When you set up a successful grow room, choosing the right extraction fan and filter is essential. A good ventilation system doesn't just bring in fresh air: it also prevents mould, odour problems and an unhealthy environment for your plants. In this article we explain how to pick the right size of extraction fan and filter, and how different types of lights affect that choice.
Why is an extraction fan important?
An extraction fan keeps the air in your grow room constantly refreshed. That supplies your plants with fresh CO2, removes excess heat and keeps humidity stable. Without proper ventilation, growth can stall and the risk of pests and diseases goes up.
How do you work out the right fan size?
To calculate the capacity your extraction fan needs, start with the volume of your grow room. Here's how:
- Calculate the room volume: multiply length, width and height. A room that is 2 m long, 2 m wide and 2 m high, for example, has a volume of 8 cubic metres (2 x 2 x 2 = 8 m³).
- Factor in the air exchange rate: ideally, the air in your grow room is replaced 20 to 30 times per hour. So you multiply the volume by 20 to 30. In our example you'd need a fan with a capacity of at least 160 m³ per hour (8 x 20 = 160).
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Factors that call for extra capacity:
- High temperatures: powerful lights such as HPS lamps generate more heat and therefore need a stronger fan.
- Carbon filters: a carbon filter reduces your fan's effective capacity by around 25%. Choose a fan with some extra capacity to make up for it.
Which filter suits your fan?
A carbon filter is essential for neutralising the odours produced while growing. The filter size needs to match your fan's capacity. These are the key points:
- Filter capacity: make sure the filter can handle at least the same airflow as your fan. A 160 m³/h fan, for example, needs a carbon filter rated for at least 160 m³/h.
- Connection diameter: the filter's diameter has to fit your fan and ducting. Common sizes are 100 mm, 125 mm and 150 mm.
- Lifespan: a carbon filter doesn't last forever, usually 12 to 18 months depending on use. Replace it in time to avoid odour problems.
How do your lights affect the choice?
The heat your lights give off has a big impact on the climate in your grow room. These are the most common lamp types and their effect:
1. HPS lamps (high-pressure sodium)
- Pros: high light output, ideal for the flowering stage.
- Cons: produce a lot of heat.
- Effect on ventilation: with HPS you need a powerful extraction fan to remove the heat effectively. Combine it with a circulation fan to spread the heat evenly.
2. LED grow lights
- Pros: energy-efficient, less heat, long lifespan.
- Cons: higher purchase price.
- Effect on ventilation: because LEDs produce less heat, you can often go for a fan with lower capacity. That saves energy and keeps things quieter.
3. CMH lamps (ceramic metal halide)
- Pros: broad, complete spectrum, suitable for both veg and flower.
- Cons: more heat than LED, but less than HPS.
- Effect on ventilation: choose a fan that can remove enough heat; it doesn't need to be as powerful as with HPS.
Example calculation
Say you have a grow tent measuring 1.5 x 1.5 x 2 m (4.5 m³) and use a 600 W HPS lamp:
- Volume: 4.5 m³.
- Capacity needed without filter: 4.5 x 20 = 90 m³/h.
- Filter compensation: 90 x 1.25 = 112.5 m³/h.
- Recommended fan: choose a model with at least 120 m³/h. Because of the heat from the HPS lamp, extra headroom is wise; with a speed controller you can simply run a stronger fan more slowly.
- Filter: use a carbon filter rated for at least 120 m³/h (or at least as much as your fan) with a connection diameter that fits your fan.
Extra tips
- Noise: consider a silencer if your fan is loud.
- Climate control: with a climate controller or fan controller, your fan responds automatically to temperature and humidity.
- Extra fans: add oscillating fans to improve air circulation in the room.
Choosing the right extraction fan and filter is a crucial step towards an optimal climate for your plants. Calculate your room volume, pick a fan with enough capacity and make sure your filter matches your ventilation system. Don't forget the heat your lights produce, as it can have a big effect on what your ventilation needs to handle. Get these factors right and you've laid the foundation for a successful grow.
