How to Make a Solar Hot Water Heater

How to Make a Solar Hot Water Heater

You wouldn't guess that in the tropics having hot water was so desirable, but at our elevation, it gets quite chilly for me in the evenings. During the rainy season the nights can get particularly cold and if you've been in the rain working, a rejuvenating warm shower is just the ticket!

Over the years we have trialed two different types of hot water systems: composting hot water and solar hot water. I've personally installed and used both systems and would like to share what I've learned.

Compost Hot Water

When we first installed the composting hot water it was revolutionary in its simplicity: one uses the heat of a decomposing compost pile to heat water. This is generally achieved by having a long tube coiled inside the compost mound. As you create the pile you slowly coil the tube amongst all the to be decomposed additions. This must be a hot, or thermophilic, compost system. When I first constructed this system it was fantastic. The water heats up within 24 hours and it stays hot night and day.

The really hot water lasts around two to four weeks, depending on the size of the pile, but over the next six weeks it does start to slowly cool down. At this point you have to build another pile for it to heat up again. At first this was easy, we would collect all of the trimmings from the occasional chopping of the soccer field to make the next pile. This would last about the same amount of time as the field took to overgrow and be cut again. It was good timing.

Once the community bought a lawn mower though, this abundant supply of nitrogen for the pile dried up. Instead of waiting six to eight weeks between cuttings, the field began to get mowed regularly and now there are no longer large amounts of grass clippings available for a hot compost pile. There were other challenges; the tubing often became twisted and was an absolute pain to dismantle and re-use with each new pile. The hot water didn't last long per shower, about five minutes, as it was limited to the amount of water held in the total length of tubing. All of this added up to a trip back to the drawing board!

Solar Hot Water

We have had several different solar hot water systems at the ranch. They all work really well as long as it is hot and sunny while you are taking a shower. Of course this is exactly the time when you don't need a hot shower, usually a cold water shower feels most refreshing. I want a hot shower when it is cold and rainy!

Until now, none of our solar hot water showers would produce hot water when I wanted hot water. This was for two main reasons. The first was that the tanks were never insulated. Even though the water in the tank would heat up, it would not stay warm into the night without insulation. The second was the lack of a functioning thermosiphon system.

A thermosiphon is created when a manifold exposed to the sun heats up, this then hats the water inside, the water expands and becomes less dense than the cool water in the system. This creates a cycle of the cooler water in the tank sinking into the manifold (area exposed to the sun) below it whilst the water in the manifold travels up into the tank. This creates a continuous cycling of hot water through the system.

But if the height differential between the tank and the manifold is not large enough or there is not a one way valve the same thing can happen in reverse, this is known as reverse thermosiphoning. This meant that the passive thermosiphon process which heats the water up while it was sunny would then reverse once it cooled down and the tank would quickly cool. This occurred in our systems because of a missing one-way valve.

Improved Hot Water Batch System

In our new solar hot water system we have made some important changes from our past systems.

  1. We increased the surface area of water in the pipe to increase speed of water heating up. By using 3/4" PVC pipe in the form seen in the pictures we are creating a larger surface area to heat water up in the sun. There is also a relatively short distance for each amount of heated water to travel in order to rise to the tank. In old systems we have had one long continuous tube.
  2. We insulated the tank to keep the water hotter for a longer period of time. By insulating the tank with about 6-7" of sawdust insulation we can now keep the water hot for a minimum of 24 hours (enough time until the sun comes out to continue heating the water again).
  3. We installed a one way valve leading to the manifold to prevent a reverse thermosiphon.

In its current form the solar hot water works as a batch heater, meaning that as you use the hot water one must manually refill the tank by opening and closing the cold water input to the tank. The overflow on the tank tells you when it is full. We do not have access to tanks that can control the very high water mains pressure that we have, therefore we use this manual system.

To improve this for a pressurized system you would want a tank that can handle the mains pressure as well as installing a heat exchanger. Instead of the sun heating the water directly, it heats an intermediary fluid that then heats the water via an exchanger in a tank. This way you can heat the water without the pressurized system affecting the thermosiphon. If you don't live so close to the equator you will also need to consider protection from freezing in the winter.

There are many forms and variations of solar hot water systems which can be designed and built. I like this system because it is passive, simple, cheap, uses easy to source materials and involves very little maintenance.

To learn more about systems like these, keep an eye out for upcoming workshops on renewable energy and appropriate technology.

Nic Donati


We have a number of life changing workshops coming up.

Please share around if you know anyone who is ready for a trip to beautiful Costa Rica, a swim in the waterfalls, nourished with homemade meals, and challenged with an intensive educational experience.


The Ranch Crew