What can be added to HDPE piping systems to prevent frost damage in underground piping?

Adding propylene glycol to HDPE piping systems is an effective method to prevent frost damage in underground piping. Propylene glycol acts as a freeze protectant due to its low freezing point and its ability to lower the freezing point of water when mixed with it. This means that even when temperatures drop below the normal freezing level, the mixture remains in a liquid state, preventing ice formation that can lead to damage or bursting of the pipes.

In contrast, the other substances suggested do not serve the same purpose effectively. Calcium chloride, while sometimes used to lower the freezing point of water, can be corrosive to certain materials and isn't suitable for use within a piping system designed for transporting liquids. Water alone would not prevent freezing; in fact, it would exacerbate the problem in cold conditions. Radon, on the other hand, is a radioactive gas and has no relevance or utility in protecting piping systems from frost damage. Thus, the use of propylene glycol stands out as the most effective and suitable solution in this scenario.