Originally Posted by gfd31
If there was a leak, or even a problem with the backflow preventer the system would have "flow" and the alarm should be sounding, and if it's connected to a fire alarm system you should have either a supervisory or a trouble on the FACP at the very least.
Flow switches generally wouldn't trip for water travelling backwards in the system. They are a spring return paddle that closes contacts when pushed. Therefore, water travelling towards the city would be pushing back to the "normal" position, and would not cause an alarm.
In a system that utilizes a pressure switch on the alarm line to detect flow, the main alarm valve would have to be open to allow water into the alarm line to create an alarm.
The backflow would be the absolute last place I would be looking. The pressure has to leak through the alarm valve to pass back into the city supply.