Passive solar thermal systems are designed to move the liquid around the system using gravity and natural thermo-siphons (hot water rising and cool water sinking provides flow through the system). These systems are also known as monobloc systems because the storage tank is usually mounted on the same structure as the collector, allowing easier natural flow.

These systems also require no pumps or pump controllers, and therefore no electricity usage. That makes the systems generally cheaper, and easier to install, but are significantly less efficient than active systems.