Designed to accommodate climates where freezing occurs more frequently, indirect systems circulate an antifreeze solution in a closed loop through the collector rather than water.  A heat exchanger within the storage tank maximizes heat transfer from the antifreeze to the coldest water in the storage tank.  The closed loop assures that the antifreeze never comes in direct contact with the water.  A differential temperature controller determines when the pump should be activated.

These systems are identical to indirect systems, except that the energy required to power the pump is provided by a photovoltaic panel that converts sunlight into electricity.

Designed to accommodate freezing climates, the collectors and pipelines of these systems never freeze because when the system is not producing heat, all the water from the collectors and pipelines drains into an insulated reservoir tank.  In addition, drain back systems are similar to closed loop
systems because they circulate an antifreeze solution or distilled water in a closed loop through the collector.  A heat exchanger within the storage tank maximizes heat transfer from the antifreeze to the coldest water in the storage tank.  The closed loop assures that the antifreeze never comes in direct contact with the water.  A differential temperature controller determines when the pump should be activated.