Warm Weather and choosing an Extraction System


Without a doubt, an effective extraction system is one of the most important components of an indoor garden.


Plants absorb carbon dioxide (CO2) from the surrounding air and then use it in the process of photosynthesis. When the levels of carbon dioxide in the surrounding air begin to drop, so does the rate of photosynthesis and therefore plant growth. It is essential that the CO2 depleted air around plants is being replaced constantly otherwise plant growth rate will slow.


HPS and other types of indoor grow lamp all produce heat. If there is inadequate air exchange in an indoor garden where grow lamps are being used, then the air temperature will rise. When the air temperature goes above 28C (83F), plant growth is affected negatively.

It is quite easy to see that the air in a grow room or tent needs to be replaced on an on-going basis. A good extraction system is a very necessary item.

What sort of extraction system do I need?

As a bare minimum, you will need a duct fan and usually some flexi-ducting to get the air away from the grow-space. If the sound of air rushing through standard flexi-ducting is too loud then acoustic ducting can be used to help reduce this problem considerably.

Problematic odour

If you are growing plants that produce a lot of odour then a carbon filter can be used on the inlet side of the fan to get rid of the smell from the air being vented to the outside. It is important to know that a carbon filter will reduce a duct fan’s extraction rate by about 25%. This fact will be used in how to choose the right size of duct fan for your set up.


How powerful does my extraction system need to be?

As a rule of thumb, the extraction system should be able to replace the air inside a grow area once every 2 minutes. One of the important specifications of a duct fan is it’s rate of extraction. If the size of the grow-space is known then the rate of extraction needed can be easily calculated.

As an example, let’s take a fairly typical 1.2m (w) x 1.2m (d) x 2m (h) grow tent. The volume of air inside this grow tent equals W x D x H = 1.2 x 1.2 x 2 = 2.88 cubic metres.

We need to extract this once every 2 minutes which is 30 times per hour.

So 2.88 x 30 = 86.4

This means that we need a system that extracts at least 86.4 m3/hr.

Adding a carbon filter means we need to multiply this figure by 1.25 to compensate for the reduced air flow: 86.4 x 1.25 = 108 m3/hr. This rate of extraction will be adequate to keep CO2 levels in the air at a satisfactory level.

If we are using High Intensity Discharge (HID) lighting such as an HPS or MH lamp then the chances are that in summer, this extraction system will still not be able to change the air quickly enough to keep the temperature down. In this case we would double the required rate of extraction. This means our system needs to be capable of removing about 216 m3/hr and we would recommend adding a fan speed controller so that the rate of extraction can be reduced when needed (for example, during winter months). Many fan speed controllers have a temperature sensor which can be placed at in an appropriate location in the grow space. The fan speed controller will then adjust the speed of the fan to keep the grow space at the user-set temperature.


As air is being extracted, there needs to be some way of replacing the air inside the tent. Many tents have ventilation panels that can be used for this purpose. However, in some situations, this is not adequate to allow enough replacement air in, and an inlet fan needs to be added to the system.

Positive & Negative Air Pressure

It is standard practice to have a more powerful fan on the outtake side of the system and a smaller fan on the intake side.

This is mainly for two reasons:

Firstly, a slightly larger fan is needed on the extraction side to compensate for the reduction of flow rate caused by the carbon filter that it has to draw air through.

Secondly, it is usually a good idea to have more air being drawn out of the grow-area than is being pushed in. This makes sure that all the air leaving the grow-area does so via the carbon filter, to remove the smells. This is called negative air pressure.

If more air is pushed into the grow-area than is being sucked out then there is often a problem of unfiltered (and therefore still smelly) air being  pushed out though gaps around doors, zips, seams, windows etc. This is called positive air pressure and should ideally be avoided because this unfiltered air that is leaking out could possibly cause an odour problem outside the grow-room.

Adding a twin fan speed controller with a temperature sensor to the setup would help keep the grow-room at the right temperature through the different seasons and the different ambient air-temperatures they bring without too much intervention by the grower. Automation like this is great for the grower that would really prefer to be sitting down and putting their feet up rather adjusting fan speeds every hour!