# Grow room dehumidifier calculator

> Free grow room dehumidifier calculator: pints per day from the water your plants use, the 2019 DOE vs old AHAM rating change, the lights-off humidity spike, and the dehumidifier's heat for your AC.

Interactive: https://genetixbay.com/knowledge/tools/dehumidifier.html

Plants breathe out almost all the water you give them. So your dehumidifier has to pull out about what you water each day, minus what drains away, plus a margin. Then check the label: since 2019, pint ratings are smaller for the same machine.

## How it works

### Water in, water out

About 97% of the water a plant takes up is transpired. Four flowering plants at 2 L a day with 10% runoff put about 7 L (15 pints) into the air every day, in line with the 3–6 L per m² a day that canopies typically use.

### Rating

Buy at least that × 1.25, on the 2019 DOE label. Old AHAM labels (tested at 80 °F) read about 1.4× higher for the same machine: a former 70-pint unit is now a 50.

### Lights-off spike

Cooler air holds less water. 55% RH at 79 °F becomes about 70% at 72 °F with the same water in it. Keep the night drop small and let the dehumidifier run in the dark.

### Heat

A dehumidifier is a heater too: its watts × 3.412 go into your cooling load.

## Constants

| Constant | Value | Unit | What it is | Evidence |
|---|---|---|---|---|
| TRANSPIRED | 0.97 | fraction | Share of the water a plant takes up that it transpires into the air | Established |
| NIGHT_RATE | 0.1 | × day rate | Night transpiration per hour as a share of the lights-on rate | Supported (other crops) |
| DEHU_MARGIN | 1.25 | × | Dehumidifier margin over the average daily water (the AC also pulls water while it runs) | Grower practice |
| PINT_PER_L | 2.11338 | US pints per L | Dehumidifier ratings are in US pints/day | Established |
| AHAM_TO_DOE | 0.7 | × | Old AHAM pints/day (80 °F, 60% RH) to the 2019 DOE rating (65 °F, 60% RH): about 70 → 50 | Established |
| DEHU_IEF_S | 1.3 | L per kWh | Federal minimum efficiency, portable units up to 25 pints/day (2019 rating); used to estimate the heat when you don't know the watts | Established (regulation) |
| DEHU_IEF_M | 1.6 | L per kWh | Federal minimum, portable units 25–50 pints/day | Established (regulation) |
| DEHU_IEF_L | 2.8 | L per kWh | Federal minimum, portable units over 50 pints/day | Established (regulation) |
| WATER_SEEDLING | 0.25 | L per plant per day | Stage helper: seedling | Grower practice (varies with pot, light and VPD) |
| WATER_VEG | 1 | L per plant per day | Stage helper: veg | Grower practice |
| WATER_FLOWER | 2 | L per plant per day | Stage helper: flower (4 plants in a 4 × 4 ft tent ≈ 5.4 L/m²/day, inside the guides' 3–6 L/m²/day) | Grower practice |

## Sources

1. Taiz L, Zeiger E, Møller IM, Murphy A. 2015. Plant Physiology and Development, 6th ed., Ch. 4 (about 97% of the water a plant absorbs is lost by transpiration). Sinauer. https://global.oup.com/academic/product/plant-physiology-and-development-9781605353531
2. Caird MA, Richards JH, Donovan LA. 2007. Nighttime stomatal conductance and transpiration in C3 and C4 plants. Plant Physiology 143(1):4–10 (night transpiration commonly 5–15% of daytime, up to ~30%). https://doi.org/10.1104/pp.106.092940
3. GenetixBay house position (founder's field experience, decades of indoor rooms): passive intake for most growers; oversize lights and dim down; replace HPS watt for watt unless the LED is top tier. https://genetixbay.com/editorial-policy
4. 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
5. US DOE. 10 CFR 430 Subpart B, Appendix X1 (dehumidifier test at 65 °F and 60% RH, required from June 2019; the older AHAM DH-1 rating used 80 °F and 60% RH) and 10 CFR 430.32(v) (minimum integrated energy factor, L/kWh). https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-B/appendix-Appendix%20X1%20to%20Subpart%20B%20of%20Part%20430
6. ENERGY STAR. Dehumidifiers: key product criteria and the 2019 capacity re-rating (a unit once sold as 70 pints/day is about 50 pints/day on the new test). https://www.energystar.gov/products/dehumidifiers
7. Alduchov OA, Eskridge RE. 1996. Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology 35(4):601–609. https://doi.org/10.1175/1520-0450(1996)035%3C0601:IMFAOS%3E2.0.CO;2