Packaged Rooftop Units: Chapter 2
Heating System
In this module, we will take a look at the heating systems used in packaged rooftop units. Skip to quiz!
Types of Heat Systems
We use different types of heating systems in packaged rooftop units. These include:
Gas furnaces,
Electric heating systems,
Heat pumps, and
Oil heating systems.
Oil heating systems are common in places where there are very few gas pipelines. On the other hand, the oil in your home must be refilled manually, just like the oil in your car. They are not so commonly used in RTUs. We’ll look at the other heating systems in detail.
Gas Furnaces
Recall that a gas furnace burns different fuel gases to heat an entire building. The heat generated by burning these gases is used to heat the air inside a building. These systems provide hot air in extremely low-temperature regions.
In RTUs, we generally use these gases for heating:
Natural gas, and
Propane gas.
These fuels are usually mixed with air and burnt.
Recall that a gas burner in a furnace works to mix gas with air in order to burn it. A gas valve controls the flow of gas going into the gas burners. The hot burnt gas that comes from burners will heat up the heat exchanger plates.
Heat exchanger tubes have the hot burnt gases flowing inside them and air flowing outside them. A blower fan then pushes air over the hot heat exchanger plates. This heats the air. Hot air is then transferred, via the ductwork, and distributed throughout the building.
Electric Heaters
Recall that electric heaters have metallic coils. When we pass electricity through these coils, they become red hot. A blower fan then forces air to flow over these coils. This heats the air. Electric heaters don’t burn anything, so they do not produce any smoke or burnt gases. There is no requirement for venting pipes or chimneys to remove smoke, as there is no burning. However, electric heating systems consume a large amount of electricity.
Electric heating systems are many times used as backup heaters in rooftop units. These systems are considered more reliable due to the 24/7 availability of electricity in many places.
Another reason electric heating systems are used a backup is that they can work in any weather conditions. They rarely have the problem of not starting or not producing enough heat in snowy weather. However, due to their high electricity consumption, they are preferred as backup heaters in most RTUs.
Heat Pumps
Recall that a heat pump provides heat by using refrigerant liquid. It does not use any fuel. The heat rejected by a condenser in air conditioning mode is used to heat a house in heating mode. A heat pump is used in the winters as a heater, and the summer season as an air conditioner.
Heat pumps use the refrigeration cycle to heat rooms. Recall that in the air conditioning cycle, an evaporator absorbs heat from the room air and cools it. Condenser coils cool refrigerant by throwing its heat outside.
Recall that a heat pump reverses the refrigeration cycle by using equipment called a reversing valve. This component reverses the direction of refrigerant flow, so an evaporator has hot refrigerant instead of cold. Instead of absorbing heat from a room, evaporator coils act as a condenser and heat the room air.
Some RTUs have two evaporator coils. They are called:
Cooling coils, and
Heating coils.
Cooling coils are used all the time for cooling, and heating coils are used for heating. These are older systems. New systems have only one coil performing both functions.
Heat pumps are used in RTUs installed in places with moderate climates. Heat pumps cannot be used to produce heat in freezing climates. In places where the temperature drops very low, below 32 F, ice forms on the outdoor coils. This creates problems in heat pumps.
Year Round Rooftop Units
Year round rooftop units are equipped with both cooling and heating coils. They are designed to work in winter, as well as in the summer.
In the summer, heating coils are made inoperative, and cooling coils are made operative. A cooling coil works to cool air. In this process, it also dehumidifies air.
Recall that when air passes through a cooling coil, moisture in the air converts into water. A condensate drain helps remove this water.
In winter, cooling coils are made inoperative, and heating coils are made operative. Recall that a heating coil operates to heat air. Some RTUs can have humidifiers to add moisture to dry air.
We use different types of heating systems in a packaged rooftop unit. These include:
Gas furnaces,
Electric heating systems,
Heat pumps, and
Oil heating systems.
RTUs with gas furnaces as heating systems are very common in places with gas pipelines.
Electric heating systems are mostly used as backup heaters in rooftop units. A heat pump provides heat by using refrigerant liquid. It does not use any fuel.
Placement of Components
In this module, we will take a look at the placement of components inside a basic rooftop unit. We’ll also look at the placement of components in an advanced rooftop unit. Skip to quiz!
Placement of Components
Recall that a rooftop unit is a packaged system. All the components of an RTU are placed inside a metal box. Components are usually placed inside an RTU in a way that they are compact and consume less space on a roof.
The main components of a basic RTU are:
Housing,
Air intake hood,
Dampers,
Air filters,
Cooling coil,
Heating coil, and
Blower.
Let’s learn about the placement and function of each of these components in detail.
Housing is the insulated enclosure of an RTU. It acts as protection for all the components. All the components of an RTU are placed inside the housing. It prevents heat transfer with outside air. It also prevents air leakage in and out of an RTU.
Housing protects the RTU components from the sun, wind, rain, snow, frost, etc. We can perform repair and maintenance work by opening the access panels on the housing of an RTU.
On one side of an RTU, we can see the presence of an air intake hood. An air intake hood allows outside fresh air to enter an RTU. It faces downwards so that rain and dust particles cannot directly enter an RTU.
Recall that dampers are plates that stop or regulate the flow of air. Dampers are placed at supply and return ducts in RTUs. As they regulate the amount of air entering and leaving, they are placed near the entry and exit of an RTU.
An intake air damper is placed at the start of an RTU. It controls the amount of fresh outdoor air coming into an RTU. An air intake hood acts as the window frame for an RTU, while the intake air damper acts as a window door that controls the quantity of air entering an RTU.
All RTUs contain air filters to clean air in a room. Recall that air filters trap dust particles and debris from the air.An air filter is placed right behind the intake air damper.
Cooling coils in RTUs cool and dehumidify the air. RTUs usually have refrigerant cooling coils. Dust accumulates over cooling coils easily. To avoid this, we place a cooling coil right behind the air filters.
Heating coils are connected with the heat pump system in an RTU. A heating coil is placed right behind the cooling coil. We only use cooling or heating, but not both, so placing the coils one after the other saves space.
A blower is placed behind the heating or cooling coils. Recall that blowers are a type of fan that forces air to move through ductwork into rooms and office spaces. We generally use centrifugal blowers in RTUs.
Other Components
Some RTUs may also recirculate return air through the ductwork. This reduces energy consumption. In these RTUs, we can find a return air damper at the end of the return air ductwork. Return air dampers regulate the amount of return air that mixes with fresh air.
In some RTUs, we reject some or all amounts of return air. This is to replace the stale air in a room with fresh outdoor air. In these RTUs, a return air exhaust is used to throw out stale return air. Return air exhausts are placed below the intake air hood in an RTU.
Advanced RTUs are made with a built-in heat wheel. These RTUs are becoming more popular due to their high energy efficiency. Heat wheels are also called energy recovery wheels (ERW), heat recovery wheels (HRW), or energy wheels.
A heat wheel rotates between fresh air and return air. It picks heat or cool from the exhaust return air, and transfers to intake fresh air without mixing them.
Some additional components of a heat wheel configuration RTU are:
Exhaust air filter,
Exhaust fan, and
Exhaust damper.
An exhaust air filter is placed before the heat wheel to filter exhaust air.
An exhaust fan is placed after the heat wheel to pull the exhaust air through it.
An exhaust damper is placed after the exhaust fan to control the amount of exhaust air released into the atmosphere.
All the components of an RTU are placed inside a metal box. Housing is the insulated enclosure of an RTU. On one side of an RTU, we can see the presence of an air hood. An intake air damper is placed at the beginning of an RTU.
An air filter is placed right after the intake air damper. A cooling coil or heating coil is placed after the cooling coil. A blower is placed after the heating or cooling coils. A heat wheel rotates between intake fresh air and exhaust return air.
Electrical Components
In this module, we will take a look at the electrical components of a rooftop unit and how they work.
Electrical Components
Recall that electrical components require a line voltage.RTUs usually work on a 3-phase power supply. They mostly work on 420V AC.
The main electrical components of an RTU are:
Compressor,
Blower motor, and
Condenser fan motor.
Some RTUs can have electrical heaters. We’ll learn about these components in upcoming slides.
Compressors have a component that runs off of electricity: Compressor Motor. Recall that a compressor changes low-pressure refrigerant to a high-pressure refrigerant with the help of a motor. Recall that a motor uses electricity to rotate.
A blower has only one component that requires electricity to operate: the blower motor. Recall that blowers are a type of fan that forces air to move through ductwork into rooms and office spaces. Recall that a motor is a device that uses electricity to rotate.
A condenser has only one component that requires electricity to operate: The condenser fan motor. Recall that a motor is a device that uses electricity to rotate. When a condenser fan motor is powered by electricity, the condenser fan will send air through its condenser coils.
3-phase electricity supplies are connected to the control panel of an RTU. From the control panel, electrical cables are connected to a motor starter or VFD. From the motor starter or VFD, electrical cables are connected to the terminals of motors.
Motor starters are electrical devices used to start and stop a motor safely. They’re similar to a contactor, but they provide low voltage and overcurrent protection. Let’s watch a video to better understand how they work.
VFD stands for ‘Variable Frequency Drive.’ It’s also used to control an electrical motor as the motor starter. It controls the speed of a motor by varying the frequency and voltage supplied to it.
VFD controls the speed of a motor. Most three-phase motors are connected to HVAC fans and operate at a fixed rate. The motor is either coupled directly to a fan, or will run the fan using a belt and pulleys.
VFD allows a motor to run at a different speed. In rooftop units, the speeds of motors are controlled by VFD. VFD minimizes the power consumption of an owner by setting different speeds for each motor.
Compressors, blower motors, and condenser fan motors have 3-phase motors as an integral part of RTUs. 3-phase motors are sealed inside compressors.
Three-phase motors also have a stator and a rotor, just like single-phase. A stator does not move and accepts electrical power. A rotor is attached to the rotating load.
Recall that some RTUs can have electric heaters. Electric heaters are devices that convert electricity into heat. This heat is used in RTUs to raise the temperature of air.
Recall that electric heaters have metallic coils. When we pass electricity through these coils, they become red hot. A blower fan then forces air to flow over these coils. This heats the air.
Other Components
Recall that transformers are devices used to convert a high voltage into a low voltage and vice versa. In rooftop units, a transformer converts the line voltage of 420 volts to 24 volts, as required for control components. It then sends the 24 volts to a thermostat and all other controls.
Recall that contactors are switches that control the compressor motor in a system. They receive control signals from a 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.
A condenser fan relay controls the power to a condenser cooling fan. When activated, a relay will turn a fan on and circulate air through the condenser coils. The fan helps cool refrigerant. By controlling the power of the fan, the relay helps control the refrigerant cooling process.
An actuator regulates the air entering and leaving an RTU by opening and closing its damper blades. The control board sends a signal to the actuator to change the position of the damper.
RTUs usually work on a 3-phase power supply. They mostly work on 420V AC. Compressors, blower motors, and condenser fan motors have 3-phase motors as an integral part of RTUs. Some RTUs can have electrical heaters.
Controls
In this module, we will learn about controls in packaged rooftop units (RTUs). Skip to quiz!
In an RTU, controls are used to:
Maintain the temperature and humidity of air,
Maintain optimum air quality,
Reduce energy consumption,
Reduce manpower,
Identify maintenance problems, and
Prevent hazards.
Recall that HVAC control systems are used to control the operations of HVAC equipment. Control systems use a small amount of voltage. In packaged rooftop units, we use 24V DC for control purposes.
Recall that thermostats are devices used to set a system to heating or cooling mode. We can set the temperature, blower speed, and many other settings on a thermostat.
HVAC systems work as per the inputs set by residents on a thermostat. All control signals for controlling a system and maintaining its safe operation are sent through a control board.
A control board is an electronic device that transfers a command to electrical components. A thermostat transfers commands to a control board according to the owner’s input. A control board transfers signals to the compressor, dampers, motorized valve, EXV, and other components.
In rooftop units, control systems can be divided into:
Safety controls, and
Flow controls.
Let’s learn more about both types of controls in RTUs.
Safety Controls
Safety controls prevent hazards. Safety control circuits are always in closed condition. If any abnormality happens, these will open to turn off an RTU.
Safety controls present in chiller systems are:
Overload relays,
LPCO switches,
HPCO switches,
Discharge temperature switches, and
Motor winding thermostats.
Let’s learn more about these safety controls in detail.
In RTUs, overload relays are found in compressors, blower motors, and condenser fan motors. Recall that overloading is a condition where a motor draws more current than the specified range.
An overload relay detects the overloading of a motor and stops the electric supply to a compressor. It protects a motor from overheating and causing damage.
LPCO switch stands for ‘low-pressure cutout switch’. Recall that these switches detect pressure on the inlet of a compressor. They stop control signals and shut off a system if the refrigerant pressure from a compressor is lower than the set value.
LPCOs are connected to a control line between a thermostat and a compressor contactor. If the refrigerant pressure from a compressor is lower than the set value, this switch stops control signals from a thermostat to the compressor contactor.
A contactor then opens and stops the line voltage from going to a compressor. No line voltage will turn off the compressor. This, in turn, will stop the refrigeration cycle in a system.
HPCO switch stands for ‘high-pressure cutout switch.’ Recall that these switches detect pressure on the outlet of a compressor. They stop control signals and shut off a system if the refrigerant pressure from a compressor is higher than the set value.
HPCOs are connected to a control line between a thermostat and a compressor contactor. If the refrigerant pressure from a compressor is higher than the set value, this switch stops the control signal from a thermostat to the compressor contactor.
A contactor then opens and stops the line voltage from going to a compressor. No line voltage will turn off the compressor. This, in turn, will stop the refrigeration cycle in a system.
A discharge temperature switch (DTS) is connected to the control line between a thermostat and a compressor contactor. At a compressor outlet, we get a high-pressure, high-temperature refrigerant vapor. The temperature of a compressor outlet may become much higher than it usually would.
We can see a DTS switch at the outlet of a compressor. Suppose the temperature from a compressor is higher than the set value. In this case, this switch stops control signals from a thermostat to the compressor contactor.
This contactor then opens and stops the line voltage from going to a compressor. No line voltage will turn off the compressor. This, in turn, will stop the refrigeration cycle in the system.
Motor winding thermostats are installed in motors to protect them. These are used to monitor the temperature of a motor winding.
They also stop control signals and shut off a motor if the temperature rises above a set threshold. This protects a motor from the excessive winding temperature that could lead to damage.
The control signal from a thermostat passes through three safety switches: the DTS, HPCO, LPCO, overload relays, and motor winding thermostats. These stop control signals from detecting any abnormal temperature or pressure in a system.
Flow Controls
Flow controls regulate the flow of refrigerant in an RTU. They’re installed in refrigerants piping.
Some of the flow controls present in RTUs are:
Solenoid valves,
Hot gas bypass valves, and
Float switches.
Let’s learn more about these flow controls in detail.
Recall that a solenoid is a type of valve that works on control signals. A solenoid valve opens or closes whenever it receives control signals. The solenoid valve in an RTU works with the reversing valve. Recall that reversing valves control the direction of refrigerant flow.
Hot gas bypass valves can also be called discharge bypass valves. They’re usually installed at the outlet of a metering device.
Float switches detect the water level in a condensate drain. Recall that when air passes through an evaporator, moisture in the air becomes water droplets. This water collects in condensate drain pans.
If a water level increases beyond the set level, float switches will cut control signals from a thermostat. Float switches are present between thermostats and control boards. This tells a user or technician that a condensate drain line is chocked.
Sensors
Some of the sensors that we find in AHUs are:
Temperature sensors,
Humidity sensors,
Pressure sensors, and
CO2 sensors.
We’ll learn about these sensors in detail.
Most RTUs have an electronic control board that receives signals from these sensors and controls different components. Temperature sensors are installed in RTUs to detect the temperature of air.
Temperature sensors send control signals to a system to control the flow of refrigerant in evaporator coils. They can also control the speed of blowers in an RTU.
We install humidity sensors in RTUs to detect the moisture content of air. They send signals to an RTU to control the humidity. They also control the operation of humidifiers or dehumidifiers if an RTU has them.
We install pressure sensors in RTUs to detect the pressure of air. Blower speed increases or decreases depending on the input given by a pressure sensor. They tell us if air filters are dirty or clean. Recall that dirty air filters will not allow air to pass through them easily.
We install CO2 sensors in RTUs to detect the amount of carbon dioxide in air. They send signals to a rooftop unit to remove stale air and add fresh air. It’s important to maintain CO2 levels below a certain amount to improve the quality of air.
Economizer Controls
Recall that an economizer is a device used to save energy consumption by using the cool air from outside a building to cool its internal space. They mix enough outdoor air with indoor air to meet the cooling temperature set on a thermostat. They mostly operate at night when outside air is cooler than inside air.
The main controls of economizers are:
Temperature sensor, and
Damper actuator.
Temperature sensors compare outdoor and return air temperatures. If the outdoor air temperature is less than the return air temperature, they send signals to a damper actuator. A damper actuator opens to allow outdoor air to enter an RTU.
In packaged rooftop units, we use 24V DC for control purposes. Safety controls prevent hazards. Flow controls regulate the flow of refrigerant in an RTU.
Complete Systems
In this module, we will take a look at the complete systems of a rooftop unit. Skip to quiz!
How Rooftop Units Work
Rooftop units start by pressing the ‘ON’ button on a room’s thermostat. We either select cooling mode or heating mode on a thermostat.
Rooftop Units - Cooling Mode
Pressing the ‘COOL’ button on a thermostat will start cooling mode. Along with pressing the ‘COOL’ button, we also set the temperature that we desire on a thermostat.
Temperature sensing bulbs in a living room detect and tell a thermostat about the room temperature. A thermostat compares the room temperature with its set temperature. A thermostat starts cooling if the room temperature is higher than the set temperature.
A thermostat sends control signals to its compressor contactor. As a compressor contactor receives a control signal, it will send line voltage to a compressor. In turn, the compressor starts the refrigeration cycle.
Along with the compressor, a blower motor and condenser fan motor also start running. Recall that blower fans suck air from outside and force it through evaporator coils. Before air enters an RTU, it passes through an air filter. The air filter traps dust particles in the air.
Recall that cold refrigerant flows through evaporator coils. As air passes through evaporator coils, the air temperature decreases.
Cold air from an RTU is circulated into a living room through ductwork by a blower. Recall that ductwork is a closed path for circulating air in HVAC systems. This process repeats to maintain comfortable temperatures in a living room.
Rooftop Units - Heating Mode
Pressing the ‘HEAT’ button on a thermostat or remote control initiates heating mode. Along with pressing the ‘HEAT’ button, we also set the temperature that we desire on a thermostat. The refrigeration system in an RTU works as a heat pump while in heating mode.
Temperature sensing bulbs in a living room detect and tell a thermostat about the room temperature. A thermostat compares the room temperature with its set temperature. A thermostat starts heating if the room temperature is lower than the set temperature.
A thermostat sends control signals to its compressor contactor. As a compressor contactor receives a control signal, it will send line voltage to the compressor. In turn, the compressor starts the refrigeration cycle.
Along with the compressor, a blower motor and condenser fan motor also start running. Recall that blower fans suck air from outside and force it through evaporator coils. Before air enters an RTU, it passes through an air filter. The air filter traps dust particles in the air.
Recall that refrigerant flows through evaporator coils after compression in heating mode. Here evaporator coils act as heating coils with the help of a reversing valve. Air temperature increases as it passes through heating coils.
Hot air from an RTU is circulated into a living room through ductwork by a blower. Recall that ductwork is a closed path for circulating air in HVAC systems. This process repeats to maintain comfortable temperatures in a living room.
Some rooftop units come with electric heaters or gas furnaces in the place of a heating coil. The process for heating mode is the same for these components. The only difference is that a gas furnace or electric heater raises the temperature of air instead of a heating coil.
Rooftop units start by pressing the ‘ON’ button on a room’s thermostat. We either select cooling mode or heating mode in a thermostat. Temperature sensing bulbs in a living room detect and tell a thermostat about the room temperature.
Question #1: Which of the following gases is not used as a fuel gas in an RTU’s gas furnace (Select all that apply)?
R22 refrigerant gas
Natural gas
Propane gas
Wood gas
Scroll down for the answer...
Answer: R22 refrigerant gas
Wood gas
In an RTU, we generally use these gases for heating:
Natural gas, and
Propane gas.
Question #2: Which component of an electric heater generates heat?
Blower fans
Metallic coils
Heat exchanger tubes
Burners
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Answer: Metallic coils
Electric heaters have metallic coils. When we pass electricity through these coils, they become red hot. A blower fan then forces air to flow over these coils. This heats the air.
Question #3: ____ are also used in many RTU’s as backup heaters.
Electric Heating Systems
Gas Furnaces
Oil Furnaces
Fire places
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Answer: Electric Heating Systems
Electric heating systems are mostly used as backup heaters in rooftop units. These systems are considered more reliable due to the 24/7 availability of electricity in many places. However, due to their high electricity consumption, they are preferred as backup heaters in most RTUs.
Question #4: An RTU with no backup heaters and a heat pump as the primary heating system is installed in a mall in Alaska. Is it a good choice?
Yes, there is no problem with this system.
No, as it will not be able to heat the mall in freezing temperatures.
No, as it will not be able to heat the mall in hot temperatures.
Can’t say anything.
Scroll down for the answer...
Answer: No, as it will not be able to heat the mall in freezing temperatures.
Heat pumps cannot be used to produce heat in freezing climates. In places where the temperature drops very low, below 32 F, ice forms on the outdoor coils. This creates problems in heat pumps. Alaska has places where the temperatures might drop well below 32 F.
Question #5: The basic refrigeration cycle is reversed in a heat pump when it provides cold air to a room.
True
False
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Answer: False
When we use heat pumps to cool offices in warm summer months, they work exactly like an air conditioner. So the basic refrigeration cycle is not reversed.
Question #6: Which of the following coils do year round RTUs have?
Cooling coil
Heating coil
Both of the above
Mosquito coil
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Answer: Both of the above
Year round RTUs have both cooling and heating coils. These coils help year round RTUs to operate in both summer and winter seasons.
Question #7: The _______ protects RTU components from the sun, wind, rain, snow, frost, etc.
Housing
Damper
Cooling coil
Condensate drain
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Answer: Housing
Housing protects RTU components from the sun, wind, rain, snow, frost, etc.
Question #8: Which of the following components allows outside fresh air to enter an RTU?
Housing
Air intake hood
Cooling coil
Condensate drain
Scroll down for the answer...
Answer: Air intake hood
An air intake hood allows outside fresh air to enter an RTU.
Question #9: Where is the intake air damper placed in a rooftop unit?
near air intake hood
near cooling coil
near blower
near condenser coil
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Answer: near air intake hood
The intake air damper is placed near the air intake hood.
Question #10: In an RTU, a cooling coil is placed behind the _________ to avoid dust accumulation.
air filter
filter drier
compressor
air intake hood
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Answer: air filter
In an RTU, a cooling coil is placed behind the air filter to avoid dust accumulation.
Question #11: Axial blowers are generally used in RTUs.
True
False
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Answer: False
Centrifugal blowers are generally used in RTUs.
Question #12: In an RTU, a blower is usually placed behind the _________.
compressor
intake air hood
cooling tower
cooling or heating coils
Scroll down for the answer...
Answer: cooling or heating coils
In an RTU, a blower is usually placed behind the cooling or heating coils.
Question #13: Which of the following regulates the amount of return air that mixes with the fresh air in an RTU?
Intake air damper
Return air damper
Exhaust air damper
Intake air hood
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Answer: Return air damper
Return air dampers regulate the amount of return air that mixes with fresh air.
Question #14: Which of the following is used to reject return air from an RTU?
Intake air damper
Return air damper
Exhaust air damper
Intake air hood
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Answer: Exhaust air damper
An exhaust air damper is used to reject return air from an RTU.
Question #15: A heat wheel rotates between __________ and ___________.
intake fresh air; exhaust return air
high pressure refrigerant; low pressure refrigerant
intake carbon dioxide; exhaust oxygen
supply cold water; return hot water
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Answer: intake fresh air; exhaust return air
A heat wheel rotates between intake fresh air and exhaust return air.
Question #16: Which of the following devices have an electrical motor as an integral part of them?
Compressors
Condensers
Air filters
Evaporators
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Answer: Compressors
Compressors have electrical motors as an integral part of them.
Question #17: Which of the following devices uses electricity to heat the air in an RTU?
Air hoods
Air filters
Electric heaters
Condensers
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Answer: Electric heaters
Electric heaters use electricity to heat the air in an RTU.
Question #18: Which of the following devices is used to convert a high voltage into a low voltage current and vice versa?
Contactors
Transformers
Thermostat
Compressor
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Answer: Transformers
Transformers are devices used to convert a high voltage into a low voltage current and vice versa.
Question #19: Which of the following control circuits are always in closed condition?
Safety controls
Flow controls
Humidity controls
All the above
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Answer: Safety controls
Safety control circuits are always in closed condition.
Question #20: Which of the following components have overload relays in RTUs? (Select all that apply)
Compressors
Evaporators
Blower motors
Air filters
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Answer: Compressors
Blower motors
In RTUs, overload relays are found in compressors, blower motors, and condenser fan motors.
Question #21: A discharge temperature switch is located at the outlet of a _______.
Condenser
Compressor
Metering device
Evaporator
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Answer: Compressor
A discharge temperature switch is located at the outlet of a compressor.
Question #22: Which of the following devices are used to monitor the temperature of a motor winding?
Discharge temperature switches
Motor winding thermostats
Hot gas bypass valves
LPCO switches
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Answer: Motor winding thermostats
Motor winding thermostats are used to monitor the temperature of a motor winding.
Question #23: Hot gas bypass valves are usually installed at the outlet of a ________.
Compressor
Evaporator
Condenser
Metering device
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Answer: Metering device
Hot gas bypass valves are usually installed at the outlet of a metering device.
Question #24: ________ detect the water level in a condensate drain.
Hot gas bypass valves
Float switches
Solenoid valves
Overload relays
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Answer: Float switches
Float switches detect the water level in a condensate drain.
Question #25: Which of the following sensors in an RTU detects the moisture content of air?
Pressure sensor
Humidity sensor
CO2 sensor
Temperature sensor
Scroll down for the answer...
Answer: Humidity sensor
CO2 sensors in an RTU detect the moisture content of air.
Question #26: Which of the following components compares the outdoor air temperature with return air temperature in the economizer of an RTU?
Pressure sensors
Humidity sensors
Damper actuators
Temperature sensors
Scroll down for the answer...
Answer: Temperature sensors
Temperature sensors compare the outdoor air temperature with return air temperature in the economizer of an RTU.
Question #27: Which of the following components detect the temperature in a living room?
Hot gas bypass valves
Motor winding thermostats
Temperature sensing bulbs
Discharge temperature switches
Scroll down for the answer...
Answer: Temperature sensing bulbs
Temperature sensing bulbs detect the temperature in a living room.
Question #28: Which of the following components cleans the air in an RTU?
Air hood
Air filter
Intake air damper
Housing
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Answer: Air filter
Before air enters an RTU, it passes through an air filter. The air filter traps dust particles in the air.
Question #29: What sends control signals to a compressor contactor?
Thermostat
Flow switch
Solenoid valve
Reversing valve
Scroll down for the answer...
Answer: Thermostat
A thermostat sends control signals to its compressor contactor.
Question #30: Which of the following components can be used to raise the temperature of the air in an RTU in heating mode?
Electric heaters
Heat pump
Gas furnace
All the above
Scroll down for the answer...
Answer: All the above
Heat pumps, gas furnaces, or electric heaters are used in an RTU to raise the temperature of the air in heating mode.