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Master the Art of Ventilation - How Air Flow Affects Your Plants

You could have the best lights, the best growing system and the best nutrients in the world, but a failure to equip your grow room with the right ventilation products will leave your plants facing a bleak future.

On a hot summer’s day one of the last places that you’d want to find yourself is cooped up inside a poorly ventilated room – the same can also be said for your plants! By regularly extracting warm, stale air away from the growing environment and replacing it with a completely fresh CO2 rich supply, you improve your ability to overcome issues such as high temperatures, unfavourable humidity levels and bug infestations. Likewise the installation of a few “supporting” ventilation-related products including air circulators and bug blockers also help plants to thrive.

This article aims to explain exactly what will happen if you successfully establish an effective ventilation system and the potential downfalls of failing to do so.

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GroWell Team

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What is ventilation and how does it affect my plants?

The ability to successfully navigate through a crop from early propagation up to the final push of flowering depends upon good ventilation being present along the way. It refers to the process of controlling airflow in and out of your grow room and has a direct effect on the management of key factors including heat and humidity, air quality and CO2 levels and anti-insect measures.


To guarantee that your plants get a good supply of CO2 rich fresh air whilst ensuring warm, stale air is efficiently removed, you need to create a reliable ventilation system. On top of this we also recommend introducing a few items that will play a supporting role, like air circulators.


What is in a ventilation system and where does it go?

A ventilation system consists of the following components:


Essential to ALL grow rooms


Extraction System

(Fan + Carbon Filter + Ducting + Connection Parts)

Takes away hot and stale air from the top of the growing area whilst fresh supplies enter from an open vent/intake fan located at the bottom of it. Complete extraction kits can be found here.

Fan Speed Controller

Automatically maintains and adjusts fan speeds (of extractor fan only or extractor fan and intake fan) based on settings predetermined by you to preserve optimum growing conditions. View our entire range of fan speed controllers here.

Different fans

Code Price Qty
Variispeed Fan Controller 2479 Variispeed Fan Controller
SMSCom 4.5 Amp Twin Fan Speed Controller Mk2 3891 SMSCom 4.5 Amp Twin Fan Speed Controller Mk2
GSE Temperature, Humidity and Negative Pressure Controller 5352 GSE Temperature, Humidity and Negative Pressure Controller


Air Circulators

These include the powerful little KlimaCool units and the rotating Pedestal Fans, and should be used to create air movement, prevent stale pockets of air forming and help with temperature control. Clip Fans also help for this job, particularly in those hard to reach spots.

Air Circulator

Code Price Qty
KlimaCool Air Circulator Fan - 30cm (12 inch) 2774 KlimaCool Air Circulator Fan - 30cm (12 inch)
KlimaCool Air Circulator Fan - 20cm (8 inch) 2773 KlimaCool Air Circulator Fan - 20cm (8 inch)
16 inch Pedestal Fan 279 16 inch Pedestal Fan
6 inch Clip-on Fan 7787 6 inch Clip-on Fan



Vital in MOST grow rooms


Intake System

(Fan + Ducting + Connection Parts)

Often required for complimenting larger extraction systems because passive intake may not prove sufficient in multi-light set ups.

Intake fan

Bug Blocker

An essential purchase if you plan on using an intake fan which effectively stops bugs from entering your growing area. Check out Bug Blockers here.

In small areas you will be both actively extracting and filtering stale air using an extractor fan, carbon filter, ducting and connection parts, and also passively drawing in fresh air through vents or holes in your growing area/tent. In larger areas, the extraction described above should be accompanied by the active intake of air via another fan at the bottom of the room/tent and ideally a small piece of ducting and the necessary connection parts.

Your extraction system will typically hang high up towards the back of the grow room or tent. Please refer to the article 'Master the Art of Ventilation - Setting Up A Complete System' for advice on installing all of the components.


So how does having a ventilation system installed let you gain control over the aforementioned key factors? Let’s see…


Beating the Heat

If you are growing plants under High Intensity Discharge lamps (SOLAAR, Grolux, Sunmaster, Sodium, Halide, etc) then the heat output of these products will have inevitably been noticed. Leaving this heat to build up without any kind of forced air exchange in your grow room can be hazardous for your plants, causing weakened structures, poor development and ultimately reduced yields. Extracting the hot air out of your grow room using an extractor fan combined with a carbon filter, ducting and connection parts helps create a much friendlier environment – ideally aim to establish a lights-on grow room temperature of 24oC – 27oC and a lights-off grow room temperature of 21oC – 24oC for optimum plant performance.

To help keep temperatures at a suitable level, try drawing in air from another room indoors (i.e. another room). This usually has more CO2 and less bugs, and in wintertime won’t be as shocking for your plants as the cold air from outside. When freezing cold weather does hit home, we also suggest investing in a grow room heater.

Code Price Qty
80 Watt KlimaHeat Tubular Grow Room Heater 611 80 Watt KlimaHeat Tubular Grow Room Heater
800W Oil-Filled Grow Room Radiator 7675 800W Oil-Filled Grow Room Radiator

Out of stock

2.5kW Oil-Filled Grow Room Radiator 7676 2.5kW Oil-Filled Grow Room Radiator


A reliable temperature and humidity meter is also absolutely essential for monitoring the grow room setting. With one glance you need to be able to tell how hot it is inside and whether or not there’s too much water vapour in the air.

Code Price Qty
Accuread Temperature and Humidity Meter 876 Accuread Temperature and Humidity Meter


Harnessing the Power of Humidity

An effective ventilation system lets you virtually eliminate the chances of any serious issues developing that relate to humidity.


Over the course of the propagation process the humidity reading needs to register somewhere between 70-85% RH – although this is easy to achieve with or without a ventilation system through use of a Hi-Top, Aeroponic or Budget Propagator. Your plants won’t yet have developed a large enough root system to absorb sufficient amounts of water and nutrients so the leaves must do most of the work.

However, for vegetative growth you should be operating a ventilation system and aiming to maintain a humidity of around 65 - 75% RH. If the level drops too low you will find that the young leaves of your plants become smaller whilst the older ones curl at the margins, appear burnt and fall off. To correct low humidity at this point and take closer control of environmental conditions we recommend that you install a suitable humidifier - see our range here.

Regularly misting plants with a good quality sprayer will also help increase humidity and actually strengthens plants as the leaves and stem are subjected to short bursts of intense spray action. The Nebuliser Foliar Fogger represents our most professional option.

Code Price Qty
Budget 750ml  Spray Gun 3654 Budget 750ml Spray Gun
2 Litre Professional Pump-Up Sprayer 3092 2 Litre Professional Pump-Up Sprayer
5 Litre Professional Lance Sprayer 3093 5 Litre Professional Lance Sprayer


The flowering stage requires a reduced humidity of 45-65% RH – anything above this figure and the threat of rotting fruits is high. By extracting away hot and humid air and replacing it with cooler less humid air from outside your grow room, high humidity will automatically become less of a problem.


As a precautionary measure, we recommend using Pythoff. Just apply in the first few weeks of flowering to build up plant immunity and protect fruits up until harvest.

Code Price Qty
Pythoff - 1 Litre 3200 Pythoff - 1 Litre


Improving air quality with CO2

Good ventilation ensures that you get a regular supply of CO2 rich air in your grow room, which plants need for photosynthesis, energy creation and growth. Since there is only a small percentage of CO2 in the atmosphere (normal air contains just 0.03%), a frequent exchange of old air for new must be facilitated to keep the gas readily available to your plants. As a general rule of thumb, you should aim to replace the air in your grow room at least 20-30 times per hour to maintain a reasonable air quality.


An intake fan will help bring fresh CO2 rich air into your grow room. Likewise, introducing an air circulator proves to be a highly effective way of achieving a more even distribution of carbon dioxide, humidity and temperature. By using one in conjunction with a ventilation system, you encourage the colder air that’s brought inside to mix evenly with the already present warmer air. They are relatively cheap to run and will help move air where there a typically dead spots.


Fighting back against the bugs

Inadequate ventilation and stale stagnant air provide airborne pests such as fungus gnats or sciarid flies a greater opportunity to invade your grow room. By keeping the air constantly moving using oscillating fans or air circulators whilst also creating a constant flow of fresh air with a ventilation/extraction system, you can provide a less hospitable environment for these pests and reduce your chances of infestation.


To prevent insects from accessing your grow room via the intake fan, simply place an appropriately sized Horti-Shield Bug Blocker over the inlet and secure it with the handy drawstring. Spraying Pyrethrum 5EC all over your growing area and plants before and during a crop will act as added insurance against the threat of unwanted pests including Spider Mites.

Code Price Qty
Pyrethrum 5 EC - 250mls Concentrate 480 Pyrethrum 5 EC - 250mls Concentrate


Now you know the reasons for ensuring that your grow room benefits from good ventilation, take a look through our guides on choosing the right ventilation products and installing a ventilation/extraction system to turn theory into practice. Also be sure to keep your eyes peeled for a follow up article outlining other ways that you can improve the process of airflow control.

This post was posted in New Growers, Ventilation and was tagged with air, airflow, ducting, exchange, exctrator, extract, extraction, fan, fans, filter, filters, humidity, temperature, vent, ventilation


9 Comments - Add Comment

  • Goof
    Posted September 17, 2017 at 8:57 am
    Can I do inlet active (with fan) and do a carbon filter with fan inside but not connected to vent out (only scrub air) and have a dehumidifier in room to take care of the humidity? So like a semi sealed room?
    • Replies

    • GroWell Team
      Posted October 13, 2017 at 4:27 pm
      Hi there,

      We would not recommend this setup unless you have an air conditioner to control the temperatures in the room, your room will get very hot if you are running powerful lights without an extraction system.

      Thanks fro your question!

  • Dai7642
    Posted August 28, 2017 at 7:30 pm
    So it's recommended to use about 3 fans - one for extracting the air, one to introduce air and another to move air around in the tent?
    • Replies

    • GroWell Team
      Posted August 31, 2017 at 12:49 pm

      You're basically correct! Although in bigger set ups you may need to use more than one air circulator fan (fortunately these tend to be a lot loss expensive than the fans designed for the extraction and intake of air).

      Hope that helps!

  • Jordan
    Posted October 25, 2016 at 7:36 pm
    Witch component has to have more power the intake fan or the extractor fan
    • Replies

    • GroWell Team
      Posted November 15, 2016 at 10:06 am
      Hi Jordan, we would recommend using a more powerful extractor fan so you can keep negative pressure in your room.

      Hope this helps
      Growell Team

  • michael connachan
    Posted January 1, 2015 at 9:03 am
    Well said Borgi, it's as though he didn't even read the article.

  • Borgi Jones
    Posted September 21, 2014 at 8:20 am
    What good was that last comment from 'plumbing'? You just stated what the whole article above is all about, but like you were introducing us to something new!

  • plumbing
    Posted July 2, 2011 at 12:38 am
    A grow room must have good ventilation to allow fresh air to get in. This is essential for your plants and crops.
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