The simplest of the active solar hot water installation, flooded open-loop systems have water in the pipes, collector array and tank at all times. They are more complex than a passive system because they use a pump and controller, but compared to other active systems they are cheap and easy to install and maintain.

A basic flooded open-loop solar thermal system consists of a collector array, a storage tank, a pump and a controller, plus all the piping to connect them. The collector is typically a flat-plate model , which gives more flexibility in placement location, though evacuated tube or even concentrating models are possible.

The collector array is connected directly to the tank (top to top and bottom to bottom). When the sun shines, the controller switches the pump on and water circulates from the bottom of the tank to the collector. It gets heated and returns to the top of the tank. At the end of the day, the system switches off.

To further simplify the process, the controller can be removed and a PV-powered pump used. This ensures that the pump only operates when there is solar radiation available to heat the water supply and reduces operating costs as a bonus.

Flooded open-loop solar thermal installations have two major disadvantages. Firstly, they have no freeze protection at all – the water in the system is the domestic supply and is not mixed with antifreeze, so these systems should never be considered where freezing weather may occur. Secondly, they are susceptible to hard-water damage over time and require the addition of a water softener in the loop (before the collector).