Many people remember the days when flushing the toilet while the shower was running could create a drastic change in water temperature or pressure for the person in the shower. Some people in older homes may still experience this. Newer homes are built with either pressure control valves or thermostatic control valves to prevent this situation from happening. With these valves installed, your shower should maintain a constant temperature regardless of what other plumbing fixtures are being used at the time.
Most homes today have pressure control valves rather than the thermostat variety, simply because the pressure control valve is lower in price. A pressure control valve makes adjustments in the outflow from your shower to adjust for any changes in pressure from either the cold or hot water pipes, so the mix of hot and cold water remains the same and the water temperature is constant.
A thermostatic control valve for plumbing fixtures is often referred to as a mixing valve. Instead of adjusting for a decrease or increase in water pressure from the two sources, a thermostatic mixing valve adjusts the water temperature by adjusting the amount of hot and cold water that are being combined, so that the temperature of the water outflow remains constant. Generally speaking, a thermostatic control valve allows for more accurate temperature adjustment than you would get with a pressure control valve.
Pressure control valves and thermostatic control valves can be installed in the water lines of older homes to remedy the changes in water temperature and pressure, however, your plumbing professional will need to assess the current plumbing system to determine how well the new valve will function in conjunction with the existing plumbing system.
