• SkillCat Team

Split Systems in Residential HVAC - Components, Control & Working

Updated: Jul 8

Complete Residential Systems : Chapter 2


Split Systems - Components


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


How They Work!


A split system is an HVAC system in which the components are divided into two units:

  • An Indoor Unit (IDU), and

  • An Outdoor Unit (ODU).

The indoor unit is found inside the house, and the outdoor unit is found outside the house.


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.


Split systems are used in residences, big size rooms, apartments, and offices. We can see split systems most commonly in multi-family houses.


As major components are installed outdoors, the indoor unit is small and occupies less indoor space. The indoor units can have different and stylish designs. So, the unit can blend with the home’s interior and is loved by many owners.


Recall that components like a compressor make noise while working. As they are installed in the outdoor unit, the unit inside the house is less noisy and silent.


As split systems do not require ductwork, they are preferred in houses with no inbuilt ducts.

Because of this, they are also called a ‘Ductless HVAC system.’


The outdoor unit of the split 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 split systems, the indoor unit will also have a separate power cable. We connect this cable to the mains power socket in the wall.


Most split ACs also acts as a heat pump to heat the house in winters. The remote control or 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 watch a video to quickly recall the working of the refrigeration cycle in a split system.


Recall the components of a refrigeration cycle.

  • Compressor,

  • Condenser,

  • Metering Device,

  • Evaporator, and

  • Reversing Valve

A split unit has all these components and uses the heat pump cycle to heat a room.


In the heating mode, the split system acts as a heat pump. Recall that a reversing valve makes the system work as an air conditioner and heat pump as required. Let us watch a video to quickly recall the working of the reversing valve.




Electrical Components


Now that we have an overview of a split system, let us discuss 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 a split system:

  • Transformer,

  • Compressor Motor,

  • Capacitors,

  • Evaporator Blower Fan Motor,

  • Condenser Fan Motor, and

  • Swing Motors.


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


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

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


Recall that motors are devices that consume electricity to produce rotation. Recall that a window AC has a single fan motor connected to both the evaporator and condenser fans. A split system has two different fan motors for both these purposes.


An evaporator blower fan motor is present in the indoor unit. A blower fan is attached to this motor that circulates the air around the evaporator coils.


The condenser fan motor is in the outdoor unit of the split system. A fan attached to this motor circulates the air around the condenser coils.


A swing motor is placed inside the indoor unit of the split system. The swing motor is attached to the air louvers or flaps. The flaps change the direction of airflow coming from the blower fan.


As the flaps rotate up and down, it spreads the cold air throughout the room. The remote controls or thermostats have a ‘Swing function’ to activate the swing motor.


Ventilation Components


Ventilation in a split unit is done by the blower fan. The evaporator 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 through the evaporator coils. The air cools down or heats up as it flows past the evaporator coils. This cold or hot air is circulated in the room by the blower fan.


In this module, we discussed the different systems in a split system. We learned about how a split system is less noisy and has a small indoor part that counts as an advantage for the system.


We also went over the role of a reversing valve in the heating mode. Lastly, we also learned about the electrical components like a swing motor and evaporator blower fan motor.

Split Systems - Control Components


In this module, we will discuss the control components in split systems. We will learn about the functioning of each control component in a split system. Skip to quiz!


Controls in a Split 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 split system:

  • Thermostat,

  • Temperature sensing bulb,

  • Relays and Contactors,

  • Safety Controls, and

  • Solenoid Valve

All these components use a 24 Volts control signal.


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 split 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 solenoid is a type of valve that works on control signals. Recall that a valve is anything that can control the flow of refrigerant in an HVAC system.


A solenoid valve opens or closes whenever it receives control signals. A solenoid valve in the split systems works with the reversing valve.


Recall that the reversing valve controls the direction of refrigerant flow. A solenoid valve receives the control signal from the thermostat for heating or cooling.


Safety controls bind the system to work within prescribed boundaries. Safety controls present in a split system are:

  • Thermal switches,

  • High-pressure switch,

  • Low-pressure switch,

  • Defrost control board, and

  • Defrost thermostat

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.


Recall that the defrost sensor measures the temperature of the outdoor unit. t sends a control signal when the temperature of the outdoor unit falls below the freezing point. The defrost sensor is usually mounted on a tube of the condenser coils.


The defrost sensor is usually mounted on a tube of the condenser coils.


Recall that a defrost control board acts as a brain for the unit when ice forms on the outdoor unit. It receives the control signal from defrost sensor. As it receives this signal, it takes action to melt the ice on the condenser coils and increase its temperature.


In this part, we discussed the control components that we see in split units. We learned the role of a solenoid valve that controls the reversing valve in a split system. We also learned about the defrost sensor and defrost control board that does not allow ice to form on the outdoor unit of the split system.

Split Systems - Working


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


Complete Working


A split 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 thermostat on the indoor unit.

Modern split systems have many different settings. We will focus on a few selected modes:

  • Cooling Mode,

  • Heating Mode, and

  • Defrost 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 split system is very similar to a window unit. 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.

Recall that a split system has a different blower and condenser fan motor.


When we press the cooling mode on the thermostat, it also 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.



Complete Working - Heating Mode


Pressing the HEAT button on the thermostat or the remote control starts the heating mode.

Recall that the split system works as a heat pump in the heating mode. The refrigeration cycle starts along with the fans in heating mode as well.


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

  • Compressor Motor,

  • Blower Fan Motor,

  • Condenser Fan Motor, and

  • Reversing Valve

Recall that the reversing valve is the component that allows the system to work as a heat pump.


In the heating mode, the thermostat sends a signal to the solenoid valve of the reversing valve. Recall that the solenoid valve controls the reversing valve.


In cooling mode, the high-temperature refrigerant flows from the compressor to the condenser coils in the outdoor unit. In heating mode, the high-temperature refrigerant flows from the compressor to the evaporator coils in the indoor unit.


In the cooling mode the solenoid does not receive any control signal. So reversing valve is in a de-energized position. In the heating mode the solenoid receives a control signal from the thermostat.


We learned how the heating and cooling in the split system works. Let us look at how it is controlled. Recall how pressing the heat or cool buttons 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 window unit. We will discuss some minor differences about how the controlling the system differs.

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


Safety switches in a split system perform the same function as in a window unit. 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 on detecting abnormal pressures.


Defrost Mode


Recall that the outdoor unit of a split system is kept in the open. Then winters are harsh, there are chances that ice forms on the outdoor unit and the condenser coils. If first is formed on the condenser coils it does not allow the transfer of heat to the outside air.


In such situations, when we turn the system on, it goes into defrost mode. A defrost mode is necessary in split systems because ice formation does not allow the system to work. The defrost mode usually lasts only for a few minutes.


Recall that a defrost sensor measures the temperature of the outdoor coils. It sends a control signal to the defrost control board when the temperature of the outdoor unit falls below 0℃, which is the freezing point.


On receiving the control signal from the defrost sensor, the defrost control board takes couple of steps. First, it sends a control signal to the solenoid valve in the reversing valve. The reversing valve gets de-energized and the system starts to work in cooling mode


Recall that in cooling mode, the hot refrigerant will go to the condensing coils. This hot refrigerant will help melt the ice on the outdoor unit.


Once the outdoor unit temperature rises above 0℃, the defrost sensor will stop sending the control signal. The defrost control board will make the system work in cooling mode till it stops receiving a signal from the defrost sensor. The system will start to work normally afterward.


Modern split systems also have an electric strip heater installed in them. We need the system to provide heat in winter, but in the defrost mode, the system works in cooling mode. So an electric strip heater provides the necessary heating for a temporary period.


In general, the thermostat and the temperature sensing bulb start the system. The safety switches stop the system if it goes out of bounds. The defrost mode does not allow ice to form on the outdoor unit of the system.


We discussed three major modes in a split system. The cooling mode, heating mode, and defrost mode.We also learned how the thermostat sends a signal to the:

  • Compressor Motor,

  • Blower Fan Motor,

  • Condenser Fan Motor, and

  • Reversing Valve in heating mode

Lastly, we learned how the split systems work in the cooling mode under defrost mode, even in winters. Also, how the defrost sensor detects the outdoor unit temperature to tell the system when to defrost.

Question #1: A split system takes up a lot of space inside the house

  1. True

  2. False

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Answer: False

The refrigeration cycle components are divided into the indoor unit and the outdoor unit. The unit inside the room has fewer components and is small in size. This does not take up a lot of space inside the rooms.

Question #2: An advantage of the split system is;

  1. These systems are quiet and produce low noise.

  2. These systems are the cheapest HVAC systems.

  3. These systems do not require a compressor to function.

  4. These systems do not require a skilled technician to install it.

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Answer: These systems are quiet and produce low noise.

Noisy components like the compressor are present in the outdoor unit. So the indoor unit inside the house is more quieter. So the house is more silent.


Question #3: We can set the mode to heating or cooling and set the temperature for the room in a split AC on the;

  1. Fan control relays

  2. Control boards

  3. Remote control

  4. Outdoor unit

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Answer: Remote control

We can set the mode to heating or cooling and set the temperature for the room in a split AC on the remote control.


Question #4: Which component in a split system plays a major role in switch from heating to cooling mode?

  1. Swing Motor

  2. Reversing Valve

  3. Compressor Contactor

  4. Evaporator Blower Motor

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Answer: Reversing Valve

The reversing valve is the component that lets the split system work in both heating and cooling modes. It does this by changing the direction of the refrigerant flow. When needed, it can send the hot refrigerant to the indoor unit so that room gets hot.


Question #5:A component that converts the line voltage to control voltage is;True

  1. Fan Motor

  2. Thermostat

  3. Compressor contactor

  4. Transformer

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Answer: Transformer

The transformer is the component that converts the 120 Volts line voltage to a 24 Volts control voltage.


Question #6: Which motor is responsible for moving the flaps so that the air is distributed evenly in the room?

  1. Fan Motor

  2. Compressor Motor

  3. Condenser Motor

  4. Swing Motor

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Answer: Swing Motor

The swing motor is attached to the flaps. As the flaps move, it changes the direction of airflow and distributing the air in the room.


Question #7: The fan that is responsible for circulating the air inside our rooms is;

  1. Compressor Fan

  2. Blower Fan

  3. Condenser Fan

  4. Swing Fan

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Answer: Blower Fan

The blower fan sucks the air from the room and forces it over the evaporator coils. As the evaporator coils have cold refrigerant flowing in them, it cools the air. This gives a jet of cold air that circulates in our rooms.


Question #8: A component that controls the compressor motor in the system is____.

  1. Contactor

  2. Relay

  3. Solenoid Valve

  4. Transformer


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Answer: Contactor

A contactor which is also called the compressor contactor, controls the compressor motor in the system.


Question #9: The reversing valve is controlled by the____.

  1. Defrost Control Board

  2. Thermal Switches

  3. Solenoid Valve

  4. Service valves

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Answer: Solenoid Valve

A solenoid valve controls the reversing valve. The reversing valve changes the direction of the refrigerant flow to make the system work in heating or cooling mode.


Question #10: The job of all the safety controls is to;

  1. Control the temperature in any room.

  2. Bind the system to work within prescribed boundaries.

  3. Bind the system to work outside the prescribed boundaries.

  4. Control all the functions of the system.

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Answer: Bind the system to work within prescribed boundaries.

The job of all the safety controls is to bind the system to work within prescribed boundaries.


Question #11: The job of which component is to not allow ice to form on the outdoor unit?


  1. Temperature sensing bulb

  2. Condenser Fans

  3. DTS

  4. Defrost Control Board

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Answer: Defrost Control Board

The defrost control board performs the task of not allowing ice to form on the outdoor unit of the split system.


Question #12: When we press cooling on the thermostat of a split system, it sends a control signal to the Airflow increases

  1. Compressor motor

  2. Compressor motor and the blower fan motor

  3. Compressor motor, blower fan motor, and condenser fan motor

  4. Blower fan motor and condenser fan motor


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Answer: Compressor motor, blower fan motor, and condenser fan motor.

When we press cooling on the thermostat of a split system, it sends a control signal to the compressor motor, blower fan motor, and condenser fan motor.


Question #13: When we press heating mode, the thermostat sends a control signal to ____ and the reversing valve is in an ____ position.

  1. heating panel; energized

  2. cooling panel; energized

  3. solenoid valve; energized

  4. heating panel; de-energized

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Answer: solenoid valve; energized

When we press heating mode, the thermostat sends a control signal to solenoid valve. The solenoid energizes the reversing valve. So we say that the reversing valve is in an energized position.


Question #14: When a split system goes into defrost mode, it does not get back to normal till

  1. The defrost sensor stops sending control signal

  2. The defrost sensor starts sending control signal

  3. The defrost sensor stops sending line voltage.

  4. The temperature sensing bulb detects the room temperature to be warm enough.

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Answer: The defrost sensor stops sending control signal

When a split system goes into defrost mode, it does not get back to normal till the defrost sensor stops sending control signal. The defrost sensor stops sending the control signal when the temperature of the outdoor coil rises above 0℃.

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