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

Grow tent too hot? A healthy grow room temperature sits roughly 20-28°C (68-82°F) with the lights on, dropping a few degrees cooler when they're off. UK summers are mild, but a sealed tent under a grow light traps heat fast. Once you push past about 28-30°C (82-86°F), growth slows and plants stress, so that's your line to take action: more extraction, cooler intake air, and smarter lighting hours.

When a summer 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 rough GBP costs and the gear that actually does the job.

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're off.

Different growth stages want slightly different temperatures, and every stage wants a cooler night than day. As a rule, 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 ~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

These ranges are widely recommended across UK and manufacturer horticulture references. Keep a reliable thermometer at canopy height — not at the floor or roof — because that's where your plants actually live. Humidity pairs with temperature to set your real climate, but that's its own topic (see our humidity guide), and the combined temperature-and-humidity sweet spot is VPD, covered in our VPD guide.

Is 32°C (90°F) too hot for a grow tent? For short spells in veg, plants will survive 30°C (86°F), but sustained temperatures above about 30°C (86°F) slow growth, and above 32-34°C (90-93°F) in a small tent you're into genuine heat-stress territory — curling leaves, stalled growth and, in flower, reduced yield. 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.

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

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

• HPS / metal halide: efficient at producing light, but they run hot and radiate a lot of that energy as heat. A 600W HPS in a small tent during a UK summer is a recipe for 32°C-plus (90°F-plus). They need significantly more extraction to stay in range.
• LED: modern LED fixtures convert more of their wattage into usable light and less into waste heat, so the same coverage runs noticeably cooler. Switching from HPS to LED is one of the single biggest heat reductions you can make — and AC Infinity's own ventilation guidance still recommends budgeting up to 50% extra extraction capacity just to handle grow-light heat, so even LEDs are not heat-free.

Other heat sources add up too: ballasts, the room the tent sits in, and warm intake air on a hot day. If your house hits 26°C (79°F), you cannot cool the tent below room temperature with airflow alone — that's the point where a portable AC enters the conversation (more on that below).

How to cool a grow tent: what actually works

Work through these in order. Each step is cheaper and simpler than the one after it, so exhaust everything you can 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 moving air out through a carbon filter (see fan sizing below).
2. Fix where your intake air comes from. Cool, fresh intake beats recycled warm air. Pull intake from the coolest part of the house — a north-facing room, a hallway, or low down near a floor — never from next to a radiator, a sunny window, or the warm exhaust of the tent itself.
3. Use the lung room. The room your tent breathes from (the "lung room") sets the floor temperature of your tent. Keep that room cool and you've cooled the tent for free. This is also where a portable AC or evaporative cooler does the most good.
4. Add a portable AC to the lung room. When the house itself is too warm, a portable air conditioner in the lung room drops your intake temperature, and the tent follows.

Cooling fix	Rough cost (GBP)	Impact
Upsize inline extraction fan + filter	from ~£60-150	High — the foundational fix; removes heat at source
Add an oscillating clip/circulation fan	from ~£20-40	Medium — evens out hot spots, prevents stagnant pockets
Re-route intake to a cooler air source	£0-15 (ducting only)	Medium-High — free if you just move the duct
Switch HPS → LED	from ~£100+	High — large, permanent heat reduction
Cool the lung room (open windows, fan)	£0-30	Medium — sets a lower baseline
Portable AC in the lung room	from ~£200+	High — the last resort when ambient is too hot

How to cool a grow tent without AC? Most UK growers never need air conditioning. Upsize extraction, route intake from your coolest room, run a circulation fan, switch to LED, and run your lights at night (next section). That stack handles a normal British summer. AC only earns its place during a genuine heatwave or in a hot loft.

What size fan do I need for a grow tent?

Start with your tent's volume in both cubic metres and cubic feet (length × width × height, in metres or feet). The cubic-foot figure gives your baseline CFM, while the cubic-metre figure gives the same baseline in m³/h — enough to exchange the tent's air roughly once a minute. Then add capacity for everything that restricts airflow.

Following AC Infinity's published sizing guidance, budget extra headroom on top of the baseline:

• Carbon filter: add ~60%
• Ducting (bends and length): add ~20%, more for sharp 90° bends
• Silencer (if fitted): add ~20%
• Grow-light heat: add up to ~50%

In AC Infinity's worked example, a 1.2 m × 0.9 m × 1.8 m tent (4' × 3' × 6') has a baseline of roughly 2.0 m³ / 72 cubic feet. With a filter, ducting and silencer, that works out at around 166 CFM / 282 m³/h — and factoring in LED heat pushes the recommendation to about 249 CFM / 423 m³/h.

Stage of the calculation	Metric airflow	Imperial airflow
Raw tent volume (1.2 m × 0.9 m × 1.8 m / 4'×3'×6')	122 m³/h	72 CFM
+ carbon filter, ducting, silencer	~282 m³/h	~166 CFM
+ LED grow-light heat	~423 m³/h	~249 CFM

The takeaway: buy a fan rated comfortably above your raw tent volume, ideally one with a controller so you can dial it back when you don't need full power. A speed-controlled or app-controlled inline fan is far more useful in summer than a single-speed unit.

How do you set up ventilation in a grow tent?

A correct summer ventilation setup is simple and consistent:

• Exhaust high. Mount the inline fan and carbon filter at the top of the tent — heat rises, so extract where it collects.
• Intake low. Bring cool fresh air in at the bottom, from the coolest room available, so it rises through the canopy before being pulled out the top.
• Circulate inside. Add an oscillating fan to keep air moving across (not directly blasting) the canopy — this breaks up hot pockets and strengthens stems.
• Keep ducting short and straight. Every bend costs airflow; a 90° bend can cut it by more than half. Use the shortest, smoothest run you can.

Should you run your lights at night in summer?

Yes — flipping your light schedule so the lights run overnight is one of the most effective free summer fixes. Your lights are your main heat source, so running them through the cool small hours (when UK night temperatures often fall to 12-16°C / 54-61°F) and keeping them off during the hot afternoon means the tent's hottest moment lines up with the coolest ambient air.

The trade-off: your dark period now falls during the day, so the room must be genuinely light-tight, and you may be running noisy fans overnight near bedrooms. But for a hobby grower fighting a July heat spike, "lights at night" can shave several degrees off your peak temperature without spending a penny.

When does a portable AC pay off?

A portable air conditioner pays off when your extraction is already maxed out and the tent still can't drop below the ambient room temperature — because on a hot day, airflow can only ever cool the tent down to the temperature of the air you're feeding it. If your lung room is sitting at 28°C (82°F), no fan on earth will get the tent to 24°C (75°F).

Reach for a portable AC when:

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

Site the unit in the lung room, not inside the tent (the exhaust heat and condensate make in-tent AC impractical for most home setups). Cool the air the tent breathes, and the tent follows. For most UK growers this is a heatwave-only tool — useful a handful of weeks a year, not a permanent fixture. If you're running a recirculating or hydroponic system, remember that hot air also warms your reservoir; see our reservoir water temperature guide for keeping roots cool.

FAQ

Is 32°C (90°F) too hot for a grow tent? Yes, for sustained periods. Plants survive brief spikes to 30°C (86°F) in veg, but above about 30°C (86°F) growth slows, and sustained 32-34°C (90-93°F) in a small tent causes heat stress — leaf curl, stalled growth and reduced flowering yield. 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 in the house, add a circulation fan, switch HPS to LED, and run your lights at night. That stack handles a normal UK summer without AC.

Why is my grow tent hotter than the room? Because the grow light dumps heat into a small sealed space faster than your extraction can remove it. Airflow can only cool the tent down to your intake air temperature — never below it — so if the tent is far hotter than the room, your fan is undersized or your ducting is too restrictive.

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 are especially important in flower, where they help protect bud density, aroma and colour.

What fan size do I need for a grow tent? Start with your tent's volume in both cubic metres and cubic feet (L × W × H), then add headroom: ~60% for a carbon filter, ~20% for ducting, and up to ~50% for grow-light heat. Compare fans in both CFM and m³/h, buy above your raw tent volume, and choose a speed-controllable fan.

Recommended kit and further reading

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

• AC Infinity ventilation & climate control — smart inline 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.
• Fans, Filters & Environment — circulation fans, carbon filters and environment kit in one place.
• Ducting & Accessories — keep runs short and smooth so your fan isn't fighting bends.
• Hygrometer Temperature & Humidity Sensor with Probe — an inexpensive max-min hygrometer with a canopy probe, so you catch heat spikes early.
• Grow tents: BudBox tents and Bloomroom tents — both feature oversized vents and ducting ports that make summer extraction easier.

Most relevant collection: if you only click one link, start with Fans, Filters & Environment — that's where the fix usually lives.

Related guides: managing grow tent humidity · understanding VPD · reservoir water temperatures.

All UK orders over £50 ship free (UK mainland, exclusions apply).

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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 in Peterborough. 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.