• SkillCat Team

A Complete Guide to Centralized HVAC Systems

Updated: Jul 9


Complete Residential Systems: Chapter 3


Centralized HVAC Systems - Components


In this module, we will discuss centralized HVAC systems. We will learn about the overall system and the functioning of centralized HVAC systems. Skip to quiz!


How They Work!


Centralized HVAC systems, like a split system, have an indoor and outdoor unit. The indoor unit can be installed in the basement or attic of the house. They distribute air to every corner of the house from the indoor unit via ductwork.


Recall that the electrical wires and refrigerant lines connect the indoor and outdoor units. The refrigerant flows in the discharge line from the outdoor to the indoor unit. The refrigerant flows in the suction line from the indoor to the outdoor unit.


Centralized HVAC systems are great for keeping large areas at a uniform temperature. They can provide cool or hot air to multiple rooms or entire houses at once.


Centralized systems are used in large areas beyond the reach or capacity of a window unit or split systems. We see centralized systems installed in big houses and mansions.


The air is circulated uniformly throughout the house in centralized HVAC systems. So, there are no cold or hot corners in the house. Homes are heated or cooled uniformly by the system.


As the air is circulated through ducts in centralized systems, we can install a different air filter in the ducts. Some modern air filters installed in the ducts even claim to clean different bacteria and viruses. This gives us much cleaner air.


The outdoor unit is located outside the house in the garden. The indoor unit in a centralized system can be located in the basement or the house's attic. So a room with a centralized HVAC system is quieter than a room installed with split systems.


A centralized HVAC system is not at all visible inside the room. We can only see air grilles and the thermostat control in the room. Recall that air grilles are the outlet or inlet of the ducts. Many owners love a fully concealed HVAC system.


The outdoor unit of the centralized HVAC system has a power cable. We connect this cable to the mains power socket in the wall. We usually call this outdoor socket the disconnect box.


In some systems, the indoor unit will also has a separate power cable. We connect this cable to the mains power socket in the wall.


All centralized HVAC systems can cool and heat the house as needed. The thermostat has a button to set the system in heating or cooling mode.


We can set the temperature inside the room by using a remote control or buttons on the indoor unit. Let us look at a video that shows the working of a centralized system.


Recall the components of a refrigeration cycle.

  • Compressor,

  • Condenser,

  • Metering Device, and

  • Evaporator

A centralized unit has all these components and uses the refrigeration cycle to cool a room.


Centralized HVAC systems have a gas furnace installed in the indoor unit. The gas furnace provides heat to the house when in heating mode. Recall that gas furnaces use natural gas to heat the system. Let us look at a video to recall the working of a gas furnace.


Natural gas is burnt in a gas furnace. The smoke from burning natural gas passes through a heat exchanger. Air then heats up as it flows over the heat exchanger plates. This hot air heats our house.



Electrical Components


Now that we have an overview about a centralized system, let us discuss about the electrical components in it. Recall that electrical components work on electricity and require a line voltage. Recall that the line voltage coming to the power socket in most residences in the US is 120 Volts.


We see the following electrical components in the centralized HVAC system:

  • Transformer,

  • Compressor Motor,

  • Capacitors,

  • Condenser Fan Motor,

  • Blower Fan Motor,

  • Condensate Pump,

  • Flame Igniter, and

  • Draft Inducer Motor.


Recall that we have learned about the following components in the earlier chapter. Transformer: Converts the line voltage of 120 Volts to control voltage of 24 Volts.


Compressor Motor: Consumes electricity and provides rotation to the compressor.

Capacitors: Store electrical energy and help the compressor to start and run smoothly.

Condenser Fan Motor: Rotates a fan that circulates the air around the condenser coils.


Blower Fan Motor: Rotates a blower fan that forces air through the evaporator coils and gives us cold air. In the heating mode, the blower fan forces air over the heat exchanger plates as well.


Recall that as air flows over the evaporator coils, water vapor condenses to liquid water. A drain pan collects this water. This water usually flows down the drain in most split systems and window units.


A centralized system has a pump that forces this water down the drain. A centralized system produces a lot of water when working in both heating and cooling modes. So a condensate drain pump is required to force this water into the drains.


Recall that a flame igniter gives the starting heat required to burn the gas in a gas furnace. The igniter works just like a lighter to provide the initial spark required for the flame to burn.


It starts to work as it receives a line voltage. On receiving the line voltage, it provides the necessary heat required to start the flame.


Recall that as the gas burns, it produces smoke and waste gases. A draft inducer fan motor acts as an exhaust fan to force this smoke out of the chimneys or vents.


Ventilation Components


Ventilation in a centralized HVAC system is done by the blower fan. The blower fan motor is connected to the blower fan and rotates it.


Recall that the blower fan is like a table fan that sucks the room air and forces it over the evaporator coils and heat exchanger. The air cools down or heats up based on the mode set by the user. The cold or hot air is circulated in the room by the blower fan through the ducts.


Recall that ductwork is a closed pathway made of thin metal sheets.

Two main purposes of the ducts are:

  • To provide a path for the hot or cold air to reach the rooms from the HVAC systems.

  • To not allow the transfer of heat between the air inside and outside the ductwork.


Centralized HVAC systems have air filters installed in ducts. Recall that air filters provide clean air by trapping the dust and other contaminants in the air. Split systems and window units also have filters, but they are not as powerful.


In this module, we discussed how a centralized HVAC system gives better air circulation and quieter operation. We also learned how a condensate pump removes the condensate water in the system.


We discussed how a flame igniter and draft inducer motor work in the gas furnace of a centralized HVAC system. Lastly, we learned about the ducts and air filters that help in ventilation in a centralized HVAC system.



Centralized HVAC Systems - Control Components


In this topic, we will discuss the controls in centralized HVAC systems. We will learn about each control component and its working in a centralized HVAC system. Skip to quiz!


Controls in a Centralized System


Recall that control systems govern the system as per the inputs required and provided by the user. Setting the mode of operation and setting room temperature are some of the ways that we control the system.


We see the following controls in a centralized system:

  • Thermostat,

  • Temperature sensing bulb,

  • Relays and Contactors,

  • Control Board,

  • Gas Valve, and

  • Safety Controls.


Recall that we have learned about the following control components in the earlier chapter. Thermostat: A device to set the system to heating or cooling mode, set the temperature, fan speed, and many such settings.


Temperature sensing bulb: Detects room temperature and sends it to the system.

Relays: A switch that controls the fans in the system. We find two fan relays in a centralized system, one for the evaporator fan motor and another for the condenser fan motor.


Contactors: A contactor is a switch that controls the compressor motor in the system. These components are similar for a window unit and a split system.


A control board is the brain of a centralized HVAC system. It controls many functions of the centralized system. All control signals for controlling the system and maintaining its safe operation are sent through the control board.


In the industry, we also call the control board a ‘Furnace Control Board’ or an ‘Electronic Control Board.’


Centralized systems supply a large amount of air. All modern systems come with many modes of heating and cooling. For example, a power mode, power saving mode, silent mode, dry mode, humid mode, and many more. An electronic control board controls all these functions.


Modern split systems and window units also come with a control board in them. However, they are not compulsorily seen in these systems.


Recall that a gas valve controls the flow of natural gas coming to the system. It allows the gas to flow only when it receives a control signal from the control board. A gas valve is just like a gas knob you would see on a gas stove.


A gas valve is one of the most important control you would find in the centralized system. A gas leakage can cause a fire in the house. A gas valve controls the flow of gas and keeps the house safe.


Safety controls present in a centralized HVAC system are:

  • Thermal switches,

  • High-pressure switch,

  • Low-pressure switch,

  • Float Switch, and

  • Pressure Switch.


Thermal switches: Detect the temperatures within the system. It stops the control signal and shuts off the system if the temperature rises above the set temperature.


High-Pressure Cutout Switch (HPCO): It detects pressure on the outlet of the compressor. It stops the control signal and shuts off the system if the refrigerant pressure from the compressor is higher than the set value.


Low-Pressure Cutout Switch (LPCO): It detects pressure on the inlet of the compressor. It stops the control signal and shuts off the system if the refrigerant pressure from the compressor is lower than the set value.


A float switch detects the water level in the condensate drain. Recall that centralized HVAC systems generate a lot of condensate water. This water collects in the condensate drain pan.


A condensate pump keeps removing the water, but sometimes the pump may stop. Sometimes the drain line may be choked. This might spill the water outside the system. A float switch detects the water level in such cases.


If the water level increases beyond a set level, the float switch cuts the control signal from the thermostat. A float switch is present between the thermostat and the control board. This tells the user or technician that the condensate drain line is chocked.


Now let us have a look at the safety switches used specifically for the heating part of the centralized HVAC system.

These safety controls are:

  • Flame Sensor,

  • Flame Roll-Out Switch,

  • Thermal Limit Switch, and

  • Exhaust Draft Pressure Switch


A flame sensor detects the flame in the furnace while the gas burns. It is also sometimes called a flame rod. Without a flame, the gas would be removed through the vent without burning. This wastes the gas. It also causes a risk of fire due to the gas leak.


Recall that the gas flame should always burn in the combustion chamber of the gas furnace. A flame roll-out means when the gas furnace flame comes out of the combustion chamber. This video shows a flame roll-out happening in a gas furnace.


The flame coming out of the combustion chamber can cause a fire or a short circuit and is dangerous. A flame roll-out switch detects this flame.


There are multiple flame roll-out switches that detect a flame outside the combustion chamber. The flame roll-out switch sends a control signal to the control board when it detects a flame. The control board stops the gas supply as it gets the control signal.


Thermal limit switches are a type of thermal switches. These switches detect the temperature of the heat exchangers in the system.


Very high temperatures in the system can cause an unwanted fire. If the temperature exceeds the set temperature of the switch, it sends a control signal to the control board to shut down the system. Let us look at a short video about the thermal limit switch.


The exhaust draft pressure switch is also called a pressure switch or a vacuum switch. It detects the flow of exhaust gases in the exhaust vent. If the exhaust vent is blocked, the smoke will gather inside the house which is a problem.


If the exhaust draft pressure switch does not detect any airflow, it sends a control signal to the control board. The control board stops the system as it receives this cont


In this module, we discussed how the control board controls most functions in a centralized HVAC system. We also learned the importance of the gas valve in controlling the gas flow.


Lastly, we also learned about how the flame sensors, thermal limit switches, and flame roll-out switches eliminate the risk of a fire in the gas furnace.




Centralized HVAC Systems - Working in Cooling Mode


In this module, we will discuss the complete step-by-step working of the centralized system in cooling mode. We will walk through each process that takes place once you switch on the unit. Skip to quiz!


Complete Working


A centralized system starts working by pressing the ON button on the remote control. You can also start the unit using the buttons on the panel or the thermostat on the indoor unit.


Modern centralized systems have many different settings.

We will focus on a few selected modes:

  • Cooling Mode,

  • Heating Mode, and

  • Backup Heating Mode.


Complete Working: Cooling Mode


Pressing the COOL button on the thermostat or the remote control starts the cooling mode. The refrigeration cycle starts along with the fans in cooling mode. Recall that the refrigeration cycle removes heat from the room.


In the cooling mode, the thermostat sends a control signal to the compressor contactor. Recall that the contactor is a type of relay that controls the compressor motor in the system. The compressor and the contactor are both present in the outdoor unit of the system.


As the compressor contactor receives the control signal, it sends the line voltage to the compressor. Starting the compressor starts the refrigeration cycle. Let us quickly look at a video to recall the refrigeration cycle.


The cooling mode in a centralized system is very similar to a split system. The refrigeration cycle and control systems are the same. We will discuss some minor differences that are seen in these systems.


In the cooling mode, the thermostat sends a signal to the:

  • Compressor Motor,

  • Blower Fan Motor, and

  • Condenser Fan Motor.

The control signal wire from the thermostat goes to both the indoor and outdoor units.


When we press the cooling mode on the thermostat, it sends a control signal to the fan relays by default. Recall that the fan relay starts the fan motor. The fan motors connected to the fans start the circulation of air.


Recall that the blower fan sucks the air from the room and forces it through the evaporator coils. In the cooling mode, the evaporator coils have cold refrigerant flowing through them. This cools the air flowing over the evaporator coils and gives us cold air.


The fan relay before the condenser fan motor also receives the signal from the thermostat. Recall that the condenser fan forces outside air through the condenser coils. This air cools the refrigerant by taking the heat from the refrigerant and throwing it out.


We learned how the cooling in the centralized system works. Let us look at how it is controlled. Recall how pressing the cool button on the thermostat starts the split systems.


When we set the mode of the system, we also set the temperature that we desire. The procedure after setting the temperature is also similar to the split system. We will discuss some minor differences about how controlling the system differs.


Recall that the temperature sensing bulb detects the temperature of the room. It sends a control signal to the thermostat when the temperature of the room reaches close to the set temperature. In the centralized system, it does the same function as a split system.


Safety switches in a centralized system perform the same function as in a split system. Recall how the Discharge Temperature Switch (DTS ) detects the temperature of the refrigerant. It cuts off the signal from the thermostat to the compressor on detecting high temperatures.


Recall how the High-Pressure Cutout Switch (HPCO) and Low-Pressure Cutout Switch (LPCO) detect the pressure of the refrigerant. These switches cut off the signal from the thermostat to the compressor to detect abnormal pressures.


A float switch detects the water level in the condensate drain. Recall that centralized HVAC systems generate a lot of condensate water. This water collects in the condensate drain pan.


A condensate pump keeps removing the water, but sometimes the pump may stop. Sometimes the drain pipe may be choked. This might spill the water outside the system.


If the water level increases beyond a set level, the float switch cuts the control signal from the thermostat. A float switch is present between the thermostat and the control board. This tells the user or technician that the condensate drain line is chocked.


These safety controls bind the system to work within prescribed boundaries. Other safety controls are also found in a centralized system that activates during the heating mode.


We discussed how a centralized system works during the cooling mode. We also learned how the gas furnace provides heat in the heating mode.


Lastly, we also learned how the safety controls like a float switch work along with the switches like a DTS, HPCO, and LPCO.




Centralized HVAC Systems - Working in Heating Mode


In this module, we will discuss the complete step-by-step working of the centralized system in heating mode. We will walk through each process that takes place once you switch on the unit. Skip to quiz!


Complete Working: Heating Mode


Pressing the HEAT button on the thermostat or the remote control starts the heating mode. Recall that the gas furnace works to heat the house in heating mode. The refrigeration cycle does not play any role in the heating mode.


When we press the HEAT button on the thermostat, it sends a signal to the control board. Recall that the control board is the brain of the system. The control board sends all control signals in a gas furnace.


The control board sends a line voltage to the draft inducer fan motor on receiving a control signal from the thermostat. Recall that the draft inducer fan motor forces any smoke or dirt from the exhaust vent.


If the furnace hasn't been used in a long time, the exhaust vent is blocked by dry leaves or garbage. Ice can also form in the exhaust vent in winters. The draft inducer fan tries to force the air in such a situation, but it does not flow through the vent as it is blocked.


As the draft inducer fan starts, the draft pressure switch measures the airflow in the exhaust vent. Recall that the draft pressure switch is also called a vacuum switch.


Vacuum switch is the safety switch that ensures that there is airflow in the exhaust vent. This is done to detect any blockages in the exhaust vent. The switch sends back a signal to the control board if the airflow is detected.


If the control board does not receive this signal from the switch, it will not start the system. This will alert the user or technician of any problems.


After receiving a confirmatory control signal from the vacuum switch, the control board sends a line voltage to the igniter. Recall that the igniter is like a lighter that helps start the flame. The igniter is set to start the flame once it receives the line voltage.


The control board now sends a control signal to the gas valve. Recall that the gas valve controls the flow of gas for burning. However, the control board waits for some time before sending this signal. This wait time is given because some igniters take time to start.


The wait time is between 5 to 10 seconds, depending on the furnace model. The gas valve allows the gas to flow as it receives the control signal. Once the gas reaches the igniter, it burns up, and we see a flame.


This flame produces heat that heats the heat exchanger in the furnace. Recall that the smoke from burning gas passes through heat exchanger tubes. The heat exchanger is metal plates that take the heat from the smoke in the tubes and transfer it to the air outside the tubes.


However, after giving a signal to the gas valve, the control board waits for about 30 seconds to 1 minute. This wait time is for the heat exchanger plates to heat up. It then sends a line voltage to the blower fan motor.


Recall that the blower fan sucks the room air and forces it over the heat exchanger tubes. Air then heats up as it flows over the heat exchanger plates. This hot air heats our house.


Now that we learned how we get hot air from the centralized system let us look at how it is controlled. Recall that pressing the heat button on the thermostat starts the gas furnace.


Recall that a temperature sensing bulb is present in the thermostat of the system. It sends a control signal to the thermostat when the temperature of the room reaches close to the set temperature.


On receiving this signal, the thermostat stops sending the control signal to the control board.



Safety Controls in Heating Mode


Recall that we have learned about these safety controls in a gas furnace:

  • Flame Sensor,

  • Flame Rollout Switch,

  • Thermal Limit Switch, and

  • Vacuum Switch

We already discussed the vacuum switch in the previous slides. Let us look at the other safety controls.

Recall that a flame sensor detects the flame and sends a signal to the control board when a flame is detected. The flame sensor is connected between the control board and the gas valve.


After the control board sends a signal to the gas valve, it waits for a control signal from the flame sensor. The control board is programmed to wait for 5-10 seconds to get a control signal from the flame sensor.


No control signal from the flame sensor means no flame is detected. The control board stops the control signal to the gas valve stopping the gas supply. This shuts off the system and gives an alert to the user or technician.


Recall that a flame roll-out means when the gas furnace flame comes out of the combustion chamber. The flame coming out of the combustion chamber can cause a fire or a short circuit and is dangerous. A flame roll-out switch detects this flame.


The flame roll-out switch is connected to the control board. It cuts off the the control signal from the control board to the gas valve when it detects a flame.


The gas valve closes, and the gas supply is stopped as the gas valve stops receiving the control signal. Multiple flame roll-out switches are present that detect flame outside the combustion chamber.


Recall that thermal limit switches detect the temperature of the heat exchangers in the system. This switch is similar to the discharge temperature switch (DTS) found in the refrigeration cycle.


This switch is connected between the control board and the gas valve. If the temperature exceeds the set temperature of the switch, it stops the control signal from the control board to the gas valve.


The gas valve closes, and the gas supply is stopped as the gas valve stops receiving the control signal. Multiple thermal limit switches are present that detect the temperatures in different places in a gas furnace.


Backup Heating Mode


Safety controls bind the gas furnace in the centralized system to work within prescribed boundaries. This is how a centralized system works in the heating mode. Sometimes the centralized system cannot provide sufficient heating in harsh winters.


Backup heating is required when the centralized system cannot provide sufficient heating in regular heat mode. This situation is usually in places where winters are extremely harsh.


Backup heating consists of electric strip heaters. Recall that electric strip heaters are metal strips that become red hot when electricity is passed through them.


In the backup heat mode, these strip heaters provide heat along with the gas furnace. Air that is blown from the blower is already hot as it has passed through the gas furnace. It becomes hotter as it flows over these red hot metal strips.


The backup heating starts when the outdoor temperature falls below a set temperature. This set temperature for backup heat to start is usually less than 35℉. This temperature is different for different centralized heating systems.


A backup heat mode is present in most modern systems. However, it is not present in older systems, and homeowners have to get them separately installed.


We discussed how a centralized system works during the heating mode, and backup heat mode. We also learned how the gas furnace provides heat in the heating mode.


We learned how the gas valve controls the flow of gas. We also discussed how the flame sensor, flame roll-out switch, vacuum switches, and thermal limit switches provide safety to the system. Lastly, we learned how the backup heat is required in harsh winters.



Question #1: We cannot find hot or cold corners in the room. This is a major advantage of which type of HVAC system?

  1. Split HVAC System

  2. Window AC

  3. Centralized HVAC System

  4. Mini and multi split HVAC systems.

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Answer: Centralized HVAC System

Hot or cold corners in the room are not generally found when the HVAC system is a centralized system. Mini split systems are small split systems. Multi split systems have multiple indoor units. These are essentially variations of a split HVAC system.


Question #2: A centralized HVAC system quieter as compared to a split HVAC system.

  1. True

  2. False

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Answer: True

Noisy components like the compressor are present in the outdoor unit. The indoor unit inside the house is present in the basement or attic of the house in a centralized HVAC system. So the house is more silent.


Question #3: The only component of a centralized system that can be seen inside the house is;

  1. Indoor Unit

  2. Outdoor Unit

  3. Air grilles

  4. Blower fan

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Answer: Air grilles

A centralized HVAC system is not at all visible inside the room. We can only see air grilles and the thermostat control in the room.


Question #4: The fuel used in gas furnaces for burning is;

  1. Fuel Oils

  2. Wood

  3. Refrigerant Gas

  4. Natural Gas

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Answer: Natural Gas

Natural gas is used as a fuel in gas furnaces for burning.


Question #5: The water generated due condensation in the heating and cooling process is sent to the drain by the;

  1. Condensate Pump

  2. Condensate Drain Pan

  3. Evaporator Coils

  4. Blower Fan Suction Line

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Answer: Condensate Pump

A condensate pump removes the condensation water and forces it down the drain. The condensate drain pan only collects the water in one place.


Question #6: The component of a centralized system that provides the necessary heat required for a flame to start in the gas furnace is;

  1. Matchstick

  2. Draft Inducer Fan Motor

  3. Flame Starter

  4. Flame Igniter

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Answer: Flame Igniter

A flame igniter is the component of a centralized system that provides the necessary heat required for a flame to start in the gas furnace.


Question #7: Which component in the centralized system ensures that the cooled or heated air reaches the rooms in the house?

  1. Ductwork

  2. Draft Inducer Fan Motor

  3. Blower Fan

  4. Air Filters

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Answer: Ductwork

Ductwork is a closed pathway made of thin metal sheets. The main purpose of the ducts is to provide a path for the hot or cold air to reach the rooms from the HVAC systems.


Question #8: A control board is also called as

  1. Furnace Control Board

  2. Electronic Control Board

  3. Both ‘1’ and ‘2’

  4. It is always referred to as the control board.

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Answer: Both ‘1’ and ‘2’

A control board is also called as a ‘Furnace Control Board’ or an ‘Electronic Control Board.’ So booth the options ‘1’ and ‘2’ are correct.


Question #9: Which control unit in a centralized system controls the flow of gas to a gas furnace?

  1. Gas Sensor

  2. Furnace Control Board

  3. Gas valve

  4. Thermostat

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Answer: Gas valve

A gas valve controls the flow inlet of gas to the furnace. The gas valve takes control signals from the control board to allow or stop the flow of gas.


Question #10: Which safety switch does not allow the water in the condensate drain pan to leak out?

  1. Float Switch

  2. Pressure Switch

  3. Condensate Drain Pump Switch

  4. Water Sensor

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Answer: Float Switch

A float switch detects the water level in the condensate drain pan. If the level of water rises above a set level, it shuts down the system and does not let water to leak out.


Question #11: The flame sensor detects;

  1. The flame in the gas furnace

  2. The burst of flames in the gas furnace

  3. The temperature of flames in the gas furnace

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Answer: The flame in the gas furnace

A flame sensor detects the flame in a gas furnace. It detects the presence of flames.


Question #12: Suppose that there is burst of flames in the gas furnace and there are flames coming out of the combustion chamber. Which safety switch will detect the flames?

  1. Thermal Limit Switch

  2. Flame Roll-Out Switch

  3. Vacuum Switch

  4. Flame Sensor

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Answer: Flame Roll-Out Switch

If there are burst of flames in the gas furnace and there are flames coming out of the combustion chamber, a flame roll-out switch will detect this.


Question #13: Pressing the COOL button on the thermostat sends out which type of signal?

  1. Control Signal

  2. Line Voltage

  3. Line Signal

  4. Alternating Signal

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Answer: Control Signal

Pressing the COOL button on the thermostat sends out a control signal.


Question #14: Which control component works to measure the temperature in the room?

  1. Thermal limit switch

  2. Temperature limit switch

  3. Temperature sensing bulb

  4. Discharge temperature switch

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Answer: Temperature sensing bulb

A temperature sensing bulb is a control component that measures the temperature in the room.


Question #15: The job of a float switch is to detect the water level in the outdoor unit pan.

  1. True

  2. False

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