A passive (thermosiphon) system relies on natural convection to circulate the hot water. Hot water naturally rises so no mechanical pump (AC or DC) or access to electricity for a pump is needed. It is essential, however, that in a passive system the storage tank be higher than the collectors. As long as the sun shines, water in the collector will heat and move slowly upward into the tank with the colder water descending to replace it. As the storage tank is usually mounted horizontally above the collectors on the roof, the added weight is a consideration.
The example shown below is for educational purposes. We do not sell or install passive systems. Why? They are less efficient than Glycol active systems because they have a shorter operating season, and they will freeze and crack in cold winters.