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HELP! 8x4 temp and vent problems

ocgreen

ocgreen

New member
Hi all,

Hope everyone is enjoying the summer.

I am struggling to control temps in my 8x4 and not sure if the answer is that I need a bigger exhaust fan or what.

But basically I just setup an 8x4x7 with 2x 315W CMH.

Here's what I have for ventilation:
  • 440 CFM fan pulling air out of top left of tent
  • 6" passive intake hole at bottom right for cold air intake
  • 2 fans in opposite corners to try and get air blowing in a circle like guide on GWE suggests
  • Fan above canopy but below lights pointing slightly upwards, as a way to push hot air from lights towards top of tent
Despite all that, I woke up to 31.8C and 27-28C reservoir temps in the buckets. And I can't seem to achieve negative pressure.

Can someone please help me understand what I'm doing wrong here? Can't seem to figure out the larger tent space.
 
I had the random idea to take off carbon filter and now I'm achieving solid negative pressure but the temperature increased (?) and RH decreased. I dont understand why the temp would go up if I'm increasing the amount of hot air being pulled out of tent.
 
What is the temperature of the room the tent is in? Assuming your exhaust is vented outside, your best bet is to cool the room the tent is in. A larger fan will move more air but it won’t cool the grow space unless the intake air is significantly cooler.
 
The tent is in a room which is much cooler (21-23C) and the exhaust vents outside. Is 21-23C cool enough or should I be aiming to keep that space even colder?

The good news is that the temperature is slowly dropping since I removed the carbon filter and achieved better negative pressure. It went up at first but started to fall slowly over time. Still hot but at least it's below that 30C upper limit now.
 
Temp slowly started dropping after it initially went up. Have to assume that it went up because it was sucking more hot air passed the sensor on the way outside.
 
21-23C is about the best you can hope for. What is the distance of the exhaust run and where is the fan located?Removing the filter allows the fan to pull closer to its rated cfm by eliminating resistance so you either need a higher cfm fan if your exhaust run is short or you need to add another fan if you have a long stretch of pipe.

I have a 440 cfm set up for my 2x4 tent with LEDs and at first glance it may seem excessive but the air has to travel 12 feet before it exits my space. By contrast I have a 200 cfm on my 3x3 with a 400w HID but the air only travels 3 feet before it exits outside.
 
I think Obi may be on to something regarding reduction in cfm. Carbon filters, every foot of duct and any bends in the ducting will reduce output. Smooth ducting is more efficient than the flexible "slinky" ducting too. There are calculators online where you plug all factors in and it will give you the required CFM rating. Then make sure your filter has a similar rating. An Alternative is to setup an AC in a window or use a portable. Run a duct from the AC output to a tent port. You may have to reduce or even eliminate your intake fan so you don't end up with positive pressure but this is a good way to cool tents.
 
Hey Obi-Wan thanks for your help here. It was about 12 ft to window. But what I came to realize after I read your reply is it had a few bends in it to get it out. Long story short I repositioned things to make it smoother line to window. Now I'm getting better pressure.
 
Hey JayGee good call man. I angled a fan I had blowing basically straight under the lights so it points more st lights towards ceiling and it definitely helps
 
You and Obi helped me realize I had some issues with my ducting. Made some adjustments and temps are a few degrees lower.

Realistically I probably need to buy a water chiller, a portable AC or a bigger fan but I'm really trying to avoid it.

After adjustments max temp for past 24 hours has been 28, so 2 degrees higher than I'm hoping to get it down to but I'm making progress.
 
Thank you to everyone who helped me here. Happy to report I woke up to 25.4C and 61RH this morning. Pretty good for early veg as far as I understand.

Key issues everyone helped me rectify:
  1. My ducting was unnecessarily long and had some bends. This was reducing CFM too much.
  2. Pointing a fan up towards light so it pushes hot air towards fan really helped.
  3. I added a 1.5' length of 6" ducting to passive intake hole and that seemed to improve amount of cold air being sucked in.
  4. I probably need a bigger fan because I can't achieve good negative pressure if i put the filter back on.
So I am going to find some time to use the calculators mentioned above and grab a bigger fan. Beats dropping major bucks on an AC and/or water chiller!

Thanks again
 
The thrifty way is to freeze water in milk jugs or juice containers at least a quart size and then you can rotate them into your reservoir don’t melt fast but Bill bring down the temperature a few degrees. Just keep rotating them and freezing them again
 
Your situation is the reason I went to no till and low wattage hi efficiency puck diy LED’s keeps the heat and the utility bills down
 
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