four color on negatives 3.5 wide 9.175 high

Wall Mount Design

The thermosyphon solar water heater is one of the simplest, most economical, and, I believe, the best type of solar water heater. It's a passive system, meaning that it has no mechanical or electrical devices that make it work. Only the sun's energy is needed to heat the water in the solar collector and circulate that water through the holding tank.

Thermosyphon is defined as "a hydraulic system whereby the circulation of fluid is caused by a temperature differential in the fluid." Figure 1 illustrates this principal. The solar collector is facing south at an angle of 30° under direct sunlight. The holding tank is behind and above the solar collector. The tank and the solar collector are connected by the piping as shown. The system is full of water under pressure with no air pockets. Standard household water pressure is about 50 psi.

Direct sunlight heats the water inside the solar collector. As the water is heated, it rises and circulation begins. The rising hot water flows up the solar collector outlet pipe and into the holding tank. Cooler water from the bottom of the tank flows down the inlet pipe and into the solar collector. The circulation of water between the solar collector and the tank will continue as long as direct sunlight falls on the solar collector, heating the water inside. The hot water will rise to the top of the tank and stay there until you use it. The holding tank should be well insulated to store the hot water for at least one day. No pumps or electronic controls are needed to make this system work, just some basic rules of physics properly applied.

The most basic principal we're using here is that heat rises. By carefully following the rules and instructions provided here, you will be able to design and build your own thermosyphon solar water heater to suit your particular hot water needs.

The Solar Collector

We've all had the experience of walking out to our car on a hot summer day as it was parked with the windows rolled up and under direct sunlight. When we opened the door, a blast of hot air was felt and perhaps the seats were too hot to sit on. That's solar energy at work, with your car being the solar collector. The sun's rays passed through the window glass and heated up the inside of the car.

The solar collector in a solar water heater works the same way, but produces much higher temperatures. The sun's energy passes through the glass cover plates and heats the water that's in the piping inside the collector. The water is passing through the solar collector very slowly, is heated, then flows to the holding tank which stores the water until you use it.


Water must circulate from the tank, to and through the solar collector, then back to the tank, and so on, until all the water in the tank is heated. If the solar collector is too small in surface area it won't be able to heat all the water in the tank. If the solar collector has more surface area than needed, it will heat up all the water faster and hotter. There are different ways to circulate the water through the solar collector. Many expensive solar water heaters on the market today use pumps and electric controls to circulate the water when the sun is shining. They work well until there's a mechanical breakdown, which is inevitable. Then it's usually a costly experience to get the part replaced so you can have solar hot water again. With the thermosyphon solar water heater, no pumps or electronic controls are needed. It is designed so that the heat energy from the sun causes circulation without the help of a mechanical device, and it is simple and maintenance free.

Sizing the Solar Water Heater for Your Needs

For maximum energy efficiency and savings, the solar water heater should heat and store enough hot water each solar day to last overnight and satisfy your needs. With this in mind, proper sizing of the holding tank and the solar collector is essential. You must calculate how much hot water your household uses each day, and

Figure 1. Piping Layout for the Thermosyphon Solar Water Heater.

Manual Air Vent Valve

Figure 1. Piping Layout for the Thermosyphon Solar Water Heater.

Manual Air Vent Valve

Standard 40 Gallon Hot water Tank

Tank is mounted higher than the solar collector.

Solar collector faces south at a 30o angle under direct sunlight.

System is full of water under pressure with no air pockets.

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