In general, solar thermal space heating installations that include storage use either water or sand as the heat storage medium. They use one big tank or a series of smaller ones plumbed together to provide the same volume of storage. If the system uses a single tank, it's generally unsealed: the tank can be opened without affecting the system, unlike pressurized tanks for domestic hot water applications. Both drainback and pressurized solar thermal systems can use storage tanks for space heating purposes.

A startling array of materials and forms has been used as storage tanks for space heating over the years. Many of them had short lives, ranging from two to fifteen years, before they developed leaks and became useless. As a rule of thumb, a 400-gallon cylindrical tank measures around 40 to 48 inches (100 to 120 centimeters) in diameter and stands 4 to 6 feet (1.2 to 1.8 meters) high – though the measurements will also depend on the materials used and the tank shape.

Before we look at tank types, there are some basic principles to remember when considering storage tanks for space heating:

• The best tanks have no seams or joins, so less chance of developing leaks.
• The best tanks are made of materials that can resist constant temperatures of 180°F (82°C).
• The tank must be insulated.
• The tank's lid must fit tightly and not drip.
• A cylindrical tank needs less reinforcement and bracing than a square or rectangular tank.
• A square or rectangular tank holds more than a cylindrical tank with the same footprint.
• A square or rectangular tank is often easier to get through doorways than a cylindrical one which contains the same volume (because it's smaller).

With all those considerations in mind, let's take a look at a variety of materials and options to see how they compare.

A wooden frame with plastic or rubber containment has been tried many times because it can be built on-site instead of having to squeeze a pre-built tank through doorways. Unfortunately, it doesn't work: the plastic or rubber is generally roofing material pressed into service where it is inappropriate. Within as few as two years, leaks develop and even in the best circumstances, the tank will never last more than ten or fifteen years.

Steel oil drums have also been tried (big ones with a 250-gallon capacity). Unfortunately, they were never made to contain water, so they develop leaks over time as well.

Stainless steel bulk milk tanks have been used with great success (usually in Dairy States, for obvious reasons). They're welded together, so they resist leakage very well and the stainless steel does not corrode from contact with water. Some people even have their own stainless steel tank built following the milk tank format, especially where the space available is very limited. Although it's a very expensive solution, the tank can be built to specification, maximizing the storage capability (which usually means a rectangular tank) and lasts a very long time. Just remember to get plenty of reinforcement and cross-bracing, especially on rectangular models.

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