Temperature in tent vs. plant

Hello All,

I’m just starting my first grow and have a question about temperature. I have a 4x4x6’ tent, with a NextLight 525w LED light, and a 6" exhaust fan/filter. I have the filter and fan inside the tent pushing air out. I have just two plants.

I understand the target temp range is stated to be around 75-80F. But where is that temp taken? I assume that we want our plants to be that temp so the temp should be measured where the tops of the plants are. I just have seedlings that haven’t germinated yet so I’m measuring on the floor of the tent where the cups are.

After doing a bunch of reading I’ve heard that the light should be lowered as low as possible as long as you can put your hand where the plants are and it’s not burning hot. Well, I have the light about 34" off the floor (about 29" above the top of the cups) and it gets to be about 98F. It just doesn’t seem possible to have both the light down low near the plants and have the temp in the 75-80 range.

In the mean time I’ve removed the cups with the seedlings out of the tent and into the outer room where the temp is about 78F.

I hope this question wasn’t too long winded (are you even still reading :slight_smile: ).

Do you have an intake fan, bringing in fresh, cooler air? Do you have any fans inside to circulate/move the air?

First thing I would do is unhook the filter from the exhaust fan. There is no reason to use the filter during seedling/veg stages, and just shortens the life of the filter. I leave mine unattached until I start to smell the plants. May or may not help with the heat issue…

The whole hand under the light thing, that is not a good measure when using LED, led puts off a lot less heat than other lights, but has a more intense light. So even though you don’t feel the physical heat, the light can still burn the plant if to low just from the light intensity. About 36 inches from plant is good for seedlings/sprouts, and then just adjust to control stretching of the plant…

@Ron330 - I do have a 14" circulating fan on the floor blowing right over the cups. I’m sure at this point it’s just pushing the hot air around. I’m sure that’s better than allowing it to sit and get hotter. I don’t have an intake fan, I do have several 6" vent holes to allow air in.

I should mention that the ‘grow room’ is a small room about 10x15’. I rely on our central air conditioner to keep it cool, but we happen to be having a cool, rainy day and the AC is therefore not running. The temp in the grow room is 84F while the rest of the house is <75F. I don’t think the central AC is going to work out for me. I’m going to shop for a window air conditioner today.

Thanks for the info about removing the air filter as well. That should definitely increase CFM.

An intake fan could be your solution, if you get some of that outside air in the tent, it will force the hotter air to rise and get sucked out. I would get one regardless, just to keep the air fresh and new.

You could try to place a house fan by one of the lower vents blowing in, and check your temps after an hour or so and see if it helps, if it lowers it, you know an intake will solve the problem…


I’m with @Ron330 - an intake fan pulling in cold air will definitely help.

In general temps in the tent will run 5-10 degrees hotter than the room. So keep that in mind.

If you can find an Ac unit to cool just that room and pull the cold air in to the bottom if the tent- I think you’ll be in a good place.

@Ron330 & @Ray4x

Well, I have the A/C unit in the window and set so the outer room stays at 75 degrees, I’ve raised the light up to 48" above the floor. My temp at the floor where the plants will be is at 86F. I have a 6" opening in the tent near the floor and another 6" opening in the roof.

I’m afraid that adding an intake fan will make it too windy in the tent. But, it seems that if others are doing it, it must be working okay. If I get another fan, would it make more sense to push the air in, or pull it in?

Once my plants break the surface of the soil, do you thing that 86F is an okay temp? If that’s too hot, I can raise the light some more or get that second fan.



If you run a slight negative pressure in the tent there shouldn’t be too much wind. It will allow for the air to exchange in the tent and would be forcing the hot air out the top.

Just keep in mind you’ll want your intake fan to be slightly less strong than your exhaust fan. Speed controllers can help with that. For instance I have a 204cfm intake fan and 404cfm exhaust- but I run a speed controller on the exhaust so it pulls more like 250 CFM. The room I have my plants in is very hot without my ac running. But I would say I average 80-84 in the tent (sometimes hits up to 100!)- and my girls seem to be doing fine. They just drink a lot of water. These temps aren’t ideal but sometimes you just gotta work with the hand you’re dealt.

Also I looked at that light and it’s crazy it runs that hot for being so expensive!

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my tent has been running around 85 most the time, hotter days towards 90. No problems yet, they dry up a bit quicker, but I didn’t have any plant issues… I use a 6 inch vornado house fan for my intake, I keep it on low just to keep fresh air going in the tent. Exhaust I run on med or high, to get negative pressure…

A breeze in the tent is good, long as it is not to strong. Besides my intake, I have 2 clip on fans blowing directly at the plants. The breeze helps strengthen the stalk of the plants. Sure, you don’t want to blast them with wind, but a gentle breeze seems to do them good…

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I hear what you’re saying about the heat. I thought that with an LED I wouldn’t have any heat issues at all. Also for that price, it would have been nice if it had a power switch and a dimmer. It has neither. :frowning:


I just ordered another intake fan. A YieldLab 6". It’s the same as the one I use for exhausting the tent. It comes with an inline speed controller (funny that they don’t mention that feature on the website). I’ll be able to dial it down so that it doesn’t over pressurize the tent.


You didnt have to buy that expensive fan for a intake. This is what I use.


As far as temps, I’m at 80 to 90degs most times at canopy height with the light on. I’ve seen it at over a 100deg at times and the girls just shrug it off. Run my light at around 40" for the seedlings. After a couple weeks old 27 to 30" seems to work well.



Probably not, but I’m hoping this will allow me to better control the temp. I’d rather over engineer it and be able to scale back if need be.


Yeah that’s pretty ridiculous! For that price I feel like is should have come with a gold dimmer! Hahahah

I just received the one @Rugar89 linked to on Thursday. It has helped reduce the tent temps by a good five degrees but I too have been running 80-84 with lights on and I just keep adding circulation fans, three now! My basement is 78 and 68% RH as I type. The NE USA has had 8 straight days of high humidity and temps in the high 80’s…hi August.

@garberfc Wooowwzers I wish I had that kind of money to drop on a light.


About half price on eBay! :yum: