LED - Heat output

@raustin - with the 135W QB LED light I ordered (and you use), how many degrees F do you see the temp rise with the light’s driver on full output? Just trying to figure what temp I need to keep my room so the light doesn’t heat the tent too much. THX

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I got a single 55 true watt cfls above my seedling in a 32x32 tent and it alone raises the temp some. Just get yourself a 4" inline fan and that’ll help with heat and air flow

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@raustin has mentored me to build a system quite much like hers. The grower’s light I ordered is a 135W LED 3000K Quantum Board. I know the LED has a reduced heat output over some other light sources, but I am sure some heat will be generated…but not sure how much?

Yes I have the 4" inline fan coming in and will be part of my system. Just waiting for the items to all come in. The light has a 10 day wait so I’m just making notes and prepping my space now.

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No worries… she will be here eventually … if your trying to match what she is doing , the only thing you need to consider is humidity and temperature differences between the both of you…
Speaking of @raustin … I need to sweep through her thread… she is awesome and always giving good advice…
So if your trying to copy her , i better get over there and see what the heck she is doing… :grin:



@MT3 i am also a @raustin follower. ill do just about anything she asks me to, lol! i got an hlg 135 v2 and i love it. at full power for flower now and it does get hot, no so hot i cant touch it but its quite warm.

i think the max operating temp of power supply is 90 celcius, which of course is just less than boiling water so if you dont have an IR thermostat you could boil some water and check if its cooler. lol jk mine has been operating fine

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Ten day wait list?! Damn, they’re really backed up.

The light raises the temp in my tent about 5°, that’s it. My tent is in an air conditioned room, so it stays about 74°F in the tent when the light is on.


Darn @raustin, the light company needs to have you on the payroll! You got 'em all backed up LOL. Got a nice email saying due to high demand, shipping for the assembled kit will be in around 10 days. That’s cool…I am gathering other gear while I wait.

I’m figuring out the temps I need to control my grow space and what it will take for me to make this happen. If I set my ambient room temp around 70F, the tent temps may be around 75F if the light and fans are running? I’m sure we will be looking a bit more into this, so I’m trying to get a handle on temperature management since this space wont be in my home. But I do have heat/AC in the space…plus I can run a digital space heater for control in the room with the tent.

I also have a dehumidifier and a humidifier depending on the situation. I’m getting this train boarded and ready to roll.


Oh wow, you’ve got everything you need. Even I don’t have all that going on. There is one thing you’re going to need that I forgot about. You’re going to need some 4" ducting to connect everything in the tent. Cheap stuff, not a big deal, but you need it.

Actually, this light company does get a lot of business from this forum thanks to our lighting guru, dbrn32. He was the one that turned me on to my quantum board.


Many of the things I have collected will work for my grow room, but were originally intended for other purposes. Of course the LED and tent were purposeful and bought for this specific reason. I can pick up a length of 4" vent duct at Home Depot or a similar store.

I’m excited, but hopeful AND realistic. If it doesn’t produce like some of the pics I have seen on ILGM, I’ll just try again and learn by the mistakes I make.


Don’t worry, you’re going to have a great first grow. I can feel it.


Had to run to a neighboring town for a quick one day business trip, and stopped by a nursery and picked up 2 bags of FF Ocean Forest soil (1.5 cf) and a 3 pak of FF Trio nutes.

Since I’ll be doing one WW Fem in my 2x2x5 grow tent, do you think I should max my grow with a 7 gallon fabric pot…or stick with a 5 gallon?

Thanks for the comforting comment, @raustin!


Nah, stick with the 5 gallon. The 7 gallon will just waste soil.


Thank you…that’s what I’ll do.


250 true watts of LEDs generates just as much heat as a 250 watt HID bulb. It just gets spread into the tent air better via the heat sink. Heat sinks don’t make heat disappear somehow, they just transfer it into the air better than an HID bulb. So looking at the tent air temperature, LED is no better, heat-wise.

They can make more light if you get good LEDs, so that can be a big plus, and the spectrum can be a lot better.

The very best thing you can do to control heat is to suck air through the heat sink and exhaust it outside. People who used HID bulbs did this same thing by putting the bulb in a clear plastic “cool tube” with a fan duct attached.


i mean absolutely no disrespect. And i need to say something about your post. I think you may have missed something about how LEDs work.

somebody (@dbrn32) please correct me if im wrong, but LEDs literally produce less IR, which is “heat”, than HID bulbs. the one point im not sure on is if the power supply makes up for that efficiency and releases enough heat to match HID emissions.

on your post you saidLEDs produce the same amount of heat but disperse it better. I think that is just wrong. But i appreciate your post because it is a good topic to discuss.

Nope: Watts are watts, a fundamental unit of thermal energy. I know exactly how LEDs work and I know exactly how most other light sources work. LEDs may produce more light in the visible spectrum than HID bulbs but that’s only if you get good LEDs. One LED bulb I just looked at produces 71 lumens per watt while HPS bulbs produce 133 lumens per watt. Good LEDs produce upwards of 160 lumens per watt.

HID bulbs put a lot of energy in the far infrared, which is not useful to plants, but in terms of heating up the grow chamber spectrum doesn’t matter! You could have a perfect lights source that makes only visible light but that light hits leaves and only a few percent is used in photosynthesis (3 to 6%). The rest is re-radiated as heat. Light that doesn’t hit leaves is also re-radiated as heat and raises the temperature of the surface it hits. This then raises the temperature of the chamber air. There is no such thing as energy “going away” by magic.

Since so little of the total energy going into the light source is used for photosynthesis, there is hardy any difference between the effect of HID versus LED versus CFL lamps raising the chamber air temperature.

Yes and no, depends on how you look at it. I’m not qualified or comfortable to explain thermodynamics. But where most people get confused here is the correlation between heat and temperature. Your canopy temps will usually be lower under a good led setup, a simple thermometer will tell you that without a background in thermodynamics.


lower canopy temp

Absolutely true. Mostly because of all that infrared the HID bulbs put out. But when you average the heat over all of the entire chamber volume of air, which happens automatically (see 2nd law of thermodynamics) all energy is conserved, so the air temp goes up the same amount. If you run a fan in there to mix up the chamber air, it makes the air temperature over the whole chamber much more uniform. So it’s harder to use the fact that hot air rises to vent the heat out at the top.

People commonly confuse temperature with energy. Temperature is a local thing. A little bit of air at a high temperature may have the same energy of a large volume of air at a lower temperature. It’s like confusing volts with watts.

I don’t believe it’s limited to that. By solely that logic an air cooled hood would have very little impact on canopy temps, when in fact it has a lot.

So taking a passively cooled led like most of stuff I work with, majority of the waste heat never has a chance to make the canopy assuming Ventilation is set up properly in the space. As opposed to hid options, where it’s all focused directly at the canopy.

I believe there’s also some give in the amount of work that takes place on multiple levels. Both by the lights and plants, that has effect on heat. But I’m simply repeating things I’ve been told and don’t fully understand.


It’s kind of a balancing act: Too much air mixing and stratification into hotter and cooler zones can’t take place. For removing heat from the chamber, it would be better to not run a fan at all. Just let the heat sinks pull air across them and send it up where it could be removed by an exhaust vent. But most growers use a pretty vigorous fan to keep mold down and strengthen the stems. So their heat sinks are not really passive! There’s not much difference between a fan on the heat sink and a fan blowing at the heat sink. They both make the heat sink work much better.

Canopy temperature is very important: That’s where all the good stuff is happening. But if your chamber air temp is 95 F, you’ve got a problem.