Psychrometric Chart: How to Read It (Beginner's Guide 2025)

For many aspiring HVAC engineers, the psychrometric chart can seem like a complex and intimidating web of lines. However, it is one of the most powerful tools in the field, providing a complete graphical representation of the thermodynamic properties of moist air.

Mastering the psychrometric chart is essential for designing, analyzing, and troubleshooting HVAC systems. This guide will break down the chart into its core components and show you how to use it with confidence.

What is the Psychrometric Chart?

The psychrometric chart is a graphical representation of the properties of air at a constant pressure (usually sea-level atmospheric pressure). It allows engineers to quickly determine the properties of air and visualize the effects of heating, cooling, humidification, and dehumidification processes.

Example: At 25°C dry-bulb ( $T_{db}$ ) and 50% RH ( $\phi$ ), you can read from the chart:

T_{wb} = 17.8°\text{C}, \quad W = 10 \text{ g/kg}, \quad h = 50.5 \text{ kJ/kg}, \quad T_{dp} = 13.9°\text{C}

Calculate these instantly with our Psychrometric Calculator.

In essence, if you know any two properties of an air sample, you can determine all the others using the chart.

Important: Standard psychrometric charts are constructed for sea-level atmospheric pressure (101.325 kPa). At higher altitudes, air density decreases and chart readings must be corrected—or altitude-specific charts used. A chart for Denver (1,609m elevation) will differ from a sea-level chart.

The Key Properties of Air on the Chart

Property	Symbol	Units	Chart Location
Dry-Bulb Temperature	Tdb	°C or °F	Horizontal axis
Wet-Bulb Temperature	Twb	°C or °F	Diagonal lines (left-downward)
Relative Humidity	RH	%	Curved lines (0-100%)
Humidity Ratio	W	g/kg or lb/lb	Vertical axis (right side)
Enthalpy	h	kJ/kg or BTU/lb	Diagonal lines (parallel to Twb)
Specific Volume	v	m³/kg or ft³/lb	Nearly vertical lines
Dew Point	Tdp	°C or °F	Follow horizontal to saturation

The chart maps out the following properties:

Dry-Bulb Temperature: The temperature of the air as measured by a standard thermometer. This is shown on the horizontal axis.
Wet-Bulb Temperature: The temperature measured by a thermometer with its bulb wrapped in a wet wick. This indicates the cooling effect of evaporation. Lines of constant wet-bulb temperature slope downwards from left to right. You can calculate wet-bulb with our Psychrometric Calculator.
Relative Humidity (RH): The amount of moisture in the air as a percentage of the maximum amount it could hold at that temperature. These are the curved lines that sweep from the bottom left to the top right. The 100% RH line is the saturation curve on the far left.
Humidity Ratio (or Absolute Humidity): The actual mass of water vapor per unit mass of dry air (e.g., in grams of water per kilogram of dry air). This is shown on the vertical axis.
Enthalpy: The total heat content of the air (including both sensible and latent heat). Lines of constant enthalpy are nearly parallel to the wet-bulb temperature lines.
Specific Volume: The volume of air per unit mass. These lines are steeper and less parallel than the enthalpy lines.
Dew Point Temperature: The temperature at which the air would become saturated (100% RH) if cooled at a constant pressure. To find it, you move horizontally to the left from your point on the chart until you hit the saturation curve. Use our Dew Point Calculator for quick calculations.

Visualizing the Chart's Axes and Lines

The chart layout consists of: dry-bulb temperature on the horizontal axis, humidity ratio on the vertical axis, relative humidity curves sweeping diagonally, wet-bulb and enthalpy lines running nearly parallel to each other, and specific volume lines at steeper angles.

How to Read the Psychrometric Chart: A Simple Example

Let's find the properties of air at:

Dry-Bulb Temperature: 25°C
Relative Humidity: 50%

Find the Dry-Bulb Temperature: Locate 25°C on the horizontal axis.
Move Up: Move vertically up from the 25°C mark.
Find the Relative Humidity Curve: Stop where your vertical line intersects the curved 50% RH line. This is your state point.
Read the Other Properties: From this single point, you can now read all other properties:
- Humidity Ratio: Move horizontally to the right to read the value on the vertical axis.
- Wet-Bulb Temperature: Follow the diagonal line up and to the left to read the temperature on the saturation curve.
- Dew Point Temperature: Move horizontally to the left to read the temperature on the saturation curve.
- Enthalpy: Read the value from the enthalpy scale, which is usually located outside the main chart area along the wet-bulb lines.

While manual chart reading is a great skill, modern engineers often use digital tools for precision. Our Psychrometric Calculator can perform these calculations instantly and accurately.

Common HVAC Processes on the Chart

The real power of the psychrometric chart is in visualizing HVAC processes.

Sensible Heating (e.g., a furnace): A horizontal line moving from left to right. Dry-bulb and wet-bulb temperatures increase, but the humidity ratio remains constant.
Sensible Cooling (e.g., cooling coils without dehumidification): A horizontal line moving from right to left. Dry-bulb and wet-bulb temperatures decrease, humidity ratio is constant.
Humidification (e.g., adding steam): A vertical line moving up. The humidity ratio increases, with a slight increase in dry-bulb temperature.
Dehumidification (e.g., using desiccants): A vertical line moving down. The humidity ratio decreases.
Cooling and Dehumidification (e.g., a standard air conditioner): A line moving from right to left and downwards. This is the most common air conditioning process. The air is cooled below its dew point, causing moisture to condense out.

Process Line Directions Reference

HVAC Process	Chart Direction	Temperature	Humidity Ratio	RH
Sensible Heating	Horizontal right	↑ Increases	Constant	↓ Decreases
Sensible Cooling	Horizontal left	↓ Decreases	Constant	↑ Increases
Humidification (steam)	Nearly vertical up	↑ Slight increase	↑ Increases	↑ Increases
Adiabatic Humidification	Along wet-bulb line	↓ Decreases	↑ Increases	↑ Increases
Dehumidification (desiccant)	Nearly vertical down	↑ Increases	↓ Decreases	↓ Decreases
Cooling + Dehumidification	Diagonal down-left	↓ Decreases	↓ Decreases	Approaches 100%
Mixing Two Air Streams	Straight line between	Between states	Between states	Between states

Calculating HVAC Process Energy

For any HVAC process, the energy required can be calculated using enthalpy change:

Q = \dot{m} \times (h_2 - h_1)

Where:

$Q$ = Heat transfer rate (kW)
$\dot{m}$ = Mass flow rate of dry air (kg/s)
$h_1$ = Initial enthalpy (kJ/kg)
$h_2$ = Final enthalpy (kJ/kg)

Sensible heat only:

Q_s = \dot{m} \times c_p \times (T_2 - T_1)

Where $c_p \approx 1.006$ kJ/(kg·K) for dry air.

Latent heat (moisture addition/removal):

Q_L = \dot{m} \times h_{fg} \times (W_2 - W_1)

Where $h_{fg} \approx 2,501$ kJ/kg (latent heat of vaporization at standard conditions).

Visualizing an Air Conditioning Process

In a typical air conditioning process, warm humid air (e.g., 30°C at 60% RH) passes through the air conditioner coil where it is cooled and dehumidified. The air exits as cool, dry air (e.g., 15°C at 90% RH) while moisture condenses out and drains away. On the psychrometric chart, this process appears as a line moving from the warm air point downward and to the left toward the cooler, drier state.

Why is This Important for HVAC Design?

The psychrometric chart is not just an academic exercise. It is used daily to:

Calculate Cooling and Heating Loads: Determine how much energy must be added or removed to bring air to the desired condition.
Design Air Conditioning Systems: Select cooling coils and size equipment to achieve the desired temperature and humidity.
Analyze Indoor Air Quality: Ensure that ventilation and conditioning processes maintain a healthy and comfortable indoor environment.
Troubleshoot System Performance: Diagnose issues like inadequate dehumidification or insufficient cooling by plotting the system's performance on the chart.

Field Tip: When plotting an air conditioning process, always check that your final state point has RH below 90%. If your cooling coil process line hits the saturation curve, you've cooled below the dew point—verify this is intentional for dehumidification.

Worked Example: Cooling Load Calculation Using Psychrometrics

Let's calculate the cooling capacity required for an office air conditioning system using psychrometric principles.

Problem Statement

An office has the following conditions:

Outside air: 35°C dry-bulb, 50% RH (summer design day)
Required indoor conditions: 24°C dry-bulb, 50% RH
Supply air temperature: 14°C (leaving cooling coil)
Airflow rate: 1,500 L/s
Outdoor air fraction: 20%

Step 1: Determine Air Properties from the Chart

Outside air (Point 1): 35°C DB, 50% RH

$h_1$ = 72 kJ/kg (from chart or calculator)
$W_1$ = 18 g/kg

Room air (Point 2): 24°C DB, 50% RH

$h_2$ = 48 kJ/kg
$W_2$ = 9.3 g/kg

Supply air (Point 3): 14°C DB, ~90% RH (after coil)

$h_3$ = 36 kJ/kg
$W_3$ = 8.8 g/kg

Step 2: Calculate Mixed Air Conditions

With 20% outdoor air and 80% return air:

h_{mix} = 0.20 \times h_1 + 0.80 \times h_2 = 0.20 \times 72 + 0.80 \times 48 = 52.8 \text{ kJ/kg}

W_{mix} = 0.20 \times W_1 + 0.80 \times W_2 = 0.20 \times 18 + 0.80 \times 9.3 = 11.0 \text{ g/kg}

Step 3: Calculate Cooling Coil Load

Mass flow rate:

\dot{m} = \frac{Q \times \rho}{1000} = \frac{1500 \times 1.2}{1000} = 1.8 \text{ kg/s}

Total cooling load (mixed air to supply air):

Q_{coil} = \dot{m} \times (h_{mix} - h_3) = 1.8 \times (52.8 - 36) = 30.2 \text{ kW}

Step 4: Separate Sensible and Latent Loads

Sensible cooling:

Q_s = \dot{m} \times c_p \times \Delta T = 1.8 \times 1.006 \times (26 - 14) = 21.7 \text{ kW}

Latent cooling (dehumidification):

Q_L = \dot{m} \times h_{fg} \times \Delta W = 1.8 \times 2501 \times (11.0 - 8.8)/1000 = 9.9 \text{ kW}

Summary

Parameter	Value	Notes
Total cooling load	30.2 kW	Mixed air to supply
Sensible heat ratio	0.72	21.7 / 30.2
Latent load	9.9 kW	Dehumidification
Required tonnage	8.6 RT	30.2 ÷ 3.517

Selection Tip: When selecting a cooling coil, verify the equipment's sensible heat ratio (SHR) matches your calculated value. A coil with SHR of 0.85 may not adequately dehumidify if your load requires SHR of 0.72.

Worked Example 2: Winter Humidification Load

Let's calculate the humidification requirement for a building in winter using psychrometric principles.

Problem Statement

An office building requires humidification during winter:

Indoor conditions: 22°C dry-bulb, 40% RH (target)
Outdoor air: -5°C dry-bulb, 80% RH
Ventilation rate: 2,000 L/s outdoor air
Question: How much steam is needed for humidification?

Step 1: Determine Air Properties

Outdoor air (Point 1): -5°C DB, 80% RH

$W_1$ = 2.0 g/kg (from chart or calculator)
Very dry in absolute terms despite high RH

Indoor target (Point 2): 22°C DB, 40% RH

$W_2$ = 6.5 g/kg
Moisture content must increase significantly

Step 2: Calculate Moisture Addition Required

\Delta W = W_2 - W_1 = 6.5 - 2.0 = 4.5 \text{ g/kg}

Step 3: Calculate Mass Flow Rate of Dry Air

\dot{m}_{air} = \frac{\dot{V} \times \rho}{1000} = \frac{2000 \times 1.2}{1000} = 2.4 \text{ kg/s}

Step 4: Calculate Steam Requirement

\dot{m}_{steam} = \dot{m}_{air} \times \Delta W = 2.4 \times 4.5 = 10.8 \text{ g/s} = 38.9 \text{ kg/hr}

Step 5: Calculate Energy for Steam Generation

If using electric humidifier (latent heat of vaporization):

P_{humidifier} = \dot{m}_{steam} \times h_{fg} = \frac{38.9}{3600} \times 2,501 = 27.0 \text{ kW}

Summary

Parameter	Value	Notes
Moisture deficit	4.5 g/kg	Outdoor vs. indoor
Steam requirement	38.9 kg/hr	For 2,000 L/s outdoor air
Electric load	27 kW	If using electric steam
Operating cost	~3.50 USD/hr	At 0.13 USD/kWh

Winter Design Tip: At -5°C outdoor, the moisture content is only 2 g/kg even at 80% RH. This demonstrates why humidification is critical in cold climates—outdoor air is essentially "dry" in absolute terms regardless of its relative humidity.

Common Psychrometric Mistakes and How to Avoid Them

Even experienced HVAC engineers make errors when working with psychrometric charts. Here are the most common mistakes and their solutions:

Mistake 1: Using Sea-Level Charts at Altitude

Problem: Standard psychrometric charts are constructed for 101.325 kPa (sea level). At Denver (1,609m), atmospheric pressure is ~83 kPa.

Impact: Air density is 18% lower, changing all humidity-related calculations.

Fix: Use altitude-corrected charts or apply correction factor:

\rho_{altitude} = \rho_{sea} \times \frac{P_{altitude}}{101.325}

Mistake 2: Confusing Wet-Bulb and Dew Point

Problem: Both involve saturation, but they're fundamentally different.

Property	Definition	Found By
Wet-bulb	Temperature if air were adiabatically saturated	Follow wet-bulb line to saturation
Dew point	Temperature where moisture condenses	Move horizontally left to saturation

Key Insight: Dew point depends only on moisture content; wet-bulb depends on both temperature and moisture.

Mistake 3: Incorrect Process Line Direction

Problem: Drawing cooling lines in the wrong direction.

Rule: Cooling and dehumidification always moves toward the saturation curve (down and left). If your process line moves away from saturation during cooling, something is wrong.

Mistake 4: Ignoring the Bypass Factor

Problem: Assuming all air contacts the coil surface during cooling.

Reality: Real cooling coils have bypass factors of 5-15%. Some air passes without contacting the cold surface.

Corrected calculation:

T_{leaving} = T_{coil} + BF \times (T_{entering} - T_{coil})

Where $BF$ = bypass factor (typically 0.05-0.15).

Quick Reference: Typical Air Conditions

Location/Condition	Dry-Bulb	RH	Humidity Ratio	Enthalpy
Summer outdoor (hot/humid)	35°C	60%	21 g/kg	89 kJ/kg
Summer outdoor (hot/dry)	38°C	20%	8 g/kg	59 kJ/kg
Winter outdoor (cold)	0°C	50%	2 g/kg	5 kJ/kg
Indoor comfort (summer)	24°C	50%	9 g/kg	48 kJ/kg
Indoor comfort (winter)	22°C	40%	6.5 g/kg	39 kJ/kg
AHU supply air	13-15°C	90-95%	8-9 g/kg	34-38 kJ/kg

Field Tip: When troubleshooting an HVAC system, take wet-bulb and dry-bulb readings at both supply and return air grilles. Plot both points on the psychrometric chart—the line connecting them shows the actual coil performance, which you can compare against the design conditions.

Quick Diagnostic: Using Psychrometrics to Troubleshoot HVAC Problems

When a system isn't performing as expected, psychrometric analysis can quickly identify the issue:

Complaint	Measure	Chart Analysis	Likely Cause
"Too humid"	High indoor RH despite AC running	Supply air RH > 95%	Coil oversized (SHR too high)
"Too dry" (winter)	Low indoor RH despite humidifier	Supply humidity ratio unchanged	Humidifier undersized or not running
"Cold drafts"	Low supply air temp	Supply DB < 12°C	Excessive outside air or failed economizer
"Stuffy"	High CO₂, normal temp/RH	Process lines show minimal outdoor air	Inadequate ventilation
"AC runs but doesn't cool"	High return air enthalpy	Compare to design enthalpy	Internal loads exceed design

Quick Property Lookup

For common HVAC scenarios, use these approximations:

Dry-bulb 24°C, 50% RH (comfort baseline):

Wet-bulb: 17°C
Dew point: 13°C
Humidity ratio: 9.3 g/kg
Enthalpy: 48 kJ/kg
Specific volume: 0.855 m³/kg

Supply air at 13°C, 95% RH (typical AHU discharge):

Wet-bulb: 12.5°C
Dew point: 12°C
Humidity ratio: 8.9 g/kg
Enthalpy: 36 kJ/kg

Industry Standards Reference

The psychrometric properties and calculations in this guide follow ASHRAE 55 (Thermal Environmental Conditions for Human Occupancy) and ASHRAE Fundamentals Handbook Chapter 1 (Psychrometrics). Air conditioning design parameters should comply with ASHRAE 62.1 (Ventilation for Acceptable Indoor Air Quality). These references contain detailed property tables and formulas used in professional HVAC calculations worldwide.

Psychrometric Design Checklist

Use this checklist for every HVAC design involving psychrometrics:

Pre-Design:

Confirm design outdoor conditions (ASHRAE weather data)
Establish indoor comfort criteria (ASHRAE 55)
Identify sensible and latent loads separately
Determine required fresh air quantity (ASHRAE 62.1)

Coil Selection:

Calculate mixed air conditions (outdoor + return)
Verify coil leaving conditions meet humidity target
Check sensible heat ratio matches equipment capability
Confirm coil can handle peak latent load

Verification:

Plot all air states on psychrometric chart
Verify process lines are physically reasonable
Check that no state exceeds 95% RH (except coil leaving)
Calculate total cooling/heating load from enthalpy change

Field Application: Quick Coil Selection Check

When selecting a cooling coil, use this quick check to verify your design:

Step 1: Calculate Sensible Heat Ratio (SHR)

SHR = \frac{Q_{sensible}}{Q_{sensible} + Q_{latent}}

Typical values:

Office buildings: 0.75-0.85
Retail spaces: 0.70-0.80
Gymnasiums: 0.50-0.65 (high latent)

Step 2: Draw Process Line

Starting from room conditions (24°C, 50% RH), draw a line toward the saturation curve. The slope represents your SHR.

Step 3: Verify Coil Leaving Conditions

The intersection with the 90-95% RH curve gives your apparatus dew point.

Field Tip: If calculated SHR doesn't match equipment SHR capability (shown in manufacturer's data), the coil will either over-dehumidify (SHR too low) or under-dehumidify (SHR too high). For humid climates, always verify that equipment SHR ≤ design SHR.

Conclusion

The psychrometric chart is a foundational tool for any HVAC professional.

Understanding its layout unlocks a powerful method for analyzing HVAC systems. Digital calculators provide quick answers. But visualizing processes on the chart provides deeper understanding—invaluable in the field.

Professional Disclaimer: The psychrometric concepts presented here are foundational educational content. For actual HVAC system design, engineers should verify calculations using ASHRAE property tables or validated software, and account for project-specific conditions including altitude, local climate data, and building requirements.

A Beginner's Guide to the Psychrometric Chart

Table of Contents

What is the Psychrometric Chart?

The Key Properties of Air on the Chart

Visualizing the Chart's Axes and Lines

How to Read the Psychrometric Chart: A Simple Example

Common HVAC Processes on the Chart

Process Line Directions Reference

Calculating HVAC Process Energy

Visualizing an Air Conditioning Process

Why is This Important for HVAC Design?

Worked Example: Cooling Load Calculation Using Psychrometrics

Problem Statement

Step 1: Determine Air Properties from the Chart

Step 2: Calculate Mixed Air Conditions

Step 3: Calculate Cooling Coil Load

Step 4: Separate Sensible and Latent Loads

Summary

Worked Example 2: Winter Humidification Load

Problem Statement

Step 1: Determine Air Properties

Step 2: Calculate Moisture Addition Required

Step 3: Calculate Mass Flow Rate of Dry Air

Step 4: Calculate Steam Requirement

Step 5: Calculate Energy for Steam Generation

Summary

Common Psychrometric Mistakes and How to Avoid Them

Mistake 1: Using Sea-Level Charts at Altitude

Mistake 2: Confusing Wet-Bulb and Dew Point

Mistake 3: Incorrect Process Line Direction

Mistake 4: Ignoring the Bypass Factor

Quick Reference: Typical Air Conditions

Quick Diagnostic: Using Psychrometrics to Troubleshoot HVAC Problems

Quick Property Lookup

Industry Standards Reference

Psychrometric Design Checklist

Field Application: Quick Coil Selection Check

Conclusion

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