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Humidity vs ventilation in a grow tent.

Iv ran out of room in my small indoor greenhouse along with wanting to upgrade my lights to LED. I have learned the shelves in my greenhouse aren't far enough apart to use the LED I want. So time for an upgrade but research first. I was wandering from all you that use grow tents how you balins ventilation with humidity. Do you put the fan on a timer so it comes on for certain periods during the day and then let the humidity build up nately?. Or is the fan on all the time and in that case how do you keep the humidity up? I was thinking of using the fan that goes right in the vent tube on the top.
 
When I used to use a grow tent I would run a small fan all day and then turn it off at night. The fan was connected to a hole in the bottom of the tent sucking air from underneath and blowing up into the tent. I wanted to put a small hole in the top to let a stream of air pass through the center of the space, letting hot air pass out, but I never got around to doing it before I dismantled it.

Depending on the size of your growing space, you might only need a tiny fan like a computer fan, in which case the humidity will not tank very hard, but it will still be noticable. Unfortunately Im not aware of a workaround and the only thing you can do is run your humidifier and burn through more water. Upgrade your humidifer to one of the ultrasonic ones if youre struggling to keep humidity up. There are some pretty beefy humidifers on the market.
 
When I used to use a grow tent I would run a small fan all day and then turn it off at night. The fan was connected to a hole in the bottom of the tent sucking air from underneath and blowing up into the tent. I wanted to put a small hole in the top to let a stream of air pass through the center of the space, letting hot air pass out, but I never got around to doing it before I dismantled it.

Depending on the size of your growing space, you might only need a tiny fan like a computer fan, in which case the humidity will not tank very hard, but it will still be noticeable. Unfortunately I'm not aware of a workaround and the only thing you can do is run your humidifier and burn through more water. Upgrade your humidifier to one of the ultrasonic ones if you're struggling to keep humidity up. There are some pretty beefy humidifiers on the market.


Is there a particular reason you stopped using a grow tent?
 
Is there a particular reason you stopped using a grow tent?

There were several reasons, the biggest one being heat control. I was using one of these

339e80c3-6e6f-40eb-98ce-acd7ff1ec39d_300.jpg



just sitting up against my southern facing window. The sun heated it up pretty quickly which made cooling it an issue. At first I tried with just the fan, which didnt seem to work enough, the growtent would still reach temperatures above 100F in the mornings, which was far far too hot for my highlanders I was trying to grow. Not only that, but running the fan ( it was a decent sized fan) would absolutely destroy all humidity in the grow tent, humidity being the only reason I was using it in the first place.


So right there it was already starting to not be worth it. I wanted a grow tent to have higher humidity, but without a fan it got way way too hot in the sun. And with the fan it still got way too hot in the sun while also losing all humidity.

The obvious solution here is to switch to grow lights, however I didnt have the money at the time and I still dont.

Then there was the issue of not enough space. There was only about 12 inches between each rack and almost all of my plants are now taller than that. It worked great in the beginning but when everything got larger and heat became uncontrollable in the spring/summer it became clear it wouldnt work.

When I moved house shortly after I just didnt bother reassembling it at my new place. Since then I've grown strictly on a windowsill and most of my plants have adapted fine (except sibuyanensis, but what can I expect from an already finnicky plant.) In the future I plan to save money for one of those 12x8 ft greenhouses I can build in my backyard, should be nice if I ever get there.
 
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You shouldnt run into the same problems as me unless you're planning on putting that up against a window like I was. Heat from grow lights is infinitely more controllable than direct sunlight turning your tent into a sauna.
 
So would you run fan and humidifier on alternate timers?
 
I would likely run a small fan 24/7 with a humidifer that turns on when it drops below a certain threshold, like 70 or 80%. I think if you alternate youll just shock your plants with periods of drastic change. When the fan is only on the humidity will drop fairly low and then when only the humidifier is on itll get very humid with no air flow. Itd be like dunking someones head in and out of water lol. Atleast that is how I think that would go, I dont have a ton of experience with these things.


Is there any particular reason you cant run a humidifer at the same time as a fan?
 
When I was using T5HO, the ventilation was a big issue (to prevent overheating). So I had a thermostat, which turns on and off the venting fan. The venting defeats the main purpose of the tent to maintain high humidity (majority of plants in my grow tents are orchids, but I do have fair number of CPs there) without damaging the house. So I had to use an active humidifier, which was controlled by a humidistat. Actually, I was actually using ZooMed HygroTherm as a combined humidistat and thermostat. Now I use high efficiency LEDs (you will still have heat issue if you use low quality, cheap LEDs), so I can maintain the optimum temp without any venting. So, I don't need the active humidifier any more. Water catch trays at the bottom of the grow tent is enough to maintain 80%RH (when ambient could be 30%). You can see the water catch trays in the photo in this link; one each for the top and bottom levels

You shouldn't confuse venting with circulation within the tent. I still use fairly strong circulating fans. Circulating fans are to break the micro-environment around the surface of leaves and roots. For example, near the stomata, O2 rich region could be created. This can interfere with carbon fixation especially in C3 plants (called photo respiration). Similarly for roots (of orchids), ventilation help to bring O2 closer to the roots. The circulating fans are on timer (e.g. 30min off 1 hour on etc), which I use to control the drying late of potting media.

Now, my ventilation fan never kicks in. You might be wondering if the grow tents have unfavorable environment of high O2 and low CO2 without the ventilation. I have tracked the CO2 concentration in the tent with an IRGA sensor. Indeed, during the day, CO2 level goes down a little bit. But it isn't something I would worry. The tent isn't too tight (i.e. I haven't made an effort to close down the vents, or seal the stitches). So the passive air exchange seems to be sufficient. Also, I usually opens the door once a day to check them. CO2 level quickly goes up.

I personally prefer 4'x2' footprint. With 4'x4', it is difficult to get to the plants in the back (unless you make it as a walk-in type).

With regard to the shelves, which aren't tall enough, you can probably use linear module style LEDs, which can be made as very low profile. Something like Samsung Q-series (an updated version of H-series, which I posted here) would be best.
 
  • #10
I didn't realize Samsung had released new models of LED strips. That might explain why several of the 2' strips had their price cut in half at Arrow. I've been thinking about stocking up since they also have a $50 off $150 coupon this week.
 
  • #11
I would likely run a small fan 24/7 with a humidifer that turns on when it drops below a certain threshold, like 70 or 80%. I think if you alternate youll just shock your plants with periods of drastic change. When the fan is only on the humidity will drop fairly low and then when only the humidifier is on itll get very humid with no air flow. Itd be like dunking someones head in and out of water lol. Atleast that is how I think that would go, I dont have a ton of experience with these things.


Is there any particular reason you can't run a humidifier at the same time as a fan?

I thought the fan would just suck the humidity right out as it's being made.

When I was using T5HO, the ventilation was a big issue (to prevent overheating). So I had a thermostat, which turns on and off the venting fan. The venting defeats the main purpose of the tent to maintain high humidity (majority of plants in my grow tents are orchids, but I do have fair number of CPs there) without damaging the house. So I had to use an active humidifier, which was controlled by a humidistat. Actually, I was actually using ZooMed HygroTherm as a combined humidistat and thermostat. Now I use high efficiency LEDs (you will still have heat issue if you use low quality, cheap LEDs), so I can maintain the optimum temp without any venting. So, I don't need the active humidifier any more. Water catch trays at the bottom of the grow tent is enough to maintain 80%RH (when ambient could be 30%). You can see the water catch trays in the photo in this link; one each for the top and bottom levels

You shouldn't confuse venting with circulation within the tent. I still use fairly strong circulating fans. Circulating fans are to break the micro-environment around the surface of leaves and roots. For example, near the stomata, O2 rich region could be created. This can interfere with carbon fixation especially in C3 plants (called photo respiration). Similarly for roots (of orchids), ventilation help to bring O2 closer to the roots. The circulating fans are on timer (e.g. 30min off 1 hour on etc), which I use to control the drying late of potting media.

Now, my ventilation fan never kicks in. You might be wondering if the grow tents have unfavorable environment of high O2 and low CO2 without the ventilation. I have tracked the CO2 concentration in the tent with an IRGA sensor. Indeed, during the day, CO2 level goes down a little bit. But it isn't something I would worry. The tent isn't too tight (i.e. I haven't made an effort to close down the vents, or seal the stitches). So the passive air exchange seems to be sufficient. Also, I usually opens the door once a day to check them. CO2 level quickly goes up.

I personally prefer 4'x2' footprint. With 4'x4', it is difficult to get to the plants in the back (unless you make it as a walk-in type).

With regard to the shelves, which aren't tall enough, you can probably use linear module style LEDs, which can be made as very low profile. Something like Samsung Q-series (an updated version of H-series, which I posted here) would be best.


I was looking at a 48x24x72. I found a rack online that is mad for damp humid places. Its 48x18x70. The lights I was looking at are mars hydro 300w. Do you think I would need a vent fan with those? Are you saying you don't use a humidifier or a foger sorry if you made this clear I just got home from a 14 hr shift.
 
  • #12
I didn't realize Samsung had released new models of LED strips. That might explain why several of the 2' strips had their price cut in half at Arrow. I've been thinking about stocking up since they also have a $50 off $150 coupon this week.

Good to know (although I still have quite a bit of H stocked up)! Q is about 10% more efficient, so H-series with lower price could be a better deal (especially with the coupon, it could be the cheapest and highest performing LEDs). Nominal current of Q is 450mA, with Meanwell, 500mA is the closest, but it is more limiting than H.
 
  • #13
I thought the fan would just suck the humidity right out as it's being made.




I was looking at a 48x24x72. I found a rack online that is mad for damp humid places. Its 48x18x70. The lights I was looking at are mars hydro 300w. Do you think I would need a vent fan with those? Are you saying you don't use a humidifier or a foger sorry if you made this clear I just got home from a 14 hr shift.

That's a good size! You can make 2 shelves or 3 shelves. I don't know if they have made any progress, but I personally would avoid Mars LEDs (but I'm sure it works ok if your electricity isn't too high). They used to be pretty inefficient, and not durable. If you put enough of those, you'll have a heat problem.

I don't need any active humidifier/fogger (to keep around 80%, fluctuate bet 70-100%). Passive humidity/water catch trays are enough for my setup.
 
  • #14
That's a good size! You can make 2 shelves or 3 shelves. I don't know if they have made any progress, but I personally would avoid Mars LEDs (but I'm sure it works ok if your electricity isn't too high). They used to be pretty inefficient, and not durable. If you put enough of those, you'll have a heat problem.

I don't need any active humidifier/fogger (to keep around 80%, fluctuate bet 70-100%). Passive humidity/water catch trays are enough for my setup.

So here is the plan.

This shelf
https://www.webstaurantstore.com/re...oxy-shelf-kit-with-74-posts/460EG1848KIT.html

With fore 300w on the two top shelves and two 45-50w for a seed starting area on the bottom. Is that overkill?
 
  • #15
Not super relevant but I've got the same shelf and its good and sturdy. You can easily just remove shelves as well for when your plants get too big and are bumping into the rack above them.
 
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