A grower in Thailand ran 18,000 BTU/h (≈5.3 kW) of air conditioning around the clock in a 250 ft² (≈23 m²) flowering room and could not get relative humidity below 69%. Nothing in the report points to a broken unit or a bad install. The result fits what happens when you size equipment using guidance written for temperate climates. This article separates the two loads an air conditioner and a dehumidifier actually handle and explains why the temperate sizing rules mislead you here. It also sets out what two grower forum threads report at the budget and commercial ends, with the warnings attached to each.
1. The report that should recalibrate your budget
This comes from a grower planning a flowering room in Thailand, posting on Rollitup in 2022 [1]: a 250 ft² (≈23 m²) room with an 18,000 BTU/h (≈5.3 kW, 1.5 refrigeration tons) split air conditioner running 24/7 could not be brought below 69% relative humidity. It is one anonymous forum report, not a measurement, but it is a useful one.
One member of that thread runs a Quest 70 in their own flowering room and recommended it. They reported that it removes about 6 US gallons (≈23 L) a day, and also said their room still reaches about 69% on rainy days. Other members advised deliberately over-specifying the dehumidifier. One member, who runs a different unit (a Trotec), warned about a second-order effect: the dehumidifier itself added about 2 °C to the room.
Compare that with the flowering-stage target in a 2024 review of greenhouse cannabis production by Buirs and Punja [3]: 23–28 °C, 50–70% RH, and a vapour pressure deficit (VPD) of about 1.13–1.4 kPa. The temperature and RH ranges are wider than the VPD range. 23 °C at 70% RH gives about 0.84 kPa, and 28 °C at 50% RH gives about 1.9 kPa, so the VPD figure is the tighter target. The thread does not give the room temperature. At 25–26 °C, 69% RH works out to roughly 1.0 kPa (air VPD, assuming leaf and air temperature are equal). That is inside the RH band but below the VPD band, and the room only got there with the A/C running continuously and nothing left in reserve.
2. Two loads, and why conflating them is the whole mistake
An air conditioner does two jobs at once, and the balance between them is what changes when you move to the tropics.
Sensible load is the heat you can measure with a thermometer: light fixtures, ballast and driver losses, motors, bodies, and heat conducted through the walls and roof. Removing it lowers the temperature.
Latent load is the water vapour: plant transpiration, evaporation from irrigated surfaces, and — the part temperate guides quietly ignore — the moisture arriving in every cubic metre of make-up air.
In a temperate grow room, most of the latent load is transpiration, and a correctly sized air conditioner removes a good share of it as condensate while it cools. That is why so much Western guidance implies that an air conditioner sized for your lights will “mostly handle” humidity, with a dehumidifier as a top-up.
In Thailand the make-up air term dominates, and it does not scale with your lights at all. Outside air at 30 °C and 85% RH holds a great deal of water. Every litre of air you bring in for CO₂ replacement, for negative pressure, or through leaks arrives loaded with moisture, and the machine has to dry it. Exchanging air does not get rid of that moisture.
This explains the failure in the report above. The air conditioner was sized for the sensible load and never had the latent capacity to reach the target. So it ran continuously at a high sensible heat ratio, and the room sat at 69%. This is our reading of the physics. The thread itself reports only that the humidity would not go lower.
3. Why the temperate rules of thumb fail here — specifically
“Air exchange fixes humidity.” True where intake air is drier than room air. In the Thai wet season the intake air is frequently more humid than the room. Air exchange then imports the problem. For the same reason, ventilation-based drying does not work here, as the companion article on drying at high ambient humidity explains.
“Size the air conditioner to the lights.” This gets sensible load approximately right and latent load entirely wrong. It is why growers end up at 69% with a working unit that is correctly sized for their lights.
“A pints-per-day (or litres-per-day) rating tells you what you need.” Dehumidifier capacity ratings are measured at a stated test condition, and a unit’s real output in your room can differ substantially from the label. The manufacturer’s own figures for the Quest 70 show this: 70 pints/day (≈33 L/day) at 80 °F/60% RH (≈27 °C), but only 43 pints/day (≈20 L/day) at 75 °F/50% RH (≈24 °C) [4]. Use the label to compare units, not as a specification for your room.
“The dehumidifier is a humidity device, not a heat device.” A refrigerant dehumidifier is a net heater. It returns the latent heat of condensation plus its own compressor work into the room. The ~2 °C one grower reported is not a defect; it is how the machine works. Your air conditioner has to remove that heat, which raises the sensible load. That leads straight to the problem described in the article on what 31 °C does to cannabinoids.
The two systems are one system. Sizing them separately is how rooms end up with the humidity solved and the temperature ruined, or the reverse.
4. Equipment growers here report using
These are two ends of the market, each from a single report in a 2022 grower forum thread. They are not tested data, but they don’t come from vendor pages either.
| Budget | Commercial | |
|---|---|---|
| Source | One reply in an ASEANNow thread on drying harvested cannabis [2] | One member’s recommendation in the Rollitup thread [1] |
| Units named | Xiaomi (Widetech-made), 18 L/day | Quest 70 |
| Price reported | about 4,700 THB on Lazada (2022) | Not given here: the model is listed as discontinued, so check current local availability and pricing |
| Rated capacity | 18 L/day (~38 US pints) | 70 pints/day (≈33 L/day) at 80 °F/60% RH; 43 pints/day (≈20 L/day) at 75 °F/50% RH [4] |
| Reported performance | Room from 55–60% → 40–45% | About 6 US gal/day (≈23 L/day) removed in the owner’s flowering room, which still reached about 69% on rainy days |
| Power | Not reported | 580 W on the manufacturer’s page [4] (older listings give 680 W); one owner reported about 480 W on average |
| Voltage | Local (220 V) | The manufacturer lists 110–120 V/60 Hz only [4]. A 220 V/50 Hz version was mentioned in the thread, but we could not confirm it from the manufacturer |
| Warning attached | Lazada quality is a gamble; the buyer also said the unit’s Wi-Fi never worked | Over-specify deliberately |
Read the budget performance figure carefully. “55–60% down to 40–45%” is a real and useful result, but it comes from a single reply in a thread about drying harvested cannabis. That points to a drying space, not a flowering room full of transpiring plants under lights, and the post gives no further detail about the room. The starting point of 55–60% also means the room was already conditioned. It is not a claim that an 18 L/day unit takes a flowering room from 85% to 45%. That distinction is where budget planning usually goes wrong.
5. A sizing method that does not pretend to be a calculation
We are not going to publish a BTU formula. Doing it honestly requires your make-up air rate, your envelope tightness, your canopy area and your irrigation volume. A formula that ignores those is the same rule of thumb that produced 69% RH. What follows is a method for arriving at your own numbers.
Step 1 — measure your envelope before buying anything. Seal the room, run nothing, and log temperature and RH for 24 hours against outdoor conditions. How fast the room drifts toward ambient is a rough proxy for how leaky your envelope is, and it is the single biggest unknown in tropical sizing. The 24-hour drift test captures heat and moisture gained through the walls and roof (and the room’s thermal mass) as well as air leakage, so treat the result as a combined envelope figure rather than a pure leak rate. A room that reaches ambient in two hours will never hold a setpoint regardless of what you install.
Step 2 — treat make-up air as a deliberate specification, not a leftover. Decide how much outside air you actually need and why. Every cubic metre is a latent cost you pay continuously. Sealing the room and recirculating, with CO₂ supplied rather than ventilated, turns that continuous load into a much smaller one. The price is that your cooling has to handle all the sensible heat with no air exchange at all.
Step 3 — size the dehumidifier from crop water, not from room volume. Your plants transpire the irrigation water you supply, minus runoff. Your daily irrigation volume (in litres), less runoff, is a defensible first estimate of the crop’s daily latent load in L/day. Add your make-up air load on top. This is the one part of tropical sizing that has an honest arithmetic basis.
Step 4 — add the dehumidifier’s own heat to the air conditioner’s sensible load. One grower reported roughly 2 °C in a small room. Size the air conditioner after this step, not before.
Step 5 — over-specify the dehumidifier, and control it on VPD. The Rollitup thread advises over-specifying. In this climate an undersized dehumidifier has no fallback, because you cannot vent your way out. Controlling on vapour pressure deficit rather than RH means the controller targets the quantity that actually matters to the plant, which combines temperature and humidity in one number.
Step 6 — accept the electricity, or change the plan. Continuous dehumidification plus continuous cooling in a monsoon climate is a real operating cost. We are not going to quote a cost per gram, because we could not find a single credible figure connecting Thai electricity tariffs to grams of cannabis. No grower thread or Thai-language source we checked publishes one. If that figure decides your project, measure your own kWh over one cycle; do not trust a number from a Western article.
6. Where the honest uncertainty sits
- The 69% report is one room, from one grower’s forum post. It is consistent with the physics and matches the general tenor of the Thailand threads we read, but it is not a measured dataset.
- The budget-unit result is one reply in one drying thread, not a flowering-room measurement.
- We have no measured Thai grow-room load data. We could not find any published energy or moisture balance for a tropical cannabis room.
- Equipment prices and availability move. The commercial unit named here is listed as discontinued, and the budget units carry an explicit reliability warning from the growers who discuss them.
- It is unresolved whether the published 50–70% flowering band is even the right target in this climate. It comes from temperate greenhouse work. The companion article on greenhouse setpoints explains why it may be unreachable here and what to do about that.
7. The short version
An air conditioner sized for your lights will not dehumidify a Thai room to target. Size the dehumidifier from crop water plus make-up air, add its own heat back into the air conditioner’s load, seal the envelope before buying anything, and control on vapour pressure deficit. Then expect to run both continuously in the wet season.
Please post four numbers: your room area (m²), your installed A/C (BTU/h or kW), your dehumidifier capacity (L/day), and the humidity you actually hold in the wet season. There is no published dataset for tropical cannabis rooms. A handful of real configurations posted here, including the ones that did not work, would help the next grower more than any sizing guide, including this one.
Related reading: the article on drying at 85–95% ambient humidity; the article on monsoon humidity and cannabinoid loss; the article on what 31 °C does to cannabinoids; the articles on growroom design and on grow room climate control and vapour pressure deficit.
References
- “Dehumidifier Planning in Humid Thailand (Flowering Room)”, Rollitup forum thread, September 2022. Dehumidifier Planning in Humid Thailand (Flowering Room) | Rollitup
- “Dehumidifier for drying”, ASEAN NOW, Thailand Cannabis Forum thread, October 2022. https://aseannow.com/topic/1275085-dehumidifier-for-drying/
- Buirs L, Punja ZK. Integrated Management of Pathogens and Microbes in Cannabis sativa L. (Cannabis) under Greenhouse Conditions. Plants. 2024;13(6):786. https://doi.org/10.3390/plants13060786
- Quest Climate, Quest 70 product specifications (manufacturer page). Quest 70 industrial dehumidifier | Quest Climate
This article is technical horticultural information for readers operating legally in their jurisdiction. Cannabis cultivation is regulated differently in every country. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis) B.E. 2568. It may be dispensed only against a prescription, it may not be sold or advertised online, and recreational use is not permitted. Cultivation and sale require licensing, and the Cannabis Control Act is still a draft before the House. Nothing here is legal advice.