Fan size for 250W MH setup?

[yosemite sam]

OK, I have a 250w MH setup on the way for my 29 gallon. I'm working on modifying the hood right now. My main question is, what size fans should I use? I have seen people using both 3" and 4" fans. I am thinking about puting one on each side of the hood, both blowing in, and have air escape out the back. Does this sound like a good idea? I have been doing some searching on the board, and hope I have it figured out how to hook the things up, but any advice on that would also be appreciated. Thanks.

 

[ryebread]

Honestly on a 29g set-up with that hot of a light............
I would probably set up three or four fans. Maybe two of the 4" fans on the left and right side of the canopy and one 4" or 5" fan on the back pulling out hot air.

 

[yosemite sam]

Thanks for the advice Rye. What kind of fan should I get? On the board I read people got 12v DC fans. I was at the radio shack today, and they had some big 4" and 4.5" fans for like $20 each, but they were 115v and 120v AC. Can I just get an adapter to conver them to DC?

 

[krux]

also if you can find some venting material, like if you look at the ones on the floor for your heater, or the wall for your airconditioner, you can mount one of those on the top of the hood so that hot air can rise directly our of it. this would also give more general ventilation.
you can find a variety of these ventilation covers, contractors call them "registers".
i live in an apartment atm, maybe someone with central heating and air can take a pic of what im talking about and post it if you dont know what i am referring to.
also, if you dont mind spending a few more bucks, i would buy fans designed to use in a salty environment, ice cap makes some, and you can find others on some of the online retailers. regular computer fans work great and are cheap, until they day that you are not at home and the salt creep or rust causes your fan to sieze. maybe having 4 will give you some back up... personally id just spend the extra cash to get fans designed for aquarium use. they will come with diagrams for wiring as an added bonus.
if you would like a site that sells aquarium fans, send me an email at krux@lanset.com

 

[yosemite sam]

Krux, I know what you mean by the registers. Where should I put it? The reflector is 24" x 9.5" and the lid it is going on is 34" x 15". I was thinking that if I put the light in the back end, which should help with cooling (since the back of the hood is open) I may not get good lighting for the tank, but if the light is in the front, it won't cool as well. As for fans, I was looking online at a few places, and they seemed a lot more expensive (like $40 instead of $20). I'll check with the good LFS here and see if he has any, or can get any in. Thanks for the help.
P.S. Nice to see a Portlander on the board. I grew up out by Beaverton/Hillsboro.

 

[krux]

yes they can be a good deal more expensive, look at the cf/m (cubic feet per minute) on the fan though, you should be getting a heck of a lot more movement than your average computer fan. this would most likely necessitate 2 5" versus 4 4" fans. not sure how long since you were in portland, but if the name souta means anything to you, you can buy fans from him.
i know you can buy the vents in a variety of widths, i used some to vent a computer desk i built once. if i remember, you can get them as small as 4 inches. i know for sure you can get them at 6 inches wide. i would just overlap them an inch or so, still a ton of room to very securely attatch your reflector. see illustration below. looking down, assuming you could see through the wood of the canopy.

 

[pyro383]

Before you go fan crazy, there are a few things you should know. 120v fans are louder than dc. CFM is the amount of air moved, the higher the better but louder. I have a 45 with open back and 194w pc and 175mh, I have 1 4" 120v fan 110cfm. This unit is loud 64 decibals. However the air temp in the canopy without a fan is 96. With fan on it 70. Water temp is 86 with fan off, when on is 80. I evoparte 1.5g a day when that used to be a week. Fan blows in. I suggest a variable speed fan, they sell them for $16 at computer stores and its 80mm or 4" and it is dc. Temp drives them and are quiet with a max of 38 decibals at 60-80 cfm. That is what I am now going to install. I would install 1 at a time starting with 4" since you can always go bigger but not smaller unless you want to redo your hood.

 

[krux]

good point, but you may have overlooked a couple things....
the issue being, you live in new york, and he lives in new mexico, at least a 20 degree difference in temperature in the summer. unless he keeps his house nice and cool this summer with his home ac, or runs a chiller, i think with a 250 over a 29 (did you take that into account, huge btu output over relatively small body of water?) he is going to see much higher temp swings than you experienced.
I suppose it never hurts to go one at a time, but in my 10 years of owning computers, i have had at least 3 fans die in that time that i can remember, and that was under optimal operating conditions, not over salt, not over water, and not running pretty much the entire time the lights are on (6-8 hours for most folks).
just a couple things to think about.
running such a big lamp over his tank, the noise factor might just be something he has to deal with. it should balance with some very beautiful sps and clam specimens.

 

[broomer5]

Here's my take on the subject.
The issue is "how do I remove heated air from the enclosure before it get's to the water. While it's true that water is cooled by the process of evaporation - there's no need in my opinion to "force" more evaporation than necessary.
Blowing hot air "around" inside the hood will not reduce the heat as well as removing it.
The logical way to do this in my mind ~ is to force the warm/hot air UP away fomr the the water surface and OUT the top. Mounting fan(s) in the ceiling of the canopy ~ allowing cooler room temperatur air to be pulled in down near the water surface.
This creates a net movement of air from the bottom of the enclosure to the top ~ where it discharges out and up.
I run (2) 250 watt Metal Halides AND 440 watts VHO over my 75 gallon tank. My water temperature does not exceed 82 F, with room temperature held between 70-75 F.

 

[broomer5]

Something like this has always seem reasonable to me.

 

[krux]

thats a great broomer, but how does one mount the reflector in that setup? do you use posts to drop the reflector down an inch or two to give room for the fans to be mounted in? in a smaller hood, like on a 29, there is a lot less room to mount fans, unlike on a 4 foot tank, where you can space the fans in between the reflectors.
maybe if you do not need to have the top of the canopy flat, you could just cut the hole, and bolt the fans on top. they will stick up an inch or so, depending on the dimension of the fan is all i guess.

 

[broomer5]

Yeah I can see where that could be a problem in a confined canopy.
I mounted the reflectors down about 6 inches from the ceiling using a "wooden beam" attached to each end of an internal frame.
You could easily drop the reflectors down as you mentioned.
I've got about 2-3 inches of air space between the top of reflector and fans. Works pretty good for me.
Lots of ways to ventilate a canopy.