In a direct flow vacuum tube design, the antifreeze liquid (used to take heat energy back to your hot water cylinder) is channelled through to the bottom of the tube and then back out. As it does so it picks up the heat collected by the absorber. The advantage of this design is that it is more efficient than the others and can be mounted at virtually any angle on your roof or wall and vertically or horizontally.

In a heat pipe design, the antifreeze liquid does not enter the vacuum tube and only makes contact with it by way of a heat exchanger. To improve the transfer of energy, some heat pipe designs still use a liquid in the tube but this is sealed in. The liquid evaporates under the heat of the sun and rises in the tube until it reaches the heat exchanger. The vapour then cools, returns to a liquid and runs back into the tube to repeat its cycle. This particular design has the disadvantage that the tubes cannot be laid flat but need an installed angle of at least 25° to operate. However, the advantage is that in the unlikely event of a tube needing to be replaced, this can be achieved without draining the complete system.

A flat plate design does not use vacuum technology and thus has the advantage of being a more economical system to install. The disadvantage is that it loses some of its collected heat energy to the atmosphere and is a little less productive. In the summer months this is really no problem but some loss of performance as against the vacuum tubes must otherwise be expected.