400 w hps heat solutions

So I installed my 400 w HPS in my indoor grow tent, 4’x3’ with three clones going. It is my first time adding the HPS light to the room and my tent increased wildly in temperature and dropped a lot of humidity. Before humidity was 40-50%, with temps 70-80 degrees. Now I am getting as low as 28% and temps as high as 90. I made some modifications to add a humidifier and a fan. Temps are still ~84 degrees and humidity as of right now is 37%. I am currently running an 80 cfm exhaust with a 4 in duct leading outside. I am also in a dry climate and in the middle of a heat wave. I’m hoping it cools off soon.

Any solutions without buying a cooling tube? The light is 18" from the plants and I don’t think it can go any higher without losing the sweet spot for lighting. Or should I just leave it?

Is it a fan cooled hood? That will help a lot. Was running a 400w mh in a 4 x 4 with a fan cooled hood, stayed 76 degrees with 50% humidity. I increased it to a 600 watt hps and it only went up 2 degrees with the same hood. I am also using a 6 inch fan. Those are my suggestions if you are going to keep the hps.


Did you exhaust from the hood out of the tent or just let it run on its own with your current exhaust system?

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just buy a six inch fan and exhaust the tent massive . A 6 inch should create enough negative pressure to suck that tent in and lower that temp


I exhaust out of the tent. Since it is my veg tent I do not have a filter in there. I run the duct work from the light to the fan then out of the tent.

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Here is how it looks.

If you use a filter, then filter, light, then fan to outside. I agree with @PoppaPuff, if this does not get you in the temperature range you want go to a 6 inch fan to help


better to push air into the light and out the light then direct out the tent. Imo easier on fan too imo.

I have a 8x4 2 air cooled 600 hps. Fan and filter inside tent pushing air into air tube one then into air tube 2 then direct outside.

Edit. Basically get another piece of ducting and push air into light and out of light and out of tent. Try that set up and see if it works. Only cost is some more ducting

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How far you all keeping your HPS from your tops? I’m currently at 18" but considering adding some height to lay off the temp a bit. I also may get a fan and hang it down to blow directly on the light towards the exhaust already installed. Once I get paid, I’ll jump on an cool tube.

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I’ve done setup with 2 1000watts too. Just harder to keep cool. My 2 x 600 air cooled hps tent stays at 25 to 26. And i could have it cooler if i wanted.

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I have had the light anywhere from 18" current height on this grow, to 30" earlier with seedlings. Look on Craigslist, or another resale site in your area. Air cooled hoods are cheap on the secondary market. $25 or less. Paid that for 2 I have. I would add a small clip fan too to blow over the tops of the plants to help keep them cool.

You can also try small bowls of water in the tent to help raise the humidity as a inexpensive alternative until a permanent solution is found.

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Some of my tops get with 14 inches. I keep them pretty close. I just lowered everything this week cause they grew alot when I was gone.


Most likely, your RH will jump back up once your plants grow some. and that’s about the extent of anything useful I have to say, but good luck!!!


I had the 600w in my 4 x 8 tent, but not in an air cooled hood. The temperature ran 7 degrees hotter then without the hood cooled


is you ballast in the tent? what type of ballast?

what are you using for a reflector?

did not see a picture of your setup

all the best



Hey man, what kind of temperatures are you dealing with in the room outside of the tent? I just use a passive 6" exhaust at 300 cfm with the holes all open and it keeps my temps under 80. The air being sucked in is usually around 70. I always have my lights on at night and off in the hottest part of the day, which helps as well. I would reccomend against buying a humidfyer, you can just put a dish or bucket of water in there to bring the humidity up if you wish.


I added a humidifier that I had lying around behind a three fan window fan. Right now temps are 82 degrees and humidity went up to 45%. However this is not the hottest part of the day so I’m going to keep a watchful eye. I tried leaving a five gallon bucket in there but it just wasn’t raising the humidity.

Outside, it is 80 degrees but im hoping temp drops outside over the next few weeks as we head to fall.

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@Stickyjones made good points about the outside environment too. A 5 gallon bucket of water may have been too much in my opinion. I would have only used a small bowl. Easier to heat the water for evaporation. Glad you had an extra humidifier to help for now.

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Cool tubes are absolute garbage; they waste an immense amount of light. Buy a bigger air cooled reflector.

It’s been said, but you absolutely 100% need air movement exchanging air outside the tent with inside. A 400 ain’t much but it’ll heat the inside of a tent tremendously if not exhausted. Plants love air as much as they love light and water.

Humidity is less important, that’s more of a fine tuning factor, get your air exchange in order first. I don’t worry about humidity…at all.


I run a 600 hps in all 5 of my 2 x 4’s.
IF You weren’t such a prick I’d tell ya how.

Good luck

That doesn’t seem to be very supportive of conversation.

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