Tanks with Integral Heat Exchangers

Water storage tanks with built-in heat exchangers are also called "indirect tanks". They used to be unique to the solar thermal installations and were consequently very expensive. However, thanks to the plumbing and heating industry popularizing these tanks for use with a separate heating source (normally a boiler), the technology has become an everyday thing. This means prices have dropped significantly – and that's good news because they have three major advantages:

• Simple installation. Indirect tanks are easier to install because you don't have to mess around with a separate heat exchanger. There's no extra mounting or plumbing work beyond the tank itself.
• Only one pump. If you install a tank with an external heat exchanger, you usually need two pumps – one on the solar side and one on the house side. That means more installation trouble, more maintenance and more power costs to run the pumps.
• They're smaller. Indirect tanks generally have a smaller footprint than using a separate heat exchanger, for the simple reason that the exchanger's already built in. Considering that water tanks are frequently installed in tight, cramped locations like cupboards and utility rooms, that can only be a good thing!

Indirect tanks come in different types, the most common of which has a knurled or rifled tube coil inside a steel tank. The tube is usually stainless steel or copper and is extremely efficient because it has a lot of surface area in direct contact with the water.

Unfortunately, the coil will slowly accumulate mineral build-up, especially in hard water areas, just like the heating element in a kettle. To prevent this, these tanks should be used only in areas with soft water and should be flushed regularly.

Some manufacturers offer tanks with a removable internal coil, accessible through a bulkhead fitting, or "wand" format coils that screw into the top of standard electric or gas heaters. These are great for hard water areas because the coils are easy to remove and maintain (or replace), but they're less efficient because the coils are necessarily smaller.

The alternative to internal heat exchangers is an external coil. In this setup, the hot water tank sits inside another tank or jacket which contains the heat exchanger. Some manufacturers use a liquid-filled jacket instead of a metal coil, which reduces corrosion problems. External heat exchangers are less efficient because the coil isn't in direct contact with the water, but they're a boon in hard-water areas.

Tanks without Heat Exchangers

The most common type of hot water storage tank used in solar thermal systems comes without a heat exchanger. These tanks are normally made of steel or fiberglass, are insulated and connect to the water supply, the drain and the heat exchanger.

That may sound familiar, and for good reason: a standard water heater works just as well, though it's usually best to convert an electric heater as they don't have a gas flue down the middle. However, there are manufacturers who make storage tanks specifically for solar thermal installations, with more ports to facilitate easy