900 watt LED into 4x2 tent

What kind of venting do you plan to use?
I’m using LED’s and heat is an issue I didn’t think I’d have to be dealing with.

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Myth: LEDs don’t produce heat,

LEDs put out just about the same amount of heat as HID bulbs, they just spread it out more. Watts are watts! LEDs may make more light, but even that light becomes heat once it hits something. Use an exhaust fan at the top of your tent or it’s going to get mighty hot in there.

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Gonna use a 6” inline to extract hot air and pull thru a filter.

Yea … this is my first time using LED’s but I don’t agree that they put out as much heat as a HID. Those things get way hotter than LED’s.

It depends on where you’re talking about measuring temps. At canopy level, led is absolutely cooler. With hid lights majority of waste heat is thrown from bulb directly toward your canopy, and is held within the hood or reflector. With led, the waste heat is usually pulled up from the diodes through the heatsink and then exhausted.

Depending on the efficiency of the led and thermal management, its possible to create more light per heat watt with leds as well. But there’s still a lot more that goes into making a blanket statement that leds will run cooler.

Sorry, it’s just a plain law of physics: 400 real watts of LEDs make exactly the same amount of heat as 400 watts of HID bulb. Heat is NOT the temperature of the surface of the bulb. That is temperature. If you blow some air over the bulb it will heat that air up as much as the air you blow over the LED heat sink. If you have an exhaust fan pulling air out of the room, that air will have the same temperature no mater what is heating it up. Heat is a product of temperature times the volume of air.

Think of a greenhouse with one 400 watt HID bulb heating it in winter. That bulb is megahot, but the greenhouse is freezing. If there are LEDs all over the greenhouse, their heatsinks may only get up to 90 F, but they heat all the air in the greenhouse.

And if you still think LEDs don’t produce heat, then forget the exhaust: You’ll bake your plants.

@1BigFella is correct about watts is watts.

Generally the main reason LEDs make less heat is because they use less watts to create the same amount of photons or micromoles, as shown in the picture I posted above.

However I do not totally agree that when those photons or micromoles hit something that they always create a lot of heat. This is mostly true if the light is made up of a lot of IR and near IR light, which is sometimes used more for the “redder” flowering spectrums.

A lot of LEDs , even in their flowering spectrum do not contain as much IR as say a HPS, and will not create as much heat on whatever they are projected at.



All light of any visible frequency that gets absorbed by anything is converted down and reradiated as long-wave IR. That’s right: Heat. The one exception is the photons that take part in photosynthesis. About 6% of the photons that actually hit a leaf are used to make ATP and sugars. But later that sugar is used to fuel respiration and CO2 is excreted, along with waste heat.

It’s just the 2nd Law of Thermodynamics: You never win. You ALWAYS end up with all energy input as waste heat. Think of all the kilowatt hours you have to pay for as input and the energy you would get from burning your dry weed. That is a very tiny fraction. Not even 1 kilowatt hour. More like a few kilowatt minutes. That’s the only difference between HID and LED, the fact that you get a little bit more growth with LEDs.

I understand the 2nd law of thermodynamics quite well, but that is not all there is to it.

A lot of that light is reflected, and just simply does not contribute to heat build up in the room as much.


This is also not entirely true.

Although newer HIDs like Ceramic Metal Halide are much more efficient and nearing or rivaling LEDS in efficient conversion of electricity to photons, even the best HPS waste a lot of the conversion of electricity to plain heat and not photons at all.


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Well, you are entitled to your opinion I guess. I’ll stick with “watts are watts”. And any light that is not reflected is absorbed and reradiated as long-wave photons or warms up the tent walls, which warm up the growth chamber air because tent walls tend to be insulated well. Any light that IS reflected just goes on to another surface that reflects, reradiates or warms up, etc. Nothing is lost. Heat does not disappear.

It all ends up as waste heat, warming up the tent. You can’t beat the 2nd law and have the heat magically go “somewhere else” unless you exhaust it.

I agree with your technical explanation, but if the light doesn’t start out as long wave radiation, it does not contribute to as much heat in the room, you are talking about apples and oranges.

Simply put, a 400 true watt LED will not contribute to as much heat in the room as a 400 watt HPS light.

There is just a lot more to it.

Also the amount of heat the plants produce through their chemical reactions and respiration is minimal, and almost not even worth considering.

And absolutely, non the less, the heat that is produced and not wanted needs to be exhausted.


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I thought the second law stated that as soon as you do something with a unit of energy the amount of energy left over will always be less? Combined with the rest to equate the more work you do per unit of energy, the less energy there is remaining?

So a 60% efficient led will have 10% less heat loss than a 50% led. Or 20% less than the typical 40% efficient hid lamp. After that, I’m not sure. I’m certainly not about to run less light regardless. So I’ll chalk that up as out of our control either way.

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But this is importrant: Until you get into nuclear mass energy reactions, energy is never destroyed. So that “less energy remaining” difference will be waste heat.

The part of the 2nd Law I’m referring to says that every energy transfer will be inefficient and generate waste heat. We were talking about lights and the ideal light would be 100% efficient. It would generate only light, no heat at all. But then 100% of that light that did not fall on a leaf would end up as waste heat, and 94% of the light that did fall on a leaf would turn into waste heat.

If we had perfect reflective surfaces in our tents, then light would just bounce around until it hit a leave, but then 94% of it would STILL turn into waste heat.

So the basic takeaway from the Second Law is that you can’t win, you can’t even break even, the House always wins in the end no matter what you do. There WILL ALWAYS be waste heat, and if you don’t exhaust it out of your tent it will get hotter and hotter. Just because your heat sink keeps your LED junction from reaching 100 C and burning up, it only does that by transferring that heat to the air moving over the fins. If the air reached 100 C, then the heat sink would no longer transfer heat below 100 C and poof go your LEDs.

But a tent is not a truly closed system.


Even if not vented at all, much of the heat radiates out the tent through the walls, and again the heat from the plants bioactivity is so negligible is shouldn’t even be really considered. And much of this “waste” heat from the light bouncing around, hitting a leaf or not, is also rather negligible.

Although in the end all be all, watts is watts and will eventually result in long wave IR, but again, that is not all there is to it.

Lets try to break it down in overly simplistic terms.

Often you hear that the plant PAR is really only or mostly made up of red and blue light. Now, many do know and understand parts of the yellow/green of the spectrum is usable and can enhance photosynthesis and the development of carotenoids, but again I am keeping this overly simplistic to illustrate the point.

And so a light that creates white light, simplified – is made up of red, green and blue light – and so, say, 50 watts of white light won’t produce as much photosynthesis as 50 watts of red and blue light only.

Also, as I said, a HPS light will create much more immediate heat than a Ceramic metal halide or LED light of the same wattage.

This is because, as I said, so much of a HPS’s energy output is in the redder side of the spectrum and creates tons of long wave IR.

Sure eventually, as you said, blue-er light can indeed turn into heat, however, even with moderate venting at the top of the tent, much of this heat will be evacuated before it has a chance to add to the overall heat of the tent, while the HPS light is contributing massive amounts of heat to the tent immediately.

This is the main difference, and is easily proven, and has been observed numerous times by many growers, by running the same wattage lights in same size tents and taking measurements.

Happy growing,


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" a HPS light will create much more immediate heat than a Ceramic metal halide or LED light of the same wattage"
Nope, you are confusing heat with temperature. That’s like confusing volts with watts. The HPS bulb creates a higher temperature, not more heat. And let’s not bring time into it with words like “immediate”. It’s simpler to consider a steady-state system.

Just an interesting tidbit:
Actually “white” has no meaning in terms of spectra. White is just something humans perceive. I can show you two narrow peaks of blue and yellow, and you will say they are white. I can show you three narrow peaks of violet, green, and red and you will say they are white. We have a pretty limited ability to perceive colors. This why looking through BluBlocker sunglasses can be so weird. You can see patterns on flowers that are invisible when you include the blue. Why are they there? For bees! Colors that do not include any blue look very much the same and colors that do include it look completely different.

Yes, yes, I understand all this, as I said, I was trying to keep it overly simplistic, or as @Niala likes to put it, “vulgarize” it, lol.

Either way, the immediately noticeable and measurable build up of heat in the tent with HPS is the same.


Their are other definition related to coarse language , but, this one i choose or prefer over popularize :wink:

Vulgarize : to make (something little known or difficult to understand) more accessible, widely known or popular among the public; popularize…, and you are a great “vulgarizator” @MacGyverStoner, lol
:+1::ok_hand: :grinning::wink:

If I may add my 2 cents, you made a great point about that the tent is not an air tight seal environnement and are not very well insulated (only
the floor are water/air thighted, the walls and the roof are not unless the grower’s made it) and the r value insulation factor is very weak, in fact the fabric material “breathe” a little… :+1::ok_hand::wink:

And for that alone, it’s make the argument of 1BigFella more or less applyable…

To be valid, it will imply that there’s no fan(s) of any kind in the tent (in the case of passive heat sink) and that the internal ventilator of the fixture suffer mechanical issues or break down… Then, maybe if temps are reaching over 100°C for an extended time, the LED diodes are gone a “poof”, it will really depend on the quality of them and time of overheat… lol :wink: :innocent:

And @dbrn32 is really knowing is “stuff” in the field of LED’s and as help a lot of poeple in that field, included me… I certainly highly value is knowledge and when the times come to build my own LED fixture, he and @MAXHeadRoom will be my references… :grinning::wink::+1::ok_hand: @MacGyverStoner

Some chinese LED fixtures doesn’t have any heat sink and exclusividly rely on the internal ventilator to be kept cool, essentialy for less building cost reason… They are usually running at the fifth of their capacity and label “equivalent watts”…

For example, I own one of these and it’s label as “equivalent” 1000 watts and draw around 200 (tested with kilowatt meter) from the wall. So it’s a real possibility that the fixture @Doughbreezy207 , enter in that kind of LED light…

They are not necessarly bad fixture, they’re usually use fairly good diodes but I am spektical that they will last the 50 000 hrs lifespan claims as I am for mine, however, I grew pretty cannabis plant and dense buds with it in a 2×2×8 closet and mine (King plus) have a very good spectrum, so is your’s @Doughbreezy207 :wink: :innocent:

So, you will be a bit shy on the foot print for flowering stage but if your budget do not allow you to buy another fixture, you’ll be just fine for now, just don’t expect the famous 1g/watt harvest lol :wink: :innocent:

Anyway, it’s just my 2 cents…

~Al :v: :innocent:


Very good points! As an extension of that, not all leds are limited to 100c operating temps either. Some are 105, some are 135c. And those are max rated case and/or junction temps. Not fail when reached temps. Granted, if you’re playing in that neighborhood you’re definitely flirting with opportunity for failure.

I think it’s more important to take a look at real world applications than it is theoretically based scenarios. You take an undersized enclosed heatsink relying on active cooling and take away that cooling, you’re going to be in bad shape relatively quickly. Compared to an oversized passive heatsink ran in open air in the event of exhaust fan failure, the temp equalization time is going to take much longer. While I agree it can happen, the scenario in which both will fail is quite a bit different.




Interesting point here: Cobs and high-current LEDs are designed to run with heat sinks. HID lamps are not! Without a heat sink the LEDs will burn up very quickly. The heat sink may not look much like a heat sink on smaller LEDs. Maybe just mounting it on a piece of aluminum rail will do, but it works as a heat sink.

There is nothing in an HPS or MH bulb that will melt without any heat sink, other than free air flow generated by the heat of the bulb. Obviously, don’t seal one in an enclosure and try to run it without air flow!