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Automatic controls are a big asset since they reduce manual control of the furnace. boilers. and auxiliary equipment. For this reason. boilerman personnel should be able to recognize and understand the basic operations of different types of boiler operating controls. The types of controls the boilerman should become familiar with are as follows: float. pressure, combustion. flame failure. and operation controls.


The float in a boiler control works on the same basic principle as the float in a flush-tank type of water closet. Float, or level, control depends on the level of fluid in a tank or boiler to indicate the balance between the flow out of and the flow into the equipment and to operate a controller to restore the balance.

A float is often used to measure the change in fluid level and to operate the controlled valve to restore the balance. It may be arranged to increase the flow when the fluid level drops. Figure 1-15 shows one of the methods used to accomplish this. Here, the float is connected to the control valve.

Figure 1-15.—Float controller.


Pressure regulating is the process of maintaining a difference of pressure between two points in a system. One type of pressure regulating maintains a definite pressure in one part of the system, while the other part fluctuates or changes within certain limits. An example of this type of control is a pressure-regulator valve (fig. 1-16) that maintains a definite pressure on the discharge side of the valve by controlling the flow of steam, air, or gas through the valve.

Figure 1-16.—Pressure regulator.

A second type of regulator maintains a definite difference in pressure between two points and also controls the flow. This type of regulator is often applied to a boiler feeding to maintain a fixed difference between the pressure of water supplied at the feed valve and the pressure in the boiler steam drum. The pressure regulator may consist of a self-contained device that operates the regulating valve directly, or it may consist of a pressure-measuring device, such as a Bourdon-tube gauge, that operates a pilot or relay valve. The valve positions the regulating valve or mechanism to maintain the desired conditions.

Pressure controls (fig. 1-17) are designed primarily for steam-heating systems but are also available for controlling air, liquids, or gases that are not chemically injurious to the control. The function of the pressure control is as follows:

  • To control the pressure in the boiler
  • To secure the fuel-burning equipment when the pressure reaches a predetermined cutout
  • To start the fuel-burning equipment when the pressure drops to the cut-in point

Figure 1-17.—Typical pressure control with a differential from 0 to 10 pounds.

There are two settings on the pressure control—the cut-in point and the differential. To find the cut-out point, you add the differential to the cut-in pressure; for example, when you were operating a boiler with a cut-in pressure of 90 pounds and a differential of 13 pounds, the cut-out pressure should be 103 pounds. When excessive vibrations are encountered, you should mount the pressure control remotely from the boiler on a solid mounting with a suitable piping connection between them. When a mercury type of switch control is used, be sure that it is mounted level and that the siphon (pigtail) has the loop extending in the direction of the back of the control and at a 90-degree angle to the front, as shown in figure 1-18. This position prevents expansion and contraction of the siphon from affecting the mercury level and accuracy of the control. Additionally, when you install any pigtail, ensure the tube is filled with water. The water will prevent hot steam from contacting the control.

The pressure control can be mounted either on a tee along with the pressure gauge on the pressure-gauge tapping, as shown in figure 1-18, or it can be mounted on the low-water cutout provided by some manufacturers. In either case, be sure that the pipe dope does NOT enter the control. The procedure you should follow is to apply the dope to the male threads, leaving the first two threads bare.

Figure 1-18.—A typical steam gauge installation.


Combustion control is the process of regulating the mixed flow of air and fuel to a furnace as necessary to supply the demand for steam. A modulating pressuretrol controls the movement of the modutrol motor which, in turn. opens or closes the oil valve and air shutters to adjust the rate of firing to suit the demands of the boiler.

A modulating motor (fig. 1-19) consists of the motor windings, a balancing relay, and a balancing potentiometer, The loading is transmitted to the winding through an oil-immersed gear train from the crank arm. The crankshaft is the double-ended type, and the crank arm may be mounted on either end of the motor. The motor works with the potentiometer coil in the modulating pressuretrol. An electrical imbalance is created by pressure change signals to the pressuretrol. This causes the motor to rotate in an attempt to rebalance the circuit. The crank arm, through linkage, positions the burner air louvers and the oil regulating valve, maintaining a balanced flow of air and oil throughout the burner firing range.

Figure 1-19.—A modulating motor.

Another process of controlling, combustion air is to use a manually adjusted air damper. A centrifugal blower, mounted on the boiler head and driven by the blower motor, furnishes combustion air. A definite amount of air must be forced into the combustion chamber to mix with the atomized oil to obtain efficient combustion. In operation, a pressure is built up in the entire head and the secondary air is forced through a diffuser to mix thoroughly with the atomized oil as combustion takes place.

The combustion airflow diagram in figure 1-20 shows a cutaway view of those components that influence most the path of the air through the burner assembly. Air is drawn into the motor-driven blower through the adjustable air damper at (A) and forced through openings (B) into the air box. Sufficient pressure is built up to force the air through openings (C) and the diffusor (D). In the area immediately beyond the diffusor (D), combustion is completed. The hot gaseous products of combustion are forced on through the remaining three passes where they give up a large portion of the heat contained to the water which completely envelopes the passes.

Figure 1-20.—Airflow diagram.

The rate at which combustion air is delivered can be changed by throttling the intake to the blower by opening or closing the air damper to obtain the exact rate of airflow required for complete combustion. Since the rate at which fuel is delivered is predetermined by the design and is not readily adjustable, setting of the air damper is the only means of obtaining the correct ratio of fuel to air to ensure the most efficient combustion.

A pressure-regulating valve is built into the pump that controls the fuel. The fuel pump (fig. 1-21) contains a two-stage gear-type pump, a suction strainer, a pressure-regulating valve, and a nozzle cutoff valve, all assembled in a single housing. Knowledge of the functional relationship of the component parts can be gained by studying the internal oil flow diagram shown in figure 1-22. Observe that the two-stage fuel unit consists essentially of two pumps operating in tandem and arranged in a common housing. The first stage develops a pressure below the atmospheric pressure level at its inlet that causes the oil to flow from storage or supply to the strainer chamber reservoir. All air drawn into the unit rises to the top of this chamber. This air and excess oil are drawn into the first-stage-pumping element and pumped back to the fuel oil storage tank. The second stage withdraws air-free oil from the strainer chamber reservoir and raises the oil pressure to that required for proper atomization at the burner nozzles. The second stage, operating against a combination pressure regulating and nozzle cutoff valve, develops atomizing pressure because of the flow restriction imposed by this valve. The pressure-regulating valve also bypasses excess second-stage oil back to the bottom of the strainer chamber reservoir. The atomizing pressure can be varied within a restricted range by adjustment of the spring-loaded pressure-regulating valve. Normal atomizing pressures generally range between 95 and 120 pounds per square inch.

Figure 1-21.—Fuel oil pump.

Figure 1-22.—The internal oil flow diagram.

An orifice is included in the fuel line to the main oil burner. as shown in figure 1-22. The orifice serves to keep the oil pressure from experiencing a sudden drop when the solenoid oil valve in that line opens. The orifice is commonly built into the solenoid oil valve (fig. 1-22, item 1). Included in the schematic diagram is a photocell (3) which, if it sights no flame, reacts to cause a switching action that results in shutting down the burner.


Frequently on fully automatic boilers, you will find an electronic type of device provided for the control of flame failure. The device provides automatic start and operation of the main burner equipment. Some controls are designed to close all fuel valves, shut down the burner equipment within 4 seconds after a flame failure. and actuate an alarm. Some controls also create a safety shutdown within 4 seconds after de-energization of ignition equipment when the main burner flame is not properly established or fails during the normal starting sequence. These controls must create a safety shutdown when the pilot flame is not established and confirmed within 7 seconds after lighting. A safety shutdown requires manual reset before operation can be resumed and prevents recycling of the burner equipment.

Q16. What is the process of maintaining a difference of pressure between two points in a system called?

Q17. The flow of the air-fuel mixture supplied to the furnace is regulated by steam demand. What is this process called?

Q18. The rate at which combustion air is delivered to the blower can be changed by throttling what device?

Q19. Once a safety shutdown of a boiler has occurred, what action must be taken before operations can resume?

David L. Heiserman, Editor

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Revised: June 06, 2015