Tools · Dew Point

Dew Point Calculator Coil Check

Dew point at a glance, plus a coil check.
Dew point at a glance from a dry-bulb and one humidity reading — RH off the space sensor or wet-bulb off a sling psychrometer — with wet-bulb, grains, and enthalpy alongside. The Coil Check field turns that dew point into the answer you actually want on an over-humidity call: drop in the supply / leaving dry-bulb and it says whether the coil is pulling the air below the entering dew point (dehumidifying) or just cooling it sensibly while the space rides up.

Inputs are seeded with an example — edit them to your numbers.

Input

Second input

Coil check — optional

Your supply / leaving-air dry-bulb (or any cold surface temperature), checked against the dew point. Leave it blank for a plain dew-point read.

Output

Dew point —
Wet-bulb (°F) —
Humidity ratio (gr/lb) —
Enthalpy (Btu/lb) —

Field notes — the over-humidity call

Read the return-air dry-bulb and RH off the space sensor (75 °F / 55 %, say) and the tool gives the return dew point (about 57.8 °F). Now compare your supply / leaving dry-bulb to it. Leaving below that dew point (55 °F) means the coil is wringing out moisture; leaving above it means you're cooling sensibly only and space humidity climbs — the failure mode behind most over-humidity complaints.

Why leaving-air dry-bulb is a fair yardstick: the coil's fin surface is colder than the air leaving it, so if the leaving air is already below the entering dew point, the surface is well below it and condensation is certain. Above the dew point it's a strong sign of little latent removal — a coil whose surface just dips under the dew point can still shed a trace. Active-dehumidification sequences hold the supply several degrees below the space dew point on purpose. For the full AHU process train and altitude-adjusted coil capacities, step up to the psychrometric chart.

Where the numbers come from: the shared ASHRAE psychrometric engine — Fundamentals (Chapter 1) IP equations, properties per pound of dry air, with relative humidity taken as the vapor partial-pressure ratio — the same math behind the psychrometric chart and the coil-sizing tool.

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