A standard single family home with NO check valve or pressure reducing valve at the meter would be an example of an open system. The water pressure can bleed back into the city water main (ever see the small movement triangle "rock" back and forth when all the water is off in the house?) Any check valve or regulator that impedes the flow of water back to the main makes it a closed system since water can only flow into the system.

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002