How grow room cooling load is calculated
Unlike residential HVAC, a grow room’s dominant heat source is artificial lighting,
not outdoor temperature. Every watt of electricity consumed by lights becomes heat in the room at a
rate of 3.412 BTU/hr per watt. Even “efficient” LEDs convert 100% of their input power
to heat — the efficiency gain is that you need fewer watts to achieve the same light intensity,
meaning less total heat for the same canopy coverage.
The four components of grow room heat load
| Heat Source |
Typical % of Total Load |
How It’s Calculated |
| Lighting (sensible heat) |
70–85% |
Total watts × 3.412 BTU/hr/W × light type factor |
| Plant transpiration (latent heat) |
8–15% |
Plant count × stage factor × latent BTU/hr per plant |
| Envelope heat gain |
5–15% |
Room area × insulation/location/climate multiplier |
| Other equipment |
3–8% |
Pump/fan/controller watts × 3.412 BTU/hr/W |
A 15% buffer is applied to the total to account for duct losses, AC cycling inefficiency, and
real-world variability.
Light type heat conversion factors
| Light Type |
Sensible Heat Factor |
Notes |
| HID / HPS / MH |
1.00 — 100% of watts to heat |
Ballast also generates significant heat; add ballast wattage |
| LED (quantum board) |
0.90 — ~90% of watts to room heat |
~10% of LED heat removed via driver/heatsink if remotely mounted |
| CMH / LEC |
0.95 — ~95% of watts to room heat |
More efficient than HPS but still high heat density |
| T5 fluorescent / CFL |
0.95 — ~95% of watts to room heat |
Low wattage per fixture; mainly used for seedlings/clones |
Ventilation sizing (CFM)
Ventilation serves two purposes in a grow room: (1) removing heat and humidity, and (2) refreshing
CO&sub2; for plant respiration. The standard approach is to size the inline fan to exchange the entire
room volume at least once per minute during lights-on:
Minimum CFM = Room volume (cu ft) × target air changes per minute
A carbon filter adds 20–25% static pressure resistance — so fans should be rated
25% higher than calculated CFM, or sized using the fan’s rated CFM at 0.25” static
pressure rather than free-air rating.
Grow room cooling & ventilation equipment
Mini split systems (recommended for rooms > 100 sq ft)
Portable AC units (tents & small rooms)
Monitoring & control
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Frequently asked questions
How many BTU do I need to cool a grow room?
A good starting rule is 3,000–4,000 BTU per 1,000 watts of HID lighting,
or 2,000–2,500 BTU per 1,000 watts of LED. Lighting accounts for 70–85%
of total cooling load. This calculator also adds plant transpiration, envelope heat gain, and
other equipment watts for a complete estimate. Use the calculator above
with your exact setup.
How many BTU to cool a 4×4 grow tent?
A 4×4 tent with a 600W LED running 18 hrs/day needs approximately 3,500–5,500
BTU/hr
of cooling, depending on climate and insulation. A 5,000–6,000 BTU portable AC or small mini split
handles most 4×4 setups. At 1,000W HPS, the same tent needs 6,000–8,000 BTU of cooling.
How many BTU to cool a 10×10 or 10×20 grow room?
A 10×10 room with 2,000W LED typically needs 12,000–18,000 BTU/hr.
A 10×20 room with 4,000W HID often needs 24,000–36,000 BTU/hr.
These are best served by a 1.5-ton to 3-ton mini split, or multiple portable units. Use the
calculator above with your exact wattage and room dimensions for the most accurate figure.
Do I need a dehumidifier in my grow room?
Yes, in most cases. Plants in flower transpire large amounts of water — a 4×4 tent with
4 flowering plants can add 1–2 pints of water per hour to the air. While the AC removes some
moisture as part of cooling, a dedicated dehumidifier is often needed in flower to keep RH below
50% and prevent powdery mildew and botrytis. See our
dehumidifier calculator for sizing.
What ventilation CFM do I need for a grow room?
The standard rule is to exchange the room’s entire volume at least once per minute:
CFM = length × width × height. A 4×4×8 ft tent (128 cu ft)
needs at least 128 CFM. Add 25% for carbon filter resistance and buy a fan rated at 160+ CFM.
For hot climates or high-wattage setups, target 2 air changes per minute.
Can I use a regular window AC in a grow room?
Yes, but with caveats. A window AC exhausts room air to cool the condenser, creating negative pressure
and pulling unfiltered outside air in through gaps. This can introduce pests, pathogens, and
temperature-inconsistent air. If you use a window AC, seal all other openings and use a sealed intake
with a HEPA filter. For sealed-room CO&sub2; enrichment, a window AC is incompatible — use a
mini split instead.
How much does it cost to run a 1,000W HPS grow room per month?
At 16¢/kWh: 1,000W lights × 12 hrs/day × 30 days = 360 kWh = ~$58/month
in lighting alone. Add AC (typically 400–600W equivalent draw), fans, pumps, and other equipment
and total monthly cost is typically $100–$180/month for a 1,000W HPS setup.
Use the calculator above with your actual wattage for a personalized estimate.