Ever looked at a psychrometric chart and thought, ‘What is this crazy spider web of lines? You’re not alone. But today, we’re going to break it down simply—no fancy math, just real-world HVAC understanding. You’ll learn how to read a psychrometric chart and use it to solve a basic problem you might face on a job site or in a mechanical plan review.
What is a Psychrometric Chart?
A psychrometric chart is a graphical representation of the properties of air. It helps HVAC pros analyze air conditioning and ventilation processes. Think of it like a map of air behavior.
Here are the key components of the chart:
Dry Bulb Temperature Lines
The horizontal axis shows Dry Bulb Temperature — the air temperature is in degrees Fahrenheit. Moving to the right the air temperature gets warmer, while moving left the air temperature gets cooler.
Relative Humidity Lines
The vertical curved lines are Relative Humidity lines, from 0 to 100 percent. These curved lines show the amount of moisture in the air compared to how much it can hold at a given temperature. It’s expressed as a percentage. When the relative humidity is 100 percent, the air is fully saturated, meaning it can’t hold any more water vapor and condensation may occur. This 100 percent relative humidity condition also happens when the dry bulb and wet bulb temperatures are the same, which is why that point lies right on the saturation curve of the chart.
For example, if we follow the 70-degree dry bulb line toward the 70-degree wet bulb the relative humidity keeps increasing until we reach this curved line which indicates the air has reached 100 percent relative humidity and is fully saturated.
Humidity Ratio
The vertical axis on the right shows Humidity Ratio, or grains of moisture per pound of dry air. The vertical axis on the psychrometric chart represents the humidity ratio—the actual amount of water vapor in the air. It’s measured in pounds of moisture per pound of dry air. As you move up this axis, the air holds more moisture.
For example, let’s look at the 70 degrees Fahrenheit dry bulb temperature line. As you move along that line and pass each increasing relative humidity curve—from 20 percent, to 40 percent, to 60 percent, and so on—the humidity ratio increases. That means there’s more water vapor in each pound of air.
When you reach the point where the dry bulb and wet bulb temperatures are both 70 degrees Fahrenheit, the air is fully saturated at 100 percent relative humidity. At that point, the humidity ratio is at its maximum for 70 degrees Fahrenheit air—about 110 grains of moisture per pound of dry air. That point lies right on the saturation curve.
Saturation Line
The curved upper boundary is the Saturation Curve — that’s 100 percent relative humidity. This outer curved boundary is called the saturation curve. It represents 100 percent relative humidity—air that is fully saturated with moisture. Along this curve, the dry bulb temperature and wet bulb temperature are equal. At any point on the saturation curve, the air cannot hold any more water vapor without condensation occurring
Wet Bulb and Dew Point Lines
Diagonal lines show Wet Bulb Temperature, Enthalpy (total heat energy), and Dew Point. The diagonal lines that slope upward to the left are wet bulb temperature lines. Wet bulb temperature reflects the lowest temperature air can reach through evaporation. On the 70 degrees Fahrenheit dry bulb line, as the wet bulb temperature increases—from, say, 55 to 65 degrees Fahrenheit —the humidity ratio also increases, meaning there’s more moisture in the air. At the same time, the relative humidity rises. When the wet bulb temperature reaches 70 degrees Fahrenheit —equal to the dry bulb—the air is fully saturated at 100 percent relative humidity and lies right on the saturation curve.
Why It Matters in HVAC – Real Use Cases
In HVAC, we use the chart to: Size dehumidification or humidification equipment. Analyze cooling coil performance. Control air mixing and ventilation and troubleshoot comfort complaints.
For example, let’s say your client is complaining about it feeling ‘muggy’ in their office—even though the thermostat reads 72 degrees Fahrenheit. The psychrometric chart can help you figure out if the humidity is the real culprit.
Let’s walk through a simple HVAC scenario.
PROBLEM: You’re conditioning air in a commercial office building. Outdoor air is coming in at 95 degrees Fahrenheit dry bulb and 60 percent relative humidity. You need to condition it to a comfortable indoor design condition of 75°F dry bulb and 50 percent relative humidity.
Step 1: Plot the Outdoor Air
This is your starting point. Use the dry bulb and move up to the 60 percent relative humidity curve to find the outdoor air condition.
Step 2: Plot the Desired Indoor Air
This is your target condition—comfortable for most people.
Step 3: Draw a Straight Line Between These Points
This represents the path air must take through cooling and dehumidification.
Step 4: Analyze What Happens Along the Line
As air moves across this line, it cools down and loses moisture. The cooling coil removes sensible heat and latent heat (humidity).
Step 5: Estimate How Much Water Is Removed
At 95 degrees Fahrenheit and 60 percent relative humidity, the humidity ratio is about 120 grains per pound.
At 75 degrees Fahrenheit and 50 percent relative humidity, it’s about 65 grains per pound.
So the cooling coil removes 55 grains of moisture per pound of dry air.
This is a simple explanation of how the psychrometric chart is used to solve problems and determine what happens to the air at various conditions.
Step 6: Total System Load (Optional Advanced)
“If you know the airflow rate—say 2,000 CFM—you can estimate total moisture removed per hour using formulas. But for now, just know this: The chart tells you how much cooling and dehumidification your system needs to achieve comfort.”
