A friend of mine build a new home and had his water pipes plumbed in a circular loop from the water heater.
The idea was that even though no water faucets are open and using water, the water is able to continually flow through the water heater and around the house to keep hot water readily available.
This way when you opened the tap to get hot water there was not a long delay for the hot water to get to the faucet. The system is closed but it is looping under pressure.
