Grow tent dehumidification advice

Hi guys,
Need some dehumidification advice here.
Its a10x5 tent in the room, with 2 600watt closed loop air cool hood and 6pcs COB panel@300watt.
Im in the area RH is pretty high. normally 70-80% can get over 80% when in those raining days.
I have window A/C on 24/7 grow tent temp high 70s to low 80s, but it doesnt bring down the RH in the tent.
RH 78% that I just read
for my ventilation
6" fan Exhaust the air out of the tent into the room on 24/7
6" fan for 2 600watt closed loop air cool hood. (intake from room and exhaust out of the room) only kick it when lights on.
6" duct direct to window A/C with no fan. in the tent side, duct come in from upper side hole direct the mid. height where.

they are pass three week after flip, I think I should have bring down the RH in this stage.
I have a dehu unit, but never use it in grow before.
Need some advice for set up Dehu for grow tent or any idea bring down RH in the tent.
appreciate all the feedback or inputs

ligrow said:

6" fan for 2 600watt closed loop air cool hood. (intake from room and exhaust out of the room) only kick it when lights on.

Click to expand...

Adding a dehumidifier is going to add a bunch of heat to your room. Which is necessary sometimes, but not if your AC can't handle the heat.

From what it sounds like the 6" fan for your air cooled hoods intake should come from either another room, or possibly outside if you live in a climate that doesn't really see winter.

With that setup the duct work from the AC to the tent is probably unnecessary.

I'd try that, if it still doesn't work, list your fan types, and list and measure separately your room/tent temps and humidity during lights on and lights off. Actually, better yet, measure your room/tent temps and humidity during lights off and on now, and then change the setup and compare them.

- Jiji

@jijiandfarmgang thanks for your feedback
Adding a dehumidifier is going to add a bunch of heat to your room. Which is necessary sometimes, but not if your AC can't handle the heat.
yes i know adding Dehu its gonna heat up my tent. Do you think Dehu is the only wat to bring down RH?

From what it sounds like the 6" fan for your air cooled hoods intake should come from either another room, or possibly outside if you live in a climate that doesn't really see winter.
Yes from what i read most closed loop air cool setup intake is from another room or outside. I was worried outside temp is not cool enough to cool down HPS temp. lights on at night time, outside temp should be around 80-85, will be hotter in upcoming season.
Do you think its good idea to have intake outside when temp high like this? i dont have another room for intake btw.

With that setup the duct work from the AC to the tent is probably unnecessary.
When I not having duct for A/C, my tent temp gets even higher, but the room is cold.

I'd try that, if it still doesn't work, list your fan types, and list and measure separately your room/tent temps and humidity during lights on and lights off. Actually, better yet, measure your room/tent temps and humidity during lights off and on now, and then change the setup and compare them.
I will have some setup change, and compare it. I didnt have temp and RH meter in the room.
6" intake I use no brand fan, Its like 440 CFM unit
TEMP when light on 78-81F RH over 70% in past few weeks
when lights off.75-78F RH I wasn't sure( but the night time temp was three weeks ago. since I flip I didnt bother them at night, except for one night I forgot to shut the tent when i left.

Any feedbacks are still welcome, would like to hear more opinion for you guys
really appreciate

The way you have your tent exhausting into the room that you are cooling is wasteful of the energy you are using and isnt working in your favor.
If you can manage to rearrange your inlets to pull air from outside to cool the hps lights, then exhaust that outside, your A/C unit will be more effective and you wont be constantly heating the room you are using to cool your tent with.
Here is a quick example of what I mean:


This should help you get your environment in check and your RH down before running a dehumidifier. The A/C unit will naturally pull humidity from the air once you have everything set up properly and the ac isnt trying to cool "new air" all the time. Hope this helps.

ligrow said:

@jijiandfarmgang
From what it sounds like the 6" fan for your air cooled hoods intake should come from either another room, or possibly outside if you live in a climate that doesn't really see winter.
Yes from what i read most closed loop air cool setup intake is from another room or outside. I was worried outside temp is not cool enough to cool down HPS temp. lights on at night time, outside temp should be around 80-85, will be hotter in upcoming season.
Do you think its good idea to have intake outside when temp high like this? i dont have another room for intake btw.

Click to expand...

Thats a great temp to pull from outside. Think of it not so much as cooling the lights down, but as getting rid of the heat.

- Jiji

@HydroRed Really appreciate all the detail with clear pic. This help me a lot.
could you advice which thermostat controlled outlet that can be use. I will see if I can find some similar model in local. only have speed controller on the fan right now. never used thermostat controlled outlet before, just check that on web.
If I hook it with tent exhaust fan, it wont work when temp is below then what I set on thermostat controlled outlet isn't it?
I thought exhaust fan should alway kick it 24/7 if is not in sealed room with CO2.
I want to know why need to use it. saving electricity bill?

This is my setup right now. I will hook the intake for air cool hood outside for sure.
but the fan is on the other end, I thought is always better pulling air than pushing?
Sorry for the bad draw.


once again really thanks for your feedback here
just want to clear thing up and have best efficiency setup in my current op

Good day@jijiandfarmgang
got it! Im sure I will have it hook up outside.

I would most certainly draw air to cool the hps hoods from outside, and vent that outside as well. Here is the same thermostatic controlled outlet I use. It will cut off once the proper temp is achieved. Considering the lights will still put off some heat in the tent, your thermostat will kick on every little while and clear out the heat pulling in cool air from the cool air conditioned room that the tent is in creating a good air exchange in the tent. This is what I use:
https://www.amazon.com/Lux-Heating-Cooling-Programmable-Thermostat/dp/B000E7NYY8/177-7383523-2503043?ie=UTF8&*Version*=1&*entries*=0
According to your drawing, you arent set up very efficiently at all like I mentioned in my first reply. Try doing like my example and you will see how less often stuff will need to run (including the expensive A/C unit) and get your whole environment under control -including the room that the a/c is in. If you do like I show in my example, it eliminates the need to run ducting directly to your tent from a/c, and you will be expelling a ton of hot air outside instead of the room you are paying $$ to cool with an air conditioner. As for fan placement, I put my fans before the source so if there are small air leaks, I wont be sucking unfiltered weed smelling air from the leak spots and sending them outside....again, unfiltered. Its really not even for performance reasons, but stealth reasons. If your fan is before the lights then if you do have small leaks anywhere like around the seal of the glass on your hps hoods, it will just blow through and put the unfiltered air that may escape right back into the tent. No worries of smell getting out to those that dont need to know what you are growing!

@HydroRed again appreciate the details
I already have the intake of air cool connect outside. Have intake remove from A/C, set it at lower side of the tent. The room is cooler, but tent is not cool enough. 76F in the room. 82f in the tent. I have my tent exhaust on with full speed.
For Controlled outlet, I can not find similar product in my location. I will get it from Amzon anyway. Have a question for the outlet. Is the temp sensor on the unit? If yes, I have to connect it in the tent that way sensor can measure temp inside of the tent right?

You got it. Temp sensor on the unit and it has to be in the tent to read the tent temp.
What cfm fan do you have in the tent. Considering the tent is a 5x10 if I remember correctly without re-reading the thread, I would imagine you need a large cfm(600cfm+) fan to properly move the air in the tent fast enough to keep temps down. With fans, I make it a standard to get a bigger fan than I need because you can always dial it down. Honestly though, a high of 82F in the tent aint a bad temp to deal with in my opinion.

HydroRed said:

You got it. Temp sensor on the unit and it has to be in the tent to read the tent temp.
What cfm fan do you have in the tent. Considering the tent is a 5x10 if I remember correctly without re-reading the thread, I would imagine you need a large cfm(600cfm+) fan to properly move the air in the tent fast enough to keep temps down. With fans, I make it a standard to get a bigger fan than I need because you can always dial it down. Honestly though, a high of 82F in the tent aint a bad temp to deal with in my opinion.

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I found a controlled outlet in local market with wire sensor on it. It's even better for me. I keep all the plugs out of the tent if possible.

After couple hours observation. Keep air cool intake out of the room does help. AC running less time. Should save me some money on bill.
But tent gets cooler when have intake duct hanging on AC. Tent temp 82 was on the coldest side of the tent. When it's 78 on intake side, I measure another end of the tent. It gets to 82. Lights just shuts off. I didn't have time measure what temp hottest side would be when it's 82 on intake side. Probably 84 85? I though in flowering stage should always under 80? If 82 is fine in tent.
What it's the highest point for you when you consider tent temp it's too high?

In inline fan I believe it's 440cfm. If I need 600+ and no fan on intake side, won't be too much negative pressure for tent? I just get in tent at shut it off stay inside for
Checking if there is any light leaks. As soon as I shut the tent, negative pressure have my tent suck in. That reduce my grow area. If runing 600+cfm fan should have more negative pressure in tent, won't be too much?

I have my tent intake duct hang back to the AC. Have the duct in the tent longer to reach out lower side of the tent for now.
I will see if I can set it like you suggest run less electricity.

Everything I posted are merely suggestions to help get you in the right direction. There may be things that just work better for you like keeping the ac duct close to the inlet or finding a thermostat with a long temp probe to keep plugs out of the tent. If it works good then run it! At the end of the day its all about what works best for your grow and the results you get from it.

HydroRed said:

Everything I posted are merely suggestions to help get you in the right direction. There may be things that just work better for you like keeping the ac duct close to the inlet or finding a thermostat with a long temp probe to keep plugs out of the tent. If it works good then run it! At the end of the day its all about what works best for your grow and the results you get from it.

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I sure try thermostat if that's the efficient way for my tent setup.

Just checked night time temp and RH. Room 76.7F RH 71%
Tent. 75.2F RH 81%
Temp should be fine, and back to the topic
What if AC can not bring down RH in tent?

Sorry for that much questions
You are a really nice guy, I remember you point me out direction on other thread too
Really appreciate it.
Happy grow

If I have everything correct, your exhaust fan is probably going to have to run 24/7 (not for co2 reasons but temp and humidity), you'll need a larger air conditioner (or less lights) and a dehumidifier.

Temps are ok now, but RH is not.

I dunno, maybe if you want to grow certain strains that are botrytis resistant, and be ready to fight pm.

- Jiji

ligrow said:

I sure try thermostat if that's the efficient way for my tent setup.

Just checked night time temp and RH. Room 76.7F RH 71%
Tent. 75.2F RH 81%
Temp should be fine, and back to the topic
What if AC can not bring down RH in tent?

Sorry for that much questions
You are a really nice guy, I remember you point me out direction on other thread too
Really appreciate it.
Happy grow

Click to expand...

Ya, that humidity is way too high. I figured the ac would have brought it down by now once everything was set up. What size(BTU) A/C do you have & what size is the room that the tent is in? Cuz like mentioned by Jiji, once you add the humidifier it will add heat to the room even more than the tent -which in turn makes your A/C run hard all the time if it isnt sufficient enough for the task. Considering it sounds like the unit you have now is just barely keeping it where it is, how will it be once hot weather really hits and you have a dehumidifier blowing hot air into the room now too. I think Jiji is on to something with the (or less lights) thing. Since your hps are vented and ducted already & using outside air to cool, that leaves your led light that isnt cooled or vented and is heating up your room considerably I'd guess. Could you possibly cut just the led for 1 grow and get yourself a few dollars together and get a larger A/C unit. They have portable A/C units that have built in dehumidifier as well so you could kill 2 birds with 1 stone. Its an all in one unit and allows you to vent ALL the heat from the A/C and the dehumidifier out the window with 1 vent. No added heat to your room!!

Heres a few questions that will help me understand your room better.
What kind of led light are you using in your tent?
Can you get by with your current grow without the led light?
How many BTU is your A/C unit?
How big is the room that the tent is in?

jijiandfarmgang said:

If I have everything correct, your exhaust fan is probably going to have to run 24/7 (not for co2 reasons but temp and humidity), you'll need a larger air conditioner (or less lights) and a dehumidifier.

Temps are ok now, but RH is not.

I dunno, maybe if you want to grow certain strains that are botrytis resistant, and be ready to fight pm.

- Jiji

Click to expand...

thanks for your feedback again. What are the strains that botrytis resistant? are those strain just more durable in high RH environment? dont want fight with pm...Still looking for some way to fix my setup.

HydroRed said:

Ya, that humidity is way too high. I figured the ac would have brought it down by now once everything was set up. What size(BTU) A/C do you have & what size is the room that the tent is in? Cuz like mentioned by Jiji, once you add the humidifier it will add heat to the room even more than the tent -which in turn makes your A/C run hard all the time if it isnt sufficient enough for the task. Considering it sounds like the unit you have now is just barely keeping it where it is, how will it be once hot weather really hits and you have a dehumidifier blowing hot air into the room now too. I think Jiji is on to something with the (or less lights) thing. Since your hps are vented and ducted already & using outside air to cool, that leaves your led light that isnt cooled or vented and is heating up your room considerably I'd guess. Could you possibly cut just the led for 1 grow and get yourself a few dollars together and get a larger A/C unit. They have portable A/C units that have built in dehumidifier as well so you could kill 2 birds with 1 stone. Its an all in one unit and allows you to vent ALL the heat from the A/C and the dehumidifier out the window with 1 vent. No added heat to your room!!

Heres a few questions that will help me understand your room better.
What kind of led light are you using in your tent?
Can you get by with your current grow without the led light?
How many BTU is your A/C unit?
How big is the room that the tent is in?

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1. LED I use 6pcs COB cxb-3590 3500K @50% 300watt for a panel
2. I have another 400watt ballast, if it really need to I can grow it without LED, but light wont be as much as right now. Those Cob panel work better than HIDs in my opinion.(havent use it harvest yet, not sure about the end result. from veg to now, plants under LED grow faster more bushy, and buds sites are closer to each other) so I want to keep LED if possible. actually I was thinking ditch HIDs if I have good result this run with LED.
3. The window A/C is 8000btu. I set it 24C, room temp 76 RH still in low 70%
4. The room its 10x16 height 8. I used to slept in that room. it feel like its more humid in there. Also my location its humid too. 60-70% over 80% when rains

Here is a thread for my current grow, have some pics for my setup
https://www.rollitup.org/t/ebb-and-grow-with-scrog.909106/

I have another 9X13 room running 12000btu single hose portable A/C for a 4x4 and 5x5 tent.
for that room I can have my RH to low 60% have a temp around 80F.
I feel portable are not good as window one. especially for single hose one.
They might not cool the room like I expect them to battle with the heat.
There were some heat days last week, portable one can only reach 82F. no matter how low I turn on the AC unit.

I want to have this OP running good as possible. I wouldn't mind spend some money on it to have the problem fix up.
Could you also advice the model you mention portable A/C with duhu function together? its i dual hose one? I heard dual hose works better.

just check on some portable A/C in my local market, I found a unit have 4 in 1 function. (Cooling, heating, Dehu, and Air Purifier)
Its dual hose model with Inverter air conditioning. its the only portable model I can find with inverter air conditioning.
Im not sure if that is like the one you mentioned. its 12000btu one. its also not cheap, about 1000USD for that unit.
Plants are look fine right now. I spent a lot of time and money on this run. Just want to make sure I can provide best environment for them.

The one I use is a Frigidaire with single hose A/C & Dehu in one. I want to say its 12K btu but I'll get a model number when lights come on. Im in a 12'x9'-ish area with 7' ceilings and this unit is overkill in summer months in the Midwest with (2) 1000W HPS. It also has a "swing" option on the louvers where the cold air comes out. It automatically moves the louvers up & down to move the air in all directions eliminating hot/cold spots.

I turn AC temp lower to 22C.
The room

HydroRed said:

The one I use is a Frigidaire with single hose A/C & Dehu in one. I want to say its 12K btu but I'll get a model number when lights come on. Im in a 12'x9'-ish area with 7' ceilings and this unit is overkill in summer months in the Midwest with (2) 1000W HPS. It also has a "swing" option on the louvers where the cold air comes out. It automatically moves the louvers up & down to move the air in all directions eliminating hot/cold spots.

Click to expand...

I have a Frigidaire Portable A/C with single hose, also have Dehu function. I thought dehu and A/C work separately.

I also wonder why A/C is cooling the room well, but RH still that high.
I try turn temp of window A/C down to 22C. room is cooler also tent RH drop a bit to 56% room RH 61%

I also use a dehumidifier since with my portable A/C unit when run on dehumidifier, it doesnt have a temp control. It will just freeze out the room until your turn the dehumidifier off and turn the controls on the A/C back to regular A/C. I use both on those hot muggy days where the RH can jump up from 50 to 80% real quick. Using both units I can drop the RH 30% and cool the room in a matter of minutes.
56% in your tent isnt a bad number at all. Certainly if you have a lot of good air circulation and movement in your tent.