The 2006 Uniform Solar Energy Code specifies that heat exchangers in SHW systems transferring heat to potable water must be double-walled—no exceptions—to prevent possible mixing from occurring in the event of a leak. I’ve always thought this rule was excessive when nontoxic propylene glycol is used and even debated a local inspector about it once (do so at your own peril).
His logic was this: Even if I always used nontoxic glycol as the system antifreeze, how could he or I ever prevent someone else from filling the system with toxic—potentially lethal—ethylene glycol in the future? Unless your local building department approves of single-wall heat exchangers in SHW systems or your favorite tag color is red, stick with the double-wall design.
