An indoor cannabis room stays stable when three things are right. Cooling and dehumidifying must match the real load. Sensors must sit where the plants are, and records must show you what went wrong. This part covers air conditioning, dehumidifiers, carbon dioxide (CO₂) enrichment, heat from the lights, sensors and climate targets, layout, hygiene, electrical safety and records.
One rule from Part 1 runs through all of it. Every watt of electricity used in the room becomes about 3.41 BTU/h (British thermal units per hour) of heat. Add the watts of every device and the heat from the building, then add 20% as a margin. Your cooling must remove that total.
Translator’s note: Paragraphs that start like this one add what changes in Thailand and Southeast Asia. They also correct errors found in the original article.
This is Part 2. Part 1: Growroom Design: Space, Airflow, HVAC, and Environmental Control for Indoor Cannabis
4. Air conditioning, dehumidifiers and CO₂
4.1 Split air conditioners
For rooms up to about 20 m², a good inverter split air conditioner is the most practical cooling in Southeast Asia. An inverter compressor adjusts its output instead of switching on and off. It holds temperature within about ±0.5 °C, compared with ±2 °C for fixed-speed units. It also uses much less electricity over a full cycle.
Check the sensible heat ratio (SHR) before you buy. An air conditioner does two jobs. It lowers the air temperature (sensible cooling) and removes water that condenses on its cold coil (latent cooling). SHR is the share of its capacity spent on lowering temperature.
Home units are built mainly to cool at normal household humidity. Plants release far more water than people do in a room of the same size. A grow room therefore needs a lower SHR, meaning more capacity for removing water. The original says commercial units rated for 100% outdoor air, or sold for greenhouses or server rooms, tend to have a better SHR. This is wrong for server rooms. Server-room (precision) air conditioners are built for a high SHR, often about 0.95, because computers release almost no water. That is the opposite of what a grow room needs.
Ask the installer for the spec sheet. If it lists sensible and total capacity, divide sensible by total to get the SHR, and compare units on that number. For sizing in Thai conditions, see Dehumidifier and Air-Conditioner Sizing for a Thai Flowering Room.
4.2 Dehumidifiers
In late flower the plants release a lot of water. Even with a correctly sized air conditioner, the room often needs a separate dehumidifier. A 70-pint compressor dehumidifier (a US rating, about 33 litres a day) handles about 2–3 m² of dense late-flower canopy at around 26 °C. A 4.8 m² canopy therefore needs two such units.
Dehumidifiers add heat. A unit drawing 700 W adds about 700 W of heat, which the air conditioner must then remove. Include every dehumidifier in the heat-load total before you choose the air conditioner.
Translator’s note: In a room nobody watches all day, run the dehumidifier’s drain hose to a floor drain instead of using the bucket. A full bucket switches most units off, and humidity then climbs without anyone noticing.
4.3 CO₂ enrichment
Normal outdoor air holds about 427 ppm (parts per million) of CO₂, the 2025 average. At that level, cannabis photosynthesis reaches its limit well below the light levels of modern LEDs or double-ended HPS lamps. Raising CO₂ to 1,000–1,500 ppm lets plants make full use of strong light.
The original cites a meta-analysis (Poorter et al., 2022) for the gain in photosynthesis. Across 200–1,200 ppm CO₂, that study found photosynthesis per leaf area more than doubled. That study pooled many C3 plant species, the large group that includes cannabis. It is not a cannabis trial.
Enrichment only works in a sealed or nearly sealed room. Enriched air that leaves through a running exhaust is money wasted. A sealed room needs cooling and dehumidifying that hold the climate with no fresh air, plus a backup unit. It also needs a CO₂ monitor with automatic shut-off, because high CO₂ pushes out oxygen and endangers anyone in the room.
For most small and medium rooms, the original concludes that better airflow and light at normal CO₂ give more return than enrichment equipment. Part 1, section 3.7, compares sealed and vented rooms. Start with airflow and light, and read CO₂ Enrichment: When It’s Needed, When It’s Wasted before you buy enrichment equipment.
5. Heat from the lights
5.1 How lamp types release heat
A 600 W HPS (high-pressure sodium) lamp sends much of its waste heat downward as radiant heat. This creates a strong column of rising warm air that keeps air moving. An LED with the same output usually draws 400–500 W for the same PPFD. PPFD (photosynthetic photon flux density) is the amount of light plants can use that reaches each square metre per second. More on this in Light Spectrum & DLI.
LEDs release most of their heat from the housing into the surrounding air, not as infrared downward. LED rooms are easier to cool, because fewer watts mean less heat. But they create no strong rising draft, so warm air pools under the ceiling. LED rooms therefore often need extra ceiling or wall fans that blow sideways (HAF fans, horizontal airflow fans).
Translator’s note: Count heat at 3.41 BTU/h per watt drawn, for LED and HPS alike. The original gives LEDs a lower figure, which is wrong. Replacing four 600 W HPS lamps with four 450 W LEDs cuts heat from 8,184 to about 6,140 BTU/h. After switching, log the ceiling and the canopy for a week. If the ceiling reads several degrees warmer, add HAF fans.
5.2 Light movers
Sometimes a fixed number of lights cannot cover the canopy evenly. A light mover carries one fixture slowly back and forth along a track, so it covers 30–40% more area. You need fewer fixtures and carry less heat. In return, the mechanism needs maintenance and hanging becomes more complex. Try one mover on one fixture before you convert a whole room.
6. Sensors, controllers and climate targets
6.1 Where to put sensors
A sensor in the wrong place gives readings that look right but are not. Place them like this:
- Temperature and humidity: at canopy height, between the lights and the top leaves, in the middle third of the room. Not on a wall, and not above a lamp. Keep it out of direct light, or it reads warm. The air right at the canopy is what the plants actually feel.
- CO₂ (if used): at canopy height. CO₂ is slightly heavier than air and spreads fairly evenly in a mixed room. Measuring at the canopy still shows what the plants really get.
- Nutrient tank temperature: in the tank, not in the feed line. The best root-zone temperature is 18–22 °C, and growers often forget to check it.
Translator’s note: A tank without a chiller can run far above 22 °C in a Thai room. Options are compared in Running Hydroponics at 31 °C Reservoir Temperature.
6.2 Controllers
Grow room controllers such as Autopilot, Trolmaster and Growlink read the sensors. From one unit, they switch the air conditioner, fans, dehumidifier, CO₂ and lights. They can also run ramp profiles. A ramp profile raises temperature or lowers humidity step by step over a 12-week cycle to match each growth stage.
At minimum, use a combined temperature and humidity controller with separate outlets for cooling and heating. Do not rely on analogue dial thermostats.
Translator’s note: Before you buy, ask the seller about warranty and local service in Thailand, or ask other growers in Equipment and Supplies. In most Thai rooms, the outlets will switch cooling and dehumidifying far more than heating. After installing a controller, cut its power for a minute and check that it keeps its settings.
6.3 Vapour pressure deficit targets
Vapour pressure deficit (VPD) is one number that combines temperature and relative humidity (RH). It shows how strongly the air pulls water out of the leaves. The original gives these targets:
| Growth stage | VPD target (kPa) | Typical temperature (°C) | Typical RH (%) |
|---|---|---|---|
| Seedling or clone | 0.4–0.8 | 23–25 | 70–80 |
| Early vegetative | 0.8–1.0 | 24–26 | 60–70 |
| Late vegetative | 1.0–1.2 | 24–26 | 55–65 |
| Early flower | 1.0–1.2 | 23–26 | 55–65 |
| Late flower | 1.2–1.6 | 22–24 | 40–50 |
When VPD is too high, the plant closes its stomata (leaf pores) to avoid drying out. It then takes in less CO₂ and grows more slowly. When VPD is too low, the water flow that carries nutrients up the plant slows down. Leaf surfaces also stay wet, and disease takes hold more easily.
Translator’s note: The original credits this table to Bugbee (2004). That paper is about nutrient management in recirculating hydroponics, not VPD. Treat the ranges as common grower targets, not as a research finding. We found no peer-reviewed source for stage-by-stage VPD targets in cannabis. In the Thai wet season, the 40–50% RH for late flower can only be reached with dehumidifiers (section 4.2).
Find the row for your current stage and set your controller to it. Check your logger against it every week.
7. Layout and hygiene
7.1 Where equipment goes
- Intake low, exhaust high, so the exhaust follows the rising warm air.
- Dehumidifier in the centre, or wherever humidity is highest. This is usually the middle of a dense canopy.
- Nutrient tanks outside the grow room if possible, or in an insulated cabinet. Their temperature is easier to control away from the room’s daily heat swings.
- Air conditioner at mid-wall height or lower, never above the lights (section 8).
7.2 Access and daily work
Growers usually underestimate how long daily work takes. Plan for:
- Aisles at least 60 cm wide on all four sides of each growing area, for inspection, training and pest scouting.
- A clear view from the door to every plant. Spotting pests and disease means checking every plant surface up close.
- Water, drain and power connections placed so they do not cross walkways.
7.3 Keeping the room clean
Pests and disease arrive mainly on clothes, tools, clones and unfiltered air. These basic steps cut the risk a lot:
- Shoes kept only for the room, or a foot bath at the door (dilute bleach or quaternary ammonium).
- Only smooth, washable surfaces. No bare wood, carpet or fabric, except insulation on equipment.
- Compressed air or a sticky lint roller to clean clothes before you go in.
- Sterilise tools between plants with 70% isopropyl alcohol or a 10% bleach solution.
Translator’s note: Clones from another farm can easily bring in spider mites, powdery mildew or hop latent viroid. Hop latent viroid (HLVd) is a tiny infectious agent that stunts plants and cuts yield. Clones can also turn out to be the wrong strain. Keep new clones in a separate space and inspect them before they enter the main room.
8. Common design mistakes
| Mistake | What happens | Fix |
|---|---|---|
| Exhaust too small | Heat and humidity build up; growth stalls | Size it at 1.3–1.5 times the calculated air-change requirement |
| Air conditioner above the lights | It draws in hot air from the lamps and works poorly | Mount it at mid-wall height or lower |
| One fixed sensor | Misses differences across the room | Add sensors at the edges and in the centre of the canopy |
| No dehumidifier | Humidity spikes in late flower; bud rot risk | Add dehumidifiers for flowering rooms |
| Uninsulated outside walls | Condensation and wasted energy | At least R-13 (about RSI 2.3), with the vapour barrier on the correct side (Part 1, section 2.2) |
| Sealed room without a backup air conditioner | CO₂ enrichment fails on the first hot day | Size cooling for the full heat load and add a backup unit |
9. Electrical planning and safety
9.1 Circuit loads
Dedicated circuits for the grow room stop breakers tripping and lower the fire risk. They also keep equipment at its rated voltage. Size each circuit and breaker for its load, following local electrical rules and a licensed electrician. Supply voltage (for example 220–240 V or 120 V) and breaker ratings differ between countries and buildings.
To check a circuit, add the watts of everything on it and divide by the voltage. Compare the result with 80% of the breaker rating. This is the usual limit for loads that run for hours (the continuous-load rule).
Example from the original, on a 220 V circuit with a 20 A breaker: maximum continuous load = 20 A × 220 V × 80% = 3,520 W. Two 600 W HPS lamps, a 350 W dehumidifier and 200 W of fans total 1,750 W, well within the limit. A third light brings it to 2,350 W, still safe. A fourth light brings it to about 3,000 W, close to the limit, so add a second circuit.
Translator’s note: The original’s example uses a 350 W dehumidifier, but its heat-load section uses a 700 W unit. With 700 W, the circuit carries 2,100 W, then 2,700 W with a third light. A fourth light brings it to 3,300 W, only 220 W below the limit. Use the watts printed on your own equipment labels.
In practice, run one circuit for lights and a second for climate equipment (air conditioner and dehumidifier). Add a third for any other high-wattage system.
9.2 Shock protection and earthing
Water and electricity sit close together in a grow room, so electric shock and fire are real risks. Take all of these measures:
- Protect every outlet with a residual current device (RCD). It is called a GFCI in North America and เครื่องตัดไฟรั่ว in Thailand. An RCD cuts the power when current leaks to earth. It matters most near tanks, misting systems and humidifiers.
- Earth (ground) all equipment properly. Ask the electrician to test that each outlet really is earthed.
- Never use extension cords as permanent wiring. Use proper power strips with surge protection rated for the load.
- Check power strips and lamp cords every month for heat damage, rat bites and water.
9.3 Backup systems
Any failure in a room nobody is watching can cost a large part of the crop.
| Component | What happens if it fails | Backup |
|---|---|---|
| Air conditioner | A heat spike can kill plants within hours | A second unit; fail-safe ventilation on a timer |
| Light timer | Lights stay on 24 hours a day and plants do not flower | Two timers, one backing up the other |
| Water pump (hydroponics) | Still water in the tank; roots die | Battery backup pump; water-level alarm |
| CO₂ system (if used) | CO₂ builds up and pushes out oxygen, a danger to people | CO₂ monitor with alarm |
Translator’s note: A power cut stops everything in this table at once. Cuts often come with storms. Three steps help:
- A UPS (uninterruptible power supply, a battery unit that bridges short cuts) for the controller, sensors, alarm and internet router, so alerts still reach you. Lights and air conditioners are usually too large for a small UPS.
- A standby generator with an automatic transfer switch for the air conditioner and fans, sized and connected by a licensed electrician.
- An alarm that messages your phone when the power fails or the temperature passes your limit.
Test your setup while the room is empty. Switch off the main breaker for a few minutes. Then check what restarts by itself, and whether timers and controllers kept their settings.
10. Records and data logging
10.1 What to record
Rooms that run well crop after crop almost always belong to growers who keep detailed notes. Over a 10–14 week cycle, you will forget details that seem obvious at the time. Examples are the feeding week that caused nutrient burn, or the temperature behind heat stress in week 6. Record at least:
- Germination date or the date clones went in
- A weekly photo of the same typical plant, from the same angle
- Weekly EC (electrical conductivity, a measure of nutrient strength) and pH of the feed and the runoff; see pH & EC
- Weekly minimum and maximum temperature at the canopy
- Weekly minimum and maximum humidity
- Every pest or disease you see, with the date
Translator’s note: If you run a licensed farm, see Good Agricultural and Collection Practice for Cannabis for the records an audit looks at. You can also post your run in Grow Diaries, so other growers can help you read your notes.
10.2 Data loggers
Low-cost digital temperature and humidity loggers with memory record minimum and maximum readings across the cycle. Review them weekly to find patterns. Typical ones are a temperature drop around 3 a.m. that pushes humidity over your limit, heat spikes at midday with lights on, and humidity rising on watering days. A week of logged data shows these patterns, while single spot readings miss them.
Bluetooth or Wi-Fi loggers that sync to a phone app send instant alerts when a reading leaves your range. They are most useful in rooms nobody watches all day.
11. Putting it together
A grow room works as one system. Cooling capacity sets how much dehumidifier heat the room can carry. Airflow decides whether your sensors show what the canopy really has. Insulation decides how steady the room stays. For the most consistent results, spend on air conditioning, insulation and monitoring before premium genetics or nutrients.
Start with these steps:
- Add up the heat load and size cooling for it plus 20% (Part 1, section 1.3).
- Size dehumidifiers for late flower, and add their heat to the load (section 4.2).
- Put a sensor at canopy height in the middle of the room and log one full week (sections 6.1 and 10.2).
- Put lights and climate equipment on separate circuits with RCD protection (sections 9.1–9.2).
- Test a power cut while the room is empty (section 9.3).
If you want your numbers checked, post your room size, equipment wattages and a week of logger readings in a reply below this topic.
References
- Bugbee, B. (2004). Nutrient management in recirculating hydroponic culture. Acta Horticulturae, 648, 99–112. NUTRIENT MANAGEMENT IN RECIRCULATING HYDROPONIC CULTURE — cited in the original for the VPD table, but the paper covers hydroponic nutrient management (see section 6.3).
- Poorter, H., Knopf, O., Wright, I. J., Temme, A. A., Hogewoning, S. W., Graf, A., Cernusak, L. A., & Pons, T. L. (2022). A meta-analysis of responses of C3 plants to atmospheric CO₂: dose–response curves for 85 traits ranging from the molecular to the whole-plant level. New Phytologist, 233(4), 1560–1596. https://doi.org/10.1111/nph.17802
- ASHRAE. (2021). ASHRAE Handbook: Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers. — listed in the original; not cited for a specific figure.
- Chandra, S., Lata, H., Mehmedic, Z., Khan, I. A., & ElSohly, M. A. (2015). Light dependence of photosynthesis and water vapor exchange characteristics in different high Δ9-THC yielding varieties of Cannabis sativa L. Journal of Applied Research on Medicinal and Aromatic Plants, 2(2), 39–47. Redirecting — listed in the original; not cited for a specific figure.