HVAC

Coil-Sizing Calculator HVAC

One coil, in isolation. Capacity tab: enter the air going in and the air coming out, get total / sensible / latent capacity and the sensible heat ratio. Leaving-state tab runs it backwards — entering air plus the load the coil has to carry, solve for the leaving-air state. Same math the psychrometric chart runs on its coil stages, surfaced on its own so a quick capacity check doesn't need a whole AHU chain.

Coil type

Input

Entering air

Dry-bulb (°F)

Define by

Wet-bulb (°F)

Leaving air

Dry-bulb (°F)

Define by

Wet-bulb (°F)

A heating coil rides humidity ratio through unchanged — leaving air carries the entering air's moisture, so only the leaving dry-bulb is needed.

Airflow

Airflow (CFM)

Output

Total capacity (MBH) —

Sensible capacity (MBH) —

Latent capacity (MBH) —

Sensible heat ratio —

Process change

Δ Dry-bulb (°F) —

Δ Humidity ratio (gr/lb) —

Δ Enthalpy (Btu/lb) —

Enter values to size the coil.

Worked example

A cooling coil — 2000 CFM of entering air at 80 °F / 67 °F WB, leaving at 55 °F / 54 °F WB.

Each air state fixes its enthalpy h and humidity ratio W from the ASHRAE chart math.
Convert volumetric flow to mass flow of dry air:
ṁ = CFM × 60 ÷ v_entering (lb dry air / h).
Total capacity is the enthalpy swing: q_total = ṁ × Δh. Sensible is the dry-bulb swing: q_sens = ṁ × (0.240 + 0.444·W) × ΔDB. Latent is whatever's left: q_total − q_sens.
The sensible heat ratio SHR = q_sens ÷ q_total says how much of the coil's work drops temperature versus wringing out water — a low SHR is a coil fighting a humid space.

Atmospheric pressure is fixed at sea level. For an altitude-adjusted answer, the psychrometric chart carries an altitude input.

Input

Entering air

Dry-bulb (°F)

Define by

Wet-bulb (°F)

Airflow

Airflow (CFM)

Coil load

Sensible load (MBH)

Latent load (MBH)

Coil capacity (MBH)

Atmospheric pressure is fixed at sea level. A heating coil's load is all sensible — humidity ratio rides through unchanged.

Output

Leaving air

Dry-bulb (°F) —

Wet-bulb (°F) —

Humidity ratio (gr/lb) —

Rel. humidity (%) —

Dew point (°F) —

Enthalpy (Btu/lb) —

Enter values to solve the leaving state.

Worked example

Run it backwards — entering air plus the load the coil has to carry, solve for what comes out.

The mass flow ṁ comes from airflow and the entering specific volume, same as the Capacity tab.
For a cooling coil, the sensible load sets the dry-bulb drop: ΔDB = −q_sens ÷ (ṁ × (0.240 + 0.444·W)). The total load (sensible + latent) sets the enthalpy drop: Δh = −q_total ÷ ṁ. Dry-bulb and enthalpy together fix the leaving point.
If the latent load is more than the air can shed at that sensible split, the leaving point lands on the saturation curve — the coil's apparatus dew point. The status line flags it.
For a heating coil it's pure sensible: the capacity sets ΔDB and humidity ratio rides through, so the leaving point is just warmer air at the same moisture.

This is the inverse of the Capacity tab — feed a leaving state from here into that tab and the capacities come back out.

← All tools