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A pressure balance valve is a sophisticated plumbing device that reacts to changes in the pressure of the delivery water. Typically, both hot and cold water sources are fed into the valve, and its job is to ensure that the volume of water that leaves it is a constant mix of these two sources, even if the delivery pressure of the two sources changes. This is very important to keep the temperature of a shower constant even when the supply is not.
Drastic changes in water temperature in the shower can be dangerous, and there have been cases of the water temperature changing from normal to scalding hot. Temperature extremes in the other direction have also caused problems, with people slipping and falling trying to avoid a suddenly very cold shower. These problems are exacerbated when very young or very old people are involved.
Since a pressure balance valve can prevent scalding, they are now required in all new home construction. Homeowners can also replace a valve in an older home, but it will likely require the assistance of a certified plumber. In older homes, the new valve may not work very well if there is an inadequate flow of water due to clogged water pipes or poor water pressure related to the age of the plumbing system. A certified plumber can help in such situations, because he or she will be able to determine beforehand the extent of the problem and what will be required to provide a functional solution.
For the ultimate in shower comfort, homeowners can also consider installing a thermostatic valve. These cost more, but they provide much better temperature control. A big disadvantage of the pressure balance valve is that it can provide limited flow rate control. A thermostatic valve can provide a full range of flow rate control, while still providing water at a constant temperature.