Thermal Expansion

The Town of Friday Harbor works hard to insure that the water we deliver to our customers meets or exceeds all health standards. All public water systems are required to develop and implement Cross-Connection Control (CCC) programs. The requirements are contained in WAC 246-290-490 of the Drinking Water Regulations. The purpose of which is to protect water quality by preventing the backflow of water from the customers plumbing into the public water system.

Backflow Prevention Device

When a backflow prevention device is placed on a water service line, the water that has been delivered to the customer's facility is prevented from returning through the meter to the water main. This creates a closed loop system within the customer's building.

Thermal Expansion Overview

Thermal Expansion (PDF) is a condition that exists whenever water is heated. The heated water cannot expand into the public water system because of the backflow device in your system. Therefore, pressure may build up in your water heater tank and household plumbing. The resulting pressure buildup or thermal expansion usually causes the pressure relief valve (T and P Valve) on your water heater to open and water is emitted from the drain piping of your water heater. Under certain conditions, such as a failure of the T and P valve, a water tank may explode.

Water Discharge

If you notice a discharge of water from your water heater you are probably experiencing thermal expansion. Current plumbing codes require the installation of thermal expansion relief devices in "closed systems." The two most common methods are to install either an expansion tank, or a thermal device installed on the cold water line to the tank. We suggest you discuss the best solution for your needs with a licensed plumber.

Thank you for your understanding as we continue to meet or exceed all regulatory requirements in providing clean, safe drinking water to all of our customers. It is our pleasure to serve you.