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Powers and Honeywell used to have a low temp. cutout/alarm (DPDT) that had a huge long bulb to string criss-cross across the face of the HX. (Freezstat) The claim was that if any two feet of the bulb went below the setpoint, the switch would trip. We used them successfully on hot water coils in 100% OA units with variable flow. But why not just use an aquastat in the idle boilers energized by OAT below freezing(or the above mentioned device)?