Is strictly exhausted Dehumidifier air good for plants?

This might be a weird question but is just exhausted dehumidifier air good for plants?

Long story short as possible, I have a tiny 2x2 grow tent that can't be fixed with a tiny dehumidifier inside to keep RH 40-45% during flower. So I bought a big nice quality dehumidifier and have it set to 35% outside the grow room and STILL I'm having a hard time getting the grow tent below 50%RH. So to fix this problem I have ducting running from the exhaust of the dehumidifier to the grow room (the tent's only air intake) and now my tent is finally 40-45% RH!

The question is though, is this good for my plants? The dehumidifier has an air filter in it on it's intake but not sure if the machine itself does anything to the air to make the exhaust have poor air quality.

Also leaving the grow tent door open and the dehumidifier outside it is not an option due to the smell. Also I cannot simply put the big dehumidifier in the grow room cause the ladies are already into flower and there isn't any space.

I'm no expert but I would think its fine as long as the heat doesn't cause problems.
I mean, people put those IN grow tents so what is the difference.
Sounds like a great idea.
What dehumidifier are you using?

jrs302 said:

I'm no expert but I would think its fine as long as the heat doesn't cause problems.
I mean, people put those IN grow tents so what is the difference.
Sounds like a great idea.
What dehumidifier are you using?

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It did raise the temp a bit by 3 degrees, 76 to 79, but should be fine in that regard. Just wasn't sure if the machine would mess with the CO2/oxygen/nitrogen levels in the air. This is what I am using:

I did a quick google search and did find an article talking about this.

Look at option 2 on that site.
Not sure of the validity of that site though.

Here is another one.
https://www.reddit.com/r/macrogrowery/comments/c9gc1u

The air is only getting blown through a cooling radiator, that's what extracts the moisture.
Yes its fine but don't blow it directly onto plants it can cause wind burn.

jrs302 said:

Here is another one.
https://www.reddit.com/r/macrogrowery/comments/c9gc1u

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Looks like I can do it. thank you

jrs302 said:

Here is another one.
https://www.reddit.com/r/macrogrowery/comments/c9gc1u

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jrs302 said:

Here is another one.
https://www.reddit.com/r/macrogrowery/comments/c9gc1u

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Thank you for doing some research. Was it just me or was the it hard to follow what he was saying for option 2 with the cardboard box? ha Here is the way I did it

I’ve been growing in a dingy basement for years. Running a dehumidifier 24/7 in/near the room you are growing in will be sufficient. Don’t worry so much about the relative humidity inside your tent; high/low RH is not really much of a problem until you go to dry your bud. Ditch the duct from your dehumidifier into the tent; it’s not helping. It’s not really hurting anything either but a much better setup is to pull off the heat with a ducted exhaust and just run your dehumidifier outside the tent for ambient Rh. Optionally you can add a filter to the duct to eliminate most of the smell.

Richard Drysift said:

I’ve been growing in a dingy basement for years. Running a dehumidifier 24/7 in/near the room you are growing in will be sufficient. Don’t worry so much about the relative humidity inside your tent; high/low RH is not really much of a problem until you go to dry your bud. Ditch the duct from your dehumidifier into the tent; it’s not helping. It’s not really hurting anything either but a much better setup is to pull off the heat with a ducted exhaust and just run your dehumidifier outside the tent for ambient Rh. Optionally you can add a filter to the duct to eliminate most of the smell.

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Thanks for the note Richard! I shouldn't be worried about RH% 55-65% during flower? I thought that would lead to mold/bud rod. The stains I'm growing now I am not sure if they'll have fat colas (5 days's into flower) but next run I am doing is Moby Dick XXL and White Widow XXL (both Dinafem) which are known to have really fat colas. Are you saying in that case as well I do not need to worry about RH% hitting 65? Thanks for the help

I’m saying you should set up your tent so there is a ducted exhaust pulling off the heat along with the humidity as opposed to pulling it in. Then you can filter it. Run your dehumidifier next to the tent; ducting it to blow on your plants is not really helping. That will be sufficient to avoid most problems associated with high humidity. Looking at your hygrometer only tells you what you already know. Plants can withstand some humidity; most problems arise when you are drying.

Richard Drysift said:

I’m saying you should set up your tent so there is a ducted exhaust pulling off the heat along with the humidity as opposed to pulling it in. Then you can filter it. Run your dehumidifier next to the tent; ducting it to blow on your plants is not really helping. That will be sufficient to avoid most problems associated with high humidity. Looking at your hygrometer only tells you what you already know. Plants can withstand some humidity; most problems arise when you are drying.

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Oh I definitely already have an exhaust with a carbon filter already (below). The problem is that even with the exhaust filter and the dehumidifier outside the tent, my RH in the tent can get in the 60% and greater range.

That is a bit high but for having plant material in a small space it is par for the course. The hygrometer in my bloom room typically reads 45 to 55% on a normal day and higher in the dead of summer. Temperatures are the real killer. Keep it in the mid 80sdegF (26-29c) and the plants will be fine. The only thing you could really do to upgrade further at this point is to add a/c.
Will you have issues with moldy bud in late flowering? It’s entirely possible... but worry about moldy bud when you have moldy bud. Of course it is better to have drier conditions late in flower and after chop but you have time to figure that out. We harvested a couple plants weeks ago & it was so steamy around here at the time that we had to move our drying plants from the basement to the attic. No bud rot no mold. You will find a solution; there’s ways around everything but don’t worry so much what RH it says on your meter. Pay closer attention to temps as high temps make for airy buds.

UrbanGreenThumb said:

The problem is that even with the exhaust filter and the dehumidifier outside the tent, my RH in the tent can get in the 60% and greater range.

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When humidity is low, the de-huey has a much harder time removing moisture from the air, for the most part you are burning electricity trying to maintain below 50%. On top of that, 80F temps running 40-45%rh is really a risk of pushing the plants too hard. They can get away with this more in the likes of hydro where water is always at hand, but in manual feeding they can die if you miss one watering.

I'm not trying to spook you, it's just to let you know that fixing one problem can create another.

Flowki said:

When humidity is low, the de-huey has a much harder time removing moisture from the air, for the most part you are burning electricity trying to maintain below 50%. On top of that, 80F temps running 40-45%rh is really a risk of pushing the plants too hard. They can get away with this more in the likes of hydro where water is always at hand, but in manual feeding they can die if you miss one watering.

I'm not trying to spook you, it's just to let you know that fixing one problem can create another.

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So are you saying that 55-60%rh is fine during flowering? And that I should not be too worried about the rh as long as the air circulation is good? Thank you for your input Flowki. It definitely feels like I'm burning electricity with little result.

UrbanGreenThumb said:

So are you saying that 55-60%rh is fine during flowering? And that I should not be too worried about the rh as long as the air circulation is good? Thank you for your input Flowki. It definitely feels like I'm burning electricity with little result.

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I'm just pointing some issues out that you could see with high temp/low humidity so you are aware of them. The thing with the De-huey is that if you run it that hard you use more electric and more ware'n tare. The cost of that would be justified if you expect a large mold problem. If at 60% humidity you may see mold, if it is a small enough loss, the cost of alternatively running the de-huey hard isn't worth it.

But I don't know your setup and situation so this is why I am not telling you to do it, you kinde have to decide for yourself. Obviously the hope is that you don't have to run the de-huey so hard, you get no mold and save energy.

Flowki said:

I'm just pointing some issues out that you could see with high temp/low humidity so you are aware of them. The thing with the De-huey is that if you run it that hard you use more electric and more ware'n tare. The cost of that would be justified if you expect a large mold problem. If at 60% humidity you may see mold, if it is a small enough loss, the cost of alternatively running the de-huey hard isn't worth it.

But I don't know your setup and situation so this is why I am not telling you to do it, you kinde have to decide for yourself. Obviously the hope is that you don't have to run the de-huey so hard, you get no mold and save energy.

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Thank you very much for your input! That gives me comfort that I don't have to obsess with getting the RH 40-45%. I am going to keep brainstorming and try to figure out a way to lower the temp and RH at the same time. Is there an RH and temp you recommend for flowering?

Richard Drysift said:

That is a bit high but for having plant material in a small space it is par for the course. The hygrometer in my bloom room typically reads 45 to 55% on a normal day and higher in the dead of summer. Temperatures are the real killer. Keep it in the mid 80sdegF (26-29c) and the plants will be fine. The only thing you could really do to upgrade further at this point is to add a/c.
Will you have issues with moldy bud in late flowering? It’s entirely possible... but worry about moldy bud when you have moldy bud. Of course it is better to have drier conditions late in flower and after chop but you have time to figure that out. We harvested a couple plants weeks ago & it was so steamy around here at the time that we had to move our drying plants from the basement to the attic. No bud rot no mold. You will find a solution; there’s ways around everything but don’t worry so much what RH it says on your meter. Pay closer attention to temps as high temps make for airy buds.

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Sorry I somehow didn't see this till now. Thank you Richard! I am very determined to find a solution and make things work. I have been brainstorming and trying many different setups. My goal is to get my, lights on, temp 80 or below and rh around 50%. I'll let you know what I come up with!

UrbanGreenThumb said:

Thank you very much for your input! That gives me comfort that I don't have to obsess with getting the RH 40-45%. I am going to keep brainstorming and try to figure out a way to lower the temp and RH at the same time. Is there an RH and temp you recommend for flowering?

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In your type of set-up a lot of people tend to fall between 74-78f for Hps, and 75-80f for Led. People do go higher than that but it does start to require a lot more attention to other details. For the most part, try to let your temps decide naturally where they lay, if it is within the ranges above then you are off to a good start. If you are going to force a certain temp then it will cost more, in some way. It can be worth it to do when you know the other elements involved, so don't think the above ranges are set in stone by any means.


Personally I'd use an out take and passive intake, with the de-huey just sitting in the room doing it's thing, just like Richard suggested. At the minute you will be creating negative pressure when the De-huey is off and it is sucking in the room air anyway, while making the out take work harder. When the de-huey is on you could end up creating positive pressure, making the scrubber less effective when ever de-huey is on. If you can open a passive intake or vent on opposite ends of the tent it will give some good natural air flow from bottom to top of the tent. Obviously you need to account for light leaks or leeks.

Again I am tip toeing around humidity suggestions ;p. ''Personally'' I would try 55% and see how it goes. If it isn't too overwhelming, look into VPD including it's effect on nutrient uptake. You can learn how humidity has a big effect, higher can be good for growth but can also do some bad things. To be honest I am half baiting you to read up on this stuff, the sooner the better because you will be able to make your own decisions.

In theory, there would be no issue because a dehumidifier runs air over a cold inner piece (which is why dehumidifiers don’t work below about 42 degrees) which condenses water from the air, the only possible issue may be cleaning the filter or an old condenser that could have contamination - if your dehumidifier is new, or relatively new, you should be fine so long as your co2 and ventilation checks out