Caleffi's iSolar 257 Series is an attractive, compact, easy-to-install, multi-functional temperature differential controller with add-on system functions. It can be used for a wide variety of applications and has inputs for four sensors. Preset factory defaults control a standard solar water heating system with a second relay (some models) to divert surplus heat. The auxiliary relay can maintain tank temperature, protect the system from overheating, or direct another source to heat the storage tank.
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