Psychrometrics Fundamentals Reference

Why this matters

Psychrometrics is the science of moist air - how temperature, humidity, and energy interact. HVAC sizing, comfort, dehumidification, ventilation, and condensation all depend on it. Reading a psychrometric chart looks intimidating but the underlying concepts are simple, and they explain why humid 78 °F feels worse than dry 82 °F and why oversized AC produces clammy comfort failures.

Air is a mixture

Atmospheric air is mostly nitrogen and oxygen plus a variable amount of water vapor. The amount of vapor in air affects almost every HVAC calculation.

Dry air properties:

• Composition: 78% N₂, 21% O₂, 1% other
• Density at sea level, 70 °F: 0.075 lb/ft³
• Specific heat: 0.24 BTU/lb·°F

Water vapor:

• Can exist in air up to a saturation limit that depends on temperature
• Carries latent heat (energy that doesn't change temperature)
• 1 lb of water vapor in air carries ~1000 BTU of latent heat (the heat of vaporization)

The five key properties

A point on a psychrometric chart is fully defined by any two of these five properties; the other three fall out of the chart geometry.

1. Dry-bulb temperature (DB): the air temperature you measure with a regular thermometer. The horizontal axis of most psych charts. 70 °F dry-bulb is "70 °F" in normal speech.

2. Wet-bulb temperature (WB): the temperature read on a thermometer wrapped in a wet wick and ventilated. Evaporation cools the wick; the equilibrium temperature is the wet-bulb. Indicates how much moisture is in the air.

• WB always ≤ DB
• WB = DB when air is at 100% RH (saturated)
• The drier the air, the larger the gap between DB and WB

3. Relative humidity (RH): current moisture / max moisture at that temperature × 100%. Expressed as percentage.

• 0% RH = bone dry
• 100% RH = saturated (condensation imminent)
• Comfort range: 30-60% RH typical

4. Dew point (DP): the temperature at which air must be cooled (at constant moisture content) for condensation to begin. Tells you the actual moisture content in absolute terms.

• DP ≤ DB always
• DP = DB when air is saturated
• Surfaces colder than DP will sweat

5. Specific humidity (W) / humidity ratio: mass of water vapor per mass of dry air, lb/lb or grains/lb.

• Independent of temperature changes (heating air doesn't change moisture content)
• Used in absolute moisture calculations (e.g., dehumidification load)

Plus enthalpy (h): total energy of the moist air mixture, BTU/lb. Used in calculating cooling capacity needed.

The psychrometric chart

A standard chart at standard atmospheric pressure (sea level, 14.696 psia) plots:

• X-axis: dry-bulb temperature
• Y-axis: humidity ratio (and sometimes vapor pressure on right side)
• Curved lines: constant relative humidity (saturation curve at 100% RH bounds the upper-left)
• Diagonal lines top-left to bottom-right: constant wet-bulb (also approximately constant enthalpy)
• Vertical lines: constant dry-bulb
• Horizontal lines: constant humidity ratio (constant moisture content)

Any point on the chart represents a specific state of moist air. Reading the chart gives you all five properties from any two known values.

Common readings

Indoor comfort target (cooling season):

• 75 °F DB, 50% RH → 64 °F WB, 55 °F DP, 65 grains/lb humidity ratio, 28.5 BTU/lb enthalpy

Outdoor cooling design (Atlanta example):

• 92 °F DB, 75 °F WB → 65% RH, 81 °F DP, 156 grains/lb, 38.5 BTU/lb

Cooled, dehumidified supply air (typical):

• 55 °F DB at saturation (100% RH) → 55 °F WB, 55 °F DP, 65 grains/lb, 23.2 BTU/lb

Why this matters for HVAC sizing

Total cooling = sensible cooling + latent cooling

When AC cools 80 °F / 50% RH air to 55 °F at the coil:

• Sensible cooling: temperature drop 80 °F → 55 °F = 25 °F sensible drop
• Latent cooling: moisture drops from 78 grains/lb to 65 grains/lb = 13 grains/lb removed (condenses on coil, drains away)

The total cooling = both. Specifically:

• Sensible (BTU/hr) = CFM × 1.08 × ΔT (dry-bulb)
• Latent (BTU/hr) = CFM × 0.68 × ΔW (humidity ratio in grains/lb)
• Total (BTU/hr) = CFM × 4.5 × Δh (enthalpy)

The "1.08 × ΔT × CFM" sensible formula is the field shortcut. Latent and total require the chart or psych software.

Humidity ratio vs relative humidity - the trap

Two air streams can have the same RH but different humidity ratios:

• 80 °F / 50% RH = 78 grains/lb
• 60 °F / 50% RH = 28 grains/lb

Cooling 80 °F / 50% RH air to 60 °F without removing moisture would result in:

• 60 °F / much higher than 50% RH (probably 80%+)

This is why simply lowering temperature in humid weather doesn't fix comfort - the same moisture content concentrates as RH rises.

Cooling coil performance

A cooling coil sized for a building extracts both sensible heat (temperature drop) and latent heat (moisture removal).

Sensible Heat Ratio (SHR): sensible cooling / total cooling.

• SHR = 1.0 (100% sensible) = dry-climate equipment (Phoenix)
• SHR = 0.75 (typical AHRI rating) = standard
• SHR = 0.6-0.7 = "high latent capacity" units (Florida-marketed)

A standard SHR 0.75 unit in humid Florida doesn't remove enough moisture; customers feel clammy at the setpoint.

Apparatus Dew Point (ADP): the coil surface temperature. Lower ADP = more moisture removal but also more sensible cooling. Modern variable-speed equipment can adjust ADP for humidity control independent of pure temperature control.

Latent load = moisture intrusion

Where does the latent load in a house come from?

• Outdoor air (infiltration + ventilation) carries moisture
• Occupants (~200 BTU/hr latent per person breathing/perspiring)
• Cooking (steam from boiling, frying)
• Showering (steam)
• Pool / aquarium evaporation
• Drying clothes indoors

A typical house has 20-40% of cooling load as latent in mid-Atlantic to Gulf climates.

Dehumidification methods

AC alone: dehumidifies as a side effect of cooling. Effective only when AC is running; problematic in shoulder seasons (mild outdoor temp but humid).

Whole-house dehumidifier: dedicated unit. Operates independently of AC. Sized in pints/day (typical residential 65-90 pints/day).

Heat-pump dehumidifier: uses refrigeration cycle to extract moisture. Reheats the dehumidified air (so room temp stays the same). Most efficient.

Two-stage / variable-speed AC: modulates capacity to run longer at lower output, maximizing dehumidification time.

Air-to-air heat exchanger / ERV: transfers moisture between incoming and outgoing air streams. Reduces latent load on the AC by pre-conditioning ventilation air.

Comfort and humidity

References

• ASHRAE Handbook - Fundamentals (psychrometric chapter)
• ASHRAE 55 (Thermal Environmental Conditions for Human Occupancy)
• ASHRAE 62.1 / 62.2 (Ventilation for Indoor Air Quality)
• Carrier Engineering Manual Volume 1 (psychrometrics for HVAC engineers)
• NIST psychrometric property calculators
• Manuall internal: Thermodynamics Basics, Heat Transfer Fundamentals