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.

photo of pipes


2 Answers 2


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:

enter image description here

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.

  • 2
    ah, didn't know about diverter tees. I had assumed that all the T connections allowed full flow of water in any direction, which made the bypass seem redundant. But with the restriction of flow in the diverter tees, the need for a bypass valve makes sense. Thanks! Oct 6, 2015 at 17:47
  • How cheaper is such a system, that forces you to use the same heating flow in all rooms? Sounds very suboptimal. Oct 7, 2015 at 12:02
  • Now I know how the diversion to my bathroom heater blower works off my hot water heating system. Thanks, I actually put a crimp in the pipe in between the tees because I didn't think enough water was being diverted. I was not able to tell whether that crimp actually helped, but it was probably unnecessary.
    – sborsher
    Oct 7, 2015 at 19:46

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 ]

  • 2
    Radiators should be a closed loop system, not related to washing machines or showers.
    – BMitch
    Oct 6, 2015 at 19:52

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