There are three basic vertical panel types for heating air from within a structure, illustrated in the cross-sectional drawing below. Each is glazed on the front (blue) to admit radiant energy, has an energy trap/exchanger (red) behind the glazing, has a top discharge opening, and has an intake opening.

The Type 1 panel has a bottom intake and a top discharge, with a straight path connecting the openings. The trap/exchanger occupies most of this pathway. The simplicity of construction and minimum material content makes the Type 1 panel the most popular choice for amateur builders. The advantage of Type 1 panels is that it is designed to heat the coolest air (near the floor). Its disadvantages are it insulates only as well as its glazing; and that, without some mechanism to prevent airflow during periods of darkness, it can operate in reverse. The most frequently used mechanism used to prevent airflow closes the top opening, which still allows cooled air to spill out the intake opening - better than nothing at all; but definitely not the best solution.

The Type 2 panel has a top intake opening and a folded air path, which, during periods of darkness, traps cold air to inhibit reverse airflow. The Type 2 panel is a popular choice for mounting below a window opening such that the discharge and intake openings can be extended through a partially-open sash-type window. Advantages of the Type 2 panel are that it improves on the reverse operation behavior of Type 1 panels without moving parts. Its disadvantage for general use is that it allows the cold air below the level of its intake opening to remain in the structure.

The Type 3 panel has a bottom intake opening and a doubly-folded air path, which solve all of the problems inherent in Type 1 and Type 2 panels. Advantages are that it provides a double "lock" to prevent reverse operation; it can be positioned to warm the coolest air in the structure, and when locked provides a triple dead air space to insulate the structure from outside cold. The disadvantage of the Type 3 panel is that it's the most difficult to construct and uses more materials than either of the other types. DeSoto Solar's standard panels are currently all Type 3 panels.