I have just bought an air cooled hood. If I had my shit together I would know what make it was but I can’t remember. My question is a simple one. There is an 8" spigot coming off both ends to attach an air flow. Now I’ve been thinking, if I push or pull air through over the top of my 600w HPS bulb (which is an arc lamp) will I cool the bulb down to such an extent that I will lose efficiency. I figure the air movement will cool the bulb and ultimately the arc. I only have a 4" inch fan on there just now and not sure whether to switch it on, thoughts anyone 
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What will be the temp of the air that you’ll be drawing into the hood? There is always some loss with air cooling a hps / mh bulb. The upside is you can keep them closer to the plants and mitigate any losses you have by cooling them .
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@Davyg. I have a 1,000 watt MH/HPS set up. it’s only 6". I have a 6" in line fan sucking the air in and a 6" in line fan blowing the air out. this is what I have
except now I draw my intake from the adjoining bedroom
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That friggin bulb produces a lot of heat
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I have no idea about rate of bulb cooling and how that applies to output of the bulb. However, the glass on hood will block some light and the fan running draws extra power but doesn’t add any light. Both of those will have negative effect on how efficient your lighting system is. You could run into a situation where that saves you on something like cooling costs though. Potentially reversing any drops in light efficiency to an increase in efficiency of your entire grow space.
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The air temperature going in is room temperatures so not cold around 70f. I was looking at it last night when I had a light bulb moment.
It was something similar that I had in mind. That 600w bulb does get hot, the 1000w must be like the sun.
So, either ditch the glass and utilize the fan elsewhere or run the fan at a reduced speed. That will keep the air moving over the bulb but not blasting past it and keep the glass spotless.
At 70f it will have minimal impact on your bulb efficiency. I run two 6” hoods in-line with a 6” fan blowing through the fixtures. I clean my glass every other week before lights on. It makes it easier if your hood has the quick release latches.
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I have a 4" fan tucked in there just now running at half speed. Glass is a bit dusty so I will clean it tonight before lights on. You’re spot on with the quick release catches, make the job so much easier. Thank you for getting back to me 
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I would use the fan with hood if you need it. I was just answering your question based on a light out vs power consumed (efficiency) perspective.
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your answer was perfect, told me what I kinda thought. Thank you again for taking the time to address my brain fart
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