In active systems, water or a heat transfer fluid is pumped through the collectors.
These are usually more expensive and a little less efficient than passive systems,
particularly if antifreeze measures are required. Additionally, active systems
are more difficult to retrofit in houses, especially where there is no basement,
because space is required for the additional equipment, such as the hot water
cylinder. Five types of systems belong in this category: direct circulation systems, indirect water heating systems, air systems, heat pump systems, and pool
heating systems. Before giving the details of these systems, the optimum flow
rate is examined.