No Water From a New Mixing Valve
Avoid this common but easily solved mistake when testing a newly installed mixing valve in a tub or shower.
I installed a new mixing valve in my tub/ shower and it doesn’t seem to work. I followed the instructions and piped in the supply lines, the tub spout, and the shower-head fitting. When done, I opened the cold-water shutoff valve to check for leaks and there were none. Then I opened the mixing valve and only a little water dribbled out. I turned the flow-diverter valve and the temperature-mixing valve in all directions and still no water.
Figuring I did something wrong or got some debris in the mixing valve, I removed the cartridge from the body—everything looked clean. After putting everything back together, I tried opening the hot-water shutoff to check for leaks and to see if water would flow through the valve. No leaks there, so I opened the mixing valve and the same thing happened as when I turned on the cold water—only a dribble came out. Could the valve be defective or is there something I may be missing?
—Harold Klempner via email
Editorial Advisor Mike Guertin replies: I had this same mixing-valve problem skunk me years ago in my own house, and I asked a plumber friend about it when he came by to borrow some tools. “You should have hired a professional,” he said, laughing. He opened both the hot- and cold-water-supply valves, then turned on the mixing valve, and water magically flowed. He laughed at me and my bewildered expression. “It’s a pressure-balancing valve; you have to open both hot- and cold-supply valves in order for the water to flow through the valve.” As soon as he said that, it made perfect sense.
The internal mechanism that balances the flow of hot and cold water through the valve did its job. A pressure-balancing mixing valve automatically adjusts the flow of hot or cold water through the valve to compensate for a change in pressure of the other one. If someone else flushes a toilet while you’re taking a shower with a direct-flow valve, the pressure in the cold water line may drop and you get a shock of hot water. Pressure-balancing valves prevent this by maintaining the temperature you adjusted the water to when you got in the shower.
When you turned on the cold water while the hot water was shut off, the valve automatically shunted the cold water because there was no pressure on the hot side of the valve. The valve needed both hot and cold supplies to be open in order to let water pass through freely.
From Fine Homebuilding #302