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

Comfort is not a temperature, it is a heat-balance problem. A body sheds heat by radiation, convection, and evaporation, and humidity controls the evaporation term. When the air is already loaded with moisture, sweat does not evaporate, the body cannot dump its share of heat, and the room feels warm at a thermostat setting that reads fine.

That is why the same dry-bulb reading produces two different complaints. Dry air at a given temperature feels cooler and lets a customer raise the setpoint without noticing. Humid air at the same temperature feels sticky and drives them to drop the setpoint chasing a comfort problem that is not about temperature at all. Every degree of setpoint they give back costs runtime, so humidity control is a comfort fix and an energy fix in the same move.

Judge the moisture by dew point, not by relative humidity. Relative humidity moves whenever temperature moves, even though nothing has been added or removed. Dew point is the absolute measure, it stays put when air is simply heated or cooled, and it maps far better to what people report. Two rooms at the same relative humidity and different temperatures do not feel the same; two rooms at the same dew point mostly do.

The bounds on either end are physical, not preference:

• Too humid. Condensation on windows, ducts, and cold surfaces. Mold and dust mites thrive. Wood swells, finishes cloud, and stored goods take damage. The smell arrives before the visible problem.
• Too dry. Static discharge, dried mucous membranes and nosebleeds, cracked trim and flooring, gapping hardwood, and instruments going out of tune. Customers usually blame the furnace for this; it is the outdoor air's moisture content plus infiltration.

Winter has a hard ceiling that summer does not. Indoor humidity in cold weather is limited by the coldest surface in the house, usually a window edge or a poorly insulated corner. Humidify past what those surfaces can tolerate and you get condensation, then staining, then rot inside the wall. As outdoor temperature falls, the safe indoor humidity target falls with it. Tell the customer that up front or they will keep turning the humidifier up until something drips.

Other comfort levers matter, and are cheap. Air movement increases evaporative cooling, so a fan buys perceived comfort without touching the setpoint. Radiant effects matter too: a cold exterior wall or a large west window pulls or adds heat regardless of air temperature, which is why one room in the house is always the complaint.

What this means on a service call. When the customer says a room is uncomfortable but the thermostat reads correctly, measure moisture before you touch capacity. High indoor dew point points to a latent problem: an oversized system short-cycling, a ventilation source dumping outdoor air, or a moisture source in the house. Adding cooling capacity to a latent problem makes it worse, because a bigger system runs shorter and removes less moisture.

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