Temps directly under 250w HPS....wow

Discussion in 'Growing Marijuana Indoors' started by bigslick7878, Jan 18, 2014.

  1. So I have been having some heat related issues lately, which is odd because it is winter but whatever. I always read "keep the plants as close to the light as possible! If you can put your hands on the top of the plant and it isn't hot it is fine!". So I usually kept the tops within 5-6 inches of a rather weak 250 watter all this time, not realizing just how incredibly hot (with very little humidity) it is near the lights. My hand can easily sit under the light at that distance without feeling much of anything. Good right?
    I did a test at varying distances just to see what the temp/humidity readings were, and this is with a 6 inch cool tube(that I feel is essentially useless after this experiment BTW) and fan blowing the hot air from the tube to a vent in the ceiling. The light is about 5 feet off the ground, it is a finished attic I am growing in. The center of the room temp (about 12x14 room) was a consistent 79 degrees, and the humidity was 34%.
    Directly under the light (not touching but within a couple inches, the temp was 113 degrees, and humidity was 11%!
    At 6 inches below, the temp was 92 only 25% humidity
    At 12 inches the temp was 89, humidity was 27%
    At 18 inches the temp was 85, humidity was 27%
    At 24 inches, the temp was 83, humidity was 27%
    As you can see very close to the light is just asking for the plants to fry, I mean that is crazy high and while some might survive others won't be able to handle it. I had no idea it would be even close to that number, and the humidity loss just makes it worse.
    I am really trying to dial my grow room in, I have been growing for a year with bag seeds (harvested over 200 plants in that time) just to get experience but now have some good strains going and don't want to mess them up. I can't even imagine what the temps would be like under a more powerful light, they have to be sky high compared to the 250 watter. Back to the cool tubes, they seem to be worthless. I have a 6 inch inline fan blowing the hot air out and it doesn't seem to make that much of a difference.
    Now here is the conundrum, if we are losing a huge amount of lumens the farther we go away from the light, where is the middle ground? What should the "room" temp be around if we are at roughly 10-15 degrees above that under the lights? Also should the "room" humidity be adjusted as well being we are losing moisture under the heat of the lamps?
    I have 2 500 watt T5's that I am going to do next to see what they come in at directly under, normally the tops of the veg plants are darn close to the bulbs in that room. Can't imagine it will be anything like the HPS but I want to see.

    What kind of exhaust do you have? I have a problem with your numbers. I just finished a grow with a 600W HPS in a 3SF space and temps at top of canopy at about 12-14 inches below the bulb were holding well at 76-80 degrees. I used an air cooled hood, not a cool-tube and a 6in, ~400 cfm vortex fan venting directly outside. With just 1 plant in that space I pulled 4oz after putting into jars to cure.
  3. So you have this very small light in the near ceiling of an attic and its too hot. Go figure.
    drop the light down. get the plants on the floor where is coolest.
    Something does not add up, I run 10 times the watts.
  4. #4 SupaaBaked, Jan 19, 2014
    Last edited by a moderator: Jan 19, 2014
    Something is wrong between the fan and where it goes out. You have too many bends or something. How many cfm's is your fan? Some colas on my plant get bleached if they are directly under the light but not burned and thats under a 600w cooltube.
  5. #5 TSoaDK, Jan 19, 2014
    Last edited by a moderator: Jan 19, 2014
    You gotta bring some outside air into that space and get the fans moving it around to counter the heat from the light.
    Do you have any type of insulation in the attic? My attic is so fucking cold in the winter time you should be having no problems especially if this cool tube is vented outside. Are you sure your inline fan is set up in the right direction and that it is blowing and not sucking?
    If all else fails...Go invest in a mobile a/c unit that does air, heat, and humidity control all in one. Set it somewhere where it will reach through and above your plants below your hood and do some trial and error to see what temperate you need to set it at to control it. At least you would be able to grow in the summertime there .
  6. #6 zippy657, Jan 19, 2014
    Last edited by a moderator: Jan 19, 2014
    You just told us your ambient temps are at 79 degrees... This is your problem. If you are cooling with 80 degree air obviously things are going to be hot. 20 degrees above ambient temps at a few inches is perfectly normal. You have to find a way to cool the room. If you are venting out the attic in the winter you should be able to use the negative pressure to draw it from anywhere (including the ridge vent lok)...

    Two things don't add up. An 80 degree attic in the winter (is it insulated or sealed off somehow??) And 200 plants in a year with this setup... LMFAO!!!!

    Pics or it didn't happen.

    Edit: I see now you say it's in a finished attic. Yea just open a Window man. All this ranting about cool tube not working and you're cooling with 80 degree air. I don't understand why you wouldn't expect 20 degrees Warner next to a 500 degree bulb... If you had no coo tube you'd be running an easy bake oven and would be at 200degrees at 2 inches. That 20 degree delta T should stay pretty consistent, so, intuitively, you should keep the ambient temps at around 60 and all will be peachy.
  7. #7 bigslick7878, Jan 19, 2014
    Last edited by a moderator: Jan 19, 2014
    OK I will try to answer all these, thanks for the responses much appreciated....
    1. What type of exhaust, is is pulling air from outside room?  - This might be one of my issues, my exhaust is not pulling from outside room, it is pulling from an open end of one of the cool tubes. Problem is the adjacent room on the top floor is my vegging room, so I can't pull from there it is warmer in that room. The only way I could really "pull" from another room is if I cut a hole in the ceiling on the floor. I don't know if I want to do that.
    2. Setup - I have 9 250 watt cool tubes all daisy chained together around perimeter on 3/4 walls of room. Think of an upside down shaped "U" if you are looking at it. I have 2 inline fans, one at the open end, and after the 5th one that suck plenty hard enough to blow the air out into the ceiling. I could easily blow the exhaust down to the second floor if need be. The air that comes out is very hot and powerful, even when room temps aren't that high.
    3. Open a window - Not possible. They are sealed and blacked out over the blinds and I can't change that easily. Also there isn't a chance in hell I am opening a window in my flower room, when I first started I tried that and you could smell it right outside. I have anywhere from 60-80 flowering at a time, it stinks.
     One really odd thing about this attic is it has a 3 inch exhaust PVC that comes up through the floor, and out the wall to the roof. (they put an addition on the house and didn't want to break the top floor drywall apart, so it runs behind the walls from first floor/second floor and comes out through the floor in the attic in the actual room, then out to roof under window). I could easily pull from outside that way, by tapping into that and exhausting either to the ceiling. I can bend the 6 inch ductwork to fit so it is sealed to the PVC and can be pullling 20-45 degree air from outside. This would probably solve my problem. I originally had plans to use that as an exhaust point in the future, but an intake for fresh cold air through the cool tubes sounds a lot better!
    Thing is I have to figure what that pipe is for. It isn't the dryer and have to make sure I am not "pulling" when it is trying to exhaust something. It is baffling as to what it goes to, hot water heater is exhausted right where it sits in basement out wall to outside, have propane for stove, no CAC, and furnace is in basement but it has nothing to do with that either. I literally have never seen an exhaust like this out to roof it is odd, no idea where it is coming from.
    Only question though, what happens when summer rolls around? I obviously can't pull that heat from outside, might have to think of something for that. Even when spring comes we will be around 60-70 degrees outside, that is probably more reasonable but definitely won't work like it will in winter now. Might have to figure it out as I go. The only way I might be able to run full steam ahead is by running an A/C in 2nd floor room very cold and pulling from there, but again that would require a hole in the ceiling.
  8. I have a small grow using cfls and even though it seems the temps dont really get as high as mh/hps they still do get warm under the canopy. My simple solution is a rotating fan to keep the air moving , a small computer server fan drawing in air and a small server fan drawing out. I use a thermo cube to fire off the double server intake and exhaust fans that rarely kick on ( except late fall or early spring on warm days.
     Even though they are cfls they still produce heat to the tune of 65 at base of pots and 80-85 at the top near the lights. Of course this has been checked by thermometer.
     In your situation its go big or stay at home,intake fan from outside and exhaust it out of the building,shouldnt be a problem since its an attic.I stayed small and at home (lol) buy using cfls to keep the temps down, nothing worse than hearing the firemarshal say "LOOK over here JIM it the accelerant"-thats my biggest fear...lol
    Yeah I have a chimney stack (not in use, no heat coming from it) that runs right through the center of the room, and there are 3 fans attatched to walls of it that hit the 3 main areas under all the lights at the top of the plants. I need to fix the exhaust issue and I think my temps under the lights will go way down. The funny thing is I have had this exact setup for 14 months now and ran it like this all through summer when it was even hotter in room and never really ran into issues like this. But this is why I spent a year growing before I got into the good stuff, I have learned so much and am still learning and dialing in my setup and environment. From every mistake comes a little bit of knowledge.
    I need to post here more and learn more, that is my goal for this year. The posters here are so much more experienced than I am and the info provided is invaluable. I have a tendency to try to "figure things out for myself", but now I at the point where I want every detail perfect.
  10. I run a NON-cooled 400w in a 3x3 tent and with my light ~12-14" away my temps are 72-73 peak. A properly match filter and fan that is rated correctly for your spaces needs does wonders.
    You have to pull air from outside the room, its coming from there somehow anyway. If your fans are pulling from the room, then the cool tubes, then exhausting, that air they're pulling has to come from somewhere, likely all the cracks around the door which doesn't replace the air in the room quickly enough. Unless you have a passive intake somewhere that you didn't mention...
    Have you thought about instead of lining all 9 lights up and putting a fan after 5 and a fan at the exhaust, why not put a fan on either end of the U and open up the round part of the U between lights 4 and 5. that way 1 fan vents 4 lights and 1 fan vents 5
  12. I think that vent is for your water drains. Without a vent the water wouldn't drain. Sent from my iPhone using Grasscity Forum mobile app
  13. #13 WDIK, Jan 19, 2014
    Last edited by a moderator: Jan 19, 2014
    That could be your sewer exhaust.  You don't want to pull air from it for obvious reasons.  Also if it is not venting to the outside, your sinks and toilet won't drain well.  Could be used for an exhaust port though.
  14. #14 bigslick7878, Jan 19, 2014
    Last edited by a moderator: Jan 19, 2014
    Ok so after some googling it appears that pipe is the ehaust from my septic tank...
    That is exactly what it does, goes up the back side of the house and above the roof. Damn.
    Ok so I am going to have to pull from down below somehow in a lower room, or what about pull from ceiling (it is not insulated, just has drywall over studs)? Hole is already there LOL. But if room is that hot won't inside ceiling be too?
    If I have to pull from the 2nd floor throught the ceiling I can, just didn't want to cut holes and tear up carpet if I didn't have to. I can crack one of the windows in the room below, and keep it ice cold in there so it sucks that air up through the tubes.
  15. If the chimney isn't in use pull the air from ther, just scrub it before it goes over your bulb and vent it into the attic. I have mine venting between my walls. As long as it has somewhere to go other than right back into your room you're good. I've also seen people exhaust and pull aur through a chimney so it shouldn't be too difficult.
    I have thought about that but the chimney is surrounded by an old plaster wall and I would have to tear up all that, plus cut through 100 year old hard stone to make a vent hole. Not really something I want to do.
  17. I wouldn't want to either. If it's something you have to do then you gotta do it dude. All you would have to do is get a sledge and bust a fat hole, cover it with plywood, hammer some dry pins through it into the brick. Use some caulking and seal it up and cut a hole the size of your ducting. I don't like breaking shit if i don't have to but i'm not seeing a lot of options. Is what you're venting to sealed off or could it come right back in the room?
  18. #18 FoundFeildofBUD, Jan 25, 2014
    Last edited by a moderator: Jan 25, 2014
    The temp gauge in direct light will give you false readings.I can't believe no one has said that. I have a 600 in 3x2 w/6in cool tube and 400 cfm turned down. No problems cooling.

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