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Airflow Properties & Affects

Airflow Systems : Chapter 1

Introduction to Airflow

In this module, we will go over what airflow is and how it affects the human body. Skip to quiz!

Human Comfort

The human body controls temperature the same way a thermostat does. The thermostat will cool or heat the house in order to keep it at a set temperature.Similarly, the temperature of the human body stays at 98.6°F in all seasons.

The range of comfortable temperatures for humans is between 67°F and 82°F. When the temperature around you is above this range, it will make your body feel hot. If it’s below this range, your body will feel cold in response.

The temperature around you determines how your body behaves. In Miami during summer, your body has a different temperature response than in the Swiss Alps in winter!

Just like your house has a thermostat that controls the temperature inside, there is a part of the brain that acts like thermostat for your body. This is called the hypothalamus. The hypothalamus recognizes temperature changes in the body and attempts to fix them.

The human body wants to stay in its range of comfortable temperatures.So if the hypothalamus realizes it’s too hot or too cold, the body will respond to fix it!

The body can produce and release heat in lots of ways. Producing heat increases the body temperature. Releasing heat decreases the body temperature. Let’s take a look at how the body does this!

These are some of the ways the body reacts when its temperature is too high:

  • Sweating, and

  • Blood Vessel Expanding

All of these reactions release heat and lower the body temperature.

These are some of the ways the body reacts when its temperature is too low:

  • Shivering, and

  • Blood Vessel Compressing

All of these reactions create heat and raise the body temperature.

Recall that the comfortable temperature range is between 67°F and 82°F. We can obtain a smaller range based on other factors like humidity, clothing season and more! For example, 75°F in the Florida humidity is a lot less comfortable than 75°F on the breezy CA coast.

Humidity is reported in percentages. A place like Florida, that is very humid would have a high percent humidity. A place like New Mexico that is not humid would have a low percent humidity.

Recall that humidity is when there is a lot of water vapor in the air. If the air is warm at 85°F and very humid with water vapor, we will feel very hot. This is because our sweat does not evaporate due to the humidity. This prevents our body from releasing heat into the surroundings.

Comfort is important in the workspace. The more comfortable you are the more productive and concentrated you will be! Workspaces uses HVAC systems to achieve this comfort. HVAC systems make sure the temperature in the room is always comfortable.

HVAC systems are heating and air conditioning systems. They control the temperature of a room or building. They increase the temperature when it’s too cold and decrease the temperature when it’s too hot.

HVAC systems not only control the temperature, but also the humidity. Let’s take a closer look at what humidity is.


Humidity is the amount of water vapor present in the air. Humidity depends on the temperature and pressure of the air around it.

Changes in humidity can seriously affect human health. High humidity can promote fungus growth that leads to harmful breathing problems. Low humidity can cause our skin to crack and feel itchy.

Just like a thermometer can measure temperature, there are devices called hygrometers to measure humidity! There are classical hygrometers that were originally used. There are also modern hygrometers with advanced technology.

One of the most common classical hygrometers is a hair hygrometer. Human hair grows in high humidity and shrinks in low humidity! This devices uses the length of a piece of hair to determine the humidity around it.

Modern hygrometers use very advanced technology to measure humidity. Some common ones are capacitive hygrometers that use electrical circuits to measure humidity.

The hygrometer readings will tell us if the humidity is too high or too low for comfort. The humidity needs to be fixed to keep the room comfortable. These adjustments can be controlled by HVAC systems.

In low humidity, we can increase the humidity by spraying water into the air. In high humidity, we can decrease the humidity by opening doors or using fans. They are both controlled by HVAC systems.

Ductwork Overview

Ductwork is a system of vents and tubing used in HVAC systems. Using ductwork, we can control the flow of air and direct cooled or warmed air to the desired room.

When there is too much airflow in a ductwork system it becomes noisy and damages the system. If there is too little airflow in the ductwork, the airflow may not make it all the way through it. So the amount of airflow needs to be just right. This is controlled by HVAC systems.

A good way to think about duct work is to think of it like the blood vessels in your body. Your blood vessels carry blood from your heart to different parts of the body. Ductwork carries air from a central location to different parts of the building.

In order to reach all rooms in a building, the ductwork must turn and change direction. We will have corners and other connection points between two lines of ductwork. These curves and connections are achieved through fittings.

Here are some of the examples of fittings used in ductwork.

  • Reducers -- change the size of the duct.

  • End Cap -- end line of duct.

  • Elbow -- allow the duct to turn.

We will talk more about how these work in a later module.

Airflow is very important to achieve human comfort. Temperature and humidity make up a large part of what determines our comfort. This is why we created HVAC systems. They allow us to achieve comfort through ductwork.


Review Air Properties

In this module, we will review basic concepts related to airflow. We will also review properties of air including pressure and humidity. Skip to quiz!

Properties of Air

Air is all around us! Even though you cannot see it, we breathe it every day. So what is it?

As seen in the video air is colorless and shapeless. It occupies space and has pressure and weight.

The best way to think about air taking up space is by blowing up a balloon. When you blow into the balloon, the air moves from your lungs into the balloon. The more air you put in the balloon, the more space it takes up inside. This shows that air occupies space

Air is a mixture of a bunch of different gases. A salad is composed of lettuce, tomato, onions and more. Just like air is composed of Nitrogen, Oxygen Carbon Dioxide and more! Each gas in air is made up of tiny gas molecules.

These molecules each have their own individual weight. When you add all these weights together you get the weight of air. Because the weight of the molecules is so small it feels like almost nothing. These molecules add up to create the weight of the entire atmosphere!

The layer of air surrounding us and the entire planet is known as the atmosphere. Recall that one property of air is that it has weight. Even though air weighs close to nothing, there is so much that it adds up. The total weight of the atmosphere is 5.5 quadrillion tons!

5.5 quadrillion tons seems heavy. The Earth is so big though, that this weight gets spread out so much we can’t feel it. This weight creates a force known as atmospheric pressure.

Force exerted by the atmosphere on the earth’s surface is called Atmospheric Pressure. The standard constant value used for atmospheric pressure at sea level is 1 atm. An atm is a unit of pressure measurement. Just like inches measure length, atm measures pressure.

When you swim to the bottom of a pool your ears may hurt because of the water pressure above. The more water above you the more pressure you feel. This is the same for air.

When you are at a high altitude there is less air above you than when you are at sea level. This means the atmospheric pressure is lower on Mount everest than in the Mojave desert.

Specific vs Relative Humidity

Recall that humidity is the amount of water vapor in the air. There are two different types of humidity:

  • Specific Humidity, and

  • Relative Humidity

Specific humidity is the ratio of water vapor to dry air. Dry air is a sample of air that has no water vapor. Specific humidity is also called the Humidity Ratio.

The specific humidity is the same at high and low temperatures. It only changes if moisture is added or removed. This is helpful in making weather predictions because it shows when more moisture is in an area.

Relative Humidity is how much water is in the air versus how much water the air can hold. Think about it this way: if you have a cup that is half-full of water, the cup contains 50% of what it can hold. Relative humidity also changes with temperature. Here’s a video to explain how.

Recall from the video that warm air can hold more water vapor than cold air. This means that 50% relative humidity in warm air has more water than in cold air. So 50% relative humidity feels hotter in warm air than cold air.

Static Pressure

Recall that ductwork is the system of vents used to distribute air in HVAC. Static pressure is defined as the force required to move air. Static pressure moves air through the HVAC system’s components and ductwork.

Another way to think of static pressure is the force that the air pushes on the inside of its container. This container may be anything from a balloon to ductwork.

Static Pressure and Airflow are related to each other. When static pressure increases, airflow decreases, and when airflow increases, static pressure decreases. But why does this happen?

Let’s take a look at the balloon example again. Because of the static pressure applied by the air at the walls of the balloon, it is inflated. Now make a small hole with the help of a needle. As the air flows out of the balloon, static pressure decreases.

CFM per Ton

CFM stands for cubic feet per minute. This measures the volume of airflow over time. The term ton is used to describe the cooling abilities of an AC unit. CFM per ton is a way of describing an HVAC system’s capabilities.

The standard airflow in cfm per ton is 400 cfm per ton. This means that a standard appliance must produce 400 cubic feet of cold air per minute to remove the heat of 1 ton.

The CFM per ton is important in deciding which HVAC system to choose. A large building with need a high CFM per ton to cool it. A single room will only need a small one.

Effects of Airflow

The airflow affects how well the HVAC system operates.How well a system operates is known as its efficiency.

If the airflow is too low, the HVAC system has to work harder to achieve temperature changes. If the airflow is too high, there will be leakage in the ductwork.

Air is the mixture of gasses all around us. Air is affected by its humidity and pressure. Airflow can be used in HVAC to maximize human comfort.


Question #1: Your body releases heat to stay warm.

  1. True

  2. False

Scroll down for the answer...

Answer: False

This is false because your body wants to keep heat inside it to stay warm.

Question #2: Which of the following does not help warm the body?

  1. Sweating

  2. Blood vessel compressing

  3. Shivering

Scroll down for the answer...

Answer: Sweating

Sweating releases water to lower the internal body temperature. This cools the body down.

Question #3: If it is 85°F out, which percent humidity will make you feel the hottest?

  1. 20%

  2. 40%

  3. 60%

  4. 80%

Scroll down for the answer...

Answer: 80%

The higher the percent humidity, the hotter it will feel.

Question #4: Which of the following devices is used to measure humidity?

  1. Thermometer

  2. Barometer

  3. Pedometer

  4. Hygrometer

Scroll down for the answer...

Answer: Hygrometer

Thermometer measures temperature, barometer measures pressure, pedometer measures steps.

Question #5:Using fans and opening doors are both ways to increase the humidity in a room.

  1. True

  2. False

Scroll down for the answer...

Answer: True

This is true because these methods allow the air to move around and evaporate the water.

Question #6: What type of airflow is needed in ductwork?

  1. High airflow

  2. Low airflow

  3. No airflow

  4. Specific amounts of airflow

Scroll down for the answer...

Answer: Specific amounts of airflow

Having too much or too little airflow can cause problems in ductwork. There is a specific amount needed for each part of each system.

Question #7: Which of the following is not a property of air?

  1. Air is see through

  2. Air has no weight

  3. Air is colorless

  4. Air has no shape

Scroll down for the answer...

Answer: Air has no weight

Even though it is not very heavy, air still has some weight to it.

Question #8: Air is made up of a mixture of ____.

  1. Solids

  2. Liquids

  3. Gasses

Scroll down for the answer...

Answer: Gasses

Air takes up a large volume and no shape therefore it is a gas.

Question #9: What are the units of atmospheric pressure

  1. Gallons

  2. Atm

  3. Meters

  4. Pounds

Scroll down for the answer...

Answer: Atm

Gallons measure volume, meters measure length, and pounds measure weight.

Question #10: Specific humidity is the ratio of water vapor to ____?

  1. Dry Air

  2. Total Air

  3. Water Molecules

  4. Liquid Water

Scroll down for the answer...

Answer: Dry Air

Specific humidity is defined as the ratio of water vapor to dry air.

Question #10: At 50% relative humidity, what temperature will the air feel the driest?

  1. 60°F

  2. 70°F

  3. 80°F

  4. 90°F

Scroll down for the answer...

Answer: 60°F

Cold air can hold less water water than warm air. So if the air can hold 50% of its maximum water, the lower temperature air will hold less. This means the air feels dry at 60°F.

Question #11: What happens when Static Pressure increases.

  1. Airflow increases

  2. Airflow decreases

  3. Airflow remains the same

Scroll down for the answer...

Answer: Airflow decreases

When static pressure increases, airflow decreases. Recall the balloon example.

Question #12: A higher CFM per ton is good for a large building that requires a lot of cooling.

  1. True

  2. False

Scroll down for the answer...

Answer: True

Larger buildings have more rooms that require cooling. This requires more air, meaning CFM per ton must be high.

Question #13: Higher airflow is always beneficial.

  1. True

  2. False

Scroll down for the answer...

Answer: False

When the airflow is too high there will be leakages in the ducts.


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