When Derek came up to me with the question “What will cool a hot room down faster, window fans that point in or out?” I had to think for a minute. It’s been a while since I lived without A/C, but I remember always pointing the fans OUT, to “push” out the warm air. But does this make sense? I know they sell window fans that spin in opposite directions, could this be the answer?

So I told Derek to model up his room and we could use Flow Simulation to run some scenarios and monitor the room temperature. We would have an answer once and for all!

We ended up creating two configurations of the room, with three Flow Simulation studies each:

  • One window

o   No fan

o   Fan pointed in

o   Fan pointed out

  • Two windows

o   Both fans pointed in

o   Both fans pointed out

o   One fan pointed in and one out

For simplicity sake, we are assuming a perfectly sealed room. Even if the room was opened up to the rest of the house, the incoming air would likely be warm anyway. We are also assuming a 12” desk fan in a fully open window. We also took the furniture out of the room. Since it does not change study-to-study, it would provide the same thermal mass, and only increase the computational time. Remember, with all simulations – Simplify as much as possible!!! The rest of the setup is as follows:

  • I created a transient study to look at this from 0-500 seconds. If I set up a steady-state study, my results would have all been identical. Thanks Thermodynamics!
  • Room temperature is 75⁰F, outside temperature is 60⁰F at 1 Atm.
  • I used a split line in the window to represent the fan and per the manufacturer it moves 800 cu.ft/min. I used an inlet flow, but could have easily used a virtual fan instead.

I repeated this in the Two-window scenario as well. Finally I added a Maximum Temperature Goal and a Mass Flow Rate Goal to assure proper convergence.

Let’s take a look at the results at 250 seconds:

Even halfway through the simulation, we can see the clear winner(s). Finally, I plotted a point chart to see the temperature in the middle of the room over time.

Again, we can see the clear winner: Two fans pointing in. This scenario effectively cooled the room in just under 6 minutes. Though, I was surprised how similar one fan pointing in and the opposite fans were.

Taking a closer look at what is happening directly inside the fan pointing out we can see why this method doesn’t help much:

The low pressure area behind the fan is sucking all the cold air entering from the window and blowing it back outside! Only a small portion of the warm air is actually getting moved out.

I’m sure that a box fan tightly enclosed in the window would produce different results, but for the purposes of this scenario, we know what to do!

So next time it’s hot inside your room and cool outside, point your fans IN. This will give you the most effective and fastest cooling. Or if you’re like me, crank up the A/C!

Thanks for reading, and as always, Happy Simulating!


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