House plumbing chatters when used w/ irrigation on

[Paul R]

I had an irrigation system installed by a local professional in June of this year. If I turn on any plumbing in the house while the sprinklers are running my pipes in the house will start chattering for about 20-30 seconds and then settle down. The hammer was initially pretty violent but I've tweaked back the inlet valve to the backlow preventer a bit to keep the house from jumping out of its skin. I still can't get rid of it entirely although cutting back on the backflow preventer has helped.

Pressure off the street varies from 80-100 lbs. Pressure to the house is a constant 60 lbs after the PRV. The irrigation is tapped into the line after the meter, but PRIOR to the PRV.... FYI - I've also got a thermal expansion tank in the house on the cold water line (after the PRV). From what I've been seeing on the Web, the no-brainer fix would be to install a 2nd Pressure Reducing Valve in the irrigation line immediately after the backflow preventer. Whatcha' think? My plumber was also considering going with a 2nd expansion tank on the city side of the existing PRV. Your thoughts?

[valveman]

May just have to much air in the expansion tank. Check the pressure on the house side when all this is happening. You need a little less air pressure in the tank than the lowest pressure you ever see on the house gauge. Can't check the tank pressure until you shut off water supply and drain all water from tank.

[Paul R]

With regard to that expansion tank. It's about 5 years old and much larger than the specs for the water heater that I have (It's an Amtrol ST-12 and I have a 50 gallon gas heater). When I replaced the water heater about 6 months ago the plumber that did the job removed the tank and reinstalled it (it's located in the line going into water heater). Since then I've noticed that it stays half full and never seems to empty or get more full for that matter. I've been think about getting the tank replaced with a new (more appropriately sized one). I'll let you know how that turns out.