaters and their connections must be of the type approved by the American Gas Association (AGA). They must also be installed according to AGA specifications. Installation factors, such as the type of gas, the capacity of the heater, and the line pressure drops, must be known to ensure proper plumbing procedures with respect to the gas service line. All newly installed piping should be tested for gas leaks.

On vented gas units, be careful to install the venting system properly to minimize the harmful effects of condensation and to ensure that the combustion products are carried away. During operat ion, the inner surface of the vent must be heated above the dew point of the combustion products. This prevents water from forming in the flue pipe. Vent sections must be installed with the male ends of the inner liner down to allow any condensation that forms to return. This is important since the burning of 1,000 cubic feet of natural gas produces approximately 12 gallons of water. For the same reason, horizontal flue pipes should have an upward pitch of at least 1-inch per running foot.

Vent pipes should be equipped with draft diverters. A diverter is a type of inverted cone through which the flue gases must pass on their way to discharge. It allows air from the heated room to be drawn into the flue pipe joining the combustion-gases. This action prevents excessive downdrafts or updrafts that are apt to extinguish the pilot light or possibly the main burner.

Oil-Fired Space Heaters

In areas where oil is the principal fuel, oil-fired space heaters are used for many space heating requirements. Oil-fired space heaters are very simple in construction. They consist of a burner, a combustion chamber and outer casing, a fuel tank, and fuel control valve. An air space is provided between the combustion chamber and the outer casing. Air enters through grilles in the bottom of the heater, is heated, and passes out through grilles in the top of the unit. Some oil-burning heaters are equipped with a blower and electric motor to force the heated air out into the room. They turn at slow speed and may be either direct drive or belt driven.

Oil-fired space heaters have atmospheric vaporizing-type burners. The burners require a light grade of fuel oil that vaporizes readily at low temperatures and leaves only small amounts of carbon and ash. Number 1 fuel oil is generally used. The two types of burners that will be discussed are the natural draft pot and the perforated sleeve.

NATURAL DRAFT POT DISTILLATE BURNERS are widely used for space heaters, room heaters, and water heaters. A cutaway view of a natural draft pot type of burner is shown in figure 4-7. In operation, the distillate (oil) is fed at the bottom of the burner, either at the center or on the sides, and is vaporized at this point by radiant heat from above. The vapors rise and mix with the air drawn through the perforated holes in the burner. During high fire conditions, the flame burns above the top combustion ring, as shown in figure 4-8; and under low fire conditions, the flame burns in the lower portion or pilot ring of the burner, as shown in figure 4-9.

Figure 4-7.—Cutaway view of a natural draft pot type of burner.

Figure 4-8.—High fire flame.

Figure 4-9.—Low fire flame.

The PERFORATED SLEEVE BURNER consists of a metal base formed of two or more circular fuel vaporizing grooves and alternate air channels (fig. 4-10). Several pairs of perforated sleeves or cylinders force the air through the perforations into the oil vapor chamber. In this way a large number of jets of air are introduced into the oil vapor, bringing about a good mixture. This mixture burns with a blue flame and is clean and odorless.

These burners usually have a short kindling wick. Some burners have a cup below the base in which alcohol is burned to provide heat for starting. The wick and alcohol are used only for lighting.

Figure 4-10.—Perforated sleeve burner.


Oil-burning heaters are portable and are easily moved from one location to another. For satisfactory operation, follow the installation procedures supplied by the manufacturer. In both pot type and perforated sleeve burners, oil is fed to the burner under control of a float-operated metering valve (fig. 4-11). Set the unit level so the oil can be properly distributed in the burner.


The fuel level control valve is the only safety device on the oil-fired space heater.

Figure 4-11.—Oil-controlled metering valve.

When several space heaters are installed in a building, an oil supply from an OUTSIDE TANK to all of the heaters is often desirable. This eliminates frequent filling of individual tanks and reduces waste from spilling. Figure 4-12 shows the principal elements of such a system and important points to consider during installation.

Figure 4-12.—Space heaters installed in series.

Be sure that the space heater is placed a safe distance from the wall. You also need a metal pan for it to sit in. This pan catches the oil if a leak occurs. Do not use a sandbox or cement as both absorb oil and create a fire hazard. In case of wood floors, place a piece of fire-retardant material, such as Gypsum board (Sheetrock) on the floor underneath the metal pan. It may also be needed on the wall behind the heater if the wall is made of wood.

Since the flow of air to a vaporizing type of burner is induced by a chimney DRAFT, pay careful attention to this feature. The draft produced by any chimney depends upon the height of the chimney and the difference in temperature between the flue gas and outside air. The cross-sectional area required depends upon the volume of flue gas to be carried. Since outside air temperature varies during the heating season, arrange the chimney or flue to produce the necessary draft under the most unfavorable conditions likely to be encountered, usually an outside temperature of 60°F. Above this temperature, heat is not usually required, and below this temperature, draft would be increased.

Install the draft REGULATOR to maintain a constant draft adjustment for the rate at which the heaters are fired. The regulator is a swinging damper or gate with provision for adjustment. Since balance and free action are the fundamentals on which its operation depends, be sure the installation provides for these features. Install the damper section with the word top at the true top position. Make sure the face is plumb. When the damper regulator is installed in a horizontal run of pipe, do not use a counterweight on the damper.

A DOWNDRAFT may seriously interfere with proper functioning of these burners. Downdraft may result when the chimney is not high enough above the roof line or is too close to other high buildings, trees, or terrain features. The chimney top must be at least 3 feet above the highest point of the building roof. If the difficulty is caused by other factors, a downdraft hood may prove effective. There are several successful designs; a simple constructed type is shown in figure 4-13.

Figure 4-13.—H-type downdraft hood.

Copper tubing is often used in an oil supply system to burners because of its high resistance to corrosion and ease of installation. The use of compression fittings or flair fittings is best for fuel supply applications. A major advantage in using copper tubing is that it can be bent easily without collapsing the tube, especially if a tubing bender is used; this cuts down on the number of fittings required for installation.


Oil-fired space heaters require periodic cleaning. Frequent checks must be made to ensure that equipment is kept clean, because accumulations of carbon and soot can cause disastrous fires. Units should be moved when they are cleaned, so they can be cleaned inside and out. Accumulations of soot must be removed from inside the fuel pipe. All piping and tubing should be kept clean and free of oil drippings.

The pot or burner assembly may be cleaned without removing the heater. When cleaning this component, remove it through the front door opening and clean all the air holes using a soft copper wire. Do not remove all the carbon from the bottom, because a small accumulation of carbon at the bottom acts as a wick and helps maintain the pilot light. In replacing the burner assembly, make sure both sides of the burner are tightened equally, so the top of the burner and the fire-retardant gasket are set firmly against the flue projection.

In checking the constant-level control valve, check the operation of the heater through a complete cycle of operation from the pilot fire position to the main fire position, and then back to the pilot fire position. Set the control valve, if it is the manual type, to high fire; and if equipped with a thermostatic device, set the thermostat above room temperature. If the heater fails to operate properly through the cycle, check the constant-level control valve and follow the manufacturer's instructions for disassembly and cleaning. A parts breakdown of the valve is shown in figure 4-14.

Figure 4-14.—Constant-level control valve.

Questions for Lesson 3

  1. What are the basic types of unit heaters?
  2. How are electrically powered space heaters generally rated?
  3. When you select and install an electric space heater, what factor should be paramount?
  4. On vented gas-fired space heaters, it is important to install the venting system properly to minimize the effects of what problem?
  5. What type of burner is used in an oil-fired space heater?
  6. What is the only safety device on an oil-fired space heater?
  7. To maintain a constant draft for the burner of an oil-fired space heater, what device should you install in the chimney?

David L. Heiserman, Editor

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Revised: June 06, 2015