The Mollier (h-x) diagram: a complete guide for HVAC engineers

The Mollier (h-x) diagram is a graphical tool for calculating and visualizing the states of moist air. It lets HVAC engineers see the temperature, humidity ratio, enthalpy and relative humidity of air at a single glance — and draw any air-treatment process (heating, cooling, humidification and their combinations).

What the Mollier diagram is

The Mollier diagram (also called the h-x or i-x diagram) shows the thermodynamic states of moist air in a two-dimensional, oblique coordinate system:

• The x axis (horizontal) — the humidity ratio $x$ [g/kg dry air] — the amount of water vapor per 1 kg of dry air
• The h axis (oblique) — the enthalpy of moist air $h$ [kJ/kg dry air] — the total heat content

The diagram is oblique (rather than rectangular) so that the lines of constant temperature and constant enthalpy do not overlap and can be read off easily.

The main constant-property lines

Line	Symbol	Physical quantity
Temperature	$t$	Dry-bulb temperature [°C]
Enthalpy	$h$	Enthalpy [kJ/kg]
Humidity ratio	$x$	Humidity ratio [g/kg dry air]
Relative humidity	$\varphi$	Relative humidity [%]
Saturation curve	$\varphi$ = 100%	Air saturation limit (the dew point lies on it)

How to read the diagram

Every state of the air is represented by a single point on the diagram. From that point you can read off:

1. The temperature — from the t line passing through the point
2. The humidity ratio — projected onto the x axis
3. The enthalpy — projected onto the h axis
4. The relative humidity — approximately, from the position between the φ = 0% and φ = 100% lines

The dew point ($t_d$) always lies on the saturation curve (φ = 100%) at the same value of $x$ as the starting state.

The basic air-treatment processes

Sensible heating

A vertical line pointing upward — the humidity ratio x does not change (constant x), temperature and enthalpy rise, relative humidity falls.

Typical use: a heating coil (electric or hot-water) in the supply air.

Cooling and dehumidification

A line descending to the left — the air is cooled below its dew point and water vapor condenses out. Temperature, enthalpy and humidity ratio all fall.

Typical use: a finned cooling coil (chilled-water coil or DX coil).

Adiabatic humidification

A line following the constant-enthalpy line diagonally down and to the right. Temperature falls and the humidity ratio rises — this holds for humidification with atomized water or for an evaporative cooler.

Isothermal humidification

A horizontal line to the right at constant temperature — the humidity ratio rises. This holds for a steam humidifier (saturated or superheated steam).

Mixing of two streams

The state of the mixed air lies on the straight segment connecting the two inlet states — at the position corresponding to the mass ratio of the two streams.

Heat recovery (HRV)

The supply air moves from point 1 (outdoor air) to point 2 (air after the heat-recovery exchanger). The air extracted from the room simultaneously cools or heats the supply stream. The efficiency of the exchanger determines how close point 2 lies to the state of the extract air.

The Mollier diagram in practice: a cascade example

A typical air handling unit in winter operation:

1. Outdoor air (t = −15 °C, φ = 80%) → enters the heat-recovery exchanger
2. After heat recovery (t = +5 °C) → sensible heating
3. After heating (t = +20 °C) → steam humidification to φ = 50%
4. Supply to the room (t = +21 °C, φ = 50%) → the room’s thermal load
5. Extract (t = +26 °C, φ = 55%) → back to heat recovery

This whole chain can be drawn as a series of connected segments in the Mollier diagram — and that is exactly what PsychroView does.

Standards and calculation accuracy

PsychroView computes psychrometric quantities per ASHRAE Fundamentals 2021, Chapter 1 (SI) using the Hyland–Wexler equations for saturation vapor pressure — within this range it is a highly accurate method (deviations from reference data on the order of hundredths to a few percent).

Design parameters for the indoor environment and ventilation are based on EN 16798-1 (European standard; indoor environmental input parameters) and EN 16798-3 (ventilation for non-residential buildings).

Mollier (SI) vs. psychrometric (Carrier) chart

In the Czech Republic and Slovakia, the Mollier diagram in SI units is the traditional choice (oblique coordinates, enthalpy on the oblique vertical axis). In Anglo-Saxon countries you meet the psychrometric (Carrier) chart in I-P or SI units with rectangular coordinates.

PsychroView supports both chart types.

Getting started

A web-based h-x tool such as PsychroView takes the same route: enter the outdoor air state (temperature and relative humidity), add the treatment processes (heating, cooling, heat recovery), and export the result to PDF or DXF for the technical report.

Frequently asked questions

What is the difference between the Mollier and the Carrier chart? Both show the same physical quantities but differ in the orientation of their axes. The Mollier (h-x) chart has oblique coordinates with enthalpy on the oblique axis — typical of Central Europe. The Carrier (psychrometric) chart has rectangular coordinates — typical of the USA.

Do I need to know the partial pressure of water vapor? No — it is enough to enter the temperature and relative humidity. PsychroView calculates all the remaining quantities automatically.

Can the diagram be used for pressures other than 101,325 Pa? Yes — PsychroView lets you enter a custom barometric pressure, which is necessary for calculations at high-altitude locations.

How many processes can be drawn at once? Without registration you can draw two processes. After registering you can chain more processes and model a whole air circuit in a single project.