Psychrometrics Basics HVAC
“This is a concept many find really scary. The first time a lot of techs look at a psych chart, they feel like they need an engineering degree just to understand it. This page aims to simplify the learning process.”
Psychrometrics is just the study of moist air — temperature, humidity, and the way those two interact when a coil, a humidifier, or an outside-air mixer changes one of them. Pull up an ASHRAE psych chart cold and it can look like a lot: seven properties tangled across two axes, a curved saturation line, and not much that says where to start. The trick is you do not have to read the whole chart at once. Get the seven words down, learn why only two of them are ever independent at a time, and the rest follows. The interactive Psychrometric Chart tool is where it all comes together once you have the vocabulary.
The Seven Properties — What Each One Tells You
Every reading you will meet on a psychrometric chart is one of seven properties of moist air. Get the words down first; the relationships come next.
DB — Dry-bulb temperature · what the thermometer reads
The plain air temperature, no moisture correction. Every space sensor, every duct probe, every supply-air RTD (resistance temperature detector) is reading dry-bulb. It is the most familiar number — and on its own, it tells you nothing about how much water is in the air. A 75 °F space is comfortable at 50% RH and sticky at 80%.
WB — Wet-bulb temperature · the evaporative-cooling floor
The temperature a thermometer reads with a wet sock over the bulb in moving air: water evaporates off the sock and pulls the reading down toward the air’s evaporative limit. Comes from a sling psychrometer or a fixed aspirated wet/dry pair. WB equals DB at saturation; the drier the air, the bigger the depression. The cooling-tower industry lives on this number — it is the lowest temperature you can ever cool water to with that air.
DP — Dew point · the glass-of-ice-water temperature
The temperature you have to cool the air to before water vapor starts condensing. Pour ice water into a glass: the surface chills near 32 °F, the air against it cools to its dew point, water condenses, the glass sweats. Any space surface below dew point sweats — windows, ducts in unconditioned ceiling cavities, cold-floor lobbies, pool glazing. This is the property the natatorium (indoor-pool) spec is really defending, even when it is written as RH.
W — Humidity ratio · grains per pound (g/kg) of dry air
Mass of water vapor per unit mass of dry air. Stated as gr/lb (US) or g/kg (metric). The mass-balance quantity: when a coil dehumidifies, this is what drops; when a humidifier adds moisture, this is what climbs. Unlike RH it does not depend on temperature, so it is the right number to compare two airstreams at different temperatures or to balance moisture loads across a system.
RH — Relative humidity · how full the air is at this temperature
The vapor pressure of the moisture in the air, divided by the saturation vapor pressure at the air’s current dry-bulb, expressed as a percent. 100% means saturated — any colder surface will condense; 0% means bone-dry. The catch: at this temperature is load-bearing. Two airstreams at 60% RH but different DB carry very different moisture and very different dew points. RH is the property people quote, and the property that bites them.
h — Enthalpy · total heat per pound of dry air
Sensible (DB-driven) plus latent (moisture-driven) heat, rolled together in one number. Btu per pound of dry air (or kJ/kg). The right basis for "how much heat is the coil moving" — multiply Δh by mass flow through the coil and you get total Btu/h, sensible and latent together. An enthalpy economizer compares OA vs RA on this number, not DB alone, because hot-but-dry OA can carry less total heat than cool-but-wet RA.
v — Specific volume · ft³ per pound (m³/kg) of dry air
The volume one pound (kilogram) of dry air occupies at its current state — ft³/lb in US units, m³/kg in metric. The translator between volumetric airflow (CFM or m³/h — what the fan delivers) and mass flow (lb/h or kg/h — what every load calc runs on). Warm humid air has a larger specific volume than cool dry air, so the same CFM moves less mass through the coil on a hot day. One reason coil performance drops off in design conditions, and one reason engineers can’t skip this number.
Two Properties Lock the Rest
Now look at those seven properties from the other direction. At a fixed pressure (which, for an indoor space at a given altitude, you can treat as constant), moist air has two independent degrees of freedom. Pick any two independent properties — say, dry-bulb and relative humidity — and the other five fall out: wet-bulb, dew point, humidity ratio, enthalpy, and specific volume all become arithmetic from those two. That is why every psychrometric chart is two-axis, and why the chart tool’s Define by dropdown is not UX whimsy: it lets you pick whichever pair you actually have a reading for and reads off the rest.
Some pairs are common and easy to come by; some are technically valid but rare. The table below is the practical short list — what the field tech and the controls engineer typically have to work with.
| Pair | Where it comes from |
|---|---|
| DB + RH | Almost every space sensor — BACnet AHU (air-handling unit) return, wall stat, IAQ (indoor-air-quality) probe. The everyday pair, and the one with the most footguns. |
| DB + WB | Sling psychrometer or aspirated wet/dry pair. The original field measurement; still the gold standard for a coil leaving-air reading. |
| DB + DP | Chilled-mirror dewpoint sensor plus a temperature element. Less common in HVAC; more common in clean rooms, labs, museums. |
| DB + W | Computed — either a transmitter that outputs grains directly, or a downstream calc from RH. Useful for mass-balance work. |
| DB + h | Computed — enthalpy economizer logic lives here. The right pair for "how much heat is moving through this coil." |
Some pairs are degenerate at the edges. At saturation (RH = 100%) DB equals WB equals DP and you only really have one number; above saturation isn’t a valid state at all — the chart’s heavy green curve is a hard ceiling.
Process Families on the Chart
Every air-handler stage moves the state point in a characteristic direction on the chart. Four families cover almost everything a controls engineer meets — naming them out loud makes it easier to read what a coil is doing the next time the leaving-air state stops making sense.
Sensible heating or cooling
A heating coil with no humidifier, or a cooling coil where the leaving DB stays above the entering dew point. The state point slides horizontally: DB moves, but humidity ratio stays put — no moisture has been added or removed. RH falls (heating drives the air away from saturation) or rises (cooling drives it toward saturation). Enthalpy follows DB.
Cooling + dehumidification
The cooling coil dips below the entering dew point. Moisture condenses out, drains away, and the leaving air has lower W and lower DP. The state-point path bends down-and-to-the-left toward the coil’s apparatus dew point — the effective coil-surface temperature the leaving air is pulled toward. Both sensible and latent enthalpy leave the airstream; on a humid day the latent share is most of the load. This is the process that does the real work in any dehumidification application — natatorium included.
Mixing two streams
Outdoor air meeting return air in an AHU mixing box, or a bypass leg meeting a coil-leaving stream. The mixed point lands on the straight line between the two source points on the chart, at the mass-weighted fraction. 20% OA + 80% RA puts you 20% of the way from RA to OA, in a straight line. The chart tool’s MA node is doing exactly this math.
Adiabatic humidification
An evaporative pad or wetted-media humidifier — water evaporates into the airstream, taking its evaporation heat from the airstream. The state point slides up-and-to-the-left along a constant wet-bulb line: W rises, DB drops, total enthalpy is essentially unchanged. The latent gain is paid for by a sensible drop. (Steam injection is different — that adds enthalpy and sits higher on the chart; the chart tool only models the adiabatic case for now.)
Each of these is a coloured segment on the interactive chart — drag the OA and RA points around and watch the process colours change as the chain re-solves.
Gotchas That Bite Controls Engineers
RH alone tells you nothing
Two airstreams at 60% RH but different DB carry very different moisture and pose very different condensation risks. A humidity spec written without a temperature spec is half a spec. When a pool engineer writes "≤ 60% RH at design conditions," they are really defending a dew-point ceiling — the RH number is convenient shorthand for it.
Dew point is the property that condenses
Whenever you are worried about water on a surface, the controlling number is dew point. The glass-of-ice-water mental model holds everywhere — pool windows, supply-duct condensation in unconditioned ceiling cavities, sweating chilled-water piping in a humid mechanical room, cold-floor lobbies in shoulder seasons. If you can back dew point out from the sensors you already have, you have the controlling number.
Enthalpy is the right basis for coil capacity
Sensible-only ṁ·Cp·ΔT misses the latent share — sometimes the bulk of cooling-coil load on a humid day. Use ṁ·Δh for the total and you will size the coil right, alarm against the right capacity, and write economizer logic that actually picks the lower-load outdoor air. The chart tool’s per-stage Δh column is the same calculation.
Specific volume is the CFM-to-mass-flow bridge
Fans deliver volumetric flow; coils, loads, and energy balances run on mass flow. ṁ = CFM · 60 / vin, so the same CFM at warmer or wetter entering air moves less mass per unit time. Coil-sizing programs handle this internally; if you are ever doing the math by hand for a sanity check, do not skip it.
Does This Air Sweat the Windows?
“I have been fighting to understand this better myself for a while now. I programmed a pool unit on a job and the engineer speced it with very tight tolerance for efficiency, and I’ve had to fight to keep zone humidity down.”
The pool-room control problem in one sentence: dew point has to stay below the coldest surface in the room, with margin to spare. Any surface colder than dew point grows water out of the air. Slide the space dry-bulb and relative humidity around below, plus a slider for the coldest surface in the room. The readouts back out humidity ratio, dew point, and enthalpy; the status panel tells you whether the glass stays dry.
Math: W = humRatioFromRH(RH, DB) → dewPoint(W) → h = enthalpy(DB, W), all from the ASHRAE Fundamentals (Chapter 1) IP equations at standard sea-level pressure. Surface temperature is your independent input — that is a thermal-bridging / envelope question, not a psychrometric one. 50 °F is a fair guess for single-pane glass on a cold winter day; double-pane and low-e will sit warmer, a poorly insulated metal frame can sit colder.