How do I stop negative airflow?

[thunderben8]

I'm using a "NZXT S340 elite" case which has 2 front 140mm fans for intake and 1 rear 120mm exhaust and 1 140mm top exhaust fan. It also has a a dust filter at the front on before the fans and has a solid front planel with 2 slots 1 at the top and one at the bottom of the front panle roughly 1x6 inches. So the front 2 fans have restricted airflow. I am getting negative airflow. Is there a way to create positive airflow without controlling the fans specifically to create positive airflow? I don't know what do do to create positive airflow?

 

[tennis2]

Before we get into complicated Bernoulli equations relating air velocity and pressure, you'll need to provide more info. The two 1"x6" vents in the front aren't necessarily inadequate.

The S340 comes with only two 120mm fans (top and rear as exhaust) which is negative air pressure because there's zero forced air intake and some amount of forced air exhaust.

What 140mm front intake fans did you add?

 

[thunderben8]

Before we get into complicated Bernoulli equations relating air velocity and pressure, you'll need to provide more info. The two 1"x6" vents in the front aren't necessarily inadequate.

The S340 comes with only two 120mm fans (top and rear as exhaust) which is negative air pressure because there's zero forced air intake and some amount of forced air exhaust.

What 140mm front intake fans did you add?

 

[CountMike]

Positive airflow was never of great help, even case manufacturers gave up on it, just too many air point entries in a case to produce any positive pressure in the case. All you have to take care of is to have even airflow over components where it counts.

 

[tennis2]

Positive pressure is of great help to prevent dust ingress! It ensures that the air entering the case is getting sucked through fan filters instead of unfiltered cracks/openings. That said, yes, the amount of pressure you can build inside a case is pretty small because there are so many places for air to escape (ie vented expansion slots).

I can feel air flowing out of the vented expansion slots in my case.

 

[CountMike]

Filters would catch 90 -99% of dust, the rest is of no consequence. My biggest concern is amount of airflow that filters greatly reduce. Even bug/mosquito screens have only 50% of holes to whole surface they cover, filters even less.

 

[tennis2]

No doubt! That's why I prefer to steer clear of cases that do anything except a nice big flat screen filter over top of the fans. I'm very dubious of cases that put the "filters" on the side vents of the front panel. Those vents have a limited area to start with, before you put a restrictive filter on them.

Of course, dust and filters is a balancing act. No point in having unrestricted airflow if your heatsinks inside the case are coated with dust. IMO, the drastic reduction in cleaning frequency is worth the slight airflow restriction.

 

[Paperdoc]

First, how do you know you have negative air flow (i.e., internal air pressure)? The way I check is to use a small source of smoke, like an incense stick or maybe a cigarette. Move that slowly around the case where there are any small cracks and see which way the smoke flows. That tells you the direction of air leakage.

Next item: check the filters on your intake fans. They need to be kept clean. Although they DO reduce air flow slightly, clogged ones are much more restrictive!

Next, let's review how the fans are being powered and controlled. The NZXT website for that case says it includes at the top and rear some model FN V2 fans, which are of the 3-pin design. So, check whether this is correct - do each of those fans have THREE wires from the motor, ending in a female connector with THREE holes? If that is true, that type of fan can have its speed controlled ONLY by a mobo fan header that uses the older Voltage Control Mode (aka DC Mode). IF a 3-pin fan's header is using the newer PWM Mode, they will always run full speed. So check in BIOS Setup for those two fans particularly whether the header is set to use DC Mode so it CAN reduce its speed as needed.

 

[tennis2]

To dumb it down even further, simply add up the CFM ratings of the intake fans and the exhaust fans (include PSU exhaust if it sucks air out of the inside of the case). More CFM in than out = positive pressure.

The biggest caveat to this method is when you have radiators (liquid cooling) since they're very restrictive to airflow. I generally cut the fan airflow rating in half ifs pushing air through a radiator.

 

[Paperdoc]

Yeah, calculating with air flow specs is useful in your initial design phase. The problem is that the actual airflow delivered by each fan is less by varying amounts, depending on things like filters and radiators that slow down air flow. That's why I like to do an actual flow direction detection with a smoke source when it's all done, to check whether it is working as planned.

OP, thanks for Best Solution.