The Idea

A small thin waterbed mattress inside a tent cot that is filled with water that's heated by the sun during the day and cooled at night.

The Details

Get this tent cot and water mattress (yes, it's meant for water, not air! It seems it comes from a medical supply company.):

Tent cot and water mattress

Then build something like this:


The idea is that the sun will heat the water in the coil of copper tubing, and by the thermosiphon effect, heat the water in the bucket.

During the day, the water in the water mattress will be cold, providing a (hopefully) nice place to lie down, even in the summer. Valves A and C will be closed to prevent my body weight from pushing the water out of the mattress, and valve B will be open to keep the water flowing into the coil of copper tubing.

Then, toward the end of the day, valve A and C will be opened and valve B closed, causing the thermosiphoned hot water to flow from the bucket into the water mattress. (Nothing can be weighing down the mattress at this time, of course.)

Now, the mattress is filled with warm water which will (hopefully) keep me warm at night.

Meanwhile, the water in the bucket is still outside the tent and cooling in the night air, ready to be put into the mattress when it gets hot during the day. I'm not sure how to do this part, but perhaps with valves A and C closed, the bucket could be lifted above the height of the mattress. Then, assuming the mattress can accommodate the extra volume of water, the water in the bucket will drain into it. Then valve B is closed, and the bucket is set down. Valve A is then opened, and the bucket will fill with water from the other side of the mattress.

Concern About the Weight of the Water

The water mattress is 32" x 68" x 4". This comes to 8704 cubic inches if it were a perfect rectangular box, which is 37.7 gallons. I think 25 gallons might be a reasonable guess as to actual volume of the mattress. Assuming 8.3 lbs per gallon, this comes to 208 lbs.

The cot is rated for 500 lbs, so it should be strong enough.

My Questions

  1. Is the design sound? Is there a better way?

  2. Is a five gallon bucket of water enough?

  3. How can I estimate how much the sun will heat the water and the temperatures that can be maintained inside the tent?

  4. How much and what diameter of copper tubing should I use? Ideally, I'd build a solar water-heating panel but portability is a concern, as is cost and labor. I'm disabled with CFS, so I have very little money and energy (for building this, assembling it, and carrying things).

(This whole project is because I want a comfortable place to stay when away from home. I cannot be exposed to mold, and most houses have mold at levels that, though unnoticeable to healthy people, will affect those who have developed sensitivity. I visit a distant doctor and would like to stay with my aunt, setting this tent up in the backyard. It might also enable me to attend an upcoming family reunion.)

I'm thinking I could make a solar cooker out of foam core and plexiglass, which, in addition to cooking food in, could also be sized to allow the coil of copper tubing to be placed inside.

  • 1
    To speed it up, In place of copper coil, you could use out-of-the-shelf solar water heater. Or DIY.... looks difficult: reuk.co.uk/DIY-Solar-Water-Heating.htm
    – J. Chomel
    Jun 23, 2016 at 6:35
  • 1
    Yesterday it was very hot, so I decided to do an experiment. I partially-filled an air mattress with water in the tent (I have the tent already, just not the water mattress, etc.). The effect was that my back got cold on the mattress, while the rest of me remained too hot due to the hot air in the tent. Passive cooling must work in a house because it's insulated (and a person doesn't lie directly on the heatsink), but without this, it doesn't work. I guess what I'll have to do is put tubing all over the tent, but then that might become too complex, expensive, and difficult to transport. Jun 28, 2016 at 16:16
  • A solution for refreshing the hot hair could be to have it circulating somehow inside the cold water?
    – J. Chomel
    Jun 29, 2016 at 6:57
  • arrrgghhh...for real? Is this for long time living or just a vacation. For real? A basic mat...what you've got going will take a plywood board 4X8 to load into one's truck. This is NOT camping. Water filled mattresses are sort of passe, sweetie, died about 1980's. Air is a luxury, not necessary for comfort in the wild or even the back yard. Learning how to clear the ground, heck, learn how to find th best place to set up one's tent...are basic skills critical skills. Water beds? Heaters? Go rent a room at a cheap motel. Simple is far far better and will entice you to 'do it again'.
    – stormy
    Jul 2, 2016 at 1:20
  • First, that's a lot of water to be trucking to any site to fill a mattress. The issues I see (mind you I'm no expert) are that you'd be "diluting" the heat gains in your 5 gal coil/bucket by a factor of 5 when you put it into 20 gallons. So if you managed to get the coil water 30 degrees above ambient (100 degrees F in 70 ambient), your net gain would only be 6 degrees above ambient. Then as long as that water is cooler than body temperature, it will draw heat from you. Insulating yourself from the mattress would help slow that, but would also kind of eleminate the benefit of warm water too.
    – That Idiot
    Oct 20, 2016 at 18:44

1 Answer 1


Never leave home without an air mattress. I've been observing the fools that sleep on the ground for years. The older you get the more likely you will do serious damage. Yes, the "macho" who sleeps on the ground, but only goes camping a few days a year because it's too cold. Unfortunately there comes a time in the fall where an air mattress also gets cold.

In answer to your question, the problem you will have with sleeping on a giant hot water bottle is too much heat. I have done something similar for cabin bunks. You are better off with the light-weight air mattress and an air space between you and the "hot water bottle". The thermal conductivity of the water bottle is critical to slow down the transmittance of the stored heat. PVC water bladders are good. It does not need to be that big 10 gallons that will last the night (at 100 F you have 2,500 BTU's above ambient). Since you are getting all high- techy, don't forget to drape a radiant barrier (space blanket on top of you). This will cut your BTU requirement in half!

It depends on overnight temperatures, but I have heard people brag about their 2 gallon water bottle and space blanket. I have 10 gallon poly under the bunks. A simple thought. A black or dark green water bladder 3 gallons would be 25 lbs. a glazed (something clear like polycarbonate) enclosure to hold said bladder and sit in sun. About a 600 to 800 btu's/sq.ft. on a sunny day. You need to be aware of overheating. 3 sq.ft. could easily be over 2000 BTU's, and that's over 150 F for 3 gallons of water. On a cloudy day, surprisingly, you still get half that. So invent a roll-up/fold-up green house, to go with the fold-up water bladder.

Better Google that though before you waste too much time.

  • Thanks. This is just the kind of answer I was looking for, at least for the heating aspect. Cooling, I seem to have discovered, is a whole different ball of wax. Oct 18, 2016 at 22:21

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