I was just asked about how to build a low-cost solar water heater. It's actually the subject for a planned how-to article but let me toss out two basic design ideas. If you are creative you should be able to construct either one. Solar water heaters used to be inexpensive but, today, they seem to have out of control prices. It just makes no sense why someone would be expected to spend $1000 or more when they are such simple devices. The two ideas I will present are a "box heater" and a "plastic pipe" heater. You can adjust each idea in terms of capacity, life-expectancy and cost.

Plastic Pipe Heater

Assuming you have enough water pressure to get water up to the roof of your house (don't laugh—I didn't have lots of times with city water in Estelí), you can get enough hot water for your shower for close to zero cost. Just go buy a roll of black plastic (probably ABS) pipe. A roll is probably 100 feet or there abouts. You will also need some fittings to connect it to your water supply and your shower head. Toss it up on the roof, connect it up, wait a couple of hours and have a hot shower. If you find you don't have enough water for your shower, just add a second roll of pipe. It's that simple. The concern you might have is the life of the pipe. It really isn't designed for hot water but if the pressure is relatively low you should be ok for at least a few years.

Box Heater

With a box heater you have more work but more options. The idea is to have a container full of water inside an insulating box whose open side points toward the sun. The best way to do this is using a low-pressure system. This decreases the engineering difficulties related to a pressurised system. To construct such a system you will need to place the heater above the point of use. This will usually mean on the roof . The box can be constructed from wood, insulated on the inside. The tank can be anything that will not rust or be damaged being exposed to the sun. The tank from an old electric or gas water heater is ideal. You will then want a water feed on the low side of the tank and the hot water outlet on the high side. Why? Because hot water will rise to the top. To keep the tank full, you need another tank located above the heater tank with a system to control filling. A unit like is used as the flush reservior for a toilet will do fine. The only other "trick" is that the hot water output needs an air escape tube. This can be done by adding a T fitting to the outlet and running a pipe up to above the reservior tank. Ok, that's it. Two ideas. When I build my box heater I will write up the details and add some photos.