I saw an interesting arrangement of pipes and valves in a house I visited. These are hot water pipes, the two vertical pipes connect to a radiator. What is the purpose of the valve between the two vertical pipes? I don't see how this could do anything useful, given the horizontal pipe that directly connects everything.
It is a bypass valve meant to be opened when the other two valves are closed.
The horizontal pipe above is connected using two diverter tees:
In a one-pipe system, each diverter tee restricts flow to direct a portion of the hot water through the local radiator and then return that flow back to the main line. If the radiator in this room were shut off using only the vertical ball valves without opening the horizontal bypass valve, then the water flow would be restricted for the entire system, hurting heat delivery to the rest of the house.
This is a hot water run.... the purpose of two pipes rather than one pipe for the run is to insure that when someone turns on the washing machine while you are in the shower you do not get a shock of hot or cold water...the value between them is to enable work on one while the other is open [ turn the shock proof on or off ]