The psychrometric chart is the most powerful single tool in HVAC engineering. It maps every possible state of moist air on a single diagram, letting you visualize cooling coil processes, mixing conditions, humidification, and dehumidification at a glance. Yet most technicians avoid it because it looks intimidating.
This guide breaks down the chart into its component lines, explains what each property means in practical HVAC terms, and shows how to use psychrometric calculations for real field work: estimating cooling loads, checking for condensation risk, sizing dehumidification equipment, and verifying commissioning measurements.
Reading the Psychrometric Chart
The psychrometric chart has dry-bulb temperature on the horizontal axis (bottom), humidity ratio on the vertical axis (right side), and a curved saturation line that forms the upper boundary. Every point inside the chart represents a unique state of moist air defined by its temperature and moisture content.
Dry-bulb temperature lines run vertically. This is the temperature measured by a standard thermometer.
Wet-bulb temperature lines run diagonally from upper left to lower right. They follow lines of constant adiabatic saturation, which is why a wet wick in moving air reads this temperature.
Relative humidity curves run roughly parallel to the saturation line. The saturation line itself is 100% RH. RH decreases as you move down and to the right from the saturation line. At any given dry-bulb temperature, moving vertically changes the humidity ratio and RH simultaneously.
Humidity ratio lines are horizontal, read from the right-side scale in grains of moisture per pound of dry air (or lb/lb). This is the absolute measure of moisture content, unlike RH which is relative to temperature.
Enthalpy lines run diagonally, approximately parallel to wet-bulb lines but read from a separate scale along the curved edge. Enthalpy represents the total heat content of the air (sensible + latent) in BTU per pound of dry air.
Key Properties and What They Mean
Dew point temperature: Follow a horizontal line from your air state leftward to the saturation curve. The temperature where you hit the curve is the dew point. If any surface in the building is below this temperature, condensation will form on it. This is how you assess window condensation, cold water pipe sweating, and slab moisture problems.
Enthalpy: The total energy content of the air per pound of dry air. The enthalpy difference between two air states tells you the total heat that must be added or removed to move between them. For a cooling coil: Q_total = 4.5 x CFM x (h_entering - h_leaving). This single calculation captures both the sensible (temperature) and latent (moisture) components of the cooling load.
Specific volume: The volume occupied by one pound of dry air plus its associated moisture. Typically 13.0-14.5 cubic feet per pound for HVAC conditions. This is used to convert between mass flow (lb/hr) and volumetric flow (CFM). At higher altitudes, specific volume increases because air density decreases.
Sensible heat ratio (SHR): The fraction of total heat that is sensible (temperature change) versus latent (moisture change). An SHR of 0.75 means 75% of the cooling load is sensible and 25% is latent. Equipment is sized based on both components. Humid climates have lower SHR (more latent load). Dry climates have higher SHR.
The Cooling Coil Process on the Chart
When air passes through a cooling coil, it moves from its entering state toward the coil surface temperature along a line that trends left and down on the chart. If the coil surface is below the entering dew point, the air hits the saturation curve and follows it down, removing both sensible heat and moisture simultaneously.
The entering condition is defined by the return air dry-bulb and wet-bulb (or RH). The leaving condition depends on the coil's bypass factor, which represents the fraction of air that passes through without contacting the coil surface. A bypass factor of 0.1 means 90% of the air contacts the coil surface and approaches the apparatus dew point (ADP), while 10% passes through unchanged.
The total cooling load is: Q_total = 4.5 x CFM x (h_in - h_out). The sensible cooling is: Q_sensible = 1.08 x CFM x (T_in - T_out). The latent cooling is the difference: Q_latent = Q_total - Q_sensible. These three numbers tell you whether the equipment can handle both the temperature and moisture load.
If your system is removing temperature but not enough moisture (high sensible ratio equipment in a humid climate), the space RH will creep up even though the thermostat is satisfied. The psychrometric chart reveals this mismatch immediately when you plot the entering and leaving conditions.
Air Mixing and Economizer Cycles
When two airstreams mix (return air and outdoor air, or two zone supplies), the resulting mixture state falls on a straight line between the two source states on the psychrometric chart. The position along that line depends on the mass flow ratio of the two streams.
For an economizer bringing in outdoor air: plot the outdoor air state and the return air state. Draw a line between them. If you bring in 25% outdoor air by mass, the mixed state is 25% of the distance from the return air point toward the outdoor air point. This tells you the mixed air temperature, humidity, and enthalpy before it reaches the coil.
This is essential for economizer design. If the outdoor air enthalpy is below the return air enthalpy, the economizer saves energy by using free cooling. If the outdoor air has lower dry-bulb but higher humidity ratio (common in mild, humid weather), the economizer may save sensible energy but add latent load. The psychrometric chart makes this tradeoff visible.
The mixed air calculation matters for coil sizing too. If your coil was sized for 80\x{00B0}F/50% RH return air and you bring in 30% outdoor air at 95\x{00B0}F/75\x{00B0}F wet-bulb, the mixed entering condition is much more demanding. Failing to account for the outdoor air load in coil selection is a common design error.
Altitude Effects on Psychrometrics
Standard psychrometric charts are printed for sea level (14.696 psia). At altitude, atmospheric pressure drops and air density decreases. At Denver (5,280 feet), pressure is about 12.2 psia. This changes every psychrometric calculation.
At altitude, the same dry-bulb and RH combination holds less total moisture by mass because there are fewer air molecules per cubic foot. The humidity ratio (grains per pound of dry air) at a given RH is lower at altitude. Enthalpy per pound of dry air is also lower. Specific volume is higher because each pound of air occupies more space.
The practical impact is that CFM-based calculations at altitude deliver less mass flow than at sea level. A 2,000 CFM system at sea level moves about 150 lb/min of air. At 5,000 feet, the same 2,000 CFM moves about 127 lb/min. If your cooling load is based on mass flow (which it should be), the coil capacity at altitude is about 15% less than the sea-level rating.
Always use altitude-corrected psychrometric data for installations above 3,000 feet. The psychrometric calculator on ToolGrit adjusts for altitude when you enter your elevation, eliminating the need for manual corrections or altitude-specific chart overlays.
Indoor Air Quality Applications
The psychrometric chart is essential for indoor air quality assessment. ASHRAE Standard 55 defines thermal comfort zones on the psychrometric chart bounded by temperature (68-78\x{00B0}F), humidity (below 0.012 lb/lb humidity ratio), and air movement. If your building conditions fall outside this zone, occupants will report discomfort regardless of what the thermostat says.
Mold risk assessment uses the dew point. If the dew point of indoor air is within 5\x{00B0}F of any surface temperature in the building, that surface is at risk for condensation and mold growth. Cold spots at thermal bridges (studs in walls, window frames, slab edges) are the most common locations. Measuring indoor conditions and comparing the dew point to known cold surface temperatures identifies mold risk before visible damage appears.
Dehumidification sizing uses the humidity ratio. If indoor conditions are 75\x{00B0}F/60% RH (humidity ratio about 0.0112 lb/lb) and the target is 75\x{00B0}F/50% RH (humidity ratio about 0.0093 lb/lb), the dehumidifier must remove 0.0019 lb of moisture per pound of air processed. Multiply by the air mass flow rate and you have the required moisture removal rate in lb/hr or pints/day.
Ventilation verification uses enthalpy. If the outdoor air enthalpy entering through the economizer or ventilation system matches the expected value for current weather conditions, the outdoor air damper is working correctly. If the measured enthalpy is significantly different, the damper may be stuck, the sensor may be failed, or return air is leaking into the outdoor air stream.