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Hydroponic Ventilation Fans Cooling CO2

Temperature of Air in Grow Room

Plants will germinate and grow better and more rapidly within a suitable temperature range. Although they will tolerate temperatures between 60°F and 92°F (17°C – 33°C) these extremes are at the limits of their range. The ideal temperature range is 70°F – 83°F (21.1°C – 28.3°C) while using CO2 you might want to be in the higher temperature range but during darkness times a lower temperature is best. Seedlings particularly will benefit from a heating mat beneath them to encourage strong root growth. Try to stick around the 72°F to 77°F (22.2°C to 25°C) range for all growth.


Planning air movement

Your plants use both light and CO2 to photosynthesise. Because of this, and also because of the heat build up from powerful lamps, your grow room needs to be well ventilated. Both intake and exit vents are essential and will bring CO2 rich air into the room whilst expelling CO2 depleted air. Ideally you will use an extraction fan high up in the room to facilitate air flow and prevent stratification. Fans are rated by the volume of air they move and are measured in either cubic feet per minute (CFM) or cubic metres per hour (M3/H). Calculate the size of your room by multiplying the width by the height by the depth and buy a fan that will change the air once every 1-5 minutes.

It is far more efficient for a fan to pull air than push air. Consequently the extraction fan should pull enough air through the inlet vent to sufficiently ventilate the room. If for some reason the supply of fresh air through the inlet vent is compromised you may want to consider using a fan to pull air into the room as well. However this is seldom necessary.

Oscillating fans

Using an oscillating fan within the grow room will help with air circulation. This is particularly important during flowering when the buds are densely packed and susceptible to mould. A gentle breeze ruffling your plants will also help strengthen the stems. In the wild, whenever a plant bends in the wind it creates tiny fissures in the stems which the plant then repairs with scar tissue. This is how stems become strong and woody. Without this movement your marijuana plants will be more fragile and have weaker stems.


If you have to move air a distance to an exterior vent, or you have a large room with fans a long way from a wall, you may need to use ducting. Ducting comes in standard sizes that will fit all of your fans and filters. Flexible ducting is the easiest to work with, but it is important to try and keep it in straight lengths. Avoid having the ducting turning too many corners as this will seriously compromise the efficiency of your ventilation system.


Fresh air has on average 390 PPM (parts per million) of CO2. Marijuana plants can effectively synthesise 1500 – 2000 PPM. Many growers augment the CO2 supply to their plants and this can dramatically increase yield. It does require a bit more expense but is considered well worth it. Don’t worry though if you can’t afford it as your plants will still produce without it.

If you do decide to use CO2, by far the most effective way is to buy a CO2 bottle and a kit from a hydroponics store which will allow you to regulate flow though a timer. Run the CO2 through rubber tubing, pierced with fine holes and arranged around the tops of your plants (CO2 is heavier than air). Remember to set your timers so that CO2 comes on when the fans go off.

There are cheaper, though less efficient, ways to create CO2 and these include; as a by-product of fermentation, using dry ice, mixing vinegar and baking soda and even spraying the leaves with soda water. Avoid using the fumes of a propane heater as these contain other noxious gasses and often represent a fire risk.

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How to Grow Marijuana Forum - Growing Marijuana Indoors & Hydroponics

18 thoughts on “Hydroponic Ventilation Fans Cooling CO2

  1. desperate says:

    Towards the end of flowering my buds get rubbery. the crystals go away and the hairs turn color early and look fried. Second time this has happened and am about to lose another crop.

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  2. ozzy1 says:

    well guys this is my first indoor grow I have been a gorilla grower for a long time. But that was when I use to live down south and the weather was better. So I just got my grow room done last week. I have the 1000 what big daddy.And let me tell ya I’m really impressed I just got done with germination I had 8 store bought seeds and they all came up.and now I cant stop wondering how much nut do they need. When you grow out side mother nature dose all the work. I’m in my first week and I am really excided and now I am thinking about getting another light so I can start cloning. The question I have what is the least wattage I need to use to keep a mother plant going.

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    Rating: -1 (from 1 vote)
  3. Dave says:

    i have a square room 2 lights in middle where is the best place up high in the room?

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  4. damo says:

    How many hours per day should I leave an oscillating fan on hydroponically grown weed? will 10 hours per day in intervals of 2 hours on, 3 hours off, 2hours on, 3 hours off etc?

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    Rating: 0 (from 4 votes)
  5. damo says:

    How many hours per day should I leave an oscillating fan on hydroponically grown weed?

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    Rating: -1 (from 1 vote)
  6. carrie says:

    What is ibest for mold

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  7. Neil says:

    Very helpful and informative got lots of thing I was doing wrong, due to lack of knowlage thanks so much for the help Neil

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  8. Big bud says:

    Iv just potted my two week cuttings they have been under light now for the first 18hours of there life they have been fed but there leaves are a little droopy and soft what could be wrong ?

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  9. dang ol stoner says:

    I’ve got myself a CO2 tank I use for making club soda and draft beer.
    So I could just mist my plants with soda water?!!

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  10. Rob says:

    I thought this stuff was easy

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    Rating: +2 (from 2 votes)

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