Ventilation and Temp Control

Ok guys. Me and @Zee we’re talking thermometer placement in a room/tent with temperature issues and when looking around I noticed that some growers/setups are running the same fan to extract/filter and to cool an HPS hood. For whatever reason, I’ve set my newly built room up with two systems. One fan pulling out through the carbon fiber filter, and another pushing fresh unfiltered air in from outside the tent, through the hood and back out. Are the two systems necessary or am I just running two fans for no reason? I ask also because the fan I’m using to cool the hood is weak and needs to be upgraded. The filter fan is fine. If I combine the systems I don’t need to buy/operate more equipment.

Subject room is home made flower chamber on the left - 18sqft. Little 3x3 veg tent on right just snuck into the photo.

Thoughts appreciated. Have a Grateful Day :v:t4:


Since I run just one exhaust. I should add this, when it was just ran through the hood vent, my temp was 82, when I added the carbon filter, it increasrd by 2 degrees. I can see the benefit of 2 exhaust for the reason of decreasing temps. If you have the time and means, I would tinker with trying just one exhaust, see if it’s worth the cost. :slight_smile:

We can see what @kellydans @Hellraiser have to add :smile:


You should only need an exhaust fan. The typical setup would be:

carbon filter → light fixture → exhaust fan

The only time you should need an inlet fan is to deal with excessive temperatures. Even in those cases though, the ambient (outside the tent) environment is what really needs adjusting.


Nice-looking set up looks like you put some time into it.I have a little different setup I only use one exhaust fan pulling hot stale air out the top. With two 6 inch pass through holes open at the bottom to allow fresh air to be pulled in the bottom up through the tent .and adjust exhaust fan to maintain a good negative pressure inside. It does seem like you should have fresh being pulled in other than just for the hood . Let’s see what other community members think ,that use hoods.


Yes, you definitely should have a fresh air intake. Have it at the bottom so it is pulling cooler air. It’s also best to have it on the opposite side of your exhaust(Remember, your exhaust will start the carbon filter, not at the exit hole) in order to promote crossflow.
Here’s a quick diagram of how one of my HID tents is set up. You can ignore the inlet fan as you shouldn’t need one.

It also looks like you are venting right outside the tent. If so, you are likely warming the air that is being pulled into the intake. You might consider putting a long extension of flexible duct to move the exhaust away from the intake. Even better would be to exhaust it out to a different room or outside the house completely.


@TommyBahama got ya covered.

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Thanks man. I actually saw this picture on a different post which is what started my muse. Today when the lights come on I’ll add an inlet vent and extend the tube on the exhaust. The underlying question still remains though. Do I really need two fans or is there at least a benefit? Second not being an inlet fan but a through fan that only services the hood separate from the filter and exhaust.

Thanks @kellydans. It was a rush job. I had to move abruptly and my 48x48x80 tent was too tall for this basement. The girls were bushing too big for the 36x36, so I hillbilly rigged that little room for them to finish.

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You do need two fans, but perhaps not what you are thinking:
You need an exhaust fan to:
evacuate your tent
pull air through your carbon filter
pull air through your air-cooled hood

You also need a fan inside the tent just to move air around. This helps with:
Moving the plant around, which strengthens its stems
Moving the air around, keeping humidity and temperature more even
Moving the air around, lessening the possibility of mildew.

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I’ve got fans in the tent. Just talking inline. My two inlines are:

There’s nothing wrong with doing it that way, but consider this if you do it the other way:
One less piece of equipment to run.
The carbon filter will keep particles from entering your HID fixture, keeping the bulb and the glass clean.

But, the way you have it is fine. I’m just letting you know some of the options.

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Yeah man. That’s what I was looking for. The exhaust is a 4 inch setup and the hood is 6 inch. So I just got a cheap Lowe’s fan and six inch ducting to cool the hood – but the little fan just don’t cut it. The 4 inch exhaust really blows hard. I’ll just get some 6-4 reducers and throw them on the filter and fan and use 6 inch ducting and cool the hood with the exhaust. I’ll use the other two holes in the wall as inlets. They’re not as low as I’d like, but they are already there . . .