If you're installing a solar thermal space heating system and including a storage tank, you need to bear several important factors in mind, beyond the basics of materials and the practicalities of installation.

The best storage tanks are those with no joints or seams to provide weak points for leaks. Of course, since the water has to get in and out somehow, a totally seamless tank is impossible. In practical terms, your tank will need:

• An entry and exit point for the water
• A drain exit
• Access to the heat exchanger inside

The first of these is simple enough: all the piping can go into and out of the tank at the top. The third is also simple: a lid can give you access to the inside of the tank, though it should be flanged on the inside to keep drips in. The drain exit is a potential problem: it's absolutely necessary but a fitting like that is a big leak concern. The best way to deal with it is to have the fitting installed by the manufacturer, at the source: fittings installed in the field are always unreliable and will give you problems, especially if you're using a cylindrical tank.

Closed loop systems

In a closed loop system, the water in the storage tank never leaves the loop. It is purely a heat storage and transfer medium.

The heat exchanger in this setup is usually a submersed model and there are usually several installed. They are best made of soft copper (type K) and can be a coil or finned tubing. You could use Pex, though it's at least four times less efficient than the copper equivalent. The coils are mounted inside the tank by suspending them or sitting them on a rack made of half-inch rigid copper tubing and a few fittings. It is important that the heat exchanger coils do not touch the edges of the tank itself. If you install a rack system, remember to put heat-resistant caps on the bottom so that it doesn't wear holes in the tank as it shifts with expansion and contraction.

The main heat exchanger is placed near the bottom of the tank as usual. This coil contains the solar fluid that passes through the collectors and brings heat to the storage medium. Near the top, two more heat exchangers hang in the cooler water. One of these takes heat out to the domestic hot water supply; the other goes to the space heating equipment.

Direct systems

To be blunt, the closed loop system is easier and has far fewer problems than a direct setup, where the hot water in the tank circulates through the space heating equipment and heats the home.

However, if you want to set up a direct system, you'll need to install a bulkhead fitting on the side of the tank. Put it at least 16 inches (41 centimeters) below the water level and use a solid brass fitting on a fiberglass tank – if possible, get the manufacturer to install it at the factory.

The bulkhead holds