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Old 07-20-2004, 12:13 PM   #1
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Fan in or out?

I just finished a DIY hood for my 35 gallon hex, and installed a little 12 v. fan in the side to prevent overheating. Nowever, I was wondering whether I should leave it pointing out so hot air is sucked out of the hood, or have it pointing in so it is blowing air in?
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Old 07-20-2004, 12:23 PM   #2
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That question has been debated many times already without reaching consensus

IMO, have a vent to allow air in and have the fan blow air out.
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Old 07-20-2004, 01:09 PM   #3
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Yep, better to use it as an exhaust fan rather than an intake fan. Unless you have two fans, which you could then point one in, one out for better overall circulation.
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Old 07-20-2004, 01:17 PM   #4
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If the hood is sitting right on top of the tank and is pretty well sealed then I would go with the exhaust idea...a fan pulling air out and a vent to allow cooler outside air in. If the fixture is a pendant type and is pretty well open then I don't think it makes much difference. I have a 175Watt MH pendant I built and I use 1 fan blowing in and 1 fan pulling air out the other side...it keeps temps pretty much at room temp in the tank.
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Old 07-20-2004, 02:02 PM   #5
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no way I am a fan guy , I built computers every which way for years.In is 100% better.The air will find a way out.I do suggest getting ain adn out though much better than jsut in
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Old 07-20-2004, 03:26 PM   #6
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The real question is what do you want to cool?

If you want to keep the water cool, you should have a fan pointed at the water. This eliminates the layer of vapor on top of the water. If this is gone, the water will keep evaporating and therefore cooling the water.

If you want to cool the lamps, then aim the fan there.

I think a fan sucking air out will not do much good at cooling the tank. Like electricity, air will follow the path of least resistance and most likely leave the vapor layer on the water.

Your best bet would be one of each. A fan aimed at the water and one on the opposite side to help exhaust the excess vapor and eliminate swirling.

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Old 07-20-2004, 03:53 PM   #7
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I'd be very surprised if, in practice, it mattered which way you did it As long as there is good circulation you should be fine.
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Old 07-20-2004, 04:18 PM   #8
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Out of curiousity I tried blowing the fan across the lights and across the water. There was a great deal more evaporation when the fan was aimed at the water. the temperature stayed right at 80ºF the entire time. When the fan was blowing across the lights, the temperature raised to 84º and the amount of evaporation dropped significantly.

I still want to build my titanium heat exchanger 8)
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Old 07-20-2004, 04:20 PM   #9
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Okay, thank guys. I guess I am just leaving it pulling hot air out of the hood.
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Old 07-20-2004, 04:52 PM   #10
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I've tested over and over and over.

Blowing drier air across the water surface increased evaporation and lowers water temperature. You can lower water temp 5 - 10 degrees blowing air at the water or 2 or 3 degrees pulling the hot air out of the canopy. Your tank your choice.

What most people don't understand is that very little hear is transferred to the water from a hot canopy. Lights heat water from the light entering the water, not the hot air around the bulbs.

Cooling flourescent bulbs does make sense though. It makes them last longer. It makes absolutely no sence for MH though. MH bulbs burn better at high temps so cooling the bulbs give you less light.

Another bonus by blowing the air in is that the fans last twice as long.
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