Use this free interactive psychrometric chart to analyze the temperature, moisture content, relative humidity, wet-bulb temperature, dew point, enthalpy, and specific volume of moist air. Move across the chart to inspect air properties, switch between IP and SI units, and zoom into any area for closer analysis.

You can also use the built-in tools to plot and label custom air conditions, illustrate HVAC processes, estimate room, return, on-coil, off-coil, and supply-air conditions, and calculate cooling-coil capacity from airflow and enthalpy difference.

How to Use This Psychrometric Chart

The chart can be used in three ways:

1. Explore air properties directly on the chart
2. Plot and connect custom air conditions
3. Calculate an HVAC cooling-coil process and coil load

Explore the Chart

Move the pointer across the chart to inspect the moist-air properties at different locations.

Depending on the selected point, the chart displays properties such as:

• Dry-bulb temperature
• Wet-bulb temperature
• Relative humidity
• Humidity ratio
• Dew-point temperature
• Enthalpy
• Specific volume

Use the chart controls to:

• Zoom in
• Zoom out
• Reset the chart
• Pan across the zoomed view
• Switch between IP and SI units

In IP mode, temperatures are shown in °F and enthalpy is shown in Btu/lb.
In SI mode, temperatures are shown in °C and enthalpy is shown in kJ/kg.

Plot Custom Air Conditions

Open the Plot the Chart section below the chart.

For each point, enter:

• Point label
• Dry-bulb temperature
• Either wet-bulb temperature or relative humidity
• Plot-line color
• A previous point to connect to, where required

The first point becomes the starting air condition.

For every subsequent point, you can either:

• Leave it unconnected
• Connect it to one of the previously created points

This is useful for plotting:

• Sensible heating
• Sensible cooling
• Cooling and dehumidification
• Reheating
• Humidification
• Evaporative cooling
• Desiccant dehumidification
• Outdoor and return-air mixing
• Measured AHU conditions
• Commissioning data
• Troubleshooting observations

After entering the points, click Apply and Plot.

The chart clears the previous process, plots the new points, and connects them according to your selections.

Calculate Cooling-Coil Capacity

Open the Calculate Coil Load section.

Enter:

• Indoor dry-bulb temperature
• Indoor relative humidity
• Room sensible heat ratio
• Total system airflow
• System heat gains before or after the coil
• Outdoor dry-bulb temperature
• Outdoor wet-bulb temperature
• Outdoor-air percentage

The calculator then plots the main air conditions and estimates:

• Room-air condition
• Return-air condition
• Outdoor-air condition
• Mixed on-coil condition
• Off-coil condition
• Supply-air condition
• On-coil enthalpy
• Off-coil enthalpy
• Cooling-coil load

The detailed calculation method is explained later in this article.

What Is a Psychrometric Chart?

A psychrometric chart is a graphical representation of the thermodynamic properties of moist air.

Atmospheric air contains:

• Dry air
• Water vapor

The amount of water vapor in the air affects comfort, cooling load, condensation, dehumidification, and HVAC system performance.

A psychrometric chart brings the main moist-air properties together in one diagram.

The chart allows engineers, technicians, students, and facility personnel to analyze how air changes during processes such as:

• Heating
• Cooling
• Humidification
• Dehumidification
• Evaporative cooling
• Air mixing
• Cooling-coil operation
• Reheating
• Ventilation-air treatment

Once any two independent moist-air properties are known, the remaining properties can generally be determined.

For example, if the air condition is:

• 75°F dry bulb
• 50% relative humidity

The chart can also be used to determine approximately:

• Wet-bulb temperature
• Humidity ratio
• Dew-point temperature
• Enthalpy
• Specific volume

What Is Psychrometrics?

Psychrometrics is the study of the physical and thermodynamic properties of moist air.

In HVAC applications, psychrometric analysis helps answer questions such as:

• How much moisture is present in the air?
• At what temperature will condensation begin?
• How much sensible and latent heat must be removed?
• What supply-air condition is required?
• How does outdoor air affect the cooling load?
• What happens when outdoor air mixes with return air?
• How much moisture is removed by a cooling coil?
• How does reheating affect relative humidity?
• Can an evaporative cooler reach the required condition?
• Is the cooling coil providing sufficient dehumidification?

The chart provides a visual method for analyzing these relationships.

Main Properties on a Psychrometric Chart

The main properties shown on a psychrometric chart are:

1. Dry-bulb temperature
2. Wet-bulb temperature
3. Relative humidity
4. Humidity ratio
5. Dew-point temperature
6. Enthalpy
7. Specific volume

Understanding these properties is the foundation for reading and using the chart correctly.

Dry-Bulb Temperature

Dry-bulb temperature is the ordinary air temperature measured by a standard thermometer.

It is called dry bulb because the thermometer bulb is dry and is not affected by evaporative cooling.

Dry-bulb temperature is normally shown along the horizontal axis at the bottom of the psychrometric chart.

Moving from left to right means the air temperature increases.

Typical units are:

• °F in IP units
• °C in SI units

A vertical line on the chart represents a constant dry-bulb temperature.

For example, all points directly above 75°F have the same dry-bulb temperature but may have different moisture contents.

Dry-bulb temperature alone is not enough to define an air condition.

A second property, such as relative humidity or wet-bulb temperature, is required.

Humidity Ratio

Humidity ratio is the mass of water vapor contained in the air relative to the mass of dry air.

It may also be called:

• Moisture content
• Mixing ratio
• Specific humidity

Common units include:

• lb of water/lb of dry air
• Grains of water/lb of dry air
• kg of water/kg of dry air
• g of water/kg of dry air

Humidity ratio is shown along the vertical axis, usually on the right side of the chart.

Horizontal lines represent constant humidity ratio.

During a purely sensible heating or cooling process, the humidity ratio remains constant.

Humidity ratio indicates the actual amount of moisture in the air.

This is different from relative humidity, which changes when the temperature changes.

Relative Humidity

Relative humidity shows how close the air is to saturation at its current temperature.

It is expressed as a percentage.

For example:

• 20% RH indicates relatively dry air
• 50% RH means the air contains approximately half the moisture required for saturation at that temperature
• 100% RH means the air is saturated

Relative-humidity lines are the curved lines extending across the chart.

The upper curved boundary is the 100% relative-humidity line, also called the saturation curve.

Relative humidity depends on both:

• Moisture content
• Air temperature

This means relative humidity can change even when no moisture is added or removed.

For example, if air is heated at constant humidity ratio, its relative humidity decreases.

If air is cooled at constant humidity ratio, its relative humidity increases until saturation is reached.

Wet-Bulb Temperature

Wet-bulb temperature is the temperature measured when air passes over a wetted thermometer bulb.

Water evaporating from the wet surface causes cooling.

The amount of cooling depends on the moisture content of the surrounding air.

Wet-bulb temperature therefore reflects the combined influence of:

• Dry-bulb temperature
• Moisture content

Wet-bulb lines run diagonally across the chart.

Wet-bulb temperature is always equal to or lower than dry-bulb temperature.

At saturation:

Dry bulb = Wet bulb = Dew point

The difference between dry-bulb and wet-bulb temperatures is called the wet-bulb depression.

A large wet-bulb depression generally indicates dry air.

A small wet-bulb depression indicates air close to saturation.

Wet-bulb temperature is important in:

• Outdoor design conditions
• Cooling-tower analysis
• Evaporative cooling
• Cooling-coil calculations
• Moist-air property calculations
• Heat-rejection equipment

Dew-Point Temperature

Dew point is the temperature at which air becomes saturated when it is cooled without changing its moisture content.

Below the dew-point temperature, some of the water vapor begins to condense into liquid water.

To find the dew point on a psychrometric chart:

1. Locate the air-condition point.
2. Move horizontally left at constant humidity ratio.
3. Stop at the saturation curve.
4. Read the temperature at that location.

For example, if a duct surface temperature is lower than the dew-point temperature of the surrounding air, condensation may form on the duct.

Dew-point temperature is important for:

• Condensation control
• Duct insulation
• Chilled-water pipe insulation
• Cold-surface analysis
• Building-envelope design
• Humidity control
• Supply-air system design

Enthalpy

Enthalpy represents the total heat content of moist air.

It includes:

• Sensible heat associated with temperature
• Latent heat associated with water vapor

Common units are:

• Btu/lb of dry air
• kJ/kg of dry air

Enthalpy lines run diagonally across the chart and are usually close to the wet-bulb lines.

The difference in enthalpy between two air conditions is useful for calculating:

• Cooling-coil load
• Heating-coil load
• Ventilation load
• Total heat removed from air
• Total heat added to air
• Energy exchanged during air mixing

A cooling coil that cools and dehumidifies air removes both sensible and latent heat.

The total reduction in air enthalpy represents the total cooling effect.

Specific Volume

Specific volume is the volume occupied by a unit mass of dry air.

Typical units are:

• ft³/lb of dry air
• m³/kg of dry air

Specific-volume lines are slightly inclined across the chart.

Specific volume is the inverse of density.

Air with a higher specific volume has a lower density.

Specific volume can be used when converting between:

• Volumetric airflow
• Dry-air mass flow
• Heating capacity
• Cooling capacity

Many preliminary HVAC calculations use standard air density.

More precise calculations can use the actual specific volume at the selected air condition.

How to Read a Psychrometric Chart

A complete air condition normally requires two independent properties.

Common combinations include:

• Dry bulb and relative humidity
• Dry bulb and wet bulb
• Dry bulb and dew point
• Dry bulb and humidity ratio
• Enthalpy and humidity ratio

The most common combinations used in HVAC are:

• Dry bulb with relative humidity
• Dry bulb with wet bulb

Example 1: Dry Bulb and Relative Humidity

Suppose the air condition is:

• 75°F dry bulb
• 50% relative humidity

To plot the condition:

1. Find 75°F on the dry-bulb axis.
2. Move vertically upward.
3. Locate the 50% relative-humidity curve.
4. Mark the intersection.

From this point, the other properties can be estimated.

To find humidity ratio, move horizontally to the right-hand scale.

To find dew point, move horizontally left to the saturation curve.

To find wet-bulb temperature, follow the diagonal wet-bulb line.

To find enthalpy, follow the corresponding diagonal enthalpy line.

To find specific volume, identify the specific-volume line passing through the point.

Example 2: Dry Bulb and Wet Bulb

Suppose the outdoor-air condition is:

• 95°F dry bulb
• 78°F wet bulb

To plot the condition:

1. Locate 95°F on the dry-bulb axis.
2. Move vertically upward.
3. Find the diagonal 78°F wet-bulb line.
4. Mark the intersection.

From this point, you can determine approximately:

• Relative humidity
• Humidity ratio
• Dew point
• Enthalpy
• Specific volume

Dry-bulb and wet-bulb temperatures are commonly used for outdoor design conditions.

In my article titled How to Read a Psychrometric Chart?, I use hardcopy chart and color highlight to provide clear visualization on each of the parameter. If you want visual explanation, check out that article.

Why Two Properties Are Required

Dry-bulb temperature alone does not define an air condition.

For example, all of the following air conditions have the same dry-bulb temperature:

• 75°F at 20% RH
• 75°F at 50% RH
• 75°F at 80% RH

However, they contain different amounts of moisture and have different enthalpy, wet-bulb temperature, and dew point.

Relative humidity alone is also insufficient.

For example, 50% RH at 50°F is not the same condition as 50% RH at 90°F.

Two independent properties are therefore needed to establish a unique air-condition point.

Understanding the Saturation Curve

The saturation curve forms the upper-left curved boundary of the standard psychrometric chart.

Every point on this curve represents 100% relative humidity.

At saturation:

• The air is holding the maximum amount of water vapor possible at that temperature and pressure
• Dry-bulb temperature equals wet-bulb temperature
• Dry-bulb temperature equals dew-point temperature

If saturated air is cooled further, moisture condenses.

This is the principle behind cooling and dehumidification at a cooling coil.

When moist air contacts a surface below its dew point, condensation forms and the humidity ratio decreases.

Common Psychrometric Processes

HVAC systems change air conditions through heating, cooling, moisture addition, moisture removal, and mixing.

These processes appear as lines moving in different directions across the chart.

Sensible Heating

Sensible heating increases dry-bulb temperature without adding moisture.

Examples include:

• Electric heating coil
• Hot-water heating coil
• Steam heating coil without steam injection
• Supply-fan heat
• Return-fan heat
• Duct heat gain

On the psychrometric chart, sensible heating moves horizontally to the right.

During sensible heating:

• Dry-bulb temperature increases
• Humidity ratio remains constant
• Relative humidity decreases
• Enthalpy increases
• Dew point remains constant

Fan heat between the cooling coil and supply-air outlet is a common example.

Sensible Cooling

Sensible cooling reduces dry-bulb temperature without removing moisture.

The process moves horizontally to the left.

During sensible cooling:

• Dry-bulb temperature decreases
• Humidity ratio remains constant
• Relative humidity increases
• Enthalpy decreases
• Dew point remains constant

Sensible cooling can continue only until the air reaches its dew point.

If cooling continues below the dew point, moisture begins to condense and the process becomes cooling with dehumidification.

In practice, perfectly sensible cooling occurs only when the cooling surface remains above the entering-air dew point.

Cooling and Dehumidification

Cooling and dehumidification is the most common process across an air-conditioning cooling coil.

The air is cooled below its dew point.

Water vapor condenses on the cold coil surface and drains away as liquid water.

The process moves downward and to the left.

During cooling and dehumidification:

• Dry-bulb temperature decreases
• Humidity ratio decreases
• Enthalpy decreases
• Moisture is removed
• Dew point decreases

The exact process depends on:

• Coil surface temperature
• Apparatus dew point
• Coil bypass factor
• Face velocity
• Number of coil rows
• Fin spacing
• Chilled-water temperature
• Refrigerant temperature
• Entering-air condition

Heating and Humidification

Heating and humidification increases both air temperature and moisture content.

The process moves upward and to the right.

Possible equipment includes:

• Steam humidifier with heating
• Heated water-spray humidifier
• Air washer with added heat
• Heating coil followed by a humidifier

This process is commonly used during winter when outdoor air is cold and dry.

During heating and humidification:

• Dry-bulb temperature increases
• Humidity ratio increases
• Enthalpy increases
• Relative humidity may increase or decrease depending on the process

Evaporative Cooling

Evaporative cooling occurs when water evaporates into the air and absorbs sensible heat.

The air temperature decreases while the humidity ratio increases.

The process generally follows an approximately constant-enthalpy or constant-wet-bulb line upward and to the left.

During direct evaporative cooling:

• Dry-bulb temperature decreases
• Humidity ratio increases
• Relative humidity increases
• Enthalpy remains approximately constant
• Wet-bulb temperature remains approximately constant

Evaporative cooling is most effective in hot and dry climates.

Its performance is limited when the outdoor wet-bulb temperature is already high.

Humidification

Humidification adds water vapor to the air.

The exact direction of the process depends on the type of humidifier.

Steam Humidification

Steam humidification adds both moisture and heat.

The process moves upward and slightly to the right.

Adiabatic Humidification

An adiabatic humidifier adds moisture while reducing sensible temperature.

The process moves upward and to the left along an approximately constant-enthalpy line.

Near-Isothermal Humidification

An idealized isothermal humidification process appears almost vertical.

In real equipment, a small temperature change usually occurs.

Chemical or Desiccant Dehumidification

Chemical dehumidification removes moisture using a hygroscopic material such as a desiccant.

The adsorption process releases heat.

As a result:

• Humidity ratio decreases
• Dry-bulb temperature increases
• Enthalpy may change depending on the process
• Relative humidity decreases

The process moves downward and to the right.

Desiccant dehumidification is useful where:

• Very low humidity is required
• Conventional cooling is insufficient
• Process control is more important than comfort cooling
• Low-dew-point air is required

Cooling with Reheat

Cooling with reheat is commonly used when the air must be dehumidified without overcooling the room.

The process occurs in two stages:

1. The air is cooled and dehumidified.
2. The air is reheated sensibly.

On the chart:

• Cooling and dehumidification moves downward and left.
• Reheating moves horizontally to the right.

Applications include:

• Hospitals
• Laboratories
• Cleanrooms
• High-humidity spaces
• Dedicated outdoor-air systems
• Process environments

Air Mixing

Air mixing occurs when two air streams combine.

Common examples include:

• Outdoor air mixed with return air
• Primary air mixed with room air
• Two return-air streams mixed together
• Bypass air mixed with conditioned air

The final mixed-air point lies on a straight line connecting the two original air conditions.

Its position depends on the relative dry-air mass flow of each stream.

If the two mass flow rates are equal, the mixed-air point lies near the middle of the line.

If one stream is much larger, the mixed point lies closer to that air condition.

For example, a system with:

• 10% outdoor air
• 90% return air

Will produce a mixed-air point much closer to the return-air condition.

Using the Custom Plotting Function

The custom plotting function allows you to create your own psychrometric-process diagram directly on the chart.

Open the Plot the Chart section.

Enter the First Point

For the first point, enter:

• A label
• Dry-bulb temperature
• Wet-bulb temperature or relative humidity
• A process-line color

For example:

This establishes the starting air condition.

Add Additional Points

Click Add Another Point.

Enter the new condition and select whether it should connect to an earlier point.

For example:

The chart connects the two points to illustrate the room air-conditioning process.

Leave Points Unconnected

Select Do not connect when you want to compare air conditions without drawing a process line.

This can be useful when comparing:

• Different rooms
• Different operating dates
• Design and actual conditions
• Multiple outdoor-air conditions
• Different AHUs
• Separate measurement locations

Practical Uses

The plotting function can be used for:

• HVAC training
• Technical presentations
• Commissioning reports
• Troubleshooting reports
• AHU analysis
• Energy-audit findings
• Comparing measured and design conditions
• Demonstrating psychrometric theory
• Explaining cooling-coil performance

Cooling-Coil Capacity Calculator

The cooling-coil calculator estimates a complete air-conditioning process based on room conditions, system airflow, sensible heat ratio, outdoor air, and system heat gains.

Open the Calculate Coil Load section.

Indoor Design Condition

Enter the target room condition.

Indoor Dry-Bulb Temperature

This is the target room temperature.

Common comfort values may be around:

• 75°F in IP units
• 24°C in SI units

The correct value depends on:

• Project requirements
• Building use
• Owner requirements
• Local design standards
• Indoor-air-quality strategy
• Energy targets

Indoor Relative Humidity

This is the target room relative humidity.

A value around 50% RH is commonly used for general comfort analysis.

However, the correct requirement depends on the application.

Spaces requiring tighter control may include:

• Hospitals
• Laboratories
• Museums
• Data centers
• Manufacturing areas
• Pharmaceutical spaces
• Food-processing areas

Sensible Heat Ratio

The sensible heat ratio, or SHR, represents the sensible portion of the room cooling load.

SHR = Sensible Cooling Load ÷ Total Cooling Load

For example, an SHR of 0.80 means:

• 80% of the room load is sensible
• 20% of the room load is latent

The SHR determines the slope of the room process on the psychrometric chart.

A high SHR produces a flatter process line.

A low SHR produces a steeper process line because more moisture must be removed.

System Airflow

Enter the total airflow passing through the cooling coil.

The calculator accepts:

• CFM in IP mode
• m³/s in SI mode

The entered airflow should represent the total supply airflow, not only the outdoor-air quantity.

System Heat Gains

The air may gain sensible heat before or after passing through the cooling coil.

Examples include:

• Supply-fan heat
• Return-fan heat
• Supply-duct heat gain
• Return-duct heat gain
• Motor heat
• Equipment heat inside the air stream

The calculator allows multiple temperature-rise inputs.

Each heat gain can be assigned as:

• Before the coil
• After the coil

Heat Gain Before the Coil

A before-coil heat gain raises the entering-air temperature and enthalpy.

This increases the cooling load imposed on the coil.

Examples may include:

• Return fan upstream of the coil
• Warm return ductwork
• Heat from equipment located before the coil

Heat Gain After the Coil

An after-coil heat gain increases the supply-air temperature after cooling.

The cooling coil must therefore produce colder leaving air to compensate.

Examples include:

• Supply fan after the coil
• Supply-duct heat gain
• Motor heat downstream of the coil

The calculator includes a default after-coil fan temperature rise of 2°F, or approximately 1.1°C.

You can revise, remove, or add more heat-gain items.

Outdoor-Air Condition

Enter:

• Outdoor dry-bulb temperature
• Outdoor wet-bulb temperature
• Outdoor-air percentage

Outdoor design conditions should be selected from an appropriate climatic source or project design criterion.

The outdoor-air percentage represents the fraction of total system airflow supplied as outdoor air.

For example:

• Total airflow = 1200 CFM
• Outdoor-air percentage = 10%

Then:

• Outdoor air = 120 CFM
• Return air = 1080 CFM

The outdoor air mixes with return air before entering the cooling coil.

Air Conditions Plotted by the Coil Calculator

Depending on the selected inputs, the calculation may plot the following conditions.

Room Air

Room air is the indoor design condition entered by the user.

It represents the condition that the HVAC system is intended to maintain.

Return Air

Return air starts at the room condition and may absorb sensible heat before reaching the cooling coil or mixing box.

Possible return-air heat gains include:

• Return-fan heat
• Return-duct heat gain
• Ceiling-plenum heat gain
• Equipment heat

If no before-coil temperature rise is entered, the return-air condition may remain the same as the room-air condition.

Outdoor Air

The outdoor-air condition is determined from:

• Outdoor dry-bulb temperature
• Outdoor wet-bulb temperature

Outdoor air may have:

• Higher temperature than room air
• Higher humidity ratio than room air
• Higher enthalpy than room air

In hot and humid climates, outdoor air can add a significant latent cooling load.

On-Coil Air

On-coil air is the air entering the cooling coil.

It is normally a mixture of:

• Return air
• Outdoor air

The mixed-air condition lies between the two original points.

Its exact position depends on the airflow ratio.

For example, with 10% outdoor air and 90% return air, the on-coil point lies much closer to the return-air condition.

Before-coil sensible heat gains also affect the on-coil condition.

Off-Coil Air

Off-coil air is the air leaving the cooling coil.

The calculator estimates the off-coil condition using a 90% relative-humidity assumption.

This is a practical approximation for preliminary analysis.

Actual off-coil conditions depend on:

• Apparatus dew point
• Coil bypass factor
• Coil face velocity
• Number of coil rows
• Fin spacing
• Chilled-water temperature
• Refrigerant temperature
• Airflow distribution
• Coil geometry
• Manufacturer selection

The calculated off-coil point should therefore be treated as a preliminary estimate.

Supply Air

Supply air is the air delivered toward the room after absorbing any after-coil sensible heat gains.

For example, if the off-coil air is 52°F and the supply fan adds 3°F:

• Off-coil temperature = 52°F
• Supply-air temperature = 55°F

Because fan heat is sensible, the humidity ratio remains approximately constant.

The process appears as a horizontal line moving to the right.

How Cooling-Coil Load Is Calculated

The cooling coil may remove:

• Sensible heat
• Latent heat

The total coil load is therefore calculated from the change in moist-air enthalpy.

In IP units, the common air-side relationship is:

Cooling-coil load = 4.5 × CFM × (On-coil enthalpy − Off-coil enthalpy)

Where:

• Cooling-coil load is in Btu/hr
• Airflow is in CFM
• Enthalpy is in Btu/lb of dry air
• 4.5 is an approximate standard-air conversion factor

The cooling load can be converted to refrigeration tons:

Cooling load in tons = Cooling load in Btu/hr ÷ 12000

In SI units:

Cooling-coil load = Air mass flow × Enthalpy difference

Where:

• Air mass flow is in kg/s
• Enthalpy difference is in kJ/kg
• Cooling load is in kW

Because enthalpy includes sensible and latent heat, this method captures both:

• Temperature reduction
• Moisture removal

Worked Cooling-Coil Example

Consider the following design inputs:

Step 1: Plot the Room Condition

The room condition is plotted at:

• 75°F dry bulb
• 50% RH

Step 2: Establish the Room Process

The 80% SHR determines the slope of the room process.

The calculator follows this process to determine the supply condition required to offset the room sensible and latent loads.

Step 3: Account for Fan Heat

The fan adds 2°F after the cooling coil.

The off-coil air must therefore be colder than the final supply air.

The line from off-coil air to supply air is sensible heating at constant humidity ratio.

Step 4: Mix Outdoor and Return Air

The system mixes:

• 90% return air
• 10% outdoor air

The mixed on-coil point lies between the return-air and outdoor-air conditions.

Because the outdoor air is hot and humid, the mixed-air enthalpy is higher than the return-air enthalpy.

Step 5: Calculate Coil Capacity

The calculator determines:

• On-coil enthalpy
• Off-coil enthalpy
• Enthalpy difference
• Cooling-coil load

The cooling-coil load is normally higher than the room cooling load because it may also include:

• Outdoor-air load
• Fan heat
• Duct heat gains
• Other air-side heat gains

I explained the decision-making and thought process behind each step, what value to use, and the limitations in my article titled How to Size AHU Cooling Coil? (Design Calculation). If you're interested to learn the manual method, check out that article.

Room Cooling Load vs Cooling-Coil Load

Room cooling load and cooling-coil load are related, but they are not always equal.

Room Cooling Load

The room cooling load represents the heat that must be removed from the conditioned space.

Typical room-load components include:

• Wall heat gain
• Roof heat gain
• Window conduction
• Solar heat gain
• People
• Lighting
• Equipment
• Infiltration
• Internal moisture generation

Cooling-Coil Load

The cooling coil may need to handle the room load plus additional system loads.

Possible additional loads include:

• Outdoor ventilation air
• Return-fan heat
• Supply-fan heat
• Return-duct heat gain
• Supply-duct heat gain
• Air leakage
• Mixing-box heat gain
• Other system heat gains

As a result, a room with a 5-ton cooling load may require a coil larger than 5 tons.

The difference can be substantial when:

• Outdoor-air percentage is high
• Outdoor humidity is high
• Fan heat is significant
• Ductwork passes through hot spaces
• Air leakage is excessive

How Outdoor Air Affects Cooling-Coil Capacity

Outdoor ventilation air may add both sensible and latent load.

Sensible Outdoor-Air Load

If the outdoor dry-bulb temperature is higher than the indoor temperature, the cooling coil must reduce the outdoor-air temperature.

Latent Outdoor-Air Load

If the outdoor humidity ratio is higher than the indoor humidity ratio, the cooling coil must remove moisture.

In hot and humid climates, the latent ventilation load can be significant.

The effect of outdoor air depends on the difference between:

• Outdoor-air enthalpy
• Return-air enthalpy

Increasing outdoor air from 10% to 20% does not always increase the coil load by exactly 10%.

The actual increase depends on the outdoor and return-air conditions.

How Sensible Heat Ratio Affects the Chart

The sensible heat ratio controls the direction of the room process line.

High Sensible Heat Ratio

A high SHR means most of the room load is sensible.

The process line is flatter because the system mainly needs to reduce temperature.

Possible examples include:

• Spaces with large envelope heat gain
• Equipment rooms with limited moisture generation
• Low-occupancy rooms
• Spaces with high lighting or electrical loads

Low Sensible Heat Ratio

A low SHR means a larger part of the load is latent.

The process line is steeper because more moisture must be removed.

Possible examples include:

• Densely occupied rooms
• Auditoriums
• Restaurants
• Commercial kitchens
• Areas with high outdoor-air rates
• Spaces with moisture-producing activities

A lower SHR generally requires:

• Lower supply-air humidity ratio
• More dehumidification
• Lower off-coil temperature
• Greater latent capacity

Why Off-Coil Air Is Not Normally at 100% RH

Air leaving a real cooling coil is often close to saturation but is not always at 100% RH.

Not all air passing through the coil makes perfect contact with the cold surface.

Some air effectively bypasses parts of the coil.

The final leaving-air condition lies between:

• The entering-air condition
• The effective coil-surface or apparatus-dew-point condition

This effect is represented by the coil bypass factor.

The calculator uses 90% RH as a simplified off-coil assumption.

Final coil selection should still be completed using manufacturer selection software.

Applications of Psychrometric Charts in HVAC

Psychrometric charts are used throughout HVAC design, operation, and troubleshooting.

Cooling-Coil Analysis

The chart can show:

• Entering-air condition
• Leaving-air condition
• Apparatus dew point
• Cooling and dehumidification path
• Enthalpy reduction
• Humidity-ratio reduction
• Moisture removal

Air-Handling Unit Analysis

A complete AHU process may include:

1. Outdoor and return-air mixing
2. Filtration
3. Cooling and dehumidification
4. Supply-fan heat
5. Reheating
6. Humidification
7. Supply to the room

Each stage can be plotted as a connected process.

Ventilation-Air Analysis

Outdoor air is required for:

• Indoor air quality
• Occupant ventilation
• Building pressurization
• Exhaust-air replacement
• Code compliance

The chart can be used to assess the sensible and latent load added by the outdoor air.

Dehumidification Analysis

The chart helps determine whether a cooling coil can achieve the required supply-air humidity ratio.

This is important for:

• Hot and humid climates
• High-occupancy spaces
• Hospitals
• Laboratories
• Museums
• Cleanrooms
• Dedicated outdoor-air systems
• Industrial applications

Heating and Humidification

Cold outdoor air often requires:

• Sensible heating
• Moisture addition

The chart can show both stages and estimate the resulting supply-air condition.

Evaporative Cooling

The chart can be used to evaluate:

• Entering dry-bulb temperature
• Entering wet-bulb temperature
• Saturation effectiveness
• Leaving dry-bulb temperature
• Leaving humidity ratio

Cooling-Tower Analysis

Cooling-tower performance is closely related to outdoor wet-bulb temperature.

The psychrometric chart helps explain:

• Why wet bulb affects tower leaving-water temperature
• Cooling-tower approach
• Evaporative heat rejection
• Air-side moisture increase

Commissioning

Measured air conditions can be plotted during commissioning.

Possible measurement points include:

• Outdoor air
• Return air
• Mixed air
• On-coil air
• Off-coil air
• Supply air
• Room air

The plotted process can be compared with design expectations.

Troubleshooting

Psychrometric plotting can help identify:

• Insufficient dehumidification
• Poor cooling-coil performance
• Excessive fan heat
• Incorrect outdoor-air percentage
• Mixed-air sensor error
• Unwanted reheating
• Duct heat gain
• Inaccurate humidity sensors
• Unexpected air leakage
• Poor mixing-box performance

Psychrometric Chart Assumptions

A psychrometric chart is prepared for a specific atmospheric pressure.

This interactive chart assumes standard sea-level pressure:

• 29.921 in. Hg
• 101.325 kPa

Air properties change with altitude because atmospheric pressure decreases as elevation increases.

For high-altitude locations, use a chart or calculation method adjusted for the local barometric pressure.

The automatic coil calculation also uses simplified assumptions, including:

• Standard air properties
• Complete outdoor and return-air mixing
• Sensible temperature-rise inputs
• Off-coil air at approximately 90% RH
• Preliminary air-side capacity estimation

The result should not replace final equipment or coil selection.

Common Psychrometric Chart Mistakes

Confusing Relative Humidity with Humidity Ratio

Relative humidity depends on temperature.

Humidity ratio represents the actual moisture content.

During sensible heating, humidity ratio remains constant while relative humidity decreases.

Reading Wet Bulb as Dew Point

Wet-bulb and dew-point temperatures are different.

Wet bulb is read along a diagonal line.

Dew point is found by moving horizontally to the saturation curve.

Assuming Relative Humidity Remains Constant During Heating

During sensible heating:

• Humidity ratio remains constant
• Relative humidity decreases

Assuming Cooling Always Removes Moisture

Cooling removes moisture only after the air reaches its dew point.

Cooling above the dew point is sensible cooling only.

Treating Room Load as Coil Load

The cooling coil may also handle:

• Outdoor-air load
• Fan heat
• Duct heat gain
• Air leakage
• Other system loads

The coil capacity may therefore be higher than the room cooling load.

Ignoring Atmospheric Pressure

A standard sea-level chart becomes less accurate at high elevations.

Reading the Chart More Precisely Than Its Scale Allows

A printed psychrometric chart is a graphical engineering tool.

It is not infinitely precise.

Use equations, software, or verified calculation tools where greater numerical accuracy is required.

Use this Psychrometric Calculator for precise numerical calculations.

Using Inaccurate Field Measurements

Small psychrometric differences may be misleading if the sensors are inaccurate.

Check:

• Temperature-sensor accuracy
• Relative-humidity-sensor accuracy
• Sensor location
• Sensor response time
• Air stratification
• Radiation effects
• Calibration status

Frequently Asked Questions

What does a psychrometric chart show?

A psychrometric chart shows the relationships between moist-air properties, including dry-bulb temperature, wet-bulb temperature, relative humidity, humidity ratio, dew point, enthalpy, and specific volume.

What two values are needed to use a psychrometric chart?

Any two independent air properties can normally establish the air condition.

The most common combinations are:

• Dry bulb and relative humidity
• Dry bulb and wet bulb

What is the curved line on a psychrometric chart?

The upper curved boundary is the saturation curve, representing 100% relative humidity.

The curved lines inside the chart represent lower relative-humidity values.

Where is dry-bulb temperature shown?

Dry-bulb temperature is shown along the horizontal axis at the bottom of the chart.

Vertical lines represent constant dry-bulb temperature.

Where is humidity ratio shown?

Humidity ratio is shown on the vertical axis, usually on the right side of the chart.

Horizontal lines represent constant humidity ratio.

How do I find dew point?

Locate the air-condition point and move horizontally left to the saturation curve.

The temperature at the intersection is the dew-point temperature.

Why are wet-bulb and enthalpy lines similar?

Wet-bulb and constant-enthalpy lines are nearly parallel under common HVAC conditions.

They are closely related but are not exactly identical.

What happens when air is sensibly heated?

The point moves horizontally to the right.

Dry-bulb temperature increases, humidity ratio remains constant, and relative humidity decreases.

What happens when air is sensibly cooled?

The point moves horizontally to the left.

Dry-bulb temperature decreases, humidity ratio remains constant, and relative humidity increases.

What happens when air is cooled below its dew point?

Water vapor condenses.

The air is cooled and dehumidified, so both dry-bulb temperature and humidity ratio decrease.

How is mixed air plotted?

Draw a straight line between the two original air conditions.

The mixed-air point lies along the line according to the relative dry-air mass flows.

Can a psychrometric chart calculate cooling-coil load?

The chart provides the entering and leaving enthalpies.

Airflow and enthalpy difference can then be used to estimate the total coil load.

The interactive tool on this page performs this calculation automatically.

Can I use this psychrometric chart in SI units?

Yes.

Use the IP/SI toggle above the chart.

Can I plot more than two points?

Yes.

The custom plotting function allows multiple points to be added and connected to previous points.

Can I use measured site conditions?

Yes.

You can enter measured dry-bulb temperature with either relative humidity or wet-bulb temperature.

Sensor accuracy should be checked before drawing conclusions.

Why is the off-coil condition assumed at 90% RH?

Real cooling-coil leaving air is often close to saturation but not exactly at 100% RH because of coil bypass and uneven contact between the air and coil surface.

The 90% RH value is used as a practical preliminary assumption.

Does the calculator select an AHU or cooling coil?

No.

It estimates air-side conditions and cooling-coil capacity.

Final coil and AHU selection should be verified using manufacturer selection software.

Can the chart be used at high altitude?

The current chart assumes standard sea-level pressure.

High-altitude projects require a chart or psychrometric calculation adjusted for the local atmospheric pressure.

Can it be used for heating calculations?

The custom plotting function can illustrate heating and humidification processes.

The automatic capacity function is specifically intended for cooling-coil analysis.

Conclusion

A psychrometric chart is one of the most useful tools for understanding air-conditioning processes.

The main properties to understand are:

• Dry-bulb temperature
• Wet-bulb temperature
• Relative humidity
• Humidity ratio
• Dew-point temperature
• Enthalpy
• Specific volume

Once these properties are understood, the chart can be used to analyze:

• Sensible heating
• Sensible cooling
• Cooling and dehumidification
• Heating and humidification
• Evaporative cooling
• Desiccant dehumidification
• Reheating
• Outdoor and return-air mixing
• Cooling-coil performance
• Complete AHU processes

The interactive chart on this page provides three practical functions:

1. Inspect moist-air properties directly
2. Plot and connect custom air conditions
3. Estimate an HVAC cooling process and cooling-coil capacity

These functions make the chart useful not only as a reference, but also as a tool for design, education, commissioning, troubleshooting, presentation, and reporting.