by Ken Denmead
We live in NorCal, in an area with a very temperate climate. It rarely gets too cold, and it rarely gets too hot. While we have a furnace for keeping warm when it’s cold, we don’t have A/C for when it gets hot, so we depend upon fans. But when we go to bed, even with fans on in the room, and even with the lightest sheets, it can still get uncomfortable. So, I went searching for a solution.
What I found was this very slick $400 solution with an app-controllable fan that could deliver either room-temperature, or warmed, air under your sheets for extra comfort. It would certainly do the job, but at that price, it was a bit expensive, we didn’t need the heat function, and the app-control was just gilding the lily. However, looking at the whole setup from my engineer’s perspective, I thought I could come up with something myself that would do the job and be a bit more economical.
Realistically, it wasn’t hard. Again, I headed to Amazon, and found the key component:
This is the Apollo Horticulture 4″ 190cfm Inline Duct Fan, and costs $60. It’s specifically made for ventilation of greenhouse setups, but could be used for all sorts of similar needs where you need to move air via ductwork. It’s reasonably quiet, and moves a decent amount of air volume (there are higher-power units available, but this one has worked well for us).
I started with just this and an 8′ length of flexible hose ($9), and pumped floor-level air under the sheets at the foot of our bed. It worked pretty well, but the end of the hose would move during the night, and make the overall cooling a little inconsistent. So, I needed to device a way to spread the cooling air out, and keep it in place.
My solution: a 5′ length of 4″ Schedule 40 PVC pipe (which I picked up at my local Ace Hardware, costing about $15). I put a cap on one end, duct taped the open end of the flexible hose to the other, and then drilled holes down the length of the pipe to create what I’m calling an “air sprinkler.” There was even a bit of engineering design that went into the size and number of the holes. I realize, to be able to deliver the same flow of air that the fan was generating, I needed the total cross-sectional area of the holes to equal that of the outlet of the fan (4″ in diameter, or a 2″ radius).
It was a pretty easy calculation. Since area = πr2, the area of the fan output was π x (2)2, or 4π, which is about 12.6 square inches. I settled on a 3/8″ drill bit, which creates a hole with the cross-sectional area of π(3/8)2, or about 0.44 square inches. With this drill bit, I could make 12.6/0.44 = about 28 holes and maintain pretty good efficiency. I then just laid out an even pattern across one side of the pipe, imagining it facing towards our feet under the covers, and evenly spraying our feet and filling the lower bed area with cooling air.
It works. It works really, really well. We’ve already had a couple very warm nights this month, and where we’d usually end up tossing and turning even with floor fans on in the room, each of these nights has been comfortable. Even though the air under the sheets is just room temperature, it does a great job of keeping thing cool and comfortable. Total price: about $85.
I’m still working on some fine details — like putting some kind of base on the pipe so it won’t roll during the night, but that’s been a minimal problem. And yes, I know it’s a little goofy to have a big plastic pipe under our sheets while we’re sleeping, but the key point is that we’re sleeping — comfortably — and that’s totally worth it.
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Ken Denmead
Ken is a husband and father from the San Francisco Bay Area, where he works as civil engineer. He became the Publisher of GeekDad in 2007, and the owner in 2010. He also wrote the NYT bestselling GeekDad series of project books for parents and kids to share.
