Backflow checkvalve installation BBFP

I am in New York city where a check valve or back flow preventer (3/4 BBFP) is required for domestic heating systems. I have a indirect hotwater tank attached to a gas boiler. My question is - What order should the back flow preventer be installed in A or B. We have had no problems but neighbors with the same set up have had a lot of leaks from the check valve



A. city water>BBFP>expansion tank>hot water tank
or B. city water>expansion tank>BBFP>hot water tank


Note: BBFP Dual Check Valves with Intermediate Atmospheric Vent prevent the reverse flow of hot or cold polluted water from entering into the potable water supply. It consists of a brass body with stainless steel internal parts, integral strainer, and two durable, tight-sealing, rubber check valves. Designed for continuous pressure, non-health hazard applications in smaller supply lines, such as laboratory equipment, processing tanks, sterilizers, dairy equipment, and specifically for boiler feed lines. Minimum Working Pressure: 25psi (172kPa), Maximum Working Pressure: 175psi (12.06 bar)

Just to clarify are you saying that the BBFP should be placed between the cold water supply and the expansion tank.

Secondly why would a BBFP be a problem if it is between the expansion tank and the heater? does water need to flow from the expansion back to the heater.

RE you confusing the "domestic heating system" with the "domestic hot water system"? The "BBFP" is to protect the domestic HEATING water from backflowing into the water system, NOT to prevent the "hot water , which is NOT "polluted", from backflowing. An expansion tank for the hot WATER system, IF you even need one, goes between the cold water shut off valve and the hot water storage tank. An expansion tank for the heating system goes at the point of "zero pressure" change, which is the boiler itself. The two systems have completely different dynamics and requirements. In addition, the heating system has its own "pressure reducing valve"/feeder. IF you do install the BBFP on the domestic hot water, it MUST go ahead of the expansion tank, otherwise the tank will have absolutely NO effect on the system's operation, and the BBFP WILL leak periodically. The "expansion" has to be able to flow from the storage tank BACK into the expansion tank and the BBFP would prevent it from happening.