• SkillCat Team

Residential Systems: Window Unit Components & Working

Updated: Jul 7

Complete Residential Systems: Chapter 1


Window Unit - Components


In this module, we will discuss the residential window units. We usually call them a window AC. We will learn about the overall system and functioning of a window unit. Skip to quiz!


How They Work!


Window units are air conditioning systems packaged in a small box.All systems and components required for cooling are stuffed into a small metal box. These are some of the smallest types of HVAC systems you will find in the market.


As window units are small systems, they have many advantages. We don’t need a skilled technician for installing a window unit. It is easy to install.


We only require an open window where we can install a window unit. There are no additional requirements like a ductwork or refrigerant piping for it.


Window units are small systems that are used to cool small spaces. It is one of the cheapest and the best options for people living in flats and apartments with windows. Window units are also used for separate individual rooms in large industrial facilities.


It is pretty easy to use a window unit once it is installed. It has only one power cable which should be connected to the mains power socket in the wall.


The temperature inside the room can be set by using a remote control or buttons on the unit. Let us look at a video that shows the working of a window unit.


Recall the components of a refrigeration cycle.

  • Compressor,

  • Condenser,

  • Metering Device, and

  • Evaporator

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



Electrical Components


Now that we have an overview about a window AC, let us discuss about the electrical components in it. We see the following electrical components in a window unit:

  • Transformer,

  • Fan Motor, and

  • Compressor Motor

  • Capacitors


Electrical components work on electricity and require a line voltage. Recall that the voltage coming to our homes via the power socket is called line voltage. The line voltage in most residences in the US is 120 Volts.


Recall that a transformer is a device used to convert a high voltage into a low voltage and vice versa. In a window unit, a transformer converts the line voltage of 120 volts to 24 volts required for control components. It sends the 24 Volts to the thermostat.


Recall that motors are devices that consume electricity to produce rotation. A fan motor is connected to the fan blades and rotates it. The fan circulates the air around the evaporator and the condenser coils.


Recall the working of a compressor. A compressor motor present in the compressor consumes electricity and provides rotation. Other parts of the compressor compress the refrigerant.


Recall that the capacitor is a device that can store electric charge. It helps the the compressor to start and run smoothly. All motors have a capacitor



Ventilation Components


Ventilation in a window unit is done by the blower fan. The fan motor is connected to the blower fan and rotates it. The blower fan is like a normal table fan that throws a forceful jet of air.


Recall that the blower fan sucks the air from the room and forces it through the evaporator coils. The air cools down as it flows past the evaporator coils. This cool air is circulated in the room by the blower fan.


New models of window units have a fresh air vent in them. Fresh air from outside is added in the room by opening the vent provided in the system.



Controls in a Window AC


Recall that control systems govern the system as per the inputs required and provided by the user. For example, when you set the system to cool a room to 70℉, you control the system via the controls in it. Control systems also ensure that the system works safely.


We see the following controls in a window unit:

  • Thermostat,

  • Temperature sensing bulb,

  • Relays and Contactors, and

  • Safety Controls


All control components use a control voltage to send their control signals. Recall that all control signals are 24 Volts.


Recall that a thermostat is a device to set the system to heating or cooling mode. We can set the temperature, fan speed, and many such settings on the thermostat. The HVAC system works as per the inputs set by us on the thermostat.


A temperature sensing bulb detects and gives feedback to the system about the room temperature. For example, consider a user sets the temperature of 65°F on the window unit. The temperature sensing bulb will tell the system to stop cooling when the room temperature reaches close to 65°F.


The fan relay is a switch that controls the fans in the system. It receives the co

ntrol signal from the thermostat to turn the fan motor on and off.


A contactor is a switch that controls the compressor motor in the system. It receives the control signal from the thermostat to turn the compressor motor on and off. Recall that a contactor is a type of relay that can handle higher voltages and current.


Safety controls bind the system to work within prescribed boundaries. Safety controls present in a window unit are

  • Thermal switches

  • High-pressure switch

  • Low-pressure switch


Thermal switches detect the temperatures within the system. If the temperature rises above a set value, this switch stops the control signal and turns off the unit. Thermal switches protect the system and user from high temperatures in the system that can cause a fire.


Pressure switches protect the system from extreme pressures. Very high pressures can cause an explosion in the system. Low pressures can sometimes cause unpredictable harm to the system.


A High-Pressure Cutout (HPCO) switch detects pressure on the outlet of the compressor. Recall that high-pressure refrigerant comes out from the compressor. If the refrigerant pressure from the compressor is higher than the set value, this switch stops the control signal and turns off the unit.


A Low-Pressure Cutout (LPCO) switch detects pressure on the inlet of the compressor. Recall that low-pressure refrigerant flows into the compressor. If the pressure of the refrigerant going to the compressor is lower than the set value, this switch stops the control signal and turns off the unit.


In this topic, we discussed the advantages of a residential window unit that include its easy of installation and no need for addition components. We also learned about the electrical components like a transformer, motor, and compressor.


Lastly we discussed the controls and safety controls like a DTS, HPCO, and LPCO that are present in a residential window unit.



Window Unit - Working


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


Complete Working


The window unit starts working by pressing the ON button on the remote control. You can also start the unit using the buttons on the thermostat on the window unit.


Modern window ACs have many different settings. We will focus on a few selected modes:

  • Fan Mode, and

  • Cooling Mode


When we press fan mode on the thermostat, it switches on the fan motor. Recall that motors are devices that consume electricity to produce rotation. The fan motor rotates the fan and circulates air in the room.


When we press the fan mode on the thermostat, it sends a control signal to the fan relay. Recall that the fan relay is a switch that controls the fans in the system. Also recall that a control signal is 24 Volts.


As the fan relay receives the control signal, it sends the line voltage to the fan motor. Recall that the line voltage is the 120 Volts coming from the power socket. The fan motor starts as it receives the line voltage.


Recall that the fan motor is connected to the blower fan. When the fan motor starts, it rotates the blower fan.


The blower fan is like a regular table fan that throws a forceful jet of air. This air is circulated in the room. This is why we feel a jet of air coming from the window unit.


The line voltage of 120 Volts can give an electric shock to humans. We touch a thermostat with our bare hands. It would become dangerous to the user if the thermostat worked on the line voltage of 120 Volts.


So the relay handles the line voltages and the thermostat handles control voltages. In the absence of a relay, the thermostat would have to send the line voltage of 120 Volts to the components. As this is dangerous, the thermostat only works on the control signal of 24 Volts.


Pressing the ON button on the thermostat starts the cooling mode. The refrigeration cycle starts along with the fans in cooling mode. Recall that the whole purpose of the refrigeration cycle is to remove heat from the room.


In the cooling mode, the thermostat sends a control signal to the compressor contactor. Recall that the compressor contactor controls the compressor motor in the system. Recall that the contactor is a type of relay that can handle higher voltages and current.


As the compressor contactor receives the control signal, it sends the line voltage to the compressor. So the contactor receives control signals from the thermostat and turns the compressor ON or OFF.


Starting the compressor starts the refrigeration cycle. Let us quickly look at a video to recall the refrigeration cycle.


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


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


The air from the blower fan gives its heat to the refrigerant. So the air is cooled as it flows over the evaporator coils. This cold air cools the room.


In the window unit, the fan motor is also connected to the condenser fan. Recall that the condenser fan forces air through the condenser coils. So, a single fan motor runs the blower fan and the condenser fan in the window unit.


Recall that the air from the condenser fan takes the heat from the refrigerant and throws it out. This cools the refrigerant, and the refrigeration cycle continues.


We learned how we get cool air from the window AC. Let us look at how to control the temperature and its safety. Recall that pressing the cool button on the thermostat starts the following:

  • Compressor motor and the refrigeration cycle,

  • Fan motor and ventilation of air.


Along with pressing the cool button, we also set the temperature that we desire on the thermostat. When we set the temperature, it sets a benchmark for the thermostat to which the room should be cooled.


Recall that the temperature sensing bulb detects the temperature of the room. The temperature sensing bulb detects and tells the thermostat about the room temperature. The thermostat compares the room temperature with the set temperature in it.


The thermostat starts cooling if the room temperature is higher than the set temperature. Recall that starting cooling means that the thermostat sends control signals to the fan relay and contactor.


Now we know how our room is cooled and how we control its temperature. Let us look at the working of the safety components in the system.


Recall that a Discharge Temperature Switch (DTS) switch detects temperatures in the system. The DTS is connected to the control line. It is connected between the thermostat and the compressor contactor.


We can see the DTS switch at the outlet of the compressor. Suppose the temperature from the compressor is higher than the set value. In that case, this switch stops the control signal from the thermostat to the compressor contactor.


The contactor opens and stops the line voltage from going to the compressor. No line voltage will turn off the compressor. This, in turn, will stop the refrigeration cycle in the system.


Recall that a High-Pressure Cutout (HPCO) switch detects pressure on the outlet of the compressor. The HPCO is connected to the control line. It is connected between the thermostat and the compressor contactor.


Suppose the refrigerant pressure from the compressor is higher than the set value. In that case, this switch stops the control signal from the thermostat to the compressor contactor.


The contactor opens and stops the line voltage from going to the compressor. No line voltage will turn off the compressor. This, in turn, will stop the refrigeration cycle in the system.


Recall that a Low-Pressure Cutout (LPCO) switch detects pressure on the inlet of the compressor. The LPCO is also connected to the control line. It is connected between the thermostat and the compressor contactor.


Suppose the refrigerant pressure going to the compressor is lower than the set value. In that case, this switch stops the control signal from the thermostat to the compressor contactor.


The contactor opens and stops the line voltage from going to the compressor. No line voltage will turn off the compressor. This, in turn, will stop the refrigeration cycle in the system.


The control signal from the thermostat passes through three safety switches: the DTS, HPCO, and LPCO. They stop the control signal on detecting any abnormal temperature or pressure in the system.


In general, the thermostat starts the system. The temperature sensing bulb controls the system as per the room temperature. The safety switches stop the system if it goes out of bounds.


In this topic, we learned how a window unit works. We discussed the fan mode and cooling and what happens when a user sets these modes on the thermostat. Lastly, we also learned about the electrical connections that coordinate with the controls to make it all work.




Question #1: A window unit is one of the best HVAC system for a/an

  1. Mansion

  2. Indoor Gym

  3. Apartment

  4. Five Star Hotel

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Answer: Apartment

A window unit will be one of the best HVAC system for a small apartment room.


Question #2: Which control unit in a window AC allows you to set the temperature of the room?

  1. Temperature sensing bulb

  2. Contactor switches

  3. Control Board

  4. Thermostat

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Answer: Thermostat

The thermostat is a component of the window unit on which you can set the temperature to which the room should be cooled.


Question #3: A window AC is designed so that the temperature in no part of the system goes above 200°C. If the temperature in the compressor outlet is 230°C which component will detect it?

  1. Thermal Switch

  2. Low Pressure Switch

  3. Temperature Sensing Bulb

  4. No component as the temperature is within the acceptable range.

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Answer: Thermal Switch

A thermal switch detects the abnormally high or low temperatures within the system. As the temperature is 230°C which is greater than the acceptable temperature of 200°C, the thermal switch will detect this temperature.


Question #4: A signal coming from the thermostat to the fan relay is a ____ signal and is of ____ Volts.

  1. control ; 24

  2. control ; 120

  3. line ; 24

  4. master ; 24

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Answer: control ; 24

A signal coming from the thermostat is a control signal and is 24 Volts. Recall that the thermostat is a control component. It sends a control signal to other components to turn them on or off. A control signal is generally 24 Volts.


Question #5: In a window unit, how many fan motors do you generally see?

  1. 1

  2. 2

  3. 3

  4. 0

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Answer: 1

In a window unit we generally see one fan motor. This single fan motor rotates the blower fan and the condenser fan.


Question #6: Pressing the cool button on the thermostat starts which of the following?

  1. The compressor motor

  2. The fan motor

  3. Both the compressor and fan motor

  4. Only the compressor motor and sometimes the fan motor.

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Answer: Both the compressor and fan motor

Pressing the cool button on the thermostat always starts the compressor motor and fan motor.


Question #7: A control component that measures the room temperature and controls the system based on it is?

  1. Discharge Temperature Switch

  2. Temperature Sensing Bulb

  3. Fan Relay

  4. Temperature buttons

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Answer: Temperature Sensing Bulb

A temperature sensing bulb measures the room temperature and controls the system based on it. It starts the system when the room needs more cooling and stops the system when the room temperature reaches close to the set temperature.


Question #8: Abnormally high temperatures in the window unit are detected by:

  1. The Discharge Temperature Switch

  2. The High-Pressure Cutout Switch

  3. The Low-Pressure Cutout Switch

  4. The Temperature Sensing Bulb

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Answer: The Discharge Temperature Switch

Abnormally high temperatures in the window unit are detected by the discharge temperature switch.


Question #9: The control signal coming from the thermostat to the compressor contactor passes through which of the controls?

  1. LPCO and HPCO.

  2. LPCO, DTS, and Temperature Sensing Bulb.

  3. LPCO, HPCO, and Temperature Sensing Bulb.

  4. LPCO, HPCO, and DTS.

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Answer: LPCO, HPCO, and DTS.

The control signal coming from the thermostat passes through the safety controls. The safety controls LPCO, HPCO, and DTS allow the control signal to pass only when the temperature and pressure are within the set limits.

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