Grow Room Temperature & Cooling Guide Australia
Grow Room Temperature & Cooling Guide Australia
Grow-room temperature control starts with a simple heat balance: heat entering or being generated inside the room must ultimately be removed somewhere else.
This guide explains sensible heat load, ventilation cooling capacity, intake temperature limits, grow-room air conditioning, dehumidifier heat and when an exhaust fan should give way to active cooling.
If cooler intake air is available, ventilation can remove substantial sensible heat. Use the Green Genius fan calculator to calculate the airflow requirement before designing the rest of the system.
Understanding Grow Room Heat Load
Indoor growing spaces contain several sources of sensible heat.
Electrical lighting is commonly the largest sensible heat source in an indoor plant room.
The fixture, driver and other electrical components ultimately release energy into the room unless part of that equipment is located outside the controlled space.
Circulation fans, pumps, controllers and other electrical equipment also add heat when their electrical consumption occurs inside the controlled space.
Walls, roofs, ceilings, doors, windows, solar exposure and warm adjacent spaces can add additional sensible heat.
Electrical Load as a Practical Starting Point
For preliminary planning, the electrical power consumed by equipment located inside the controlled space provides a useful starting estimate of internal sensible heat.
For example:
A complete HVAC calculation can also include:
- roof and wall heat gain;
- solar load;
- outside-air ventilation;
- infiltration;
- people;
- dehumidification equipment;
- and latent moisture load.
Do LED Drivers Add Heat to the Grow Room?
If the driver is physically located inside the controlled room, its electrical losses ultimately contribute heat to that room.
If the driver is genuinely located outside the controlled thermal boundary, its own heat losses should not automatically be added to the room's internal equipment load.
Sensible Heat and Plant Transpiration
Plants also change the way energy exists inside the room.
During transpiration, water evaporates from plant surfaces. Evaporation converts sensible energy from the surroundings into latent energy carried by water vapour.
Transpiration converts part of the existing sensible energy into latent energy. Professional indoor-plant HVAC sizing reconciles both sides of the energy balance so the same energy is not counted twice.
The practical consequence is that grow-room HVAC equipment must deal with both:
- sensible temperature load;
- and latent moisture load.
How Much Heat Can Ventilation Remove?
When intake air is cooler than the room, ventilation can carry sensible heat out of the space.
A useful SI approximation is:
Rearranged for airflow:
Why Intake Temperature Matters So Much
Consider 500 m³/h of delivered ventilation airflow with a room target of 28°C.
| Intake Temperature | Temperature Difference | Approx. Sensible Cooling Capacity |
|---|---|---|
| 20°C | 8°C | 1367 W |
| 22°C | 6°C | 1025 W |
| 24°C | 4°C | 683 W |
| 26°C | 2°C | 342 W |
| 28°C | 0°C | 0 W |
| 30°C | -2°C | Ventilation adds sensible heat relative to a 28°C target |
Ventilation Cooling Capacity Calculator
Estimate how much sensible heat a known delivered airflow can remove at the entered intake and target temperatures.
This calculator addresses sensible heat only. It does not calculate plant moisture load, latent cooling, building cooling load or final air-conditioner capacity.
The Intake-Air Temperature Limit
Ventilation cooling depends on a temperature difference.
If the incoming air is the same temperature as the desired room temperature:
then the incoming air has no sensible cooling headroom relative to that target.
If the incoming air is hotter:
then outside-air ventilation adds sensible heat relative to the target rather than removing it.
Once the intake air is too warm, the available options include:
- using a cooler intake source;
- conditioning the intake air;
- reducing internal heat load;
- changing the operating schedule;
- or using active refrigeration.
Measure the Actual Intake Air
Weather data does not necessarily describe the air entering the grow room.
For example, outdoor air might be relatively mild while the intake is drawing from:
- a sun-heated garage;
- a warehouse roof space;
- a closed spare room;
- an equipment room;
- or another poorly ventilated internal space.
The Room Around a Grow Tent Is Part of the Cooling System
A grow tent may successfully exhaust warm air from inside the enclosure but still create a heat problem if that air is dumped into the same small room that supplies the tent intake.
The heat must eventually cross a larger system boundary.
The surrounding room needs its own path for heat rejection or active cooling.
Running Lights During Cooler Hours
Where operating schedules allow, running high heat-load equipment during cooler hours can increase the temperature difference available for ventilation.
For example:
This can reduce peak ventilation or refrigeration demand.
Does Insulation Help Cool a Grow Room?
Insulation can reduce unwanted heat transfer through the room envelope.
It is particularly useful where the room is exposed to:
- hot roofs;
- sun-heated walls;
- warm adjacent rooms;
- or conditioned indoor air that needs to be retained.
A well-insulated room containing 1500 W of internal electrical load still needs a way to reject that energy.
When Should You Add Air Conditioning?
Active cooling becomes increasingly appropriate when:
- intake temperature regularly approaches the room target;
- summer ventilation cannot maintain temperature;
- required exhaust airflow becomes impractically large;
- noise limits make very high ventilation rates unsuitable;
- conditioned air is already available;
- the room operates year-round;
- or temperature stability matters more than outdoor conditions allow.
Grow Room Air Conditioning Options
An indoor evaporator conditions recirculated room air while an outdoor condenser rejects heat outside the controlled space.
Variable-capacity inverter models can modulate output as room load changes.
A self-contained indoor unit rejects condenser heat through a duct or other outlet.
Single-duct portable systems can create room depressurisation and draw warm replacement air into the space.
Larger controlled rooms may use equipment designed to combine several temperature and humidity functions within one environmental system.
Fixed Split-System Air Conditioning
A split system moves heat from the indoor space to an outdoor condenser.
This is fundamentally different from ventilation.
Ventilation replaces room air with another air source, whereas the split system primarily recirculates and conditions indoor air.
Single-Duct Portable Air Conditioners
A typical single-duct portable refrigerated air conditioner sits inside the room and uses room air to reject condenser heat through an exhaust duct.
Because air is being mechanically removed from the room, replacement air has to enter.
If that replacement air is hot and humid, the portable unit has to deal with some of the load created by its own exhaust process.
Air Conditioning Moves Heat — It Does Not Destroy It
A refrigeration system removes heat from the controlled space and rejects it somewhere else.
If the hot side of the cooling system warms the same small room that supplies the grow-space intake, the larger heat problem remains.
Do Not Size Air Conditioning From Grow-Light Wattage Alone
Lighting is often the largest internal load and therefore an important input.
But:
A full load calculation may also consider:
- other electrical equipment;
- drivers located inside the room;
- roof and wall heat transfer;
- solar gain;
- outside ventilation air;
- building leakage;
- dehumidification heat;
- and latent moisture load.
For substantial permanent rooms, use an appropriate HVAC load calculation based on the actual building and operating conditions.
Cooling Capacity Is Not Electrical Power Consumption
A refrigeration air conditioner transfers heat.
The cooling capacity shown on an air conditioner is therefore not the same thing as the electrical power consumed by that air conditioner.
For example, a unit rated to provide several kilowatts of cooling does not necessarily consume the same number of kilowatts of electrical power.
- rated cooling capacity;
- electrical input;
- efficiency;
- and cooling performance at the actual operating conditions.
Is a Bigger Air Conditioner Better?
Not necessarily.
Cooling equipment needs sufficient capacity for difficult conditions, but extreme oversizing can create poor part-load behaviour.
A large fixed-output unit may pull room temperature down quickly and then shut off.
If moisture removal is also required, short run periods can reduce the time available for the cooling coil to remove water.
Variable-capacity equipment can be useful where the room load changes substantially between lights-on and lights-off conditions.
Dehumidifiers Add Heat to the Room
A conventional self-contained refrigeration dehumidifier removes water vapour from the air by condensing it on a cold coil.
Unless the equipment has remote heat rejection, the heat associated with the process and the electrical energy consumed by the machine ultimately returns to the room as sensible heat.
The larger the latent moisture-control requirement, the more important it becomes to design cooling and dehumidification as one environmental system.
Lights-On and Lights-Off Cooling Are Different
Indoor plant rooms can have very different load conditions across the daily operating cycle.
| Condition | Sensible Load | Cooling Requirement | Humidity Consideration |
|---|---|---|---|
| Lights On | Usually high | Often highest sensible cooling requirement | Plant transpiration also creates latent load |
| Lights-Off Transition | Falls rapidly | Cooling demand can drop quickly | Moisture load may remain temporarily high |
| Lights Off | Lower | Often much lower sensible requirement | Humidity control can remain necessary |
Conditioning the Room Around a Grow Tent
One practical strategy is to air-condition the room containing the grow tent and let the tent draw its intake air from that conditioned space.
This gives the tent a more stable intake temperature.
However, if the tent exhaust is discharged outdoors:
The building air conditioner then carries the cooling load associated with conditioning that replacement air.
As ventilation rate increases, the building HVAC system may need to cool and dehumidify more replacement air.
Ventilated, Hybrid and Recirculating Cooling Strategies
| Strategy | Outside-Air Exchange | Primary Cooling Method | Typical Application |
|---|---|---|---|
| Ventilation Dominant | Relatively high | Cooler intake air | Small tents and rooms where outside/intake conditions are favourable |
| Hybrid | Controlled or reduced | Ventilation + refrigeration | Rooms where outside air is useful only some of the time |
| Highly Recirculating | Low process-air exchange | Active HVAC | Controlled rooms requiring consistent year-round conditions |
Reducing ventilation can lower the amount of conditioned air being discarded, but temperature, humidity, air quality and pressure must then be controlled by other systems.
Cooling Grow Rooms in Brisbane
Brisbane's climate makes intake-air temperature particularly important during the warmer part of the year.
A ventilation system that has substantial cooling capacity during winter can have very little sensible cooling headroom on a warm summer afternoon.
Grow Room Cooling Design Sequence
Grow Room Temperature Troubleshooting
| Symptom | Likely Area to Check | What It May Mean |
|---|---|---|
| Fan at maximum but temperature keeps rising | Intake temperature, actual airflow, heat load | Ventilation capacity is below the room load |
| Opening tent door lowers temperature | Passive intake | Existing intake may be restrictive |
| Works in winter but not summer | Seasonal intake temperature | Available ΔT has fallen |
| Larger exhaust fan makes little difference | Intake temperature | Air source may already be too warm |
| Room outside tent gets hotter through the day | Exhaust destination | Tent heat is being recirculated into its own intake environment |
| AC runs continuously after dehumidifier is added | Dehumidifier heat load | Cooling load has increased |
| Temperature varies significantly across canopy | Air distribution | Cooling exists but is poorly mixed through the space |
| Temperature sensor looks normal but plants feel much hotter | Sensor placement and radiant load | Single air-temperature reading may not represent all conditions |
Continue Designing Your Environment
Calculate the delivered airflow required for your room and heat load.
Understand humidity ratio, dew point, transpiration and dehumidification.
See how Brisbane temperature and humidity affect ventilation performance.
Shop Grow Room Cooling & Environmental Equipment
Green Genius stocks ventilation, dehumidification and environmental-control equipment for grow tents and larger indoor growing spaces.
Grow Room Temperature & Cooling FAQs
Can an exhaust fan cool below the intake-air temperature?
Not by ordinary sensible ventilation alone. Once the incoming air reaches the desired room temperature, it has no remaining sensible cooling headroom relative to that target. Cooling below the intake dry-bulb temperature requires another cooling process or a cooler air source.
How much airflow do I need to remove grow-light heat?
The answer depends on the sensible heat load and the temperature difference between intake air and the maximum room temperature. Use:
Then account for carbon filters, ducting and other system resistance when selecting the fan.
Does all grow-light wattage become heat?
Electrical energy consumed within the controlled environment ultimately forms part of its energy balance. However, plants can convert some sensible energy into latent energy through transpiration, so complete HVAC sizing should reconcile sensible and latent loads rather than simply stacking them together.
Does moving LED drivers outside the room reduce heat?
It can reduce the driver heat released inside the controlled space. Only the portion of equipment physically outside the room's thermal boundary should be excluded from that room's internal equipment heat estimate.
Is a bigger exhaust fan always better for cooling?
No. Additional airflow is useful only while the intake air can provide cooling and the complete duct/filter system can deliver that airflow efficiently. Once intake air is too warm, a larger fan cannot maintain a temperature below that air source.
Is a portable air conditioner suitable for a grow room?
It can provide cooling in some smaller or temporary installations, but single-duct portable units exhaust room air and draw replacement air back into the space. A properly sized fixed split system is generally more efficient for permanent cooling.
Does a dehumidifier heat the grow room?
A conventional self-contained refrigeration dehumidifier normally returns sensible heat to the room while removing water. Its heat contribution should be included when checking the cooling requirement.
Should I size an air conditioner from grow-light wattage?
Lighting wattage is an important starting load but not a complete HVAC calculation. Building heat gain, ventilation, leakage, other electrical equipment, dehumidification and latent load can all affect final equipment selection.