HERE is a white paper on cross-connection hazards of the two main types of solar thermal systems. In my research on the subject, I found the two main solar system types fit the two main categories for conventional cross connections. The paper shows they are the same, with the same solutions.
This is especially important for the non pressurized drainback type of solar system (single pressure, backsiphonage), which has been lumped into the category of double pressure systems (backpressure), causing unnecessary expense and performance reduction.
The goal of this paper is to show that non pressurized solar systems fall under the backsiphonage classification and should be treated as such in any codes.
In fact, non pressurized solar systems can be viewed as a special case where the probability of cross-connection hazard is so low that backsiphon equipment does not add to the safety.
A statement reflecting this fact is:
1) Non pressurized drainback tanks with unobstructed vent/overflow pipes and single wall heat exchangers meet the leak detection requirement and cannot be operated at a negative pressure differential sufficient to mix tank fluid with potable water.
A statement requiring the same backsiphonage equipment as conventional equipment is:
2) To guarantee that a negative pressure cannot be established across a single wall exchanger in a non pressurized drainback system, a backflow preventer (vacuum breaker) shall be installed on the street side of the heat exchanger.
While statement 1) is preferable, statement 2) is also acceptable.