The simple answer is that it does exactly as the names says; it reduces the incoming water pressure. It allows the municipality to pump in water at higher than normal pressures, and/or more than a typical domestic water system is designed to handle. There are two primary reasons for this increased pressure. One is to serve larger square footage homes, and the other is so that residents and businesses don’t experience significant or noticeable pressure decreases during high-demand times of the day and night. For example, most neighborhoods in Frisco and McKinney are in the 90-100 PSI (pounds per square inch) range at the street. Some areas on the western side of Frisco (ex: Teel Parkway between Lebanon & Eldorado) often exceed 125 PSI. There are some areas in Plano (North along 121 and northwest along the NDT) where the homes exceed 85 PSI. In that case, you might not have a PRV but need one, and you may be eligible for a $500 rebate through the city. Normal or ideal water pressure is usually around 75-80 PSI. Anything above 85 PSI is too much pressure for the house and the various fixtures and/or appliances to handle. Toilet fill valves are especially affected when the cold water pressure is over 85 PSI. So, the PRV is required to reduce the incoming water pressure before it enters the home. This reduces the likelihood of pressure-induced leaks and/or ruptured water lines that can cause catastrophic damage—as well as the weird phenomenon known as “water hammer.” Staying with our example cities of Frisco and McKinney, any home that was built in 2000 or later should have a Pressure Reducing Valve (PRV) by city plumbing code. Some larger homes that were built in the late 90s (ex: Starwood) will also have them. Back in the early days of the PRV requirements, the builders installed them on the main water lines right before water entered the home. Because of this, they are often buried and/or obscured by mature landscape. Most homeowners do not even know that PRVs exist until they experience a high- or low-pressure problem, or they form a leak. In fact, you may be one of the many people reading about PRVs for the first time here.