Stagnation is a common problem in other closed loop glycol systems because glycol can break down when it reaches high temperatures. But our system will never suffer from "stagnation", where fluid is "trapped" under high pressure and temperatures in the collectors because of our overheat protection mechanism. This makes the Sunnovations system safer and will prevent chemical breakdown of the glycol-based heat transfer fluid. Unlike the Sunnovations system, other closed loop glycol systems risk the glycol turning acidic and losing its freeze-protection properties. Also in other systems, acidic glycol corrodes the tubes and causes leaks over time, which will ruin collector panels. Yes, other systems have a backup power source that will still provide hot water, but this doesn’t make the problem go away and will eventually render their solar hot water system useless.  Annual inspection and possible replacement of the glycol is mandated for other systems to prevent corrosion, but not for a Sunnovations system!