Temperature, Pressure, and Heat (Full ver.)

1.1 Hot Air Rises

Temperature, Heat, and Heat Transfer

In HVAC, temperature is not explained as something being “hot” or “cold” in a physical sense. Cold is not a form of energy. An object or a fluid does not contain cold; it contains less heat. Heat is energy, and because it is energy, it can be measured, compared, and controlled.

 

What people often describe as “heat moving to cold” is called heat transfer. Heat always moves from an area with more heat energy to an area with less heat energy. This movement continues until both areas reach the same temperature. When there is no temperature difference, heat transfer stops.

 

Heat does not move through all materials the same way.

• Some materials allow heat to pass through easily.
• Other materials slow the movement of heat.

Insulation slows heat movement, but it does not stop it completely.

 

Heat Transfer in a Building

A house is an easy way to understand heat transfer. On a hot summer day, the temperature outside may be much higher than inside. Because there is more heat energy outside, heat moves through the walls into the house.

 

In winter, the direction of heat movement is reversed. Heat moves from the warmer indoor space to the colder outdoor space. If the heating system stops working, the indoor temperature will continue to drop. When the indoor and outdoor temperatures become the same, heat transfer stops because there is no longer a temperature difference.

 

Why Warm Air Rises

The statement “warm air rises” is true, but it helps to understand why. When air is heated, it expands. This expansion makes the air lighter. Lighter air moves upward.

 

When air cools, it becomes smaller and heavier. Heavier air moves downward. This behavior is a basic rule of fluids and applies to both air and liquids.

• Warm air is lighter and rises
• Cool air is heavier and sinks

Air Movement in Multi-Story Buildings

In a multi-story building, air weight affects how temperatures change between floors. When an HVAC system is running and air is constantly moving, temperatures can stay balanced throughout the building.

 

When the fan is turned off, air moves naturally.

• Warmer air rises toward the upper floors
• Cooler air settles toward the lower floors

This happens in both summer and winter. The system may be heating or cooling, but air behavior stays the same.

 

Stratification

This natural layering of air by temperature is called stratification. It occurs when air is not being mixed and is allowed to settle into layers. A similar effect can be seen in water, where deeper water is often colder than water near the surface.

 

One of the main jobs of an HVAC system is to reduce stratification. By keeping air moving, the system helps maintain even temperatures and comfort throughout the space.

 

1.2 Temperature Scales

Temperature Scales and Water as a Reference In HVAC and science, temperature is measured using standard scales. The two most common ones are the Fahrenheit scale and the Celsius scale. Both scales use water as a reference point because water behaves consistently and is easy to observe.

 

On the Fahrenheit scale, water freezes at 32°F and boils at 212°F. That means there are 180 degrees between the freezing point and boiling point of water.

 

On the Celsius scale, water freezes at 0°C and boils at 100°C. This gives us 100 degrees between freezing and boiling.

 

Because both scales are based on the same physical event—water freezing and boiling—water becomes the benchmark that connects the two systems.

 

Why the Numbers Are Different

The key difference between the two scales is how many steps they use between freezing and boiling.

• Fahrenheit uses 180 steps
• Celsius uses 100 steps

This difference in spacing is why temperature conversions are needed and why the numbers 1.8, 9/5, and 5/9 appear in the formulas.

 

Converting Celsius to Fahrenheit

To convert a temperature from Celsius to Fahrenheit, the scale difference and the starting point must both be adjusted.

 

The process works like this:
You first scale the temperature by multiplying by 1.8, then shift it by adding 32.

 

For example, when converting 100°C:

• Multiply 100 by 1.8 to get 180
• Add 32
• The result is 212°F, which matches the boiling point of water

 

Converting Fahrenheit to Celsius

Converting from Fahrenheit to Celsius follows the same logic, but in reverse. Since the Fahrenheit scale starts at 32 instead of zero, that offset must be removed first.

 

The steps are:

• Subtract 32 from the Fahrenheit temperature
• Multiply the result by 5/9 (about 0.55)

Using 212°F as an example:

• Subtracting 32 gives 180
• Multiplying by 5/9 converts it to 100°C

 

2.1 Barometric Pressure

Barometric Pressure and the Atmosphere

This lesson focuses on barometric pressure, which is another name for atmospheric pressure. Barometric pressure is the pressure created by the air around us, and it is measured using a device called a barometer.

 

At sea level, atmospheric pressure measures 29.92 inches of mercury. This value represents how much force the air above us is pushing downward. When pressure is written as “inches of mercury,” the abbreviation Hg is used. Hg comes from an older name for mercury and refers to its silver, liquid appearance.

 

How a Barometer Works

A basic barometer uses a tube filled with mercury. Inside the tube is a vacuum, and at the bottom is a pool of mercury. When the barometer is placed at sea level, the weight of the air pushes down on the mercury pool and forces the mercury to rise inside the tube.

 

The mercury rises to almost 30 inches, which matches the atmospheric pressure at sea level. That pressure is equal to about 15 pounds per square inch. Even though the atmosphere is always pushing down with this force, we do not feel it because the pressure acts evenly in all directions.

 

Why Atmospheric Pressure Exists

Atmospheric pressure exists because air has mass. A good way to understand this is by thinking about water.

 

When you dive into a swimming pool, the deeper you go, the more pressure you feel. This happens because more water is stacked above you, and its weight pushes down on your body. The deeper you go, the greater the pressure.

 

The same idea applies to air. The Earth is surrounded by layers of air, and the lowest layer extends about 7 miles above the surface. When you stand at sea level, you are standing at the bottom of this “ocean of air.”

• Air above you has weight
• That weight pushes downward
• This creates atmospheric pressure

 

Pressure and Boiling Point

At sea level, water boils at 212°F. Sea level pressure is also described as 0 psi gauge or 29 inches of mercury.

 

An important rule in HVAC is that lower pressure lowers the boiling point. Pressure and temperature are directly related. When pressure goes down, temperature goes down as well. This is especially important when talking about boiling.

 

This same idea applies to refrigerants. Lower refrigerant pressure results in lower refrigerant temperature.

 

Elevation and Boiling Temperature

Pressure changes with elevation because there is less air above you as you go higher.

 

At sea level, water boils at 212°F because the air pressure pushing down on it is high. As elevation increases, atmospheric pressure decreases, and water boils at lower temperatures.

• At about 5,000 feet, pressure drops to around 25 inches of mercury
• At this elevation, water boils at about 203°F
• At around 30,000 feet, pressure drops to about 1 inch of mercury
• At that height, water can boil at 100°F

This happens because there is less pressure pushing down on the surface of the water, allowing it to boil more easily.

 

2.2 Lowering Pressure with a Pump

Using Pressure to Change the Boiling Point

In the previous lesson, we saw that the boiling point of water can be lowered by increasing elevation. This happens because higher elevation means less air above the water and therefore lower pressure. Lower pressure allows water to boil at a lower temperature.

 

The same result can be achieved without changing elevation by using a mechanical device called a vacuum pump. A vacuum pump lowers the pressure inside a closed system, which lowers the boiling point of a liquid inside that system.

 

Vacuum Chamber Example

In this example, water is placed inside a sealed vacuum chamber along with a mercury barometer. At the start, the chamber is at normal atmospheric pressure. The barometer reads close to 30 inches of mercury, and the air temperature inside the chamber is 80°F.

 

At this pressure, the boiling point of water is still 212°F, so the water remains calm and in a liquid state. Nothing unusual happens yet because pressure has not changed.

 

Alongside the inches-of-mercury scale, another pressure scale is shown: the micron scale. Microns are a much smaller unit of pressure.
29.92 inches of mercury equals about 760,000 microns.

 

Lowering Pressure with a Vacuum Pump

When the vacuum pump is turned on, it begins removing air from the chamber. As pressure drops, the boiling point of water drops with it.

 

As pressure is reduced step by step, the boiling point changes as follows:

• At 10 inches of mercury (about 258,000 microns), water boils at 162°F
• At 2 inches of mercury (about 52,000 microns), water boils at 102°F
• At 1 inch of mercury (about 25,000 microns), water boils at 80°F

At this point, water can boil even though the temperature is only 80°F. This clearly shows that lower pressure means a lower boiling point.

 

Raising the Boiling Point with Pressure

The boiling point can also be increased by doing the opposite: raising pressure. Pressure and temperature have a direct relationship.

• When pressure goes up, temperature goes up.
• When pressure goes down, temperature goes down.

A simple and familiar example of this is a pressure cooker.

 

How a Pressure Cooker Works

In an open pot, vapor can escape freely as water heats up. In a pressure cooker, the lid seals the pot, trapping the vapor inside. As the water heats, pressure builds inside the cooker.

At normal pressure, water boils at 212°F. As heating continues and pressure rises, the boiling point increases.

For example:

• At 30 psi gauge pressure, water does not boil until it reaches about 271°F

This happens because the increased pressure pushes down on the surface of the water, making it harder for vapor bubbles to form. When pressure is removed, boiling happens more easily.

 

2.3 The Compound (Low Side) Gauge

Absolute Pressure and Gauge Pressure

In this lesson, we introduce the idea of absolute pressure, written as PSIA. PSIA means pounds per square inch absolute. It is called “absolute” because it includes all pressure, including the pressure from the atmosphere.

 

When pressure is measured as 0 PSIA, that means a perfect vacuum. All air has been removed, including atmospheric pressure. At sea level, normal atmospheric pressure is 14.7 PSIA. This means that even when we think pressure is “zero,” there is still air pressure acting on everything around us unless we remove it completely.

 

Absolute Pressure and Inches of Mercury

Atmospheric pressure can be shown in two common ways.

• 14.7 PSIA
• 29.92 inches of mercury

 

These two values describe the same pressure. A mercury barometer shows this by how high the mercury rises in a tube. The weight of the air pushes the mercury up to about 29.92 inches, which equals the same force as 14.7 pounds per square inch.

 

This is the pressure we live under every day, even though we do not feel it.

 

Why We Do Not Use Barometers in the Field

While barometers work well to show atmospheric pressure, they are not practical for HVAC work. Mercury is hazardous, and carrying a mercury-filled device in a truck would be unsafe and unrealistic.

 

Instead, HVAC technicians use pressure gauges, which are safer, easier to use, and designed for daily field work.

 

The Low-Side (Compound) Gauge

The low-side gauge is also called a compound gauge because it measures two types of pressure.

• Positive pressure, shown in PSIG
• Vacuum, shown in inches of mercury

 

PSIG stands for pounds per square inch gauge. Gauge pressure does not include atmospheric pressure. It only shows pressure above or below the surrounding air.

 

Relationship Between PSIA and PSIG

To keep pressures straight, one rule is critical:

 

PSIA = PSIG + 14.7

 

This means gauge pressure must always have atmospheric pressure added to it to become absolute pressure.

For example, when a low-side gauge reads 0 PSIG, the system is not at zero pressure. It is actually at 14.7 PSIA, because atmospheric pressure is still present.

 

Reading the Compound Gauge

On a low-side gauge, the outer numbers show PSIG. These are the main numbers used to read system pressure.

 

The lower part of the gauge shows inches of mercury vacuum. When the gauge is pulled into a full vacuum, it reads close to 30 inches of mercury vacuum, which represents 0 PSIA.

 

As pressure increases from a vacuum:

• 15 inches of mercury vacuum equals about 7.4 PSIA
• Atmospheric pressure appears as 0 PSIG on the gauge, even though absolute pressure is still 14.7 PSIA

 

2.4 The Gauge Manifold

Low-Side and High-Side Gauges

Different gauges are used for different parts of an HVAC system. The low-side gauge is used on the suction side of the system, which is the cold side. For that reason, the gauge body is always blue. You will often hear it called the low side gauge or the suction gauge.

 

The low-side gauge is called a compound gauge because it does two jobs. It measures pressure when the system is running, and it also measures vacuum when the system is being evacuated.

 

The high-side gauge is connected to the discharge side of the system, which is the hot side. Its gauge body is always red. This color difference makes it easy to quickly tell which side of the system you are working on.

 

 

What the Low-Side Gauge Tells You

The low-side gauge does more than just show suction pressure. It also shows suction temperature and vacuum.

 

The temperature information comes from additional scales printed on the gauge face. These scales are labeled with different refrigerants, such as R-134a, R-22, and R-404A. Each refrigerant has its own color-coded scale.

 

These temperature scales show the boiling temperature, also called the saturation temperature, of the refrigerant at a given pressure. As pressure changes, the boiling temperature changes, and the gauge shows that relationship directly.

 

This means the gauge is giving you the same information you would find on a pressure–temperature chart, without needing to look one up.

 

What the High-Side Gauge Tells You

The high-side gauge works the same way, but on the hot side of the system. It shows condensing pressure and condensing temperature.

 

In the example shown, the high-side pressure is about 278 PSIG. This is just above the halfway mark between 250 and 300. The temperature scale for the refrigerant shows the temperature at which the refrigerant is condensing at that pressure.

 

Again, pressure and temperature move together. As condensing pressure rises, condensing temperature rises with it.

 

2.5 Pressure-Temperature Chart

 

Measuring Heat: Temperature vs Quantity

Up to this point, the focus has been on temperature. Temperature describes how hot or cold something is. It represents the intensity of heat and is measured in degrees.

 

Heat can also be measured by quantity, which tells us how much heat energy is present or transferred. This is measured in BTUs.

 

Sensible Heat and Latent Heat

Heat is divided into two main types: sensible heat and latent heat.

 

Sensible heat is heat that causes a change in temperature and can be measured with a thermometer. When sensible heat is added or removed, the temperature changes but the state of the material does not.

 

Latent heat causes a change of state but does not change temperature. It is often called “hidden heat.”

 

Examples of Latent Heat

At 32°F, ice melts into water. During this process:

• The ice changes from solid to liquid
• The temperature stays at 32°F until all ice is melted

The same idea applies to boiling water. When water reaches 212°F, it begins to boil.

• The water changes from liquid to vapor
• The temperature stays at 212°F until the water fully turns to steam

Only after the state change is complete can the temperature rise further.

 

3.1 Heat of Vaporization

BTU and Specific Heat

This lesson introduces one of the most important physical ideas behind the refrigeration cycle. To explain it clearly, we start with water.

 

A BTU is a unit used to measure the quantity of heat. By definition, one BTU is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. This definition is the reason water is used as the reference point for many heat calculations.

 

From this, we get another important term called specific heat. The specific heat of water is 1.0, and all other materials are compared to water using this value.

 

Ice has a specific heat of 0.5. This means it takes only half as much heat to change the temperature of ice compared to water.

 

In practical terms, one BTU raises one pound of ice by two degrees, while that same BTU raises water by only one degree.

 

Sensible Heat in Ice

Now we look at sensible heat, which is heat that changes temperature and can be measured with a thermometer.

 

If we start with one pound of ice at 0°F and warm it to 32°F, we are changing only the temperature, not the state.

 

Because ice has a specific heat of 0.5, this temperature increase of 32 degrees requires 16 BTUs of sensible heat.

At this point, the ice is still ice. Only the temperature has changed.

 

Latent Heat and the Heat of Fusion

To turn ice into water, we must change its state, not just its temperature. Heat used to change state is called latent heat, often described as hidden heat because the temperature does not change during the process.

 

The change from solid ice to liquid water is called the heat of fusion. It takes 144 BTUs to change one pound of ice at 32°F into one pound of water at 32°F.

 

During this entire process:

• The temperature stays at 32°F
• The substance changes state
• A large amount of heat is absorbed

The same amount of heat is released when water freezes back into ice.

 

Sensible Heat in Water

Once the ice has fully melted, heat added to the water again becomes sensible heat.

 

Raising one pound of water from 32°F to 212°F requires a temperature increase of 180 degrees, which means 180 BTUs of sensible heat.

 

This changes the temperature a lot, but the water remains liquid the entire time.

 

Latent Heat and the Heat of Vaporization

Now comes the most important part.

 

To change one pound of water at 212°F into steam at 212°F, the temperature does not increase at all. Instead, the state changes from liquid to vapor. This process is called the heat of vaporization.

 

The amount of heat required for this change is 970 BTUs.

 

This is far more energy than what was required to:

• Raise ice to melting temperature
• Melt ice into water
• Raise water to boiling temperature

Even though the temperature does not change, the energy involved is enormous.

 

3.2 Condensing and Subcooling

Reviewing Latent and Sensible Heat with Ice and Water

We begin by reviewing latent heat of fusion using a simple example. Imagine a one-pound block of ice at 30°F. To raise its temperature to 32°F, we need to add heat. Because ice has a lower specific heat than water, one BTU is enough to raise the temperature of that ice by two degrees.

 

This is a sensible heat change, because the temperature changes and can be measured.

 

Latent Heat of Fusion

Once the ice reaches 32°F, adding more heat no longer raises the temperature. Instead, the ice begins to melt.

 

To change one pound of ice at 32°F into one pound of water at 32°F, we must add 144 BTUs.

 

During this process:

• The temperature stays at 32°F
• The state changes from solid to liquid
• The heat added is latent heat

This process is called the latent heat of fusion.

 

Sensible Heat in Water

After the ice has completely melted, the water can now increase in temperature. Raising one pound of water from 32°F to 212°F requires 180 BTUs. This is again a sensible heat change, because the temperature increases and can be measured with a thermometer.

 

At this point, the water is hot, but it is still liquid.

 

Latent Heat of Vaporization

To change water into vapor, temperature increase is no longer enough. Turning one pound of water at 212°F into steam at 212°F requires 970 BTUs.

 

This heat does not raise the temperature. Instead, it changes the state of the substance from liquid to vapor. This process is called the latent heat of vaporization.

 

Compared to sensible heat, the amount of energy involved here is enormous. Far more heat is required to change state than to change temperature.

 

 

Condensing: The Reverse Process

Condensing is the opposite of vaporization. Instead of adding heat, heat is removed. When vapor loses latent heat, it changes back into a liquid.

 

This process happens inside a refrigeration condenser. The refrigerant enters the condenser as a hot vapor. Air is blown across the condenser coil. Even on a hot day, this air is cooler than the refrigerant.

 

From the refrigerant’s point of view, the air is cool. Heat moves from the refrigerant into the air. As latent heat is removed, the vapor reaches its condensing temperature and turns into liquid inside the coil.

 

Water Example for Condensing

Think again about boiling water. When water turns into steam at 212°F, it absorbs latent heat. If that steam were captured and cooled by colder air, it would give up that latent heat and condense back into liquid water.

 

This is exactly what happens in a refrigeration condenser.

 

Introduction to Subcooling

Subcooling is the next concept. Subcooling means cooling a liquid below its boiling or condensing temperature.

 

If water condenses at 212°F and then continues to cool to 211°F, it is said to be subcooled by 1 degree. The water is already fully liquid, but additional heat has been removed.

 

In HVAC systems, subcooling tells us that the refrigerant leaving the condenser is fully condensed and has extra cooling, which is important for proper system operation.

 

3.3 Three States of a Refrigerant

Refrigerant State and Sensible Heat

The focus shifts back to sensible heat and how the state of a refrigerant is determined. The key idea is simple:
the temperature of the refrigerant decides its state.

 

There are only three possible conditions, based on the refrigerant’s boiling point at that pressure.

 

Three Possible Refrigerant States

A refrigerant can only be in one of three conditions:

• Below its boiling point
• At its boiling point
• Above its boiling point

Each condition directly tells us the state of the refrigerant.

 

Below the Boiling Point: Subcooled

If a refrigerant is below its boiling point, it is said to be subcooled. In this condition, the refrigerant can only exist as a liquid.

 

There is no vapor present. Once the refrigerant has fully condensed into a liquid, any further cooling simply lowers its temperature below the boiling point.

 

At the Boiling Point: Saturated

If a refrigerant is at its boiling point, it is called saturated.

In this condition:

• Liquid and vapor exist together
• The refrigerant is in the process of changing state

At saturation, temperature stays constant while the refrigerant changes from liquid to vapor or from vapor to liquid. It will remain in this mixed state until the change of state is complete.

 

Above the Boiling Point: Superheated

If a refrigerant is above its boiling point, it is said to be superheated.

 

At this point:

• All liquid has already turned into vapor
• The refrigerant exists as vapor only

Any added heat now raises the temperature of the vapor instead of changing its state.

 

Water Examples to Make It Clear

Imagine a pot of water at normal pressure.

At 211°F, the water is below its boiling point. It must be a liquid.

At 212°F, the water is at its boiling point. Liquid and vapor are present together. This is the saturated condition.

At 213°F, the water is above its boiling point. All liquid has turned into vapor. This is superheated vapor.

 

The Rule That Keeps Everything Straight

This rule is critical and should be memorized:

• Only liquid can be subcooled
• Only vapor can be superheated

Until a substance is 100 percent vapor, it cannot be superheated. Until it is 100 percent liquid, it cannot be subcooled.