Newbie with temp and humidity problems


I am on my first grow, my plants are about 4.5 weeks since germinating the seeds.

I have a 2m x 2m x2m tent, I currently have 13 plants under 2 x 600w HPS lights and intend to have them under 4 lights when they are a little bigger. The lights are about 2.5 feet above the canopy at the moment. I have 2 problems, which I assume are linked.

  1. The temp in my tent is too hot. It was fine before I put the 2nd light on, now the heat is climbing to around 30 - 32celcius. I have ventilation (an sms dual controller) with a 5 inch intake fan and an 8inch exhaust fan. The exhaust fan is working extremely hard but the intake doesnt seem to be blowing much. I have plugged the fan directly into the wall and it comes on full, but when plugged into the controller it seems to work not much better than a passive intake.

The room outside the tent is cold and the intake fan is near an open window (i put tights over the outside of the fan to stop beasties).

The air coming out of the exhaust fan feels cold, which is odd because it is up high and the tent is very warm).

I have taken the controller back to the shop and changed it for a new 1, this hasnt helped. Do I have the correct sized fans? Also I have read that maybe I should just use passive intakes (but more of them), what are your thoughts on this? I;ve tried having both vents open, so the negative pressure is quite small. How much negative pressure should there be?

I had to fit my own wires and plugs to the fans, and thought I may have used wire that was too thick (ie too much resistance) and i have changed it for thinner wire but this is no different.

The plants are in 2.5 gallon pots on the ground the sensor is canopy height as well as my 2 thermometer/hygrometers.

The ducts are quite short and not bendy.

Any suggestions as to what is wrong and how I can get the temp down. I believe it needs to be a steady 25 celcius (ish) in the day.

  1. The humidity is too low. I think this may be because the exhaust fan is working so hard. I have 2 small humidifiers on constantly. I also have oscillating fan, up high, facing toward the canopy and on low. I realise the fans may reduce the humidity but I want the plants to grow strong stems.

I have hung wet towels in the grow tent to get the humidity up, this is inefficient and makes a mess. The humidity rises and then falls again quite quickly, and water gets everywhere. Placing bowls of water in the tent doesnt seem to make a difference.

If i get the ventilation system working correctly will this sort itself out. I realise that a 2m x 2m tent is too big for these small plants but its that only space I have at the moment and the issue is that its too hot not too cold and i only have 2 lights on, surely the controller and fans should be able to handle this. I also realise that humidity ought to be less of a problem as the plants grow.

In summary

  1. Should I need passive intakes or should the intake fan just work better, and that this is basically calculated by the controller?

  2. Do I need to raise the plants off the ground, and in turn raise the sensors and thermometers?

  3. Any idea why the controller isn’t regulating the temperature correctly.

  4. Should the negative pressure be so much that the tent is sucked in very strongly, so much so that the floor is 6 inches of the ground in places, or is this a sign that something is not right?

  5. Should I turn the oscillating fan off to get the humidity up, which is the worst of 2 evils: having no air blowing on the plants to make the stems strong or the tent not being humid enough (its currently around 30-40 %)

  6. Any other suggestions for getting the humidity up?

  7. If I get the ventilation correct, will the humidity issue resolve itself?

Thanks all for your help.

are you using cool tubes or any type of vented shades if you are going to run lots of lights it’s the only way to keep lights from producing huge amounts of heat based on the fact you said 2 fans I am guessing you aren’t? Vented shades let you pull heat directly off lights and make a huge difference in an 8’x8’ space as described without ac I wouldn’t add 4 lights without vented shades the vented shades also let you exhaust slower so humidity is easier to control

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Hi, I no i’m not, but I’ll investigate. i still cant help feeling like something isn’t working. All the equipment was bought together from an experienced guy in my local hydroponics shop. He seems to think it should work perfectly.

Thanks for your input though, Ill look into vented shades.

Hi, also I just wanted to clarify, I’m only running 2 600w hps lights at the moment.

Humidity will rise and fall depending on how many and how big the planets are but you really need to look into vented hoods, I don’t think you’re going to be able to handle the heat without them


those lights u have are not made to run in a tent they need to b in a open room b/c of the heat if u are gonna run 2 600watt hps bulbs you need a cool tube or a vented hood

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Thanks for your input, I will look into the vented hoods. Can anyone suggest why the intake fan is not coming on?

speed controller is shot try bypassing it cut the cord and wire plug after controller if it works then go buy a light dimmer and wire it in they are cheap 9-10 bucks any dimmer rated for house lights will work

Hi, thanks for your comment. I have replaced it with a new 1 as it had a guarantee. It doesnt seem to have fixed it thought. I have been researching the vented hoods though. I think I will invest in them.