A. A pressure balancing valve will prevent you from being scalded if someone flushes a toilet since it keeps the ratio of hot to cold consistent. However, if someone shoves the handle all the way to the "all hot" position, there's virtually no cold water tempering the hot water. If your water heater is set very high, or if members of your household have sensitive skin, this can be an accident waiting to happen. For this reason, these pressure balance mixing valves have a stop screw that will allow you to set the maximum hot to cold ratio, effectively setting the maximum temperature. To take advantage of this feature, simply rotate the knob to set the water to the hottest that you'd ever want out of the shower, then turn the set screw until it bottoms out. This prevents the temperature knob from turning past the position you selected, effectively limiting the maximum temperature allowed from that valve.