Grow Tent Too Hot? Summer Temperature Control for UK Grow Rooms

Grow tent too hot? A healthy grow room temperature sits at roughly 20-28°C (68-82°F) with the lights on, a few degrees cooler with them off. Above about 30°C (86°F), growth slows and plants stress, so that's your line to take action: more extraction, cooler intake air, and smarter lighting hours.

UK summers are mild, but a sealed tent under a grow light traps heat fast. When a heatwave hits Peterborough and the rest of the UK, the tent is usually the hottest room in the house. The good news is that most heat problems are fixable with airflow tweaks rather than expensive kit. Below is a UK-specific, by-stage temperature guide and a ranked set of cooling fixes, with the gear that actually does the job.

The lung room is the room your tent breathes from. Its air temperature sets the floor for your tent, because fans can only cool the tent down to the temperature of the air you feed them.

The three fixes that matter most:

• Upsize extraction first, and pull intake air from the coolest room you have.
• Cut heat at source: LED instead of HPS, drivers outside the tent, lights dimmed or raised.
• Run lights overnight in a heatwave, and keep a portable AC for the lung room as the last resort.

What temperature should a grow tent be? (by stage)

A grow tent should sit at roughly 20-28°C (68-82°F) with the lights on, dropping a few degrees cooler when they are off.

Every stage wants a cooler night than day, so aim for a lights-off temperature roughly 2-8°C (4-14°F) below your lights-on figure, depending on stage. Younger plants like it a touch warmer and steadier; flowering plants prefer cooler nights to protect bud quality.

Stage	Day (lights on)	Night (lights off)	Why
Seedling / clone	22-25°C (72-77°F)	20-22°C (68-72°F)	Warm, stable conditions encourage rooting; big swings stall fragile young plants
Vegetative	24-28°C (75-82°F)	20-22°C (68-72°F)	The warmest stage: fast growth tolerates more heat, but keep it under about 28°C (82°F)
Flowering	24-26°C (75-79°F)	18-20°C (64-68°F)	Cooler nights protect bud density, aroma and colour, and reduce heat stress on developing flowers

Keep a reliable thermometer at canopy height, not at the floor or roof, because that's where your plants actually live. Temperature works together with humidity (see our humidity guide), and the combined sweet spot is VPD, covered in our VPD guide.

Treat 30°C (86°F) as your "fix it now" trigger and never let a summer tent sit at 32°C (90°F) for hours. Sustained temperatures above 32-34°C (90-93°F) in a small tent are genuine heat-stress territory.

How can you tell a grow tent is too hot?

Plants show heat stress late, so check the thermometer first. Typical signs:

• Leaf curl or "taco-ing": leaves fold upwards along the midrib, worst nearest the light.
• Bleached or crispy patches: pale or burnt tissue under the light, which can be heat as well as light intensity.
• Foxtailing: flowers stacking into tall, spiky towers late in flower under intense light and heat.
• Stalled growth: slow new growth, or stretched plants with wide gaps between nodes.

Why is my grow tent so hot? LED vs HPS heat

Almost all of your tent's heat comes from the grow light, so the fix starts with understanding how much heat your light dumps into the space.

• HPS / metal halide: they radiate a lot of their energy as heat. A 600W HPS in a small tent in a UK summer is a recipe for 32°C-plus (90°F-plus) and needs far more extraction.
• LED: modern fixtures turn more of their wattage into usable light, so the same coverage runs noticeably cooler. Moving from HPS to LED grow lights is one of the biggest permanent heat reductions you can make, though LEDs still need extra extraction.

Drivers, ballasts and warm intake air add heat too. Move drivers and ballasts outside the tent if you can, and raise or dim the light until the canopy is no longer hot to the touch.

How do you cool down a grow tent (or grow room)?

Work through these in order, cheapest first, before reaching for AC.

1. Upsize your extraction. An undersized exhaust fan is the most common cause of an overheating tent. Get a properly rated inline fan pulling air out through a carbon filter. Good starting points are the Systemair RVK InLine Fan - Sileo and the WhispAIR Mixed Flow Fan, or a ready-matched fan and filter kit.
2. Add a speed controller or thermostat. A controller lets you ramp extraction up as the tent warms and back down at night. A model with a temperature probe, such as the SMS Smart Controller, does it automatically; browse all fan speed controllers.
3. Fix where your intake air comes from. Pull intake from the coolest part of the house, such as a north-facing room, a hallway or low down near a floor. Never draw it from next to a radiator, a sunny window or the tent's own warm exhaust.
4. Replace a worn carbon filter before summer. A loaded filter restricts airflow and quietly cuts your extraction, so check your carbon filters at the start of each warm season. Extracting harder? See keeping the smell contained when you extract more air.
5. Circulate inside the tent. A clip-on fan from the air circulators range, such as the CLOUDRAY S6 Gen 2 clip fan, breaks up hot pockets under the light.
6. Cool the lung room. Open a window, run a fan, or add a portable AC to that room and the tent follows for free.

Cooling fix	Impact	Effort
Upsize inline extraction fan and filter	High: the foundational fix, removes heat at source	Medium: fit a fan, filter and ducting
Speed controller or thermostat	Medium: matches extraction to the heat	Low: plug in and set
Re-route intake to a cooler air source	Medium-High: free if you just move the duct	Low
Switch HPS to LED	High: large, permanent heat reduction	Medium to high: new fixture
Portable AC in the lung room	High: the last resort when ambient is too hot	Medium: kit, power and drainage

How to cool a grow tent without AC? Most UK growers never need air conditioning: the steps above, plus LED and lights at night (below), handle a normal British summer. Fabric pots help too, because the breathable wall lets heat escape (see why fabric pots run cooler). Silicon is one additive that can help plants cope with heat stress, but it never replaces fixing the climate; the root-zone health guide explains what it does. Running CO₂? Read running CO₂ in a hot tent first.

What size fan do I need for a grow tent?

Start with your tent's volume in cubic metres and cubic feet, enough to exchange its air roughly once a minute, then add capacity for the carbon filter, ducting and the light's heat. Buy a fan rated comfortably above your raw tent volume, ideally speed-controlled. Our guide on how to size an extraction fan gives the full method, a worked example and intake sizing.

Set it up simply: exhaust high (heat rises), intake low from the coolest room, and keep ducting short and straight, since every bend costs airflow. A hygrometer such as the Hygrometer Temperature & Humidity Sensor with Probe reads temperature and humidity at canopy height, and a unit that states min/max memory records overnight spikes you would otherwise miss. More in our monitors and controllers guide.

Should you run your lights at night in summer?

Yes. Running the lights overnight is one of the most effective free summer fixes. The light is your main heat source, so running it through the cool small hours (UK nights often fall to 12-16°C / 54-61°F) and keeping it off in the hot afternoon puts the tent's hottest moment alongside the coolest air.

The trade-off: your dark period now falls in daytime, so the room must be genuinely light-tight.

When does a portable AC pay off?

A portable air conditioner pays off when extraction is maxed out and the tent still cannot drop below the room temperature. If your lung room sits at 28°C (82°F), no fan will get the tent to 24°C (75°F).

Reach for a portable AC when:

• Your house or loft regularly exceeds about 28°C (82°F) in summer.
• You have already upsized extraction, sorted intake and run lights at night.
• You are flowering and need tight night-time temperatures to protect quality.

Site the unit in the lung room, not inside the tent, because exhaust heat and condensate make in-tent AC impractical for most home setups. For most UK growers this is a heatwave-only tool. Hot air also warms a hydroponic reservoir, so keep it cool in a hot spell, and spider mites thrive in summer heat, so check leaf undersides in any hot spell.

What changes in autumn and winter?

Cool nights flip the problem. With the lights off, a fan left at full speed can pull the tent below your 18-20°C (64-68°F) flowering night target, so turn it down on the controller. Our winter grow room tips cover heating and condensation, and the grow room setup checklist pulls the whole climate plan together.

FAQ

Is 32°C (90°F) too hot for a grow tent? Yes, if it is sustained. Plants survive brief spikes to 30°C (86°F) in veg, but above that growth slows, and 32-34°C (90-93°F) in a small tent causes heat stress. Treat 30°C (86°F) as your action threshold.

How do I cool a grow tent without air conditioning? Upsize your extraction fan, route intake air from the coolest room, add a controller and a circulation fan, switch HPS to LED and run your lights at night. That combination handles a normal UK summer.

Why is my grow tent hotter than the room? The grow light dumps heat into a small sealed space faster than extraction removes it. Airflow can only cool the tent to your intake air temperature, so a tent far hotter than the room means an undersized fan or restrictive ducting.

Should the grow tent be cooler at night? Yes. Aim for a lights-off temperature roughly 2-8°C (4-14°F) below your lights-on figure, depending on stage. Cooler nights matter most in flower, where they help protect bud density, aroma and colour.

Recommended kit and further reading

Here's the gear we'd point a UK grower to when summer heat strikes:

• Fans, Filters and Environment: circulation fans, carbon filters and environment kit in one place.
• AC Infinity ventilation and climate control: fans and controllers that let you dial extraction up in a heatwave and back down at night.
• Intake & Outtake Fans: properly rated inline fans, the foundation of any heat fix.
• Ducting & Accessories: keep runs short and smooth so your fan isn't fighting bends.
• Grow tents: BudBox tents and the Trojan Gold Series: oversized vents and ducting ports make summer extraction easier.

Most relevant collection: if you only click one link, start with Fans, Filters and Environment, where the fix usually lives.

Related guides: managing grow tent humidity · understanding VPD · sizing grow room ventilation.

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Why trust The Grow Superstore

We've been helping UK indoor growers since 2009, both online and from our physical store at 6 Wulfric Square, Bretton, Peterborough PE3 8RF. Our team sets up and runs grow tents every day, so the advice above comes from real summers spent fighting real heat, not theory. Pop into the store for a free in-store consultation, or get in touch and we'll help you size the right fan and ventilation kit for your space and your climate.