Input and Output Fans - Native Growers Guide

Posted by Daniel Taylor on

Hi and welcome to our second Native Growers Guide blog in the series we will be looking at input and output fans as well as working out grow room air volume and what size intake and outtake fan a room would need. It may all sound a bit complicated but hopefully we will try and break it down here...

Before choosing an extractor fan you need to work out what size you require. Generally speaking the air in your grow room should be exchanged out at least once every 3 minutes. In this blog post we will look at how to calculate your air flow exchange needs as well as different methods of grow area extraction and the best practices for maximising your air flow.

Passive Air Flow vs Active Air Flow

I remember when someone first tried to explain passive and active air flow and all the varying factors that come with it - my head was so fried I thought I would never get it. However, once broken down it is really not that complicated. Passive Air Flow in your grow area simply means that you don't have an intake fan just a vent hole for clean air to enter the grow area. Active Air Flow is therefore a system which has both an outtake fan and an intake fan that pushes clean air into your grow room - thus minimising the negative pressure putting less strain on your extractor fan which will also help to prolong it's life. When using Passive Air Flow method ensure your passive intake ducts are twice the size of your extractor fan diameter. This will still create adequate negative pressure but not too much as to put strain on your zips and stitching if using a grow tent for example. Too much inntake air cause the tent to blow up creating pressure on seams and pushing unwanted odours and possibly damp air from your grow area through gaps such as under the doors. For example, when growing some strains of chillies the odour can be that strong that everyone in your house will be complaining of the smell! In all seriousness this can be a health risk for people who are vulnerable to vapourous odours. 

How to Calculate Grow Space

Now for the sciencey part - calculating your grow room area. The simple formula to get the total volume (metres cubed - m3) for your room is

Width x Length x Height = Grow Room Volume

Weather Conditions and Other Environmental Factors

This is just to calculate the total physical volume, however there are other factors to take into consideration which will affect your ability to control and maintain a healthy grow room environment. The area which you grow in, whether it's house, garage or attic, will have different factors to consider.

Ambient Outdoor Temperatures

Ambient Indoor Temperatures 

So if we were to be growing in a warm/ambient climate our input air will be changed due to these factors and that will lead to additional climate changes within the grow space. The hot temperatures and dry air being sucked through your intake fan could make temperatures unbearable for plants and with the change in humidity can be detrimental to your crop - temperatures above 29C or 85F will slow or stop plant growth as will temperatures below 13C or 55F. Maintaining a stable day and night temperature will ensure your crop reaches its full potential. We will cover grow area temperatures and variables in a future post. 


Your extraction system setup will also have an affect on your extractor fan size needed as it will reduce the capacity of the output fan as follows;

Air Cooled Light - Minus 5% 

Non Air Cooled Light - Minus 10 - 15%

CO2 Enrichment - Minus 5%

Carbon Filter - Minus 20%

Hot/Dry Climates - Minus up to 25%

Hot/Humid Climates - Minus up to 40%

(Please note these are all estimated calculations based on the article by


Example Calculation

If you had a grow tent of 2m wide x 2m long x 2m high you would work out your grow room volume as follows:

2 x 2 x 2 = 8m3 (8 metres cubed)

Therefore you need to exchange 8m3 every 3 minutes or 20 times an hour. The minimum you would want from your fan is 160m3/h

Choosing your Fan Size

Now you have calculated your grow area and have a rough idea of the fan size you will need.

Let's start with the outtake fan - once you know the extraction rate you need you can choose your fan size, every extraction fan should be provided with air volume capacity and this is usually put in m3/h. Take a look at the example from our Vortex Inline Fans below

Vortex Inline Fan Air Exchange Rate

We would recommend always choosing an output extraction fan that is bigger than the size you need so the fan can cope with fluctuations in temperature. The other reason is so the fan can run at a lower speed - let me explain - in most situations, a larger fan can run at half speed with the same efficiency as a small fan running on full speed. This can be beneficial as a larger fan on half speed is quieter than a small fan on full speed. 

Moving on to our air intake - as we mentioned before you can use either a passive intake system or an active intake system. With a passive system you will simply have an input vent opening for your intake air flow, we recommend the size of the passive intake gap is twice the size of your extraction output size. With active intake systems you will also need to have an input fan, ideally this wants to be smaller than the size of your outtake fan to stop positive pressure in the grow area.


Fans/Extraction Equipment

So you know what fan sizes you need and the amount of air exchange required in your grow area, now onto the fans. We have currently got 2 types of extraction fans available to buy - each fan has the full specifications listed for you to choose the size you need.

Vortex Inline Fan - a lower cost option this little fan has a great exchange capability without using much power and is super easy to install.

Vortex Super Silenced Acoustic Fan - this is more of an investment as these fans come with a high price tag and probably for the more serious grower - however, they are well worth the money if you are a serious grower due to their efficient running and low noise output

We also have a wide range of extraction and environment products available in our online hydroponics shop such as ducting, ducting works, air moving fans and carbon filters.

Native Top Tips

Carbon filters and long ducting will reduce output air flow - take into consideration when choosing your input and output extraction fans for your grow room.

Have your larger extraction fan running on reduced speed and match this with a temperature controlled fan speed controller to maintain a constant and ideal environment and tweak as needed from there. 

Long ducting reduces the air flow, particularly if you have many bends, so keep your extraction ducting as short as possible and pull the ducting tight to create a smoother area for better air flow.

Input air flow should ideally come from the opposite side of the grow area to the extraction fan to ensure fresh air flows up through the plants.

Central heating is a big factor to take into consideration when growing in your house as this will not only add heat to your grow area but will also dry the air. 

Example Calculation

2m x 2m x 2m tent with a short run of ducting which we know need 160m3/h of air exchange.

If you were using the 8in Vortex Inline Fan which has a capacity of 1050m3/h this may seem like a lot for such a small need, however, when you take into account your other factors as we mentioned earlier the extractor fan capacity will be significantly reduced.

Original Exchange Needed: 160m3/h

8in Inline Fan Capacity: 1050m3/h

Now we need to factor in our other variables. 

4x 600w Bulbs in Non Air Cooled Reflectors: Take of 15% for each bulb = 60% fan capacity reduction

1x Carbon Filter: Take off 20% fan capacity reduction 

1050m3/h minus 80% = 210m3/h

As you can see with the added environmental factors the 8 inch Inline Fan which seemed way too much at the beginning is still able to exchange the air flow correctly with a slight buffer but the fan's capacity is drastically reduced by our lights and carbon filter. 

Final Note

So that is it from us on the topic of grow room air flow and extraction - we hope you got some new tips and facts to apply to your own grow room. 

Check out more of our grow room guides on our Native Growers Guide Blog

Share this post

← Older Post Newer Post →

Leave a comment

Please note, comments must be approved before they are published.