A grow room stays on its climate targets only if the equipment is sized and placed correctly. This guide covers dehumidifiers, air conditioning, fans and carbon filters, and climate controllers. It shows how to keep bud rot and powdery mildew out of a flowering room. It ends with a summary table and a daily checklist.
Two terms appear often. Vapour pressure deficit (VPD) combines temperature and relative humidity (RH) into one number, given in kilopascals (kPa). The higher the VPD, the faster plants lose water through their leaves. Dew point is the temperature at which water in the air condenses. Any leaf or bud colder than the dew point gets wet.
This is Part 2. Part 1: Grow Room Climate Control: VPD, Temperature, Humidity — Guide 2026
Dehumidifiers
Three types
| Type | How it works | Best for | Water removed per kWh | Working range |
|---|---|---|---|---|
| Refrigerant (compressor) | Pulls air over cold coils so water condenses | Most grow rooms, 18–30 °C | 2.5–4.0 L | 18–35 °C |
| Desiccant (rotary wheel) | A drying material absorbs water, then is heated to dry it out again | Cold rooms below 18 °C, drying rooms | 1.5–2.5 L | 1–40 °C |
| Whole-house / ducted | One central unit with ducts | Large commercial facilities | 3.0–5.0 L | 15–35 °C |
How much water you need to remove
Plants release through transpiration about 97% of the water their roots take up. So:
Daily water removal (L) = daily irrigation volume (L) × 0.97
A room of 20 plants, each given 4 L a day, needs about 77.6 L of water removed per day. At 2.5–4.0 L per kWh, a refrigerant dehumidifier uses about 19–31 kWh a day to remove that water.
The formula assumes the plants take up all the water you give. Water that drains out of the room does not add to the load. Runoff left standing in trays inside the room evaporates, so it does add to the load.
Translator’s note: the litres per day printed on a dehumidifier box are measured at the test conditions in its specification sheet. At your room’s temperature and RH, it may remove less. Read the test conditions and buy with spare capacity. Dehumidifier and Air-Conditioner Sizing for a Thai Flowering Room shows why one air conditioner cannot hold flowering RH in a Thai room.
What to do now: measure the water you give the room in one day. Include runoff that stays inside the room. Multiply by 0.97 and compare the result with the rated capacity of your dehumidifiers. Then measure the water actually collected over 24 hours. This shows whether your units keep up at your room’s real conditions.
Air Conditioning and Cooling Capacity
The cooling system must remove the heat from lights, equipment and the building. Heat is counted in BTU (British thermal units), and air conditioner capacity in BTU per hour. In Thailand, air conditioners are commonly sold by their BTU size. The full heating, ventilation and air-conditioning system is often called HVAC. Room layout is covered in Growroom Design: Space, Airflow, HVAC.
The heat-load formula
Total BTU = (lamp watts × BTU multiplier) + environmental load + dehumidifier heat
The environmental load covers heat coming in through poorly insulated walls and roof. It also depends on the gap between outside and inside temperature, and on the number of air exchanges. As a rule of thumb, add 20–30% on top of the lamp heat. A refrigerant dehumidifier also releases heat into the room. Count at least its power draw in watts × 3.41.
Heat per 1,000 W by light type, as given in the original
| Light type | BTU per 1,000 W | Note in the original |
|---|---|---|
| HPS (high-pressure sodium) | 3,410 | Nearly 100% of input becomes heat |
| LED (full spectrum) | 2,046–2,730 | 60–80% heat, the rest is light |
| CMH (ceramic metal halide) | 3,100 | Slightly less than HPS |
| Fluorescent T5 | 3,200 | High heat relative to light output |
Correction for sizing: the LED row understates the cooling load. Plants, walls and floor absorb the light from any lamp, and it turns into heat inside the room. Plants store only a small share of it. So for cooling, count every light type at about 3,410 BTU per hour for each 1,000 W drawn from the wall. LEDs heat the room less than HPS only because they need fewer watts for the same light.
Example: one LED fixture drawing 1,000 W from the wall gives 3,410 BTU per hour. Adding 20–30% gives about 4,100–4,400 BTU per hour, plus the dehumidifier heat.
Wall-mounted split unit versus a central system
A mini-split is a wall-mounted split air conditioner. SEER (seasonal energy efficiency ratio) shows how much cooling you get per unit of electricity. A higher SEER means a lower power bill.
| Feature | Mini-split | Central system |
|---|---|---|
| Installation | Wall-mounted, simple | Ducted, needs professional installation |
| Cost per ton (US prices) | USD 1,500–3,000 | USD 3,000–6,000 |
| Efficiency (SEER) | 18–25 | 14–20 |
| Dehumidification | Limited, needs a dehumidifier as well | Better with a variable-speed air handler |
| Zone control | Each indoor head is a zone | Needs zone dampers |
| Expansion | Add heads as needed | Full redesign to expand |
| Best for | Home grows, small commercial | Large commercial, several rooms |
One “ton” of cooling is 12,000 BTU per hour, about 3.5 kW.
Translator’s note: the prices above are US prices from the original. Installed prices in Thailand are different, so get quotes from several local installers. The 20–30% rule of thumb may be too low for a room under a sun-heated roof or with walls facing the afternoon sun. Insulating the ceiling and walls reduces this load. If the air conditioner runs non-stop and the room is still above target late in the light period, the cooling is too small.
What to do now: add up the wall watts of all lights, using a plug-in power meter or the fixture’s specification. Multiply by 3.41, add 20–30%, then add the dehumidifier heat. Compare the total with the BTU rating of your air conditioners. This tells you before the hot season whether the room will overheat.
Airflow, Carbon Filters and Negative Pressure
Fan size
The original uses CFM (cubic feet per minute), a US airflow unit. One CFM is about 1.7 m³ per hour.
CFM = room volume (ft³) / exchange time (minutes)
Metric: airflow (m³/h) = room volume (m³) × 60 / exchange time (minutes)
The standard target is a full air exchange every 1–3 minutes. For rooms with added CO₂, lower the exchange rate and recirculate the air inside the room. For example, a 1.2 × 1.2 × 2 m tent holds 2.88 m³. One exchange every 2 minutes needs about 86 m³/h (about 51 CFM).
Carbon filter sizing
| Filter size (inches, diameter × length) | Rated airflow | Room volume | Exchange |
|---|---|---|---|
| 4 × 12 | 200 CFM (≈340 m³/h) | up to 400 ft³ (≈11.3 m³) | every 2 min |
| 6 × 16 | 400 CFM (≈680 m³/h) | up to 800 ft³ (≈22.7 m³) | every 2 min |
| 6 × 24 | 550 CFM (≈934 m³/h) | up to 1,100 ft³ (≈31.1 m³) | every 2 min |
| 8 × 24 | 750 CFM (≈1,274 m³/h) | up to 1,500 ft³ (≈42.5 m³) | every 2 min |
| 8 × 39 | 1,000 CFM (≈1,699 m³/h) | up to 2,000 ft³ (≈56.6 m³) | every 2 min |
| 10 × 39 | 1,400 CFM (≈2,379 m³/h) | up to 2,800 ft³ (≈79.3 m³) | every 2 min |
| 12 × 39 | 1,700+ CFM (≈2,888+ m³/h) | up to 3,400+ ft³ (≈96.3+ m³) | every 2 min |
Negative pressure
Set the exhaust fan to move 10–15% more air than the intake. The air pressure inside then stays slightly lower than outside. Outside air leaks in, not out, and all air leaving the room passes through the carbon filter. This stops odour leaks. To check, look for tent walls that bend slightly inward. Or hold a tissue near an intake vent and see whether it is pulled in.
Translator’s note: in the rainy season, every air exchange with outside air brings in hot, humid air. The air conditioner and dehumidifier must then treat that air. A tent placed inside an air-conditioned room takes in cooler, drier room air. If you seal a room and rely on air conditioning and dehumidifiers, you must still control odour. Run the room air through a carbon filter, and check for smell outside the door.
What to do now: measure your room and calculate the airflow needed for one exchange every 1–3 minutes. Compare it with your exhaust fan and carbon filter ratings. Then do the tissue test at the intake. This confirms that odour leaves only through the filter.
Preventing Bud Rot and Powdery Mildew
| Botrytis (bud rot) | Powdery mildew (PM) | |
|---|---|---|
| Pathogen | Botrytis cinerea (fungus) | Order Erysiphales (fungi) |
| Moisture need | High: free water, RH above 60% | Moderate: thrives at 40–70% RH |
| RH threshold | Above 60%, critical above 70% | 40–70%; spore release peaks at low RH |
| Best temperature | 15–25 °C | 20–30 °C |
| Where it starts | Inside dense buds | On leaf surfaces |
| What you see | Brown or grey rot inside the bud | White powder on top of leaves |
| Airflow | Helps | Helps |
| Most dangerous period | Late flower, dense colas | Veg to mid-flower |
Botrytis is covered in detail in its own topic.
Two diseases with opposite needs
Pushing RH very low to stop Botrytis can favour powdery mildew. Powdery mildew thrives in drier air (40–70% RH) and spreads its spores more easily when humidity is low. The safe zone for both diseases is 45–50% RH in mid to late flower, with strong airflow of 0.5–1.0 m/s at the canopy.
RH targets by flower week
| Flower week | Target RH (%) | VPD target (kPa) | Main risk |
|---|---|---|---|
| Weeks 1–3 | 55–60 | 1.3–1.5 | Powdery mildew |
| Weeks 4–5 | 50–55 | 1.5–1.6 | Powdery mildew and early Botrytis |
| Weeks 6–7 | 45–50 | 1.5–1.6 | Botrytis in dense buds |
| Week 8 on | 40–45 | 1.5–1.6 | Botrytis, mould in the harvest |
These VPD targets are air VPD, like all the stage tables in this guide.
Keep leaf temperature at least 2 °C above the dew point at all times. Below the dew point, water condenses and the disease risk becomes critical. Part 1 has the dew-point table and explains why the hour after lights-off is the riskiest time.
Translator’s note: in the Thai rainy season, outside air is very humid, and power cuts can stop dehumidifiers overnight. So Botrytis is the main late-flower risk in most Thai rooms. If you need a spray, use only products registered with the Department of Agriculture (กรมวิชาการเกษตร), and follow the label. See What You May Legally Spray Under Thai GACP.
What to do now: measure airflow at several points across the canopy with a handheld anemometer (a small wind meter). Fix any point below 0.5 m/s. Set the RH target for your current flower week. Once a week, check the upper and lower sides of the leaves, and open a few dense buds. You will catch either disease while it is still small.
Climate Controllers
Feature comparison from the original
| Feature | Pulse Pro | Trolmaster Hydro-X | AC Infinity 69 Pro | Inkbird ITC-608T |
|---|---|---|---|---|
| Type | Monitor + cloud | Full controller | Controller + monitor | Basic controller |
| VPD | Display | Control | Control | No |
| CO₂ monitoring | Yes | Yes (add-on sensor) | Yes (add-on) | No |
| PPFD monitoring | Yes (add-on) | No | No | No |
| Device control | Via API, no relay | 0–10 V, relay, PWM | 0–10 V, relay | Relay only |
| Mobile app | Yes | Yes | Yes | No |
| Cloud data logging | Yes, unlimited | Yes, limited | Yes | No |
PPFD is the light intensity reaching the canopy. A relay is an on/off switch for a device. A 0–10 V signal sets light dimming or fan speed. PWM is a pulse signal for fan speed. An API lets other software read the data and send commands.
What to look for at each scale
| Scale | What to look for | Why |
|---|---|---|
| Hobby (1–2 lights) | Basic controller with temperature and humidity relays | Affordable, enough control |
| Serious home grow with CO₂ | VPD control and a CO₂ sensor input | Controls humidity and CO₂ together |
| Small commercial (10–20 lights) | Multi-zone, 0–10 V dimming, CO₂ control | Several rooms from one system |
| Large commercial (50+ lights) | Data logging linked to building automation | Cloud monitoring, API links, data analysis |
Translator’s note: this forum does not recommend any controller brand. The table above is a feature list from the original, not a test. Before you buy, ask the official Thai distributor some questions. Is the model sold in Thailand with a local warranty? Are spare sensors available? Does it run on Thai mains power without an adapter? Does it keep controlling, and keep its data, when the internet drops? Does it restart with its settings after a power cut? For other growers’ experience, ask in the Equipment and Supplies section.
Where to place sensors
A controller acts only on what its sensor measures. Bad placement is one of the most common reasons why the display shows a good VPD while the plants show stress.
- At canopy height. Mount the temperature and RH sensor at the top of the canopy, and move it up as plants grow. Near the floor, wet substrate makes readings too humid. Close to the light, heat makes them too dry.
- Out of direct light. A sensor in direct light reads warmer than the air. Use a shaded housing or a radiation shield.
- Away from equipment outlets. Keep the sensor out of the airflow from the dehumidifier, air conditioner, intake or humidifier. There it measures the machine’s output, not the room.
- More than one sensor in larger rooms. Temperature and humidity form layers from floor to ceiling and differ from wall to wall. In rooms with several lights, log at least two canopy positions and compare them.
- Check calibration. RH sensors drift over time. Check them now and then against a reference, for example a two-point salt test. In this test the sensor sits in a sealed box with a damp salt that holds a known RH. Saturated sodium chloride (table salt) holds about 75% RH, and magnesium chloride about 33% RH, at room temperature. Replace sensors that read far off.
- Leaf temperature. For leaf VPD, point an infrared thermometer at the upper side of leaves in the middle to upper canopy, under steady light. Do not measure right after misting or watering.
What to do now: hang a second, recently checked sensor at canopy height next to your controller’s sensor for three days. Compare the two readings. This shows whether the controller is working from true canopy conditions.
Summary and Daily Checklist
All targets by stage
| Parameter | Clone | Veg | Early flower | Late flower | Ripening |
|---|---|---|---|---|---|
| Day temp (°C) | 24–26 | 25–28 | 24–27 | 22–26 | 20–24 |
| Night temp (°C) | 22–24 | 20–24 | 20–23 | 18–22 | 16–20 |
| DIF (°C) | +2 to +4 | +4 to +6 | +4 to +6 | +4 to +6 | +4 to +8 |
| RH (%) | 70–80 | 55–70 | 50–60 | 40–55 | 35–45 |
| VPD (kPa) | 0.6–1.0 | 1.0–1.6 | 1.3–1.6 | 1.3–1.8 | 1.5–1.7 |
| CO₂ (ppm) | 400–600 | 800–1200 | 1000–1400 | 800–1200 | 400 (outside air) |
| Airflow (m/s) | 0.3–0.5 | 0.5–1.0 | 0.5–1.0 | 0.8–1.2 | 0.5–1.0 |
DIF is the day temperature minus the night temperature. The ripening night range of 16–20 °C is the standard. Nights of 14–16 °C apply only to the optional cool-night finish in Part 1, for cultivars that carry colour genes.
Daily climate checklist
- Check that the day–night temperature difference (DIF) matches the target for the current stage. This keeps plant height and the finish on plan.
- Check VPD with the canopy-level sensor, not a wall sensor. The wall sensor shows the room, not the plants.
- Look at the RH record for the last 24 hours. During flower, there should be no peaks above 65%. A peak shows when the mould risk rose, usually after lights-off.
- Check dehumidifier drain lines and tanks for blockages. A blocked drain can stop the unit or flood the floor.
- Check negative pressure by eye or with a differential pressure gauge (a gauge that compares inside and outside air pressure). This confirms that air leaves only through the carbon filter.
- Check that the oscillating fans are running and that airflow at the canopy is 0.5–1.0 m/s. Moving air keeps leaf and bud surfaces dry.
- Check CO₂ levels, and make sure the controller adds CO₂ only while the lights are on. Plants use CO₂ only in light, so CO₂ added in the dark is wasted.
- Look under the leaves for early powdery mildew (white spots). Small spots are easier to deal with than a spread infection.
- Record all readings in a grow diary or a digital monitoring system. The record shows trends and helps you trace the cause of a problem.
Translator’s note: in Thailand, add two checks. After any power cut, check that every device restarted with the right settings. In the rainy season, read the overnight RH record every morning during late flower.
References
- Backer, R. et al. (2019). “Closing the Yield Gap for Cannabis: A Meta-Analysis of Factors Determining Cannabis Yield.” Frontiers in Plant Science, 10, 495.
- Caplan, D., Stemeroff, J., Dixon, M. and Zheng, Y. (2018). “Vegetative propagation of cannabis by stem cuttings: effects of leaf number, cutting position, rooting hormone, and leaf tip removal.” Canadian Journal of Plant Science, 98(5), 1126–1132.
- Chandra, S. et al. (2011). “Temperature response of photosynthesis in different drug and fiber varieties of Cannabis sativa L.” Physiology and Molecular Biology of Plants.
- Rodriguez-Morrison, V., Llewellyn, D. and Zheng, Y. (2021). “Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment.” Frontiers in Plant Science, 12, 646020.
- Eichhorn Bilodeau, S. et al. (2019). “An Update on Plant Photobiology and Implications for Cannabis Production.” Frontiers in Plant Science, 10, 296.
- Jin, D. et al. (2020). “Secondary Metabolites Profiled in Cannabis Inflorescences, Leaves, Stem Barks, and Roots for Medicinal Purposes.” Scientific Reports.
- Punja, Z.K. (2021). “Emerging diseases of Cannabis sativa and sustainable management.” Pest Management Science.
- Saloner, A. and Bernstein, N. (2021). “Nitrogen supply affects cannabinoid and terpenoid profile in medical cannabis (Cannabis sativa L.).” Industrial Crops and Products, 167, 113516.
- ASHRAE (2023). ASHRAE Handbook — HVAC Applications, chapter on environmental control for animals and plants.
Legal Note
This article is for education only and is not legal advice. In Thailand, cannabis flower (ช่อดอกกัญชา) is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The Notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and has been in force since 26 June 2025 (explained here). Flower may be sold only with a PT 33 (ภ.ท.33) prescription, for at most 30 days. Advertising and online sale of flower are prohibited. The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569 came into force on 30 April 2026 and tightens licence criteria for cannabis flower (see the 2026 regulations). Check every licence requirement with the Thai authorities before you start growing. The draft Cannabis Control Act is not yet law. This forum does not help anyone buy or sell controlled substances.
Have a question about sizing a dehumidifier or air conditioner for your room? Post it below this topic. Include the room size, light wattage, plant count, daily water use and your night RH readings. Other growers can then work through the numbers with you.