The collector is a multiple-size water tank with inlet and outlet for water circulation. Vacuum tubes are filled with water. They can be used only for open split or drain-back solar systems. Maximum working pressure 0,2 Bar.
They are not distributed in Bulgaria and are accepted with distrust in contrast to other countries where they are preferred. One of the reasons is their ignorance and the opinion that in winter conditions it is imperative to use an antifreeze fluid. They are used for both small and high capacity systems.
The advantages are their low cost and lower installation costs.