Please help... Problem with night/dark time humidity

Hello once again!

I am about to switch from 18/6 to 12/12. I thought I would get a head start on my flowering stage heat and humidity control.

My research for flowering stage showed…
heat should be in range: 65F-80F
Humidity range of: 40RH-(45-50RH)

My humidity keeps spiking at night to about 80RH. I remedied by adding a heater to up the dark period heat. It was working well for the veg stage at 18/6 however I am having issues with keeping stable Temps and RH after the switch.

My equipment:

48"x48"x81" vivasun grow tent.
2x 1200w Greengo Led lights at 18" above plant tops.
4" ducting with vivasun charcoal filter and inline fan controlled by an bdlink temp controller.
4" ducting for intake.
Vivosun heat mat under plants (northern unheated basement gets cold) with vivasun temp controller.
700 watt radiator heater controlled by a vivasun temp controller.
Vivasun mini dehumidifier and a Everlasting Comfort humidifier both controlled by an ink bird humidity controller.
2x vivasun 6" oscillating clip fans, one above the canopy the other below.

At this point watering occurs at first light in the morning 16 fluid Oz per plant every other day.
9 plants in 3 gallon pots with soil… But will be removing 3 soon as I had a better than expected female to male development from bag seeds. Unknown strains… However all indicia or indica dominant hybrids.

In the morning my digital hydrometer shows the humidy can reach 80RH.

Thank you for taking the time to read all that!

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I would run a dehumidifier at night. Eighty RH is going to eventually led to problems in flowering.

Watch your temps if you do. Dehumidifiers can put out quite a bit of heat.


Dehumidifier or larger exhaust fan. 4” is really small for a 4x4. You could also run both of your fans as exhaust fans and use the vents for passive intake. See if that helps your situation


do you mean adding a small fan blowing air into my intake duct?

i was afraid i should have gotten the 6" exhaust system

You could or just let the exhaust fans pull it in naturally.

Ahhhh I understand… Open up the square vents to allow the 6" oscillating clip fans suck in fresh intake air.

If you move your other 4” to exhaust as well, they will pull air through the lower vents. It’s called passive intake. The oscillating fans inside the tent are there to move air around inside the tent.

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I did more looking into the passive air intake…

Not only is my exhaust system too small at 4" diameter… For my 4x4x6 tent… But also… My intake duct… Which is fanless/passive… Is too small at 4" diameter and should be 2-4 times the size of the exhaust system to allow proper air flow.

I really cannot afford a new 6" exhaust system and larger intake duct for proper air flow… Also… If I open the vents I will get light bleed during dark period of flowering… Soooo…

I am going to buy a 4" duct fan to force more air into the 4" intake duct hopefully helping the issue. Cost on Amazon is:

Both the existing exhaust system and the new intake duct fan will be controlled by the same bdlink controller.

If that doesn’t help I will try to have the intake on a separate outlet for a constant intake air flow. Maybe a different controller. Or added to the humidity controller.

Does this plan seem sound or is my thinking very flawed?

Oh shit, I misread your 1st post entirely. I thought you wrote you were running a 4” exhaust and a 4” intake. I was saying run both 4” fans as exhaust but I see that’s not the case.

Your intake isn’t the issue right now. It’s your exhaust. It’s not able to even move enough air to cause an issue with intake. If you do go with that inline fan, I’d run it as an exhaust as well on the opposite side of the tent. You want to pull that moist air out of your tent.

like this:

That fan you linked I bought for just this reason. It’s junk. It blows soooo soft.

With the inline fan only being 100 cfm, not sure if it’s going to do much for you if it’s connected to a filter. Are you running the other fan full speed at all times and does it have a filter on it as well?

Thank you for the input… I cancelled the order… Am possibly going to order another equal to the original exhaust and filter.

I cancelled the 100 cfm fan order.

The original exhaust system is 203cfm:

I was thinking of ordering another filter(do to odors of flowering) and fan of same cfm.

The original is running on low as it already sucks in my tent sides. Any higher… It is real bad.

Wow i can’t get that much vacuum in m 2x4x5 with exact same fan/filter. But my RH does run the other way. The highest I can get them in veg is like 55 with a humidifier running full time.

So with flower I’m a little worried cause it gets toasty in tent without some humidity Being piped in. Up to 88-90 without humidifier running. Maybe now with cooler air outside I can drop room temp to help.

I ran that same fan for a few years. I think they underrate it because it feels like it moves more than 205cfm. Something that can help with the negative pressure collapse is getting some cheap shower curtain rods or some 1/2” pvc and attaching it across the sides of the tent. Basically making a middle crossbar (something a 4x4 should have been built with) so the sides don’t cave in as much. It would allow you to crank the fan up.


here is the ol shower curtain rod trick. 5da191a8fad440c9d12f082005b258962bb61220_2_666x500



Thanks for the pipe tips… If I turn up the fan it sucks the sides in so much that it interferes with the oscillating fans making them click from their oscillating being restricted.

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Does the tent really only have 1 4” port on the bottom? If so, the pic you showed shows two exhaust ports, you could open the unused one and should alleviate some of the negative pressure.

Actually I think they are 6" ports… I have a 4" flex duct with a mesh cover to help with light bleed… I will try to add one to the other bottom port to help with suck-in.

I had one extra vivosun 4ft rod… That has helped with the click so far.

Original exhaust:

New exhaust pre-setup/ordering of new fan and filter:

I also cranked up the fan on the original.