# Grow room AC and cooling load calculator

> Free grow room cooling calculator: BTU/h and tons from your lights and every other device (3.412 BTU/h per watt), people and wall gains, with sealed vs vented explained and the single-hose portable warning.

Interactive: https://genetixbay.com/knowledge/tools/cooling-load.html

Every watt you plug in becomes heat: 1 W = 3.412 BTU/h, whether it's an LED, an HPS, a fan or a computer. Add up the wall watts of everything in the room, add people and the heat leaking through the walls, then add a margin. That's your AC size.

## How it works

### Watts to heat

BTU/h = wall watts × 3.412. Four 1,000 W double-ended HPS draw about 1,045 W each, 4,180 W in all: 14,262 BTU/h before anything else.

### People and walls

About 450 BTU/h per person working inside. Walls and ceiling: U-value × area × the temperature difference. A tent inside the house adds nothing here.

### Tons

One ton = 12,000 BTU/h. Add about 20% margin and round up to a real size.

### Sealed vs vented

A vented room swaps its air with the exhaust fan, which only works while the intake air is cooler than the room. A sealed room keeps the exhaust off, holds CO₂, and lets the AC and dehumidifier do all the work. Sealed is more control; vented is cheaper and simpler.

### The dehumidifier

Count its watts. Don't add the heat it releases condensing water a second time: that energy came from the lights, when the plants evaporated it.

## Constants

| Constant | Value | Unit | What it is | Evidence |
|---|---|---|---|---|
| BTU_PER_W | 3.412 | BTU/h per W | Every watt drawn from the wall becomes 3.412 BTU/h of heat in the room, whatever the device | Established |
| BTU_PER_TON | 12000 | BTU/h per ton | One ton of cooling | Established |
| PERSON_BTU | 450 | BTU/h per person | Heat from a person doing light work (ASHRAE: about 250 sensible + 200 latent) | Established |
| U_GOOD | 0.06 | BTU/h·ft²·°F | Well-insulated room (about R-16 overall) | Established (typical assembly) |
| U_AVG | 0.1 | BTU/h·ft²·°F | Average insulated stud walls (about R-10 overall) | Established (typical assembly) |
| U_POOR | 0.25 | BTU/h·ft²·°F | Uninsulated framed walls (about R-4) | Established (typical assembly) |
| U_SHED | 0.6 | BTU/h·ft²·°F | Bare garage, shed or metal building | Estimate |
| AC_MARGIN | 0.2 | + | Cooling margin over the calculated load (homes keep within ~15%; grow rooms run near flat out, so 20% is common) | Grower practice |

## Sources

1. NIST. Guide for the Use of the International System of Units (SP 811), Appendix B: conversion factors (1 W = 3.412 142 BTU/h; 1 ft³ = 0.028 316 8 m³; 1 US gal = 3.785 412 L). https://www.nist.gov/pml/special-publication-811
2. ASHRAE. 2021. ASHRAE Handbook: Fundamentals. Ch. 1 (psychrometrics: sensible heat of air, q = 1.08 × cfm × ΔT at standard air), Ch. 18 (heat gain from people, lighting and equipment), Ch. 21 (duct design: fitting and friction losses), Ch. 26–27 (thermal resistance of building assemblies). ASHRAE, Atlanta. https://www.ashrae.org/technical-resources/ashrae-handbook
3. GenetixBay Knowledge Base. LED vs HPS: the heat and power-bill hype, and an LED buyer-beware guide (fixture µmol/J tiers, 85% canopy capture, watt-for-watt house advice, 1 W = 3.412 BTU/h). https://genetixbay.com/knowledge/equipment/led-vs-hps-hype-and-buyer-beware.html
4. ACCA. Manual S: Residential Equipment Selection (cooling equipment kept close to the calculated load; oversizing short-cycles and dehumidifies poorly). https://www.acca.org/standards/technical-manuals/manual-s
5. US DOE. 10 CFR 430 Subpart B, Appendix CC: Uniform test method for portable air conditioners (seasonally adjusted cooling capacity, which counts the infiltration a single-hose unit causes). https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-B/appendix-Appendix%20CC%20to%20Subpart%20B%20of%20Part%20430