Push/Pull Config: Matched Fans?

When putting dual fans on a radiator, is it necessary to have matched fans or is it OK to have fans with different CFM ratings? I found a really inexpensive Sunon "Quiet" 120mm fan in a FS/FT forum and I hope I can use it with the Panaflo already installed.

It's best to have the fans match that way one is not over powering the oter.

I think if you are planning on doing that I would suggest using the weaker fan as the puller and the stronger fan as the pusher. In my eyes it seems that the strong fan would considerably slow down once it hit hc and the fan on the other side would help it by guiding it through and getting the best possible airflow or maybe I am just totally off.

But a stronger pusher will cause the slower one to work harder since more air needs to get through, making more noise because of the turbulence.
Seems that matched fans are the best since they keep a steady and equal flow, just like your pump in the loop.

It won't matter... neither fan is going to be pushing close to their maximum (heater cores have one hell of a pressure drop compared to the pressure generated by a typical fan).

I recommend staying away from fans that are not matched as both fans will have to work harder than if they were matched. This will reduce the life of your fans and cause them to make tons of noise in the long run

It is true that neither fan will be pushing it's rated freeflow CFM, but using matched fans should be quietest.

Matched fans in a push pull setup should have as close to an equal flow rate from each fan - that is ideal for quiet operation.

IMOG said:

... Matched fans in a push pull setup should have as close to an equal flow rate from each fan - that is ideal for quiet operation.

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Unless you have a leak somewhere, the flow through both fans, and the core itself, will be the same. It's exactly the same thing as running two mismatched (water) pumps in series... I'm not sure what's making this so hard for everyone. It's not going to make tons extra noise, nor will it kill the fans... it's just two axial pumps working happily together in series.

Skulemate said:

Unless you have a leak somewhere, the flow through both fans, and the core itself, will be the same. It's exactly the same thing as running two mismatched (water) pumps in series... I'm not sure what's making this so hard for everyone. It's not going to make tons extra noise, nor will it kill the fans... it's just two axial pumps working happily together in series.

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Im not saying it will kill the fan immediately but it will in long run decrease the life of the fan. The reason being the lower CFM fan would be in conflict with the higher CFM fan as higher CFM is trying to do more than lower CFM can handle. This will shorten the life of the lower CFM fan.

Skulemate said:

Unless you have a leak somewhere, the flow through both fans, and the core itself, will be the same. It's exactly the same thing as running two mismatched (water) pumps in series... I'm not sure what's making this so hard for everyone. It's not going to make tons extra noise, nor will it kill the fans... it's just two axial pumps working happily together in series.

Click to expand...

Two fans with different ratings will be louder... That's what I was saying.

It's obvious that without leaks the actual air flow will be the same.

So given that the airflow will be the same through each, it is ideal that the ratings for each fan are the same so that the fans are closer to being in harmony with one another. This will make them operate more quietly - whether this is important to you or not is besides the point I was making.

The only thing that makes this hard for anyone is when someone misinterprets what someone else said.

ILikeMy240sx said:

Im not saying it will kill the fan immediately but it will in long run decrease the life of the fan. The reason being the lower CFM fan would be in conflict with the higher CFM fan as higher CFM is trying to do more than lower CFM can handle. This will shorten the life of the lower CFM fan.

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I seriously doubt that. It depends on the pair of fans used, sure, but I doubt that you'd be able to get enough flow through the core with a single fan to push the slower fan off its P-Q curve. Maybe if you were using a cage blower and an L1A...

I'm not sure what was misintrepreted... you explain what you meant, if not that the two fans will move different amounts of air:

IMOG said:

... Matched fans in a push pull setup should have as close to an equal flow rate from each fan - that is ideal for quiet operation.

Click to expand...

If it's so obvious, then why the initial confusion?

Bensa said:

But a stronger pusher will cause the slower one to work harder since more air needs to get through, making more noise because of the turbulence.
Seems that matched fans are the best since they keep a steady and equal flow, just like your pump in the loop.

Click to expand...

Having the weaker fan as the "puller" depending on exactly HOW much weaker actually could the the best noise~balance for airflow. The heatercore reduces the cfm that reaches the other side much more than you may realize. Preferably though would be to have the same size of fans. A 80cfm then 50ish cfm 120mm fan would not cause the 50cfm fan that is pulling to be spun up faster...because the 80cfm 120mm fan wouldnt get even 50cfm though the heatercore. Hope this helps out some.

Skulemate said:

If it's so obvious, then why the initial confusion?

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There wasn't any initial confusion, I think I just need to explain what I was saying to you more thoroughly.

Keep in mind the air pressure of the heatercore unit, if the first fan sucking air in is more powerful, it will create a higher pressure within the heatercore because it will be overpowering the second fan blowing air out. Now under constant operation the flow rate through each fan will still be equal, but only because of backflow and the air pressure.

Two matched fans of equal flow rate will create close to nuetral pressure and create the least noise - axial fans are far noiser when working against air pressure and backflow. Two matched fans should help to minimize the pulling fan from being choked, and the pushing fan from being backed up.

It would be nice to have a huge test of fans with CFM & loudness at specified pressure - let's say 5 air pressure levels.

This way it would be easier to find proper fan.

Anyway, I'm for using matched fans. Still one blower should be nicer, but if someone really likes fans - going with matched pair will be better than using 2 different.

I'm not sure I follow. Where is this additional back-pressure coming from? I thought that we were reducing the load on each fan by running them in series... for the given flow rate, one pump will be contributing X pressure, and the other will produce Y pressure (since their P-Q curves are different; the total pressure produced by the two fans will be the pressure drop required by the radiator for the given flow rate), but they are not hindering each other as you suggest.

Oh, and I commend you on a civil reply. (My last response was perhaps a bit snippy. ) I am interested in learning the reasoning behind your stance on this, but I'm not sure that I am following completely.

Your right, we are reducing the load on each fan by running them in series. However, considering the use of two push/pull fans in series there is an additional back-pressure relatively:

A) Two equal powered fans

B) Stronger pusher, weaker puller.

B will create more backpressure for the pusher than A will.

(I can't take credit for the civil response - the blue stars come with mandatory daily tranqilizer injections)

Well, my Sunon came today. That gives me one Sunon mounted as a side blowhole, one Panaflo on the radiator and one very quiet and IMO very weak Vantec Stealth as a spare. Since a matched pair is not possible in my situation, I'm going to experiment a bit with my three fans. We'll see how it goes. I don't have the longer mounting rods or the second shroud, so it might be a while before I can really experiment.

BTW, here's what I have:

Panaflo FBA12G12M (Radiator Fan, Pulling)
Air Flow: 86.5 CFM
Noise: 35.5 (dBA)
Came with my used water cooling setup

Sunon KD1212PTB3-6A (Now a side blow-hole fan)
Air Flow: 69 CFM
Noise: 35.5 dB(A)
$7 shipped

Vantec Stealth SF12025L (Now a spare)
Air Flow: 53 CFM
Noise: 28 dB(A)
$6 shipped and it came with an 80mm stealth too!

I just couldn't pass up these fans for the price. So, if I've read everyone's posts correctly, it looks like I should eventually have the Panaflo pushing, the Stealth pulling and the Sunon in the side blow hole keeping the case cool? I've measured my system's sound output with a RatShack SPL meter and I'm getting 56 dBA from my system right now.