do I have a closed system?

[Pman6]

if i shut off the cold water supply valve at the water heater, but not the main shutoff to the house, water flows back through the hot water supply into the water heater.

does this suggest I have an open system?

My current Rheem wh is 20 years old, never had an expansion tank.

 

[Weekend Handyman]

I am not a plumber. I would say you have a single handle faucet or shower valve open and the water is crossing over.

 

[Sponsor]

 

[Tuttles Revenge]

"Open System" refers to having a domestic water heater that also supplies heated water to radiators or in floor heating loops without a physical barrier between the two systems.

A "Cross Connection" refers to having a cold water pipe and a hot water pipe connected at a point after the water heater which allows water to pass between the two piping systems in an uncontrolled manner. Generally undesirable.

We have experienced cross over caused by sensor activated kitchen faucets that have a failed check valve in them. Or sometimes in a pressure balance shower valve where a shut off valve has been installed downstream of the mixing valve.