If you are in a house with interconnected smoke detectors (hardwired or hardwired/battery combo), can you interconnect them with the waterflow switch for a home fire sprinkler system, or do you need to upgrade to a full fire alarm control panel at this point?
(It seems that since most (all?) hardwired smoke detectors use 9V on their interconnect line (as per this and this) relative to the neutral to activate the interconnect circuit, one could make a very simple interconnect setup using a 9V battery, a 9V wall-wart adapter, and a couple of garden-variety Schottky diode rectifiers to provide a 9V supply to the waterflow switch; but would such a contraption violate the fire alarm or fire sprinkler codes?)
Best Answer
I can't speak to the regulatory side of this, other than to say I'd be very surprised if a custom-built circuit like you're describing would pass an inspection just for the fact that it's a non-UL/CSA approved device on a life safety circuit. Being in a private residence at least will relax the requirements compared to a commercial or multi-tenant building, but connecting non-listed circuits and devices is tough in any case.
Rather than using a custom circuit, there are interconnect relays on the market that are designed for doing precisely this type of thing. For example, Kidde has a SM120X module that is designed to interface to external strobes/sirens/etc, as well as accept activation input for the interconnect circuit (using pull stations and rate-of-rise heat detectors as examples).
You should be able to use one of these and hook up your flow sensor in place of the pull station in the diagram above.
To me, it seems there is a low risk of impact: