One of the most important concepts to understand when learning to be an HVAC Technician is temperature and theory of heat. Our HVAC Boot Camp Course will prepare you to become a highly sought after entry level HVAC Technician. We also help with job placement for our outstanding students.

HEAT - Simply put, it is molecules in motion. The faster the molecules the more heat is generated. Rub your hands together really fast, do you feel the heat? Why is that? It is because, as you rub your hands together the molecules in your skin begin to move faster which creates the heat.

From now on we are not going to use the terms “hot” or “cold” we will use “more heat” or “less heat”.

Even ice has heat because it has molecules in motion (heat). It just has less heat than we do, so we consider it cold.

GAS LAWS - In an air conditioning or refrigeration system, all of the gas laws are in play at the same time.

Boyle's Law - As the volume of refrigerant decreases the pressure increases. As the volume of refrigeration increases the pressure decreases.

Charles’ Law - As the pressure of refrigerant increases, so does the temperature. As pressure decreases in refrigerant, so does the temperature.

Dalton’s Law - If you combine two different gases, ie. refrigerant and nitrogen, their individual pressures add to each other, increasing pressure.

TYPES OF HEAT TRANSFER - There are three methods of heat transfer, Conduction, Convection, and Radiation. Here are examples of each:

Conduction - Is when heat is transferred by direct contact. When you grab the handle of a hot pan the heat immediately transfers to your hand and you get burned.

Convection - The transfer of heat through a medium like vapor or liquid. A space heater blows air (vapor) over heating elements. The air is blown onto your hand and your hand is warmed up. Or, your water heater which transfers the heat from the water heater elements to the water which warms your hands.

Radiation - Heats by waves or rays. The sun radiates heat. On a 50 degree day, standing in the sun keeps you warm from radiation. A microwave oven heats your food by radiation using microwaves.

HEAT TRANSFER - Using the hot pan in the example above, if we place it on a granite countertop, the heat will be transferred from the hot pan with more heat to the countertop. The transfer of heat continues until the pan and the countertop reach the same temperature. This is another example of heat transfer by conduction.

STATES OF MATTER - There are only three states or forms of matter: solid, liquid, and vapor.

Examples: ice, water, and steam. Changing of state happens when ice melts or freezes it also happens when water boils or condenses. Just like not using hot or cold we must not use boil as when we think of boiling we think of extremely hot water. When refrigerant boils it does so at very low temperatures which can be confusing.

Heat transfer happens no matter what the objects are made of or their state (solid, liquid, or vapor). As long as they are at different levels of heat, heat transfer happens.

Heating the water from room temperature to boiling requires very little heat energy. But, when the state changes from liquid to vapor, it requires massive amounts of heat energy.

LATENT HEAT - The heat energy required or released that causes a change of state.

The change of state from:

  • liquid (water) to (vapor) - requires a massive transfer of heat energy.

  • vapor back to water - releases all of the heat energy that created the vapor.

SENSIBLE HEAT - The heat energy gained or released that causes a change in temperature and which can be measured which can be measured.

  • Raising the temperature of room temperature water to the boiling point - requires very little heat energy.

Note - As we move forward to discussing refrigerants, we will only talk about liquid and vapor. We will no longer talk of solids. 

SATURATION TEMPERATURE - The temperature of a liquid or vapor where if any heat is added or removed, a change of state will take place. Both liquid and vapor are present.

Note - The saturation temperature will be different if the pressure is different. Here is an example - Water in the mountains will boil at 200 F because the air pressure is less than sea level where water boils at 212 F.

At saturation temperature, both liquid and vapor are present and are very sensitive to addition or removal of heat. If we add heat it is absorbed and the liquid changes state to vapor. If heat is removed vapor releases the heat and changes state to liquid.

There are pressure temperature (PT) charts for refrigerants that give us the saturation temperature at different pressures.

Basically if the refrigerant pressure is low, the saturation temperature is low. If the pressure is high, the saturation temperature is high. This is called the Pressure Temperature Relationship.

Using  the PT chart for R-410A (the refrigerant most used today in home air conditioning) as an example:

  • At  a gauge pressure of 78 PSIG, the saturation temperature of the refrigerant is 9 F. If we expose the refrigerant to 75F air it will change state from liquid to vapor, absorbing massive amounts of heat energy. COOLING

At 364 PSIG, the saturation temperature of R-410A is now up to 110 degrees.  If we expose the refrigerant to 75F air it will change state from vapor to liquid, releasing massive amounts of heat energy. HEATING


Ron Walker
Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.