HVAC Calculator: Psychrometrics, Coil Load, Duct Size

Standard HVAC calculations, with every step shown, in IP or SI units:

• Psychrometrics: a full air state (dew point, relative humidity, humidity ratio, enthalpy, specific volume, wet bulb) from any two readings, corrected for elevation.
• Coil load: sensible, latent and total load (MBH and tons, or kW), sensible heat ratio and condensate, from airflow plus entering and leaving air (optionally outdoor/return air mixing).
• Duct size: round duct diameter (with nearby standard sizes) and an equivalent rectangular size for an airflow at a friction rate, with velocity, velocity pressure and Reynolds number.

Each result lists the formulas and intermediate values (steps), plus notes and references, so it can be checked by hand.

Who it is for

• Technicians, students and estimators who want quick, transparent numbers.
• Developers and AI agents that need HVAC math as a tool call instead of re-implementing it.

Input

Field	Required	What it does
calculation	yes	psychrometrics, coil_load or duct_size.
parameters	yes	That calculation's inputs (see below). Common spellings are understood, e.g. db/dry_bulb, rh/relative_humidity. Add "units": "SI" for metric.

Parameters by calculation:

• Psychrometrics: {"dry_bulb": 95, "wet_bulb": 75, "elevation": 0}. Any two of dry bulb, wet bulb, dew point, relative humidity, humidity ratio or enthalpy.
• Coil load: {"airflow": 10000, "entering": {"dry_bulb": 80, "wet_bulb": 67}, "leaving": {"dry_bulb": 55, "wet_bulb": 54}}. Airflow is in CFM, or L/s with SI units.
• Duct size: {"airflow": 1000, "friction_rate": 0.08}. Friction rate is in in. w.c. per 100 ft, or Pa/m with SI units.

Example:

{ "calculation": "psychrometrics", "parameters": { "dry_bulb": 95, "wet_bulb": 75, "elevation": 0 } }

Output

One dataset item per run: calculation, units, unit_labels, the inputs used, the results (state, or loads and sizes), steps, notes and references.

Pricing

$0.002 per calculation (pay per event). Apify platform usage is included, and Apify adds its standard run-start fee of $0.00005.

Limitations (please read)

• Not engineering design. Results are textbook calculations for quick estimates and checking. They are not a stamped engineering design and do not replace a licensed engineer's judgement or local code requirements.
• Duct sizing: Darcy-Weisbach with the Colebrook-White friction factor and galvanized steel roughness (ε = 0.0003 ft), per ASHRAE Handbook—Fundamentals Ch. 21.
• Psychrometrics: uses the ASHRAE Handbook—Fundamentals (2021) psychrometric equations for moist air at the pressure for your elevation; it is meant for building HVAC conditions.

Use it from an AI agent (MCP)

This Actor runs with Apify's default limited permissions, so it is available through Apify's MCP server. Add it to Claude, Cursor or any MCP client with:

https://mcp.apify.com?tools=<publisher>/hvac-calculator

The agent then calls it with input like {"calculation": "duct_size", "parameters": {"airflow": 1000, "friction_rate": 0.08}}. Results land in the run's dataset; the MCP server's get-dataset-items tool reads them.

How it works

Pure calculation. No outside data sources or network calls are used.