AC Sizing for Grow Rooms

What AC sizing actually means

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

Two loads matter:

• Sensible load: heat that raises air temperature (lights, ballasts, pumps, dehumidifiers, motors, bodies).
• Latent load: energy needed to condense water vapor out of the air. Plants transpire a lot of water — a mature flowering canopy can release several liters per day per kW of light. An AC that only handles sensible load will leave you with cool, soggy air and mold risk Strong evidence.

Residential AC sizing tools (like Manual J) are built for houses, not sealed rooms full of HPS or LED arrays. They will underestimate grow loads badly ^[1].

Why growers care

Cannabis has a fairly narrow comfort window. Above roughly 28–30 °C (82–86 °F) leaf surface temperature, photosynthesis efficiency drops and terpene loss accelerates ^[2] Strong evidence. Above ~75% RH in flower, Botrytis (bud rot) risk climbs sharply ^[3] Strong evidence.

Undersized AC means:

• Lights-on temps creep up, especially in summer.
• Stretch, foxtailing, and reduced cannabinoid/terpene content.
• Forced light dimming or shorter photoperiods to cope.

Oversized AC is just as bad:

• Short cycling: the unit hits temp setpoint fast, shuts off, and never runs long enough to dehumidify.
• High RH despite cool air. Bud rot. Powdery mildew.
• Wasted capital and higher electrical demand.

When to start

Before you buy lights. Lighting choice is the single biggest input to your heat load, and you can't pick an AC without knowing it. If your room is already built, do the calculation before your next light upgrade or before sealing the room.

Also re-run the math if you:

• Change from HID to LED (LEDs reduce sensible load but latent load stays similar — plants still transpire).
• Add CO2 supplementation and seal the room.
• Add or remove a dehumidifier (dehus add sensible heat — typically the wattage they draw becomes room heat) Strong evidence.

How to size your AC: step by step

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

Lights, drivers, pumps, fans, dehumidifier, CO2 controller, etc. Use nameplate watts or measure with a Kill-A-Watt. For LEDs, use actual draw, not advertised 'equivalent' wattage.

Step 2: Convert watts to BTU/h.

1 watt of electrical input ≈ 3.41 BTU/h of heat output ^[4]. Essentially all the electricity you put into a sealed room ends up as heat (this is conservation of energy — light energy gets absorbed and re-emitted as heat) Strong evidence.

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

Step 3: Add envelope load (if applicable).

If your room shares walls with hot attics, garages, or outdoors, add heat gain through the walls. A rough number for an insulated interior room is small (often <10% of light load). For a poorly insulated outbuilding in summer, it can be huge — use a Manual J calculator or hire an HVAC tech.

Step 4: Estimate latent load.

A flowering canopy transpires roughly 1.0–1.5 L of water per kW of light per 12-hour photoperiod, varying with VPD ^[5] Weak / limited. Each liter of water vapor condensed represents ~2,260 kJ, or about 2,140 BTU. So 2.4 kW of light × 1.25 L/kW = 3 L/day ÷ 12 h = 0.25 L/h × 2,140 ≈ 535 BTU/h latent. This is small compared to sensible load but it's the part that residential sizing ignores — and if your AC SHR (sensible heat ratio) is too high, you'll need a separate dehumidifier.

Step 5: Add a safety margin.

Add 15–25%. Not 100%. Oversizing is a real problem, not a free safety net Strong evidence.

Step 6: Pick a unit.

Match or slightly exceed your total BTU/h. Prefer units with variable-speed (inverter) compressors — they modulate output and dehumidify better at part load. Mini-split heat pumps are the standard pick for small to mid-size rooms.

Worked example (small flower room):

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

Common mistakes

• Using square-foot rules. '20 BTU per sq ft' is residential guidance. A 10×10 grow room with 2 kW of light needs roughly 3× what a 10×10 bedroom needs Strong evidence.
• Forgetting the dehumidifier's own heat. Standalone dehus dump their wattage as sensible heat into the room. Size your AC including the dehu running.
• Believing LEDs run cool. A 600 W LED produces the same ~2,050 BTU/h as a 600 W HPS. The difference is spectral, not thermal Strong evidence. LEDs do reduce radiant heat on the leaf surface, which lets you run higher room temps — but the AC still has to remove the same kilowatts.
• Oversizing 'just in case.' Short cycling kills both efficiency and humidity control. Better to size correctly and add a dehumidifier than to oversize the AC.
• Ignoring the lights-off load. Lights-off heat load is much lower. A single-stage AC that's right for lights-on will be wildly oversized at night. Inverter units handle this; single-stage units may need a smaller secondary system or a wider deadband.

Related techniques

AC sizing is one piece of environmental control. See also:

• VPD Management — once your AC and dehu are right-sized, you can actually hit VPD targets.
• Sealed Room vs Vented Room — sealed rooms need full AC capacity; vented rooms can offload some heat with exhaust fans.
• CO2 Supplementation — requires a sealed room, which requires correctly sized AC.
• Dehumidifier Sizing — the latent-load companion to this article.

Sources

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