How To Air Condition Your Altitude Tent

So the altitude tent works on the principle of a machine that pushes in de-oxygenated air and that pushes out whatever was in there to begin with.  So once it's run for a couple hours, your oxygen level in the tent is dropped to something resembling what you'd find at altitude.  Once you get in it, your body breathes that air and expels carbon dioxide and moisture.  Most of the carbon dioxide is pushed out by the continual pumping of the air machine, but much of the moisture stays in the form of humidity.  What's worse, your normal body heat as well as some extra heat generated by the air machine cause the temperature in the tent to increase several degrees over the temperature outside the tent.  So it can get warm and sticky.

That's a problem for me, as I'm a light sleeper.  To get good sleep, I like to be bundled up (even if it's just a sheet) and I need it to be cool.  If it's not cool enough, I get too hot and wake up.  What's worse is by the time I wake up, the bed is fairly warm, too, and I have to just get out of it for twenty minutes or more.  Worse than that is the fact that it usually takes me another thirty minutes to go back to sleep.  So the best case is I lose around an hour of sleep, usually more.

This was happening with some regularity, so I knew I needed to do something to cool the inside of the tent.  The only commercial offering I've seen to handle this is actually to use a tiny "split system" HVAC unit.  That's expensive (around $2,000) and probably not a great solution, either, since it probably cools too much for a small tent.  It would basically freeze you for the few minutes it would need to run.  Oh, and it requires you to do some significant plumbing to an outside compressor that also requires an electrician to wire it in.  Too much for a tent.

So I decided to try another way...my own custom "heat exchanger."  The idea was to take two small radiators and plumb them together with some long hose.  In between would be a pump to circulate fluid between them.  On each radiator would be a fan.  While it's winter, I'd put one of these outside the bedroom window and the other inside the tent.  I didn't do any math, but just did some guessing on sizes of things.

My inside radiator with fan.

So here's the material list:

• 2 Mazda Miata heater cores
• 50' of 5/8" vinyl hose from Lowes
• five hose clamps
• Sunterra Fountain Pump (130GPH)
• 1 bucket with lid
• 2 small fans
• half gallon of automotive anti-freeze

You can find about any older heater core from a car for about $25 (local junk yards or online at places like car-part.com).  The Miata ones came with two long aluminum tubes on each outlet that were easy to cut off to about 1" long, and the 5/8" vinyl hose will fit on the tube snugly.  Perfect.  The pump says it's a 1/2" outlet, but I found the larger part of the outlet works fine to clamp the 5/8" hose on as well. I found the pump at a local Northern Tool (for $20), but Lowes or Home Depot probably have something very similar (if not identical).  I've seen it online for less than $15.  The fans I used are larger computer style fans that are 110V.  Just about any small and quiet fan will do.

The assembly is pretty simple.  The pump is submersible, which means you need to drop it in a tank of fluid.  That's where the bucket comes in.  You don't need anything much larger than the pump itself, and the lid needs to be large enough to handle a hole for two pieces of hose and the cord.  I actually used a plastic container that some of my nutrition product came in.

Tank shown with shoe boxes to keep the hose from kinking.

(A note about the cord: I'm an electrical kind of guy, so I drilled a hole in the lid just big enough for the cord itself, but that doesn't leave any way to assemble it since it has a plug on one end and a pump on the other.  So I cut the cord, fed it through, and spliced it back together.  Do this at your own peril.  Should be safe as long as you do your splicing on the side that's going to be outside the tank.)

So, cut a piece of hose long enough to reach from one radiator to the other, and clamp each end of that hose to an outlet on each radiator.  I placed my tank inside the tent (seemed more efficient that way, since a tank of cool fluid will also help cool the space).  So then cut another piece of tube to run from your tank to the OUTSIDE radiator.  Clamp one end to the outlet of the pump (inside the tank) and the other to the outside radiator.  Then cut another piece of tube to connect the inside radiator to the tank.  It only needs to reach well inside the lid as it's just going to dump return fluid from the system into the reservoir.

Lay this entire system out on a flat floor and put about a half gallon of water in your system.  Plug the pump in until it pumps the tank dry and unplug it quickly (these pumps should never be run long dry...it will kill them).  Then use your anti-freeze to fill the rest of the system.  You don't need anti-freeze if you live in a climate where it never gets below freezing outside, but it does actually transfer heat better, so it's worth using anyway.   Once you can leave the pump running and not have the tank go dry, check for leaks at your clamps. 

Outside radiator and pool noodle, shown from inside.

Now you'll want to attach your fans to your heater cores.  I oriented mine so that the one going outside was pointing out when in the window.  That way it shouldn't suck in rain water when it rains and kill the fan.  Then you can install your system in your altitude tent and an outside window.  To install in the window, I used a swimming pool "noodle" to seal off the window and not pinch the hoses.  I simply cut a slit lengthwise from the end and then two holes through the noodle to match the hose locations.  Then close the window against the noodle with the radiator/fan combo outside.

Outside radiator shown from outside the house.

There's also the matter of "burping" the air out of the lines once your system is setup.  With the pump running, you start at the pump supply end and lift the hose to advance the first air bubble toward the outside radiator.  Work that bubble to the radiator, and then move the radiator around to work it through the radiator and out the other side.  Then work that bubble back down that hose all the way back to your tank.  It should expel the bubble into the tank and you should be set.

One note here about head pressure.  Head pressure is the amount of "lift" a given pump can create.  The pump I chose has a lift maximum of four feet.  So that means that no point in this system can be more than four feet higher than the level the pump is sitting at.  I also believe that anti-freeze is heavier by volume than water, which probably reduces the lift height by some amount, so I wouldn't go more than three feet higher than your tank.  It's okay if your hoses go down and then back up, so if you are putting your radiator in a higher window, you'll need to also elevate your tank inside your tent somehow so that your radiator isn't more than about three feet above your pump (assuming you use a pump with a four foot lift capability for water, anyway).

I've found this system to work quite well, but I got pretty lucky with the fans I had on-hand.  The one on the outside radiator is high volume and loud, but it's outside the window and doesn't bother anything.  The one inside is lower volume and quiet.  The pump is very quiet.  All in all, this system makes nearly zero noise inside the house.  It's great in that respect.  It's not quite as efficient as I thought it might be, but a bit more fan inside would probably do the trick.  Right now with it getting below 40F at night, it works quite well without over-cooling at all.  I've not bothered to turn it off at any point, but it keeps the tent several degrees cooler.  It's not so good, however, that it causes any condensation of note, which may be a problem if you have lower outside temps and/or a better inside fan.  If so, the easiest solution is probably to put the inside radiator over a small bucket to catch the condensation and just empty it regularly.

 So what to do when it gets warm outside?  I've got a couple months to fix that, but one suggested solution is to try this same situation except remove the fan from the outside radiator and place that radiator in the freezer section of a small dorm refrigerator.  That will require drilling some holes in the door and re-routing your hoses through the door (and sealing those holes somehow).  But it may work.  The other solution, and the one I'm hoping to try, is to use a small salt water aquarium chiller.  I believe the 1/10th horsepower is about the proper size for this.  New they are about $400, which isn't good, but I hope to find one for MUCH less than that used.  We'll see if I can pull that off.  That will still require the use of the pump and tank, but you will simply remove the outside radiator and tie your two lines into the inlet and outlet on the chiller.  At that size most of them have both 1/2" and 5/8" plumbing, so it should be VERY simple to do (it'll take longer to bleed the system again than to re-do the plumbing!). 

If you try this, good luck, and leave some feedback here on how it goes!