Psychrometric calculator

What a psychrometric calculator actually computes

Moist air at any point in an HVAC system is fully described by two independent state variables plus site barometric pressure. Pick any two of dry bulb temperature, wet bulb temperature, dew point, relative humidity, or humidity ratio, and every other property is determined. The calculator above uses dry bulb plus one of the others because dry bulb is the easiest to measure (any thermometer reads it directly) and the most familiar to everyone reading a thermostat. The second variable is whatever your meter or chart actually gives you: a sling psychrometer reads wet bulb, a capacitive hygrometer reads RH, an Espy gauge or chilled mirror reads dew point, and a Pitot grid plus enthalpy probe reads humidity ratio.

The eight returned properties are what every cooling-coil selection, dehumidification load, evaporative-cooler effectiveness check, and humidification calculation needs as input. Doing them by hand off a paper ASHRAE psychrometric chart is slow and error-prone past the third decimal place, especially at altitudes that depart from sea-level pressure (the standard chart is plotted at 14.696 psi). Doing them repeatedly during a commissioning trip means a calculator pays for itself on the first job.

Hyland-Wexler 1983: the math under the chart

Every modern psychrometric calculator runs on the same saturation vapor pressure formulation: Hyland and Wexler 1983, published in ASHRAE Transactions volume 89 part 2A. The formula has two branches: a six-coefficient log-polynomial for liquid water above 32 F, and a seven-coefficient version for ice below 32 F. Both branches are accurate to within 0.01 percent of the underlying thermodynamic surface across the entire HVAC operating range from minus 100 F to plus 200 F. ASHRAE adopted it as the reference standard in the Handbook of Fundamentals and has kept it as the basis for every subsequent edition.

Why does the formula matter? Because the lookup-table psychrometric calculators that were common before about 2005 cut corners by linearly interpolating between sparse chart values. At altitude, in the heat-pump cold-climate range below 20 F, or near the saturation curve where the vapor-pressure derivative is steep, that error compounds. A dehumidifier sized off a 5 percent vapor-pressure error can be a full pint-class too small. A cooling coil sized off a 2 percent enthalpy error can miss the sensible heat ratio enough to leave a room muggy. The Hyland-Wexler coefficient set is what the load-calc industry has standardized on.

The eight properties and what they mean for a job

• Dry bulb (F): the temperature any thermometer reads. The horizontal axis on the chart.
• Wet bulb (F): the temperature a wet thermometer reads in moving air, where evaporation balances sensible heat gain. Always less than or equal to dry bulb. Used directly in AHRI 210/240 ARI rating points (80 DB / 67 WB indoor, 95 DB outdoor for cooling).
• Dew point (F): the temperature at which moisture starts condensing out of the air. The number that drives surface-condensation risk on cold pipes, windows, and uninsulated ductwork.
• Relative humidity (percent): ratio of actual vapor pressure to saturation vapor pressure at the same dry bulb. Most often-cited and most often-misread number: 50 percent RH at 75 F is comfortable, 50 percent RH at 95 F is suffocating.
• Humidity ratio (grains per pound): absolute moisture content per pound of dry air. The straight-line variable on the chart, and the one that does not change when you sensibly heat or cool air. Latent loads are expressed as a change in humidity ratio.
• Enthalpy (BTU per pound dry air): total thermal energy per pound of mixture, sensible plus latent. The total-cooling-load number. Cooling coil capacity = mass flow rate times (entering enthalpy minus leaving enthalpy).
• Specific volume (ft³ per pound dry air): reciprocal of density. Converts between volumetric airflow (CFM) and mass airflow (lb/hr). Standard air at 70 F dry, 14.7 psi has v of 13.33 ft³/lb. Hot or high-altitude air takes more volume per pound, which is why CFM derating at altitude is real.
• Vapor pressure (psi): partial pressure of water vapor in the air mixture. The thermodynamic driver behind humidity ratio, dew point, and condensation risk.

Coil process mode: the cooling coil selection check

Switch the calculator to coil mode and enter entering air conditions (typically 80 F / 50 percent RH in a residential return) and leaving air conditions (typically 55 F at near-saturated 95 percent RH off a properly cold coil). The result returns:

• Total cooling (BTU per hour per CFM): the full enthalpy delta the coil has to deliver. Multiply by the actual blower CFM to get total cooling capacity.
• Sensible cooling (BTU per hour per CFM): the portion that drops the dry bulb. Equals 1.08 times the dry-bulb delta at standard density (the familiar HVAC rule of thumb).
• Latent cooling (BTU per hour per CFM): the portion that removes moisture. Equals total minus sensible.
• Sensible heat ratio (SHR): sensible divided by total. 0.70-0.75 is typical for residential cooling. SHR below 0.65 means the system is doing significant dehumidification (good in humid climates, oversized for dry climates). SHR above 0.85 means the coil is barely dehumidifying (typical of dry climates, high entering RH, or a coil that is running too warm).

SHR is the single most useful number for diagnosing a system that "feels muggy even at 72 F." If indoor RH is creeping above 55 percent in summer and SHR is above 0.80, the coil is short-cycling, the blower speed is too high, or the coil itself is undersized for the latent load. The fix is usually slowing the blower from 400 to 350 CFM per ton, not lowering the thermostat.

Altitude correction: why the standard chart lies above 3,000 feet

Every printed psychrometric chart is plotted at 14.696 psi (sea level). Barometric pressure drops about 0.5 psi per 1,000 feet of elevation, which shifts the whole chart. At Denver (5,280 ft, ~12.2 psi) the humidity ratio at 75 F / 50 percent RH is about 8 percent higher than the sea-level chart shows. The enthalpy of the same air state is about 4 percent higher. Cooling coil capacity rated at sea-level standard air drops by roughly the inverse of the density ratio: a 3 ton sea-level coil delivers about 2.65 tons of equivalent sensible cooling at Denver altitude with the same air side and refrigerant side conditions.

The calculator above asks for site altitude and computes barometric pressure from the ASHRAE eq 3 formula p = 14.696 * (1 - 6.8754e-6 * Z)^5.2559. Every downstream property is then consistent with site pressure. If you are sizing equipment for Denver, Salt Lake, Albuquerque, Reno, Cheyenne, or any of the mountain ski-town markets, the altitude input changes the answer meaningfully. Manufacturers publish altitude derating tables for this; the calculator gives you the raw thermodynamic numbers you need to apply them.

Common air-state benchmarks worth memorizing

A few air states show up repeatedly in HVAC work. Useful as a sanity check when the calculator gives you a number you do not trust:

• AHRI cooling rating point: 80 F DB / 67 F WB indoor, 95 F DB / 75 F WB outdoor. RH about 51 percent indoor. Used for all SEER2, EER2, and capacity ratings.
• Standard comfort: 75 F / 50 percent RH. WB about 63 F, DP about 55 F, h about 28 BTU/lb. The ASHRAE 55 comfort target.
• Hot-humid summer design (Houston / Miami / New Orleans): 95 F / 78 F WB. DP about 76 F, RH about 56 percent, h about 41 BTU/lb. Latent load dominates.
• Hot-dry summer design (Phoenix / Las Vegas / Albuquerque): 110 F / 70 F WB. DP about 58 F, RH about 16 percent, h about 34 BTU/lb. Sensible load dominates; evaporative cooling becomes viable.
• Winter design (Minneapolis / Burlington): minus 10 F DB / minus 12 F WB. Air is essentially dry: humidity ratio about 0.0004 lb/lb. Humidification is the entire moisture problem in winter.
• Cold-climate heat pump entering air (cooling mode rare): in winter cooling mode is replaced by heating; the comparable point for heating capacity is 47 F outdoor / 70 F indoor per AHRI 210/240 high-temperature heating rating.

Verifying the calculator against ASHRAE chart values

A quick way to confirm the math: enter 80 F dry bulb, 50 percent RH, 0 ft altitude. The calculator should return wet bulb close to 66.7 F, dew point close to 59.7 F, humidity ratio close to 0.0111 lb/lb (78 grains/lb), and enthalpy close to 31.4 BTU/lb. Those are the same numbers you would read off any printed sea-level psychrometric chart at the 80 / 50 intersection. If you get materially different numbers, the issue is a wet bulb input that exceeds the dry bulb (impossible) or an altitude input that does not match the chart you are comparing against.