Short answer
Lower grow-tent temperature by moving the light's heat out faster than it builds: more exhaust from a cooler room, drivers outside the tent, lights dimmed 10–20 %, and lights on during the coolest hours. Target 24–28 °C (75–82 °F) in veg and 22–26 °C (72–79 °F) in flower; above about 30 °C (86 °F) without CO₂, photosynthesis falls [1]. Supported.
Why
Nearly every watt the light draws ends up as heat in the tent. The exhaust fan can only replace hot air with room air, so the room temperature sets your floor.
In cannabis, net photosynthesis and water-use efficiency drop above about 30 °C leaf temperature unless light and CO₂ are both high [1]. CO₂ enrichment shifts the optimum warmer [2], which is why commercial enriched rooms run hotter than a tent should.
Leaf temperature is what counts. Under LED, well-watered leaves usually run a little below air; under HPS they run warmer [3].
Hot rooms also warm the root zone. Above 24 °C (75 °F) in DWC, root-rot risk rises sharply [4].
The numbers
| Item | Target |
|---|---|
| Veg, lights on | 24–28 °C (75–82 °F) |
| Flower, lights on | 22–26 °C (72–79 °F) |
| Late flower | 20–24 °C (68–75 °F) |
| Hard limit, no CO₂ | 30–32 °C (86–90 °F) sustained |
| Exhaust, lit tent | 1 full volume every 1–3 minutes, +25–50 % for filter and ducting |
| Reservoir | 18–22 °C (64–72 °F) |
What to do
- Measure at canopy height with a shaded probe, plus leaf temperature with an IR thermometer.
- Move LED drivers or ballasts outside the tent.
- Raise exhaust speed; pull intake air from the coolest place you can.
- Run the lights at night in summer.
- Dim or raise the light 10–20 % if leaves read above about 28 °C (82 °F).
- If the room itself is hot, cool the room (air conditioning) or run less light.
Common mistakes
- Copying a 28–30 °C CO₂-room setpoint into a tent without CO₂.
- A bigger fan pulling 30 °C air from a hot attic.
- Ignoring the reservoir in summer.
- Plugging an air conditioner into the circuit already carrying the lights.
Related
- Full article: Temperature by stage
- Should I run my room warmer with LEDs than with HPS?
- Do I need an intake fan?
- Heat and light stress · Airflow
- Terms: heat stress
