Negative vs. Positive pressure in tent

Currently I have negative pressure in my tent, but I plan to add an intake fan that may cause me to have positive pressure. Will this cause any harm to the plants? I can’t imagine it’s more than a lb or two of pressure.

2 Likes

I’m not sure to be honest but @Countryboyjvd1971 knows about negative pressure I believe and might be able to answer your question.

2 Likes

Positive pressure is better , stops mold from farming

1 Like

I assume you have an exhaust and carbon filter set up?

If so, the only harm positive pressure would create would be unwanted smells external to the tent. The neg pressure insures all of your tent air is being sucked out the exhaust system.

I just added a 240 cfm inline fan for intake at the bottom of the tent. I have a 190cfm exhaust fan and the intake has not effected my neg pressure (which I need to control smell. I’m sure there are more knowledgeable folks who will add/subtract from this but hopefully this helps.

3 Likes

I don’t agree.
With positive pressure you get ballooning which causes stale air to sit in one spot without moving, such as in the corners of the tent. It wont give you 100% refreshed air like you get with negative pressure which causes a vacuum affect.

4 Likes

And to add to that, Gorilla Grow Tents even sells extra frame tubing that you can install to keep the tent sides from sucking in to bad when you have lots of neg. pressure.

3 Likes

Negative or positive pressure doesn’t control smell a carbon filter does.

I’m with @Rugar89, on the positive pressure.

Having a positive pressure situation will indeed cause unfiltered air to leave your grow area without being filtered. When you exhaust through a carbon filter, you want any and all air leaving your tent to be through the filter. If you’re bringing more air in than you’re exhausting through your filter, that’s not happening.

2 Likes

But it does Bro. With pos. pressure it will cause excess air to leave the tent thru the seams. The tent is not air tight like a ballon, so it has to go somewhere. With neg. pressure all the air in the tent is forced to be sucked out thru the filter.

5 Likes

@garberfc I would suggest making sure your intake fan has a speed controller. You’re going to want to maintain the negative pressure, even if it’s just a slight amount. Being able to tweak your fan speeds will give you a lot of control.

2 Likes

Cool, I was trying to figure out how to keep my tent from sucking in when vents were closed. I’d thought about curtain rods placed inside but haven’t figured out how. This would be much easier

@dbrn32, @Rugar89, @Wishingilivedina420state, @Screwauger, @Jmesser80

Let me answer a few of the questions first; Yes, you’re absolutely right, I don’t want odors leaving the tent and that would happen with positive pressure, I hadn’t thought of that. I’m in my seedling stage so that’s not an issue right now. Being in the seedling stage, I removed the carbon filter in an attempt to move more air through the tent. I’m trying to lower the temp in the lower / floor of the tent (and at the suggestion that I save some $$$ by not using it if it’s not needed). Right now the temp is at 86-88F.

I have a 4x4x80 tent and I’m exhausting the tent with a 6", 440cfm fan (no filter, but I’m going to add it back). I have my light raised to about 48" from the floor.

Here’s a question for the group: I’m exhausting the air from the floor of the tent. Would it be more effective in lowering the floor temp if I moved the exhaust fan to the ceiling where all the hot air goes? And keep the inlets at the floor open?

Thanks in advance for your help,
F

4 Likes

I do think that is standard, to vent the hottest air out of the tent and since colder air is heavier, an exhaust near the top portion of your grow space would naturally draw the cooler air up due to the circulation. I say, intake cooler air at bottom, vent warmer air top.

1 Like

yes

hot air rises

3 Likes

@garberfc
From a thermodynamics standpoint, I’d say yes you would want to exhaust the top of the tent, as hot air rises. The colder air would circulate up through the tent, lowering temps and also stabilizing them. @Countryboyjvd1971 should be able to give you the best answer though. He’s the HVAC guy.

4 Likes

In mine, I have exhaust fan up top, intake is underneath. My box is raised up 3 inches on a platform so intake comes in though it. I keep negative pressure, but it is not extreme. :sunglasses:

1 Like

My temp can be controlled by fan speed. :sunglasses:

1 Like

@Wishingilivedina420state

Sorry bro but I’m with @Rugar89 and @dbrn32

You want negative pressure.

It will pull all exhaust air out through your filter and also allows for all air to be continually exchanged in the tent and avoiding stale air.

This is the best way to prevent mold imo.

3 Likes

@garberfc

For sure you want fresh air in the bottom and exhaust out the highest point.

3 Likes

With the filter/fan at the top of the tent the temp at the bottom of the tent has dropped a few degrees to 84F. I have a window A/C in the grow room and I’ve just dropped it’s temp by 2 degrees to see if that’ll help drop the tent temp further.

My seedlings are just breaking through the surface of the soil, are they hardy enough to handle the temps / light of the tent? Or, should I give them a few more days of growth? The lights are 48" from them.

2 Likes