400 Watt MH/HPS questions

Discussion in 'Lighting' started by Hecktorharris1, May 10, 2011.

  1. I just got a 400 watt MH/HPS for my birthday. Im going to pick it up tomorrow. I will post pictures then.

    I was wondering what kind of ventilation would I need a large fan and a small fan. Also the room I'm growing in right now is fairly large. I will be putting in a grow box soon tho.

    I also need to know how far away from the plant the light will have to be.

    Does anyone have any good mounting idea's I cant screw it into the ceiling.

    I'm using a coco medium too I'm not sure if that makes a difference.

    Thank you for any help :D
  2. Make a light horse out of 2x4's
  3. #3 Jellyman, May 10, 2011
    Last edited by a moderator: May 10, 2011
    Any good 10" axial fan at local stores will work well enough to cool the lamp if the airflow is fairly unobstructed. Or, a good 4" centrifugal fan dedicated to the lamp is also plenty of cooling and will pull air past obstructions better. Ideally, the lamp can run at its coolest when it has at least one dedicated fan pushing or pulling air through intake & exhaust ducts that are separate from the grow area's airflow. It's cheaper to design the lamp's cooling airflow as the exhaust for the grow area but you end up cooling the lamp with air already heated by the lamp.

    In addition to lamp cooling, the other airflows you should consider are fresh air in & out of the grow area and air blowing on the plants. Just like with CO2 enrichment, fresh air helps the plants grow bigger faster. Enough air blowing on the plants to cause them to shake a bit will encourage thicker stems for transporting more nutrients and supporting large buds.

    A 400w needs to be a minimum of 10" from any plant parts at all times. Setting it to 12" each day or at least 13" every other day should provide enough room for the plants' vertical growth between adjustments.

    If you're talking about a full lamp and not just a bulb & ballast, and can't cause any damage to the ceiling, you'll need a stand of some sort for it. The most basic stand might be two stacks of (books, wood, etc.) items on either side of the grow area, with one board, broom handle or something resting on top. The lamp would hang from the middle.

    For coco coir, you'll want to set your pH's to about 6.
  4. For your light you want a centrifugal(vortec)fan. Axial fans and duct booster fans do not do a good job for exhausting heat from the tube. You can get away with axial fans for bringing in fresh air and exhausting stale air but for the most part the vortec fans do best all around and cost about the same.

    Another way to cool the tube is with a water cooled heat exchanger system that hooks to your light and a water source.

    Is best to run your light on one dedicated fan and other exhaust fans(scrubber etc)with another.
    You can push or, pull cool outside air thru the tube but, if you are using only one fan(not recommended) on a scrubber you want to pull the air thru and out.

    The interior of your ducting, length and bends determine how well your system exhaust heat.

    In a large area you may have no problems with heat(ambient temps)issues but, if you go to a grow box or, tent you will and want a fan with sufficient cfm that can handle both situations.
  5. What if I want to keep her nice and short would I leave it at about 10"s from the plant at all times?

    Thank you guys sooooooo much. Please stop by and check out my grow :D
  6. #6 colafarmer, May 10, 2011
    Last edited by a moderator: May 10, 2011
    If you can put your hand between the light and your plant and is comfortable will be just fine....Closer is better as seen in this chart

  7. #7 Jellyman, May 10, 2011
    Last edited by a moderator: May 10, 2011
    Every H.I.D. bulb of enough wattage needs to be spaced away from the plants, regardless of cooling. This is not because of heat but because of light burn. The average 400w needs to be at least ten inches away at all times because any closer (at least for an entire photoperiod) would cause light burn on the top leaves. Keep in mind that a light mover allows the grower to set lamps closer than the minimum because the more intense light isn't held on the plants for as long.

    Now, to get to this minimum distance, heat must be handled. A 400w without proper cooling won't likely be able to get anywhere near that 10" from the plants, but for a different reason that light burn; heat.

    Here, check out the 10th reply in this thread, and its attachment. It's similar to the graph Cola posted: http://forum.grasscity.com/indoor-marijuana-growing/808743-closer-lights-=-more-thc.html

    With a lamp cooled very well by cold outside air, you'd be able to stick your hand on the lamp's glass itself (do not attempt) without any discomfort.
  8. If these are centrifugal inline fans, sure.

    When using multiple fans in series, each fan needs to move air as well as each other fan. Even with the restrictions, your 6" fan should still move air at the place you plan to mount the 4". If the air at that point is already moving faster than the 4" can manage, then installing the 4" fan will actually decrease airflow.
  9. so two 6in or two 4in?
  10. Thanks for all the info it has really helped me quite a lot.

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