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[The Art of Ventilation] Why & How to Move Air (Part 1 of 3)

[The Art of Ventilation] Why & How to Move Air (Part 1 of 3)

Alex Grady

On a hot summer’s day, the last place you’d want to find yourself is stuck in a hot, humid, poorly ventilated room – your plants feel the same.

Don't let your air turn stale - get your ventilation in order. If you don't, plants suffer and pests thrive.

This is part 1 of a 3 part series.

Here you learn why you need to move air, and how to do it.

Why Air Turns Stale

In your grow room, several things happen:

  • Plants consume CO2 and release moisture
  • Grow room equipment produces heat

This is a problem because

  • Too much moisture and heat inhibits growth
  • CO2 is needed for photosynthesis
  • Mould thrives where there's moisture

Not good. To stop your air stagnating, you need a ventilation system.

What's Ventilation?

It's the process of extracting stale, hot, humid, CO2 depleted air out of your grow room, while drawing fresh, CO2 rich air in.

Doing this will help you control your:

  • Temperature
  • Humidity
  • Bugs
  • CO2 levels
  • Overall air quality

To do this, you need to set up a ventilation (a.k.a. air exchange) system. We also recommend adding a few supporting items - like air circulation fans.

You Definitely Need

All ventilation systems need:

Extraction System

Draws hot, stale air out of your growing area, removing odours in the process.

Make sure that you extract more air than you draw in - otherwise your tent will become too full of air. When this happens, air will start seeping out your tent before odours have been removed.

Fit your extractor fan at the top of your growing area.

Complete extraction kits can be found here.

Intake vents

If you've only got a small set-up, you only need a few vents at the bottom of your tent. Fresh, CO2 rich air will enter your grow room passively.

Fan controllers

They speed fans up if your grow room becomes too hot. They slow fans down if your grow room needs heating. They really make life easy.

Aim for:

  • Lights on temperature of 24oC - 27oC
  • Lights off temperature of 21oC - 24oC

Remember that extracting air isn't all about temperature.

So make sure you extract enough air to keep your CO2 level and humidity under control, and maintain negative pressure - that's where more air's extracted than is drawn in.

  • For most people, a good minimum speed is 10 - 30%.

Air circulators

Air circulator fans are used to move air inside your grow room.

You can point them at corners, nutrient chillers, reflectors, ballasts - any areas where you tend to get heat build ups.

Just make sure you don't aim them directly at plants, because this can cause windburn.

Grow Room Heaters

When you extract air in winter, your grow room can become too cold. A grow room heater is the answer.

The most convenient one is a KlimaHeat Electric Grow Room Heater. They heat large areas quickly, and have an inbuilt thermostat so will heat and cool your grow room automatically.

You'll probably need

Most systems will also need

Intake system

If you're extractig lots of air, vents won't cut it. You need an intake system which will actively draw air into you grow room. Pick a fan that's 20% less powerful than your extractor fan. Or run a similar fan at 20% speed.

For a stable temperature, draw air into your grow room from another indoor room - not from outside. In winter, the temperature will be a bit warmer so won't shock plants. On top of this, indoor air carries fewer bugs and more CO2 (because you release CO2 when you breathe).

Bug Blockers

When drawing air in, creepy crawlies sometimes get dragged in too. Keep the little nasties out with Bug Blockers.

Temperature and humidity meter

You can tell how hot and humid your grow room is with a simple glance. If you want plants to be at their best, it's a must!

As a rule, you should aim for a humidity of:

  • 70 - 85% RH in propagation
  • 65 - 75% RH in veg
  • 45 - 65% in flowering

And a temperature of:

  • Lights on temperature of 24oC - 27oC
  • Lights off temperature of 21oC - 24oC

You may also need


Plants actually like a high humidity during propagation and vegetative growth. If you're extracting lots of air to control your temperature and CO2 level, your humidity could drop too low. This is most likely to happen in summer.

If this happens, a humidifier should do the trick. Failing that, be sure to mist plants with a good quality sprayer , like the Nebuliser Foliar Fogger.


If you're hardly extracting any air, and your grow room becomes too humid, you may need a dehumidifier. This is most likely to happen in winter, during flowering.

CO2 Bags

Plants need CO2 for photosynthesis.

The problem with a ventilation system, is you can only give plants as much CO2 as is being drawn in, which tends to be 500ppm - 600ppm.

Plants can actually manage a CO2 level of 1000 - 1200ppm. The only way to reach that level is by adding it artificially.

Don't forget

Your intake fan or vents should be at the bottom of your tent.

Your extractor fan should be high, towards the back of your grow room, on the opposite side of your tent to your intake fan or vent.

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