Usually a back-up heating system has to support a solar hot water system when heating needs are greater than the supply, i.e. for a wood stove or a conventional central heating system.
Active Solar Hot Water Systems
Active solar hot water systems can be either direct or indirect circulation systems. In both types of active hot water solar systems, a pump distributes the fluid that runs through the collectors. In a direct circulation system specifically, household water is heated directly in the collectors. A direct system is suitable for warmer climates. In indirect circulation systems, pumps circulate a non-freezing heat transfer fluid through the collectors and a heat exchanger that heats the water. An indirect circulation active solar hot water system is ideal for colder climates.
Source: U.S. Department of Energy - Click to EnlargeAdvantages of an active solar hot water system, both direct and indirect, are that they offer integral protection against freezing, overheat control, optimal rate of heat transfer from the collector, a wider choice of collectors, and a pipe layout.
Active hot water solar systems are more complex and expensive than passive ones, but they are more efficient. Once installed, an active solar hot water system is quite economical and provides 40-80% of a home´s heating needs.
Passive Solar Hot Water Systems
Passive solar hot water systems, or passive circulation systems, rely on gravity and let water naturally circulate as it is heated. Water or heat transfer fluid moves through the system without pumps.
Source: U.S. Department of Energy - Click to Enlarge
One of the advantages of passive hot water solar systems is that they work without any electricity components. They are generally more reliable, easier to maintain, and possibly longer-lasting than active solar hot water systems.
Professional planning and installation is indispensable for optimal functioning of a solar hot water system. A poorly installed system will not provide optimized heating control. Thus, it may be beneficial to seek the advice of a solar professional when considering a solar hot water system.