Refrigeration for HVAC 101

Refrigeration 101 - Refrigeration and Air Conditioning Technology is a textbook and will help you follow this material.

You must understand heat and heat transfer principles and theory. That includes the fundamental physics of heat, including thermodynamics. If you have the recommended textbook, read Chapter One in the textbook (Refrigeration and Air Conditioning Technology). A technician needs to comprehend these principles and practice them as they ply the trade of refrigeration.

These fundamentals are necessary for a technician to fully grasp to be their best. By studying these principles and comprehending them, you will be able to solve problems and be the best at what you do. Whether you do installation or service, this knowledge will help you go further than those who have not studied and learned these fundamentals.

What is covered in this lesson:

  • Definition of Refrigeration
  • Fundamentals of Heat
  • Temperature Measurement
  • Temperature Conversion
  • What is a Btu?
  • Transfer and Flow of Heat - Conduction, Convection, and Radiation
  • Sensible and Latent heat as applied to HVACR
  • The Properties of Air
  • Specific Heat for Various Substances
  • Superheat and Subcooling
  • Refrigeration System Component Identification
  • Lesson One Quiz

Definition of Refrigeration - Refrigeration for HVAC 101

The following video is approximately 20 minutes long. Everything in this video is covered in this course along with additional information and knowledge to help you learn refrigeration for HVAC. The video is old but the information is valid and will give you a basic overview of how refrigeration works in illustrative form. It would be good to take notes as the basics of refrigeration are covered in this video.

The classic definition of refrigeration is a process of moving heat from one location to another in controlled conditions. Most people will define refrigeration as subjecting people or things to cold to keep the people or things cool or cold. As you study refrigeration, you will come to understand that in refrigeration, we are removing heat from one place and moving it to another place as the definition above states. Refrigeration is a process of moving heat, and in relative terms (at least to most humans when they in an air-conditioned space), we are making the space cool or cold depending on the temperature setting and the process.

If you put a warm bottle of soda in the refrigerator to cool it down, the refrigerator will work to make it cool by removing the heat from the soda and the soda bottle. The heat is absorbed on the inside of the refrigerator at the evaporator. Then it is rejected on the outside of the refrigerator by the condenser. After a given amount of time, the bottle of soda assumes the same temperature as the inside of the refrigerator. The heat was removed from the bottle of soda.

So when I am in my house and my wife says we need to turn the air conditioner on to make it cool in the house I am thinking we turn the air conditioner on to remove heat from the house thereby making it cool (in relative human comfort terms) in the house. Remember, the definition of refrigeration is a process of moving heat from one location to another in controlled conditions. An air conditioner is like a big refrigerator. In the case of the air conditioner in my house, the fan sucks the air into the return ductwork and sends it to the evaporator coil.

That is where the heat in the air is absorbed. That heat then travels outside through the process of refrigeration and is rejected into the outside atmosphere. The air that passed over the evaporator coil is now cooler than what it was when it entered the evaporator. Then the fan blows the air throughout the supply duct and into the house where the cool air is distributed, making my wife cool and happy. Of course, there is more to the process of refrigeration, and we will cover that in a later module within this course. For now, we begin with the basics.

Fundamentals of Heat - Refrigeration for HVAC 101

Heat is energy whether produced naturally or whether produced by another means - usually mechanically or chemically. It is crucial to understand that heat always flows to something cooler. If you are outside in the winter, the heat from your body will flow out of your body and into the outside air. The same is true in any circumstance where something is cooler, and something else is warmer. The heat will naturally flow to the cooler in any form.

All matter is made up of atoms that combine to make molecules. The warmer or, the hotter the matter, the faster the atoms move inside that matter or substance. The cooler the substance or matter, the slower the atoms move. The warmer atoms give up their energy (or heat) to the cooler atoms and begin to slow the cooler they get in the matter or substance.

Theoretically, absolute zero is the coldest temperature and is where all motion stops inside the matter or substance. Absolute zero as measured on the thermometer is -460 degrees Fahrenheit or -273 Celsius. That is the true definition of cold. Any temperature in this neighborhood can be considered cold, scientifically, so anything above that temperature can be considered warm or hot, which is relative depending on what you are describing. Absolute has never been achieved as of yet. However, researchers at MIT have come very close to cooling sodium gas to absolute zero; it is a temperature that remains elusive to produce in nature or a lab. Perhaps, by the time you are reading this, science will have achieved that milestone.

Temperature Measurement - Refrigeration for HVAC 101

Since this is a US-based course, we use the Fahrenheit scale of measurement, but for the rest of the world, they use the Celsius scale of measurement to measure temperature. For this course, we will use the Fahrenheit scale. If you work in a laboratory, you may use other forms of measuring temperatures, such as Rankine or Kelvin. These temperature measurements are typically used in labs and some processes that deal with very low/high temperatures, so it is not normal for the average technician to use this form of measurement.

In measuring temperature in the HVACR trade, we use various devices. The old mainstay is a thermometer, but most technicians use infrared temperature guns and electronic temperature thermometers that use thermistors to read temperatures. The electronic thermometers have a digital readout with a temperature probe that is placed in the area where you want to measure the temperature. The infrared temperature gun can be pointed at an object, and when the trigger is pulled, a laser light shines on the object or area where you want to measure the temperature.

It is essential to understand the use of the infrared temperature gun and read the instructions as corrections need to be taken for distance. Most infrared guns come with a correction table for more precise temperatures. In some cases, the correction is not necessary, especially when you only want a temperature differential, and the distance is not changed from one point to the next.

Whether you use Celsius or Fahrenheit for temperature readings is only essential for your geographical location. What is truly important is that you have proper tools that are correctly calibrated to give you accurate readings. The readings you get will be used in service and troubleshooting to make decisions that affect the performance of the equipment. Inaccurate readings will give you poor results and possibly lead to failure of the equipment or not functioning as designed. More on proper tools will be covered in a later module for this course.

In HVACR, there will be a time when you will need to measure the temperatures both inside the structure and outside the structure. You will also need to be able to measure two similar but different temperatures. One is the dry-bulb temperature, and the other is the wet-bulb temperature. These temperatures are critical and necessary when we are charging the refrigeration system. They are needed for the charts we use so that we know precisely how much refrigerant charge the system needs.

Dry Bulb Temperature - it is the temperature you read on every thermostat. If the weatherman says tomorrow the temperature will be 30° F., then we know it is going to be cold. If he says, it is going to be 90° F. we know it is going to be hot. That is the dry-bulb temperature as everyone knows it. The dry bulb temperature is measured with an ordinary thermometer.

Wet Bulb Temperature - is the lowest level of temperature that can be obtained through evaporative cooling of a ventilated surface covered with ice or wet with water. It indicates a temperature that is near thermodynamic or real temperature. It is the lowest level of temperature that can be reached under present ambient conditions brought by water evaporation.

Using both the dry bulb temperature and the wet-bulb temperature, we can determine several factors about the air, such as relative humidity, grains of moisture, dew point, and enthalpy. This information is used for the calculation of the refrigerant charge based on a chart provided by the manufacturer of the air conditioner or heat pump system. It is necessary data collected by you, and it is important to get accurate data for the calculation.

Temperature Conversion - Refrigeration for HVAC 101

Sometimes you must convert temperature measurements in the field from Celsius to Fahrenheit or Fahrenheit to Celsius. In a few jobs available in HVACR, you will also need to convert between those two, Celsius and Fahrenheit, along with converting to Kelvin and Rankine. Therefore you need to know how to convert the various temperature scales from one to another.

Perhaps you are in the field and installing a new air conditioner. The installation book that came with the equipment is all in Celsius, but you use Fahrenheit for temperature measurement. You will need to know how to convert the Celsius to Fahrenheit so that you can properly install the equipment according to the manufacturer’s guidelines. Below are the formulas for converting various temperature measurement scales from one to another.

Celsius to Fahrenheit - Multiply by 1.8 (or 9/5) and add 32. For example - let’s say you need to convert 40° Celsius to Fahrenheit. Pull out your cell phone and open the calculator app. 1.8 x 40 = 72. Take the 72 and add 32 so 72 + 32 = 104° Fahrenheit when converted from 40° Celsius.

Fahrenheit to Celsius - Deduct 32, then multiply by 5, then divide by 9. For example - let’s say we need to convert 104° F. to Celsius. 104 - 32 = 72, then we multiply 72 x 5 = 360, then simply divide by 9 so 360/9 = 40° Celsius when converted from Fahrenheit.

For Rankine and Kelvin, conversions follow along in the textbook.

What is a Btu?

Btu is a British Thermal Unit and is a way of measuring the volume of heat in HVACR. One Btu equals the amount of heat needed to raise one pound of water one degree Fahrenheit. Whether the one degree of heat is removed from the one pound of water or whether the one degree of heat is added to the water, it is considered a Btu.

Twelve thousand (12,000) Btu’s equals one ton of air conditioning. That is important to know as most air conditioning systems are sized by the ton in Btu’s. So if someone tells you it is a three-ton unit, then you know it is 36,000 Btu’s. What does this mean, and how do you use this information. It makes a huge difference in sizing parts and the initial installation of the system. The ductwork needs to be sized according to the amount of Btu’s the system will deliver. On the standard air conditioner, the Cubic Feet per Minute of airflow or CFM’s will need to be at least 1200 CFM’s unless it is a high-velocity system, which is rare to find. So the ductwork needs to be sized for the proper amount of CFM’s, or you will have problems with the system.

Btu’s will be converted to other forms of energy, which are important in other factors of the sizing of air conditioning and heating equipment. That will be included in other advanced courses as necessary for the subject.

Transfer and Flow of Heat - Conduction, Convection, and Radiation - Refrigeration for HVAC 101

Conduction - is the movement of heat in solid substances or matter. The substances can be similar or dissimilar. An example of this is a pot on a stove. As the bottom of the pot is heated, the heat flows into other parts of the pot, including the handle of the pot. Some pots have ceramic handles, so while the pot is made of metal, the handle is made of ceramic. Since the ceramic conducts heat poorly (natural insulator), it will not get as hot as quickly as the rest of the pot. That allows one to hold the handle of the pot without burning their hand, whereas if the handle was metal, it would quickly conduct the heat much faster than the ceramic handle would and make it nearly unbearable to hold without a kitchen glove, another natural insulator.

Convection - is the movement of heat through the air or through a liquid. The key is that fluids and gases are fluid and can flow. At the same time, substances of mass can’t flow because substances of mass are solid so any heat transfer through solids is done by conduction from one molecule to another while heat transfer in fluids and gases are facilitated by flow throughout the fluid or gases by the flow of the gases or liquids. So with convection, the heat transfer is done on a higher scale than conduction because of the flow of gases and fluids.

Within the convection definition, you will have “natural heat convection” and “forced heat convection.” An example of forced heat convection can be applied to the process of refrigeration because most of the heat is being mechanically forced to move through the laws of convection. An example of natural heat convection can be used to describe the world’s oceans and the natural currents of the water in the oceans caused by convection.

Radiant HeaterRadiation - Radiation or more precisely thermal radiation is the movement or transfer of heat by heat rays. This can be a very complex subject when you get serious about the science of thermal radiation, but a good example of this is an asphalt road in the summer or even the winter. The asphalt absorbs the rays of the sun much easier than the surrounding area of the road. If you had bare feet and stepped off a gravel road onto an asphalt road in the summer, you would likely find it very uncomfortable to stand on the asphalt with bare feet. The road became hot because of thermal radiation from the sun.

Radiation does not depend on any medium to move or flow and can travel in a vacuum such as space. In HVACR, when we discuss radiation, we generally are speaking of radiant heat and not the same radiation generated in a nuclear power plant. Radiant heaters are used for both residential and commercial applications and are important to understand how it works. As the radiant heat is moving through an open space, it is not absorbed, but when it hits the first solid object it encounters, the radiant heat is absorbed into the object.

Such as in the photo, the radiant heat from the heaters installed in this garage will be absorbed by the floor in the garage (unless, of course, a vehicle is parked there). The floor absorbs this heat, and using the laws of thermodynamics, the heat will rise using convection, and another law we discussed above - heat always flows to something cooler. Using radiation, the people and the vehicles stay warmer in the garage when it is cold outside.

Sensible and Latent Heat as Applied to HVACR - Refrigeration for HVAC 101

change of state BTU chart

Sensible Heat - As heat is added to a substance, the substance changes temperature. Likewise, if heat is removed from a substance, it changes temperature. This is referred to as Sensible Heat. In other words, Sensible Heat is a form of energy that can be measured with a thermometer. That is contrasted by the next term that is important to understand in thermodynamics and refrigeration, and that term is Latent Heat.

Latent Heat - Latent Heat is often referred to as hidden heat because it is not measured with a thermometer but rather the amount of Btu’s necessary to bring a change of state to a substance. For example, when water changes from a solid (ice) to a liquid, the water changes state. The amount of energy needed to change the water from a solid to a liquid is more than is necessary to raise the temperature of the water in solid form, a liquid form, or as a vapor form (steam).

Following the chart, above, you can see that at 32° F., the temperature did not change even though more Btu’s were added to melt the ice. Follow along with the chart, and you can see that from 0° F. to 32° it took 16 Btu’s, but when it hit 32° F. mark, it took an additional 144 Btu’s to melt the ice or change it from a solid to a liquid. The 16 Btu’s is referred to as Sensible Heat while the 144 Btu’s is referred to as Latent Heat or hidden heat because it cannot be measured on the temperature scale with a thermometer.

Going further up the scale we can also see when we change the state again from a liquid (water) to a vapor (steam) it took only 180 Btu’s to change the temperature from 32° F. to 212° F., but we had to add 970 Btu’s to change the state from a liquid to a vapor or change water to steam. Again, as with the melting ice, the 180 Btu’s (from 32° F. to 212° F.) is Sensible Heat while the 970 Btu’s is Latent Heat and the 970 Btu’s did not change the temperature but instead changed water (solid) to steam (vapor). So more energy is expended when you change the state of something versus merely changing the temperature.

Understanding that, while Latent Heat does not cause a change on the thermometer or a change in temperature that there is a change in the state of the substance, in this case (the chart above) water. Whether you go up the scale or down the scale when a substance changes state, it requires more Btu’s either being added or taken away to change the state of the substance. In the case of making ice, you are taking heat away from the water but not observing any change in the temperature. At the same time, if you melt the ice, you will not observe any change in temperature while adding more Btu’s.

It is important to comprehend that all these temperatures and changes are done at the atmospheric pressure at sea level because when we change the pressure, we change the boiling point and freezing point at which we observe changes of state as noted above. If we lower the pressure, we lower the boiling point. Example - In Denver, Colorado the boiling point of water is approximately 203° F. At the same time, at sea level it is 212° F.. Hence, there is a pressure-temperature relation pressure-temperature explained in another segment of this lesson however it is important, in refrigeration, to understand the difference between sensible heat and latent heat as these principles in refrigeration are used for sizing equipment and for troubleshooting problems that may arise with equipment from time to time.

Basic Properties of Air - Refrigeration for HVAC 101

This subject dovetails with the previous segment of Sensible and Latent Heat and the next segment of Specific Heat. It will also be used in a later module to demonstrate the process of refrigeration, so it is important to offer a limited lesson on the Properties of Air. Since most evaporator coils come into contact with air, it is necessary to understand some of the properties of air since we are conditioning the air using the evaporator coil in the process of refrigeration. Air is comprised of 78% nitrogen, 21% oxygen, and one percent of other gases, including water vapor. It is the heat contained in the air along with the water vapor that we are dealing with in the typical refrigeration system (aside from chilled water systems which condition water rather than the air).

Dew Point - Sit a glass of ice water on the table on a hot summer day, and what happens to the outside of the glass? It forms moisture on the outside of the glass that will eventually drip down the side of the glass and get the table wet. That is the moisture in the air condensing on the side of the glass because the Dew Point Temperature of the glass causes the moisture in the air to lose its heat. So the moisture in the air, water vapor changes state from a vapor back to a liquid. As from the lesson above on Latent Heat, whenever we have a change of state of a substance, we are using Latent Heat to change that state. And what did it take in Btu’s to change the state of water from the above lesson on Latent Heat? It took more Btu’s to change the state of water than it does to change the temperature of the water (or any substance for that matter). So removing moisture from the air takes more Btu’s than changing the temperature of the air whether we remove the heat from the air or add heat to the air.



  • Specific Heat - the heat required to raise the temperature of the unit mass of a given substance by a given amount (usually one degree).
  • Dewpoint Temperature - The dewpoint temperature is the temperature at which the air can no longer “hold” all of the water vapor which is mixed with it, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature.
  • Sensible Heat (dry-bulb temperature) - this is the reading you get on a thermometer or thermostat.
  • Latent Heat - Latent heat is the energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be understood as energy in a hidden form that is supplied or extracted to change the state of a substance without changing its temperature. In HVAC, the humidity or moisture content of air is added heat to the air.
  • Wet-Bulb Temperature - The wet-bulb temperature is the temperature read by a thermometer covered in water-soaked cloth over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature, and it is lower at lower humidity.

Specific Heat for Various Substances - Refrigeration for HVAC 101

Different materials or substances have different reactions when the heat is applied. As discussed previously, when one Btu of heat is added to one pound of water, the temperature of the water increases by one degree F. This is not true for other substances; for example, to raise the temperature of wood by one degree, F. would require only one-third of a Btu. Other substances require more, and others require less Btu’s to increase the temperature one degree F. Specific Heat can be defined as the amount of heat (in Btu’s) that is required to raise the temperature of that particular substance one degree F. or the amount of heat released from the substance to lower the temperature one degree Fahrenheit.

It works both ways on the temperature scale and is important to understand in HVACR for making load calculations and sizing equipment. Many factors are included in sizing HVAC and refrigeration equipment. Still, the specific heat of various substances is essential because structures are made of various materials, and a part or factor in the calculation is made based on the type of material that needs to be heated or cooled. That is especially true in industry, for manufacturing. Let’s say a bicycle manufacturer stores materials for producing bicycles in an unconditioned warehouse, and it is winter time. The temperature outside is 40° F., but for the workers to begin the process of manufacturing the bicycles, the materials need to be at least 60° F. Using the specific heat calculation for the materials the manufacturer will know the amount of time needed to raise the temperature of the materials based the weight and type of the materials and the capacity of the HVAC system to heat the space where those materials are stored before they are needed for the manufacturing process. That was likely a big factor in sizing HVAC equipment for the contractor that installed the heating and cooling (if equipped with cooling) equipment.

Another reason to understand specific heat is used with chilled water systems that operate in the wintertime or when anti-freeze (glycol) is added to a piping system to prevent freezing. When anti-freeze or glycol is added to the water, it changes the specific heat of the water or the ability of the water to absorb heat. Depending on the piping arrangement and the equipment will depend on the limitations of how much glycol can be added to the water to maintain peak load conditions. In most circumstances, a chart is used based on those factors to help technicians add the correct amount of anti-freeze (glycol) to the system with causing issues with performance. Do you add 10% or 20%, and how will that affect the system at peak load when the demand is high?

The following video will help you understand Specific Heat and Specific Heat Capacity; even though the video uses the metric system, you can still follow along and gain a better understanding of Specific Heat.

If you do not know:

0° Celsius = 32° Fahrenheit

100° Celsius = 212° Fahrenheit

As discussed in the above video, substances have three separate states: Solid, liquid, and vapor (gas). These substances move into these different states depending on the temperature (and pressure). For now, we’ll only focus on temperature and how it changes substances into different states.

Change of State BTU Chart

change of state BTU chart

Furthermore, pay close attention to the BTUs needed to change the water (H2O) to various states.

Superheat and Subcooling



It is essential to understand the definition of “superheat” as it relates to refrigeration. You will frequently be required to calculate “superheat” to charge an HVAC or refrigeration system. The easiest way to explain “superheat” is to use water as an example. We know the boiling point of water is 212° Fahrenheit (sea level). Water changes state from a liquid to a vapor at that temperature (at sea level). If we keep adding heat to the steam is gets hotter. So as we add more BTUs to the steam (vaporized water), it gets warmer in temperature (and pressure). The additional temperature we record as more heat is added is called “superheat.”

That is applicable to refrigeration because refrigerants boil and absorb more heat over its vaporization temperature. Using a “superheat calculation along with some other variables such as wet bulb and dry bulb temperatures for the load, we can determine the correct amount of refrigerant an HVAC system needs to function efficiently. If we get it wrong, the system will be overcharged or undercharged. “Superheat” can be used for specific applications like air conditioning and some refrigeration units such as walk-in coolers to set a proper charge on the refrigeration system. Study this concept and understand it because you will use it when working as a technician.


subcooling definition

Subcooling is another term that is important to understand because you also need this in the field as an HVAC/R technician. When water vapor cools, it changes state to a liquid. That temperature is 212° Fahrenheit. As the temperature of the liquid cools even further, the reference to the temperature can be made as subcooling below its change of state point or condensing point on the temperature scale.

Study these terms and make sure to comprehend them. Research these terms and concepts of science until you understand them. If necessary, use our forum here to ask questions. We have instructors monitoring the site to answer questions and guide you through your learning experience. All of these principles, concepts, and laws will be used as you work as an HVAC/R technician.

Refrigeration System Component Identification - Refrigeration for HVAC 101

Refrigeration System Component Identification

Above is a basic refrigeration system diagram. There are other designs for refrigeration systems and their components however this is the basic design you need to become familiar with before moving on to learning different types of refrigeration system designs. We will start at the top and work our way around clockwise.

  1. Suction Line - this is the line leaving the evaporator and terminating at the suction port on the compressor. This line carries low-pressure refrigerant vapor is the larger line that is insulated.
  2. Compressor - there are different types of compressors used in HVAC. The basic and most common compressors used in HVAC are the scroll compressor followed by the reciprocating compressor and the rotary compressor. More details on this in Lesson 2.
  3. Discharge Line - the vapor refrigerant leaves the compressor as high-pressure vapor on its way to the condenser.
  4. Condenser Coils - the high-pressure vapor refrigerant enters the condenser coils where a specific amount of heat is removed changing the vapor to a liquid.
  5. Liquid Line - The refrigerant leaves the condenser and passes through a filter diet to remove any possible moisture in the system (desiccants) and then through a site glass (an optional component in some refrigeration systems) where it journeys towards the metering device.
  6. Metering Device - the liquid refrigerant enters the metering device where it changes from a high-pressure liquid to a low-pressure/temperature mix of gas and liquid.
  7. Evaporator Coil - The evaporator coil takes the low-pressure/temperature vapor/liquid and absorbs heat where the liquid boils as it absorbs the heat. This changes all the refrigerant into a vapor where it enters the suction line.

There will be more detail later on the various components of a refrigeration system but it is important to familiarize yourself with these basic components. As a technician, you need to be very knowledgeable about refrigeration systems including their design and operation.

Apply yourself and learn this information so you can apply it in the field and be a successful HVAC/R technician.

That is foundation knowledge that is imperative to know and understand so you can apply it as you advance through learning and practicing refrigeration and other areas of HVAC.

Refrigeration for HVAC 101

Refrigeration for HVAC 101 Part 2