# fans blow onto heat sink or suck air off?



## Larry Grenier (Apr 19, 2005)

You'll find most installations have fans blowing in and possibly a vent somewhere for an exhaust. Small problem with fans pulling air is moisture on the fan itself.


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## Dave-H (Jul 29, 2010)

I don't think it will matter how the fans are oriented. What matters is the amount of air that is moving across the heatsink and taking away heat, regardless of direction.

Also keep in mind that if you were to set up a fan to blow air on the heatsink, you won't see much benefit unless that air has somewhere to go and cooler air has a way of being introduced. So, you really want to have either a completely open system (i.e. no canopy/lid) with a fan moving hot air away from the light and cooler air onto the light OR if your system is closed you want to establish an intake and an exhaust duct/opening then use the fan to push the air through.

You may not need to over engineer it - in general you can expect because a heat sink is specifically produced to allow heat to exchange with surrounding air the addition of a fan to move that air will have a significant effect. Try out a simple configuration and see how it goes.

Just remember that you want to move the air AWAY from the fans, not just blow it around.


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## IWANNAGOFAST (Jan 14, 2008)

oh, I should have mentioned this will be in an open top system. It's a rimless tank and the fixture will be hung from the ceiling or a light bar 

The top of the fixture is enclosed but the bottom is open

So, should I have option a or b?


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## Dave-H (Jul 29, 2010)

It won't matter - imagine which one will result in the MOST air moving away from the lights and the MOST fresh air moving towards them.

With an open system, there is so much air that can come in you might be successful regardless of how you do it so you could just consider cosmetics.

Heat rises, so maybe blowing the hotter air UP would be a good approach, for lack of a better rationale.


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## Baadboy11 (Oct 28, 2009)

I would blow it up simply because....I don't have a reason thats just how I would do it..lolz


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## HolyAngel (Oct 18, 2010)

On a computer, the heatsink fan always pulls the air up an blows it away from the heatsink, there are some configurations with heat pipes that have the fans in a push/pull configuration which you may be able to do for the whole unit, but for the fan sitting right on top of the heatsink and mounted to it, it blows out.


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## IWANNAGOFAST (Jan 14, 2008)

HolyAngel said:


> On a computer, the heatsink fan always pulls the air up an blows it away from the heatsink, there are some configurations with heat pipes that have the fans in a push/pull configuration which you may be able to do for the whole unit, but for the fan sitting right on top of the heatsink and mounted to it, it blows out.


That was my plan too, but the thing is, the fan won't be sitting right on top of the heatsink, there will be a 2 inch gap between them, and the heat sink is 36 inches long I thought it might be better to have cool air hitting the heat sinks to cool them off?


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## wmsvn (Mar 23, 2009)

It depends whether you are in a close space or not. If it is in enclosed space then you want to sucking the air off to push it out of the enclosed space. But if it is an open space, then it depends on how the position the fan too. If possible you want the fan to blow the air into the heat sink assumed that there are places for the hot air to escape easily.


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## Dave-H (Jul 29, 2010)

In computers, they put the fan directly on the hottest chip which is usually horizontally sitting on the motherboard. I don't think there is much else to the orientation of those fans.

You want to simply have the maximum airflow moving across the largest area of the heatsink possible. The heatsink is made to exchange heat with surrounding air, and it doesn't matter where the fan is at all. It only matters that there is cool air coming across the heatsink and that that air, once warmed by the heatsink, moves away from the lights.

If I had a 36 inch heatsink, I would try to configure the fan so that there is constant air flow across the entire heatsink - that would probably involve mounting the fan off the heatsink rather than right on top of it.


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## HolyAngel (Oct 18, 2010)

Yeah if you could make like a sealed tunnel with a fan on each end, one pushing air in, the other pulling air out, that would probably work most effectively.

If the fans will be off the heatsink 2" or so and you cant do the above, I'd have the fans blowing on the heatsink.


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## redman88 (Dec 12, 2008)

with an open bottom light. i would mount the fans on the left and right side of the cannopy and have them pushing air out of the cannopy. so that the cool air come up and under the lights and the hot air gets pushed out of the sides.


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## redfishsc (Aug 29, 2010)

I prefer to mount the fans right to the heatsink, blowing air onto them. 

If you have them attached right onto the fins of the heatsink, it probably doesn't matter. 


If they are not touching the heatsink, definitely have them blowing onto the heatsink otherwise you can't be sure it's getting sufficient air movement.


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## IWANNAGOFAST (Jan 14, 2008)

hmm, I may need to get more fan controllers then. I was planning on just having 2 fans at the top of the fixture, but maybe I'll get some smaller ones on either end.

This is what the fixture housing looks like. This is the view if you were looking at it from the bottom of the housing. 









I'll cut 2 holes for the fans on top, and the bottom will be open. I could add some fans to the ends as well but would prefer not to have too many.


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## redman88 (Dec 12, 2008)

if you could give the top a curve then you could put one fan on each end and you would end up with good circulation


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## IWANNAGOFAST (Jan 14, 2008)

Ok, so here is the LEDs mounted on the heatsink









And here are the fans I will be using (I'll have 2)









My dad said he thinks they should be used to pull air off, but since they're not in contact with the heat sinks I think it may be better to have air blow on them? 

My thinking is, you blow on hot soup to cool it down, you don't suck the hot air off.


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## HolyAngel (Oct 18, 2010)

Nice! I have those same fans on my pc case ^^

Did you use thermal paste when you mounted the LEDs to the heatsink? I didn't catch it in my quick glimpse over the thread, but that will *definitely* make a difference, especially if you get Arctic Silver and don't waste your money on the cheap stuff at radio shack or elsewhere FYI


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## Hoppy (Dec 24, 2005)

I feel very storngly that the fans should blow onto the heat sink, not try to suck air around them. The fans can either blow along the fins, or be directed at the face of the finned side. The latter is most likely to be the most effective for shorter heatsinks. But, I have seen no data on the subject.


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