• SkillCat Team

Control Systems & Control Components

Updated: Jul 8

Basic Electrical Systems and Controls: Chapter 2


Introduction to Control Systems


In this module, we will introduce you to control systems. We will study the different types of HVAC control systems and how they function. Skip to quiz!


Overview


HVAC control systems are used to control the operations of heating, ventilation, and air conditioning equipment. An example of a control device is a thermostat. By lowering the thermostat, we control the functions of the air conditioner unit.


Control systems manage devices or components based on the desired result. Control systems have inputs and outputs. Inputs are used to gather and respond to information. Outputs are used to control an action.


Let’s revisit the thermostat example. Temperature is the input to the system. We set this input by changing the thermostat to the desired temperature. The system output will respond to this information by either turning on the air conditioning or the heater.


The first control systems were called pneumatic control systems. These control systems relied on pressurized air and gas to send and receive signals. These controls were sold as commercial HVAC systems by large companies that dominated the industry.


As time went on, smaller companies joined the industry with electrical control systems. These control systems relied on sending and receiving electrical signals in the form of voltages. The larger companies quickly adopted these control systems.


Control systems are broken down into two categories:

  • Open-loop systems, and

  • Closed-loop systems

We will see what classifies a control system as open-loop or closed-loop.


In an open-loop control system, the input is independent of the output. The input will not change regardless of what the output of the system is.


A toaster is an example of an open-loop system. This system runs on a timer. The set time is the input to the system. The toaster heating up and staying hot for that set time is the output.

The system is open-loop because the toaster will not take into account if the bread is toasted or not.


In a closed-loop system, the input is dependent on the output. The input will change according to what the output of the system is. Another name for closed-loop control is feedback because the output of the system is fed back as part of the input.


An electric iron is an example of a closed-loop system. This system is used to generate heat but will stop heating when a specific temperature is reached. Here, the output of the system is temperature. The input to the system depends on the output temperature.


Control is essential for systems to remain stable. Stability is when a system maintains desirable outputs. If a system were unstable then the inputs would force the outputs to dangerous conditions.


Let’s look back to the electric iron example. With control, we reach the desired temperature and maintain it. Without control, the system will eventually overheat and catch fire. Next, we will see how control systems are powered.



Power Sources

Recall that a power source is used to deliver electrical power. An example of a power source is an electrical socket. Without an electrical socket, household electronics with plugs will not function. Similarly, a control system will not function without a power source.


There are two ways to classify controls systems:

  • Based on their dependency on feedback, or

  • Based on how they are powered.

We have already discussed the differences between control systems based on feedback dependency.


Now we will study control systems based on their power sources. There are four control systems that we will focus on:

  • Electromechanical control systems,

  • Pneumatic control systems,

  • Electronic control systems, and

  • Direct digital control systems

Electromechanical systems operate on both electrical and mechanical properties. Electromechanical control systems have two states. These states are “on” and “off.”


An example of an electromechanical control device is a relay. A relay is a switch that turns circuits on and off. These switches operate off electrical input signals and control mechanical outputs.


Pneumatic control systems use pressurized air to send and receive signals. Pneumatic control systems operate off an air supply. These systems have an air compressor to pressurize the air.

Pressurized air is sent through tubes to control mechanical components.


An example of a pneumatic control system is a pneumatic actuator. A pneumatic actuator takes pressurized air as input and controls a valve as an output. Depending on the strength of the air, the valve can open either partially or fully.

Electronic control systems provide two-position control: “on” or “off.” These systems use electrical inputs to control electrical outputs. Electronic systems sometimes feature visual displays that show system status and operation.


An example of an electrical control system is a temperature sensor. Temperature sensors generate a voltage in response to a room’s temperature. The input is the temperature, and the output is the control voltage.


Direct digital control systems are the most widely used control systems. A direct digital control system is made up of a variety of electronic controls. These electronic controls are networked together into a central computer system.


Direct digital control systems receive inputs from sensors. These inputs are sent to the central computer system. This central system allows us to manage and control all functions throughout the network easily.


Pneumatic controls move pressurized air, and electronic controls send and receive electrical signals. Pneumatic controls can partially open devices, whereas electronic controls must fully open or close them. Electronic controls provide precise control. They are more efficient and dominate the HVAC industry.


Electronic controls provide the option to collect and send data to a network, whereas pneumatic controls cannot. Electronic controls manage a few specific functions, but direct digital control can manage many electronic functions over an extensive network.


System Cycling

We have just seen how control systems depend and operate based on their power sources. Now, we will see how closed-loop control systems depend on cycling for the feedback process. Cycling is defined as a process that starts and stops continuously.


An example of a cycling system is an air conditioner. When we turn on the air conditioner, it begins to cool our home. When the desired temperature is reached, the air conditioner shuts off. The air conditioner will start back up again when the temperature in the room is higher than the temperature set by the thermostat.


Swing is defined as the difference between the maximum and minimum allowed values. In the air conditioning example, the system will start the cycle again when the temperature swings past a max or min value.


If we set our swing to be small, the system will have to cycle more frequently to stay in the desired range. This system will be inefficient as it constantly has to turn on and off. More energy and money will be consumed. The higher the swing, the more efficient the system.


Superheat hunting is when a valve constantly opens and closes to maintain a constant operating point. Superheat hunting results in an inefficient system. Energy and money are both wasted.


As we have seen, control systems are essential to maintaining desired outcomes. Control systems are used in system cycling to tell the system when to start and stop.


The most common of these control systems is the electronic control system. Recall that this system has two states: on and off. Mechanical control systems are also implemented for system cycling, although they are less accurate.


Control systems gather information in the form of inputs, process this information, and control an output. Control systems are either open-loop or closed-loop. The systems power source will categorize it as either an:

  • Electromechanical system,

  • Pneumatic system,

  • Electronic system, or

  • Direct digital system


System cycling is when a process continuously starts and stops to maintain the desired condition. Control systems are used to perform system cycling. Electronic control is the most common and precise form of system cycling.



Component Controls - Part 1


In this module will show how controls are used in the electrical components we studied in a previous module. We will analyze specific control components and how they work. Skip to quiz!


Overview

Recall that control systems play a vital role in HVAC systems. These systems control the operations of heating, ventilation, and air conditioning. Controls allow for automation of the HVAC system, making the system both efficient and intelligent.


Think back to the example of a thermostat controlling the temperature of a room. Without this control system, we could not reach our desired temperature. The system will not know when to turn off or on. Energy and money are wasted.


Control systems are used to carry out a variety of functions. The most common control functions are:

  • Temperature,

  • Mode,

  • Speed, and

  • Timers


Temperature control is used to control the temperature of a room. Modes are used to control a mode of operation: heating, fan, cooling, etc. Speed is used to control the speed of a motor: low, medium, and high. Timers are used to control how long a system stays in operation.


The following subtopics will demonstrate how control is used in the following electrical components:

  • Evaporator,

  • Metering device,

  • Air handler, and

  • Compressor


Evaporator

Recall that the evaporator is used to absorb heat from the surrounding air or water and transfer it to the refrigerant. The evaporator relies on the evaporator fan and fan motor to blow air over the evaporator coils.


The evaporator fan motor is the only component of the evaporator that operates on controls. The fan motor will output one of three speeds:

  • Low,

  • Medium, or

  • High

The thermostat controls the fan motor with a selector switch. A selector switch is a device that runs a specific output depending on its input. The input from the thermostat will tell the selector switch whether to switch to a low, medium, or high speed. The fan motor will spin accordingly.


Metering Device

Recall that the metering device controls the flow of refrigerant into the evaporator coils.

The refrigerant comes from the condenser coils as high-pressure and high-temperature.


Most metering devices do not require the use of control components because they have thermal expansion valves. Thermal expansion valves convert the high-pressure and high-temperature refrigerant to lower pressure and temperature.


Some metering devices use a control component known as an electronic expansion valve. Electronic expansion valves perform the same function as thermal expansion valves. The temperature set by the thermostat controls these valves. We will now study the air handler and how it relates to the metering device.


Air Handler

Air handlers are used to keep room air fresh. They use both recycled air from indoors and fresh outdoor air. These systems remove low-quality indoor air from enclosed spaces. This air is then mixed with outdoor air and passed through an air filter. The air is then heated or cooled with a heat exchanger before returning indoors.


The air handler requires the use of several control components to perform its function. These control components are:

  • Fan monitoring controls, and

  • The control board


Fan monitoring controls tell the blower fan of the air handler how to operate. Inputs are received from pressure switches in the air ducts. If the pressure is too low or too high, the fan monitoring controls will adjust the speed of the blower fan.


The control board also operates on the blower fan. The control board instructs the blower fan to delay when switching between on and off. Delay is used when the system switches between heating and cooling.


When a system swaps between heating or cooling, the remaining air in the vents will be the opposite of temperature of what is desired. The control board delays the blower fan so that remaining hot or cold air in the vents is not passed into a system that desires the opposite.

We have just seen how the control board plays an important role by controlling the blower fan. Now we will see how the control board also controls the components of the evaporator and the metering device.


Recall that some metering devices use electronic expansion valves rather than thermal ones. The air handler control board controls these electronic control valves. The valve will open partially or fully, depending on what the control board instructs.


If the temperature set on the thermostat is far off from the measured temperature of the room, the control board will tell the valve to open fully. If the temperature is off by a small amount, the control board will tell the valve to open partially.


The control board also controls the evaporator fan motor. If we are far off from our desired temperature, the control board will tell the fan to blow at high speed. Recall that the control board also operates off signals received from pressure switches.


The control board will also know when air filters need replacement. By interpreting signals sent from the pressure sensors, the control board will sense when the air pressure is too low. Low air pressure is usually the result of a clogged air filter. We will now study the control components in the compressor.


Compressor

Recall that the role of the compressor is to compress the refrigerant to a higher temperature and pressure. Control systems are vital to maintaining the stable operation of the compressor. They allow the compressor to become more efficient and durable.


The compressor control components are made up of a collection of sensors and electrical components. The main compressor control components are:

  • Temperature sensors, and

  • Solenoid valves

Temperature sensors are used to control whether or not to turn on the compressor motor. If the room temperature is different from what is set by the thermostat, the motor will turn on. When the desired temperature is reached, the compressor motor will turn off again.


Recall that a solenoid valve opens and closes depending on the electrical signal it receives. The solenoid valve is controlled to open or close depending on whether the compressor requires more refrigerant.


Control systems are essential to regulate the temperature of a room. These systems are used to control many different functions in the HVAC process, but the most common are:

  • Temperature,

  • Mode,

  • Speed, and

  • Timers


The evaporator, metering device, air handler, and compressor all require control components to function. The air handler control board helps control the control components of the air handler, evaporator, and metering device.




Component Controls - Part 2


In this module, we will continue to study electrical component controls in the HVAC system.

We will analyze specific control components and how they work. Skip to quiz!


Condenser

Recall that condensers are used to exchange heat with the outside air. This exchange of heat is used to cool the refrigerant and release the heat outdoors.


The condenser utilizes a control system that helps it perform its function. There are three control components used in the condenser:

  • Control board,

  • Fan relay, and

  • Fan cycling switch

Condenser control boards are only found in heat pumps and large commercial HVAC systems. The control board’s function is to ensure that ice does not form on the condenser coils. The control board manages this task by measuring the temperature of the coils.


The condenser control board is also known as the defrost control board because it controls the defrost sequence. When the condenser coils are close to freezing or frozen, defrost sensors signal the control board.


The control board will instruct the system to go into heating mode. Hot refrigerant will be supplied, and the fan motor will turn off.


The hot refrigerant will melt the ice, and the fan will turn off. The system will be in defrost. Once the defrost sensors no longer detect frost on the coils, it will send a signal to the control board. The board will terminate the defrost process.


The condenser fan relay controls the power to the condenser cooling fan. When activated, the relay will turn the fan on and circulate air through the condenser coils. The fan helps cool the refrigerant. By controlling the power of the fan, the relay helps control the refrigerant cooling process.


The fan cycling switch controls turning the condenser fan on or off. The fan cycling switch receives inputs from pressure sensors.


If the refrigerant has too much pressure, the fan is turned on. Conversely, if the refrigerant pressure is too low, the fan is turned off. Now we will see how compressor and condenser controls work together.


Condensing Unit

Recall that the condensing unit is the name given to the outdoor unit of the HVAC system.

A condensing unit contains the condenser coils and the compressor.


The condensing unit relies on the following control components:

  • Solenoid valves,

  • Pressure switches,

  • The control board,

  • Temperature sensors,

  • VSD circuits, and

  • Overcurrent sensors

We have already studied solenoid valves, pressure switches, the control board, and temperature sensors.


A VSD circuit, or variable speed drive circuit, accurately controls the speed of any motor. The compressor, blower fan, and compressor fan all require motors to function. VSD circuits control each of their fans and thereby control the overall condensing unit.


Overcurrent sensors are used to measure the current supplied to the system. Too low of a current will result in the system not operating. Too high of a current may cause a system to burn up. These sensors will cut the power of the system if current levels are too high.


We will now summarize the flow of control in the condensing unit. Let’s say that we turn the thermostat down and call for cooling. So, long as the pressure and temperature sensors do not detect abnormal measurement, a control signal will activate the contactor.


If the current to the contactor is too high, the overcurrent sensors will shut off the power. If the current is normal, the control signal will pass to the compressor. VSD circuits will control the motors of the blower fan, compressor, and condenser fan based on the room temperature.


The control board will also interpret the room temperature and send signals to the solenoid valve. The solenoid valve will control the flow of refrigerant. We can see how the flow of control makes the system precise and energy-efficient. Now we will study control components in the heating system.


Heating Systems

The heating system operates exactly the opposite of the cooling system. An air conditioner supplies cool air indoors by exchanging heat and passing hot air outdoors. Conversely, a heat pump supplies warm air indoors by exchanging heat and passing cool air outdoors.


A heat pump accomplishes its function by reversing the flow of the refrigerant. We supply the high-temperature, high-pressure refrigerant from the compressor to the evaporator rather than the condenser.


The hot refrigerant sent to the evaporator will heat the evaporator coils. The evaporator fan will circulate air and pass it over the coils. Heat will transfer from the coils to the air, and this warm air will pass indoors and heat the home.


The heat pump relies on three main control components to function:

  • The reversing valve,

  • Second stage thermostat bulb, and

  • Sequencing relay

Let’s take a closer look at the roles of these control components.


Recall that the heat pump reverses the flow of refrigerant in the HVAC system. The heat pump accomplishes this with the reversing valve. The reversing valve controls whether or not the system is in a heating or cooling mode of operation by changing the direction of refrigerant flow.


In extremely cold climates, additional sources of heating are required to reach the desired temperatures. A stage thermostat bulb is a component in the thermometer that detects if the room’s temperature cannot reach the set temperature.


If we cannot reach the set temperature, the component gives a control signal to turn on the system’s additional heating coils. Because these heating coils consume large amounts of electricity, a sequencing relay is used to help control power consumption.


Sequencing relays control the sequence in which electric coils begin heating. The relay provides power to the heating coils one by one with a few seconds of delay in between.

The sequencing relay makes the system energy-efficient.


We will now take a look at the sequence of operation in a heating pump. Setting the thermostat to the heating mode will trigger the reversing valve and compressor to pump high-temperature and high-pressure refrigerant to the evaporator.


The high-pressure, high-temperature refrigerant will pass through the evaporator coils and exchange heat with the air that passes over them. The heated air will pass indoors, and the cooled refrigerant will continue to move throughout the system. The refrigerant will eventually return to the compressor, and the cycle will begin again.


We will now take a look at control systems in gas heating systems. Recall that gas heating makes the use of burning fuel to produce heat. The heating process utilizes control components to perform its function.


Gas heating relies on the following control components:

  • Thermostats,

  • Gas valves,

  • Pressure switches,

  • Ignitors, and

  • The furnace control board

We have already studied thermostats, valves, and pressure switches.


The furnace control board controls all of the operations of the furnace. The control board operates on signals sent from the thermostat and temperature sensors. Suppose the room temperature is different than what is desired. The control board will either turn on or turn off the furnace.


The control board is also responsible for controlling various functions. The control board handles gas valves, ignition, the blower, and the heat exchanger. The control board also performs safety checks, such as preventing gas leaks. Let’s take a look at the flow of control.


The thermostat receives a set temperature from the user and sends a control signal to the control board. The temperature sensor will read the room temperature and also send a signal to the control board. The board will interpret both of these signals and decide whether or not to start or stop the system.


If the system needs to start, the control board will instruct the fuel valve to supply gas to the system. The control board will also send a signal to the igniter to start a flame. When the gas reaches the igniters, a fire will start, and the heating system will begin heating.


Other Components

We have studied a wide variety of control components across many different components in the HVAC system. Here are a few last control components that are worth mentioning:

  • Discharge temperature switch,

  • Float switch, and

  • Carbon monoxide sensors

The discharge switch is located at the output of the compressor. This control component is used for safety. The compressor may experience refrigerant blockage, which will result in temperature increases. If the temperature reaches too high of a level, the discharge temperature switch will alert the user or turn off the system.


Float switches are located in the condensate drain pan. Recall the drain pan collects water droplets that fall from the evaporator coils. If the water-flow from the drain pan out of the system becomes clogged, the water level will rise. If the water level is too high, the float switch will turn off the system to prevent water from overflowing.


Carbon monoxide sensors are present in fuel heating systems. Recall that the burning of heat creates both carbon dioxide and carbon monoxide. Carbon monoxide is a deadly gas. Carbon monoxide sensors detect the level of carbon monoxide in a room. If the levels are too high, the user is warned.


Control components in the condenser and the condensing unit help control the process of heating and cooling. The condensing unit control board is the brain behind the condensing unit and interprets and sends the majority of the control signals.


Heat pumps work in the exact opposite manner as air conditioners. Heat pumps accomplish this by reversing the flow of the refrigerant. Control systems also perform safety checks on HVAC components. By interpreting and controlling signals in the area, control components protect both the equipment and the user.




Question #1: Another name for a closed-loop system is feedback because the output is fed back as part of the input.

  1. True

  2. False

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Answer: True

True, another name for a closed-loop system is feedback because the output is fed back as part of the input.


Question #2: Control is essential for systems to remain stable.

  1. True

  2. False

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Answer: True

True, control is essential for systems to remain stable.


Question #3: Which of the following is not one of the four categories of control systems discussed?

  1. Mechanical controls

  2. Pneumatic controls

  3. Electronic controls

  4. Direct digital controls

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Answer: Mechanical controls

Mechanical controls are not one of the four categories of control systems discussed.


Question #4: Electronic controls are more efficient and precise than pneumatic controls.

  1. True

  2. False

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Answer: True

True, electronic controls are more efficient and precise than pneumatic controls.


Question #5: Superheat hunting is when a valve constantly opens and closes to maintain a constant operating point.

  1. True

  2. False

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Answer: True

True, superheat hunting is when a valve constantly opens and closes to maintain a constant operating point.


Question #6: Which of the following control systems is the most precise at system cycling?

  1. Electronic

  2. Pneumatic

  3. Mechanical

  4. Open-loop

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Answer: Electronic

Electronic control is the most precise control system for system cycling.


Question #7: Controls systems are important because they allow for efficiency.

  1. True

  2. False

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Answer: True

True, controls allow for automation, making the system both efficient and intelligent.


Question #8: Which of the common control functions are used to control the evaporator fan motor?

  1. Speed

  2. Temperature

  3. Mode

  4. Timer

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Answer: Speed

Speed is the common control function used to control the evaporator fan motor.


Question #9: Most metering devices do not require the use of control components.

  1. True

  2. False

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Answer: True

True, most metering devices do not require the use of control components.


Question #10: The control components of the air handler are fan monitoring controls and the control board.

  1. True

  2. False

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Answer: True

True, the control components of the air handler are fan monitoring controls and the control board.


Question #11: The air handler control board helps control all but which of the following.

  1. Compressor

  2. Evaporator

  3. Metering Device

  4. Air handler

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Answer: Compressor

Compressor, the air handler control board helps control the components of the evaporator, metering device, and air handler.


Question #12: The main control components of the compressor are temperature sensors and solenoid valves.

  1. True

  2. False

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Answer: True

True, the main control components of the compressor are temperature sensors and solenoid valves.


Question #13: Which of the following is not a control component in the condenser.

  1. Solenoid valve

  2. Fan cycling switch

  3. Fan relay

  4. Control Board

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Answer: Solenoid valve

Solenoid valve, a solenoid valve is not a control component in the condenser.


Question #14: Which of the following control components in the condenser controls the defrost sequence.

  1. Control board

  2. Fan relay

  3. Fan cycling switch

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Answer: Control board

Control board, the control board controls the defrost sequence in the condenser.


Question #15: VSD circuits accurately control the speed of any motor.

  1. True

  2. False

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Answer: True

True, VSD circuits accurately control the speed of any motor.


Question #16: The reversing valve changes the direction of refrigerant flow.

  1. True

  2. False

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Answer: True

True, the reversing valve changes the direction of refrigerant flow.


Question #17: The furnace control board controls all of the operations of the furnace.

  1. True

  2. False

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Answer: True

True, the furnace control board controls all of the operations of the furnace.


Question #18: Which of the following control components ensures the compressor doesn’t reach too high of a temperature.

  1. Discharge temperature switch

  2. Float switch

  3. Carbon monoxide sensor

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