Sentry Guard is patented design of all fluid coils (water and steam). It is a burst-proof design and does not allow damage from a freeze event. Simply, as a coil goes under a freeze condition (stagnate fluid with cold air across the fins) then the coil is prone to tube and return bend leaks. It can be a matter of a few minutes before the pressure builds to a point inside the heat exchanger whereby tubes and or bends bloat trying to hold the pressure and finally split to relieve the pressure.
Many systems have pump, valve or damper problems that can cause a freeze event. Many steam coil systems have sitting condensate in a coil due to problems of removing condensate. Sentry Guard is an easy answer for problem systems.
Sentry Guard design simply pinpoints where this pressure is relieved by patented freeze relief plugs at each end of the coil. Once a few of these plugs fail then the coil is out of harms way. The service personnel screw off the failed plugs and screw news ones back on the coil.