AC Sizing for Grow Rooms

Growing, buying or possessing cannabis and seeds is legal only in some places. Check the law where you live before acting on this information.

What AC sizing actually means

AC sizing is the process of calculating the cooling capacity (measured in BTU per hour, or tons — 1 ton = 12,000 BTU/h) your grow space needs to maintain target temperature and humidity under full lights-on load.

Two heat loads must be managed:

Sensible load: heat that raises air temperature. This includes lights, drivers, ballasts, pumps, dehumidifiers, fans, and any other electrical equipment. In a sealed room, virtually all your electrical input becomes sensible heat Strong evidence.

Latent load: energy required to condense water vapor out of the air. Plants transpire continuously—a mature flowering canopy can release 1–2 liters of water vapor per kW of light per 12-hour photoperiod Weak / limited^[1]. An AC that ignores latent load will cool the room but leave it humid, creating mold risk and bud rot Strong evidence^[2].

Residential AC sizing tools (like Manual J) are built for houses with people, lights, and normal building heat gain. They do not account for the high sensible load density of LED or HID arrays in a small sealed space, nor do they typically model latent load. Applying residential rules directly to grow rooms will produce undersized estimates.

Why growers care

Cannabis has a narrow environmental comfort window. Exceeding it means lost yield, potency, and terpene content—or forced compromises.

Temperature: Leaf surface temperature above roughly 28–30 °C (82–86 °F) reduces photosynthetic efficiency and accelerates terpene loss Strong evidence^[1]. Chronic heat stress causes stretch, foxtailing, and reduced cannabinoid production.

Humidity: Relative humidity above ~75% in flower, especially with poor air movement, sharply increases Botrytis (bud rot) risk Strong evidence^[2].

Undersized AC means:

• Lights-on temperatures climb, especially in summer.
• Forced compromises: dimming lights, cutting the photoperiod, or running supplemental dehumidifiers that add more heat.
• Visible stress and lower quality.

Oversized AC is equally problematic:

• Short cycling: the unit reaches setpoint quickly, shuts off before running long enough to dehumidify, then restarts.
• High RH despite cool air—bud rot, powdery mildew.
• Wasted capital, higher electrical demand, poor part-load efficiency.

When to start sizing

Before you buy lights. Lighting choice is the single largest input to your heat load. You cannot pick an AC without knowing your light wattage and type.

If your room is already built, perform the sizing calculation:

• Before your next light upgrade.
• Before sealing the room for CO2 supplementation.
• Before adding or upgrading a dehumidifier (standalone dehus dump their wattage as sensible heat, increasing room load) Strong evidence.
• When changing from HID to LED, or vice versa. LEDs and HPS produce similar total heat per watt, but LEDs reduce radiant heat on the leaf surface, which is a different optimization problem Strong evidence^[3].

How to size your AC: step by step

Step 1: List every watt that runs during lights-on

Include lights, drivers, ballasts, fans, pumps, standalone dehumidifiers, CO2 controllers, and any other powered equipment. Use nameplate wattage or measure actual draw with a Kill-A-Watt meter. For LEDs, use measured electrical input, not advertised 'equivalent' wattage.

Step 2: Convert watts to BTU/h

One watt of electrical input equals approximately 3.41 BTU/h of heat output Strong evidence. In a sealed room, almost all electricity becomes heat via conservation of energy—light energy is absorbed by plants and surfaces, then re-emitted as heat.

Example: 4× 600W LED bars (2,400W) + 200W fans/pumps + 800W dehumidifier = 3,400W total → 3,400 × 3.41 ≈ 11,600 BTU/h sensible load.

Step 3: Add envelope load (if applicable)

If your room shares walls with hot attics, garages, or the outdoors, heat enters through the walls. For a well-insulated interior room, this is often <10% of light load. For a poorly insulated outbuilding in summer, it can be substantial. Use a Manual J calculator or consult an HVAC technician if this applies to you.

Step 4: Estimate latent load

A flowering canopy transpires roughly 1.0–1.5 liters of water per kW of light per 12-hour photoperiod, depending on vapor pressure deficit (VPD) Weak / limited^[1]. Each liter of condensed water represents ~2,140 BTU.

Example: 2.4 kW of light × 1.25 L/kW = 3 L/day ÷ 12 hours = 0.25 L/h × 2,140 BTU/L ≈ 535 BTU/h latent load. This is smaller than sensible load but is the part most residential sizing tools ignore. It determines whether you need a separate dehumidifier or whether your AC's sensible heat ratio (SHR) is sufficient to dehumidify on its own.

Step 5: Add a safety margin

Add 15–25%, not 100%. Oversizing beyond this range causes short-cycling and humidity control failure Strong evidence. A modest margin accounts for seasonal variation and future upgrades without triggering the short-cycle trap.

Step 6: Pick a unit

Select an AC unit that matches or slightly exceeds your total BTU/h. Prefer variable-speed (inverter) compressors—they modulate capacity and dehumidify effectively at part load, avoiding short-cycling. Mini-split heat pumps are the industry standard for small to mid-size grow rooms (under ~30,000 BTU/h).

Worked example (small flower room)

• 2,400 W LED + 300 W accessories + 700 W dehumidifier = 3,400 W → 11,600 BTU/h sensible
• Latent load: ~600 BTU/h
• Total: ~12,200 BTU/h
• Safety margin (×1.20): ~14,600 BTU/h
• Recommendation: a 1.25-ton (15,000 BTU/h) mini-split is appropriate.

Common mistakes

Using square-foot rules. The '20 BTU per sq ft' rule is residential guidance. A 10×10 grow room with 2 kW of light needs roughly 3× the cooling of a 10×10 bedroom Strong evidence. Density of equipment, not floor area, drives grow-room load.

Forgetting the dehumidifier's own heat. Standalone dehumidifiers consume electrical power and dump it all as sensible heat into the room. Always include the dehu's wattage in your total load. If you don't, your AC will be undersized.

Believing LEDs run "cool." A 600W LED produces the same ~2,050 BTU/h as a 600W HPS Strong evidence^[3]. LEDs do reduce radiant heat on the leaf surface, which lets you run higher room temperatures and often improves spectral efficiency—but the AC still has to remove the same kilowatts. The benefit is not thermal; it is spectral and operational.

Oversizing 'just in case.' Short-cycling—where the AC hits setpoint and shuts off before running long enough to dehumidify—is worse than undersizing by a small margin. A correctly sized AC with a modest safety margin, paired with a separate dehumidifier if needed, is more reliable than an oversized unit fighting humidity.

Ignoring lights-off load. Lights-off heat load is much lower (fans, pumps, maybe a dehu). A single-stage AC sized for lights-on will be massively oversized at night, guaranteeing short-cycling and humidity swings. Inverter-based ACs handle this gracefully; older single-stage units may require a secondary smaller system or a wider temperature deadband.

Related techniques

AC sizing is one component of integrated environmental control:

• VPD (Vapor Pressure Deficit) Basics — Once your AC and dehumidifier are correctly sized, you can manage VPD actively to optimize transpiration and growth.
• Inline Fan Sizing — Exhaust airflow complements AC; oversized fans waste energy and can interfere with humidity control.
• CO₂ supplementation — Requires a sealed room, which requires correctly sized AC to handle the added latent load from higher plant transpiration.
• Dehumidifier choice and placement — The latent-load companion to this article. Standalone dehus add sensible heat; whole-room AC handles both, but may need a secondary dehu for fine humidity control.

Sources

How this page was made

Related

• Datalogger for Grow Rooms — Record temperature and humidity trends to diagnose and prevent environmental problems.
• First Grow Checklist — A structured order of operations to get from seed to harvest safely and legally.
• How to Grow Animal Cookies — Indoor and outdoor cultivation guide for the Animal Cookies hybrid strain.
• How to Grow Cannatonic — Growing the high-CBD hybrid: pheno-hunting, environment, and lab testing essentials.
• Inline Fan Sizing — Choosing the right exhaust fan capacity for heat, humidity and odor control.
• Insulating a Grow Space — Seal walls, ceiling and air leaks to stabilize temperature and cut heating costs.
• Setting Up a 5x5 Grow Tent — Complete guide to assembling, configuring, and commissioning a 5x5 grow tent for indoor ca...
• VPD (Vapor Pressure Deficit) Basics — How to use vapor pressure deficit to manage plant transpiration and prevent humidity-relat...