Water pressure and your plumbing system

Above: a pressure reducing valve

Highlights

  • The pressure in our water pipes results from the weight of water in our elevated tanks in various places in the community. 
  • The greater the height or elevation of water in our tank compared to the elevation of a house or business, the greater the pressure. 
  • The water pressure in a private plumbing system is either:

(a) determined by the height from the plumbing system to the water level in our storage tank, so the plumbing system pressure is normally very close to what it is in our nearby public water pipe; or

(b) reduced to a controlled level by a plumbing system device called a pressure reducing valve (PRV).

  • Lower pressure in your plumbing system means less chance of leaks in pipes and fixtures. 
  • Lower pressure reduces water use at showerheads, faucets and spigots, so it is a way to conserve water.

Introduction

Each of us uses plumbing systems at home and at work without necessarily knowing in detail how our water pipes, drains and fixtures work.

Below is some basic information about water pressure and devices called “pressure reducing valves.”

We hope this information will help you save time, money and inconvenience through a better understanding of your plumbing system.

What determines the pressure in a public water system?

The pressure in our water pipes results from the weight of water in elevated water storage tanks in various areas of the community. The greater the height or elevation of water in our tank compared to the elevation of our pipes serving a house or business, the greater the pressure. Water is heavy: a cubic foot of it (7.48 gallons) weighs about 62 pounds.

Because of variations in the lay of the land or “topography” in our community, water pressures vary. Pressure in low-lying areas may exceed 150 pounds per square inch (psi); pressures in our water lines in higher areas such as hills or ridges may be 35 to 40 psi in some neighborhoods. Because we keep the height of water in our tanks in a consistent range (under normal conditions) the pressure in our water lines at a given location will change very little during the course of a day, but a major water main leak could affect pressure, for example.

In our community, the elevation of the land is generally higher to the west, and the land slopes down to the east. (This topography is the reason that most of the creeks in Chapel Hill and Carrboro flow from west to east.) Therefore, pressures in our water lines are normally higher on the low-lying east side of Chapel Hill than on the west side of Carrboro.

What determines the pressure in a private plumbing system?

The water pressure in a private plumbing system is either:

(a) determined by the vertical height from the plumbing system up to the water level in our storage tank, so the pressure is normally very close to what it is in our water lines near a building; or

(b) reduced to a controlled level by the operation of a plumbing system device called a Pressure Reducing Valve (PRV). In a plumbing system with a PRV, the pressure is often set between 35 and 60 psi.

What is a Pressure Reducing Device (PRV)?

A PRV is a device that reduces and stabilizes the water pressure in a home, business or other building to a level typically between 35 and 60 psi. This pressure reduction is important because the pressure in some of OWASA’s public water mains can be much higher. Plumbing pipes, hoses and fixtures tend to last longer and have fewer leaks if they are used at lower pressures. 

Lower pressure also helps conserve water!

When does the Plumbing Code require a PRV?

Under the NC plumbing code, a PRV is required if the pressure in the OWASA water main serving a house or building is above 80 psi. Even if not required, a PRV may be a good idea if you need to adjust or limit pressure in your plumbing system.

How can I find out whether my plumbing system has a PRV?

PRVs are usually installed at a point near where your private water supply line (from our water meter) comes into the house or other building. For example, if your residence has a crawlspace, the PRV may be installed just inside the foundation wall. 

If there is no crawlspace, the PRV may be in a closet  at the front side of a residence, in an equipment room at a business, etc. 

DO PRVs fail? What indicates a PRV is failing?

Like other devices, PRVs can fail due to age and other factors. If you notice fluctuations in water pressure, a failing PRV may be the cause. As a first step, we invite you to call us at 919-968-4421 to schedule a free water pressure check. If repair or replacement of your PRV is needed, we recommend having a licensed plumber advise you about the type of PRV needed and install it.