Choosing your fan


Choosing a fan for your grow room is an essential part of growing indoors getting it wrong could be valuable to your time, money & effort.

Have you spent hours searching for the correct size for your area?

Getting your extraction set up takes a little maths but it is a necessity, which can seem like a massive deal for new time growers.

Hydro grow are here to save you in your time of need!

Here are a few pointer’s in choosing the fan for you grow room.

To start with you need a basic idea of the habits of air and relation to temperature, along with high and low pressure within the earth atmosphere; or you could take it as read that hot air goes up and cold air goes down. Your bulb produces heat, which you need to reduce. So it helps the have your extraction near the bulbs near to the top and the fresh air intake at the bottom.
Before you do that you need to figure out the right fan for you.

Please follow the simple steps – We’ll be working out 2.4m x 2.4m x 2.0m tent as an example throughout as an example, which is based on the standard positioning of a 600w over 1 – 1.2m.

Before you commence you need to commence with an accurate measurement of your growing area, in this case the measurement’s are in cubic metres m3.


This is the best way of calculating and describing volume. You will have been sold your tent with measurements like this above.

TENT EXAMPLE – 2.4M X 2.4M X 2.0M = 11.25M3

Be sure that your area is measured in metre’s if it’s below 1 Metre it will be measured in centimetres for example 75CM is classed as 0.75M, because one metre is classed as 100CM.


Plants take in Carbon Dioxide (C02) and release Oxygen (O2) through the process of photosynthesis. Just as humans will suffocate in Co2 due to lack of oxygen, plants will conversely suffocate in O2 due to lack of Carbon Dioxide. So as well as the primary need to remove excess heat, you also need to give you plant’s a steady supply of fresh air.

Exactly how much air they need exchanged is in constant flux, depending on factors such as (primarily) outside temperatures or even the size of your plants. It’s one of the aspects of growing that you’ll be adjusting in many years to come. Eventually arriving at your own perfection, then deciding it’s not good enough and starting all over again to gain the perfection.

As a rule, the maximum exchange rate you should ever need is every minute per hour.

That means that every minute you replace the entire the volume of your growing space with fresh air. Every minute per hour should be ample to control temperatures except maybe when your in the peak of summer months. There will be times when you don’t need this amount of air moved. For example when lights are off, or cold winter months, therefore it is wise to use fan speed controllers to regulate and maintain a steady temperature.

The amount, frequency and temperature of air coming into your growing space will alter the temperature within that space as well as, to an extent, the size of your plants.

The only effective method of maximising the efficiency of your volume exchange rate is through frequent monitoring and trial and error.

Going back to the example tent and using the every minute per hour rule, we can work out the volume exchange rate by multiplying the volume of the growing space by 60 (there being 60 minutes in an hour):

Example Tent: 11.25M3 X 60 = 691M3/HOUR (HR)

So our volume exchange rate 691m3/hr


There are a number of mitigating factors that will affect the type of capacity or size of fan that you’ll need. Silencers, the length and number of bends in the ducting used, ambient air temperature and humidity all have a potential effect, but some of them are normal, especially in a smaller growing environment.

The most prominent mitigating factor in Hydroponics is almost always the carbon filter. Carbon filters reduce unwanted odours, and even the most seemingly innocent plants can smell a little funky when grown using an indoor set up. Most reputable manufacturers will provide pressure graphs that give exact figures ;

As a rule you should plan for at least 25% or 1/4 reduction in the fan’s capacity.

Returning to our example:

Example Tent: 691m3/hr X 1.33 = 919.03M3 / HR

So, volume exchange rate multiplied by 1.33 (One and 1/3rd) equals the total volume of air that your extractor fan needs to be able to move, or to put it another way, this is the requisite capacity of your fan.



It’s always better to over spec your extractor fan, which is to say it’s better to buy a fan that has a greater capacity than you need.

If you expand the size of your growing space at a later date, a fan with a greater capacity may be able to compensate for the added load, but if you buy one that’s too small, you’re going to buy a new one. Fan speed controllers regulate the speed at which your fan operates, so you can adjust them down to the necessary speed.

Returning to our example:

An RVK 10″ L fan has a capacity of 1020m3/hr, meaning it will comfortably manage our load of 919m3/hr.


Once you’ve chosen your extraction fan, you can move onto choosing an intake fan. The idea is to create negative air pressure within your growing environment.

Negative air pressure is achieved by extracting more air from a space then is allowed into it. This allows you to create and maintain a contained environment, and will direct all the extracted air through the filter, rather than allowing it to escape randomly. In order to achieve negative air pressure, you need to aim for an input of 15% less than your output, whilst remembering to account for the mitigating factors like the carbon filter.

When choosing your filter, be sure to check that it can deal with a volume of air equal to above or that of your fan. If the filter is too small, you will get pressure build up which will reduce the output of the fan.

To get an exact idea of how a filter will affect a plan you can refer to pressure graphs from manufacturers and cross reference the one for the fan against the filter. Also, be sure that your fan against the one for the filter. Also, be sure that your fan, filter and ducting all slot together comfortably. Ideally visit us just to be sure.

We offer fast ducting clips and acoustic ducting clamps.

Reference Point: Negative Air Pressure Equation
Extractor Fan Capacity X Mitigating Factors (25%) X Negative Air Pressure (15%)

So returning once again to our faithful example –
Example – 1020m3/h X 0.75 X 0.85 = 650.25m3H

So the necessary capacity for our intake fan is 650m3/hr. In this case an RVK 6″L would be a good choice, with it’s capacity of 660m3/hr. Worthwhile fan speed controllers have two input’s meaning you can adjust both fan speeds accordingly. Ideally you want to see the sides of your tent sucking inwards slightly.

So there you have it, you’re now a qualified air conditioning engineer! (not really).

We wish you all the best with your projects and the hunt for the ideal set up, Happy hunting!

The HG Team

If you require any further assistance, please contact a member of the team direct.












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