Have 4-zone h.w. baseboard system. Hydro-Therm furnace piped according instructions has worked fine for 15 years. Recently, the backflow prev. atmospheric vent began to discharge a small but significant amount of water. City w.p. is 60psi (regulated). Supply goes to backflow preventer > auto fill/regulator > furnace. There is also an exp. tank in the loop (new). All the parts listed are new, Watts brand. After new backflow prev. installed, water flow from vent has been reduced but not stopped; dripping steadily. Furnace pressure when firing is high, 16-18 lbs. Hydro-Therm support agrees operating pressure is high but had no real solution to offer.