Town of Friday Harbor
San Juan Island

Find Tax Parcel# Forms Maps Municipal Code


Getting Here
About the Town
Whom To Call
What's New
Town Documents
Links & Resources

60 Second Street
PO Box 219






Thermal Expansion        Thermal Expansion (.pdf)

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. You are receiving this letter because our records indicate that you have or are about to install a backflow prevention device in your system.

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. This letter and enclosed brochure are being sent to you via certified mail to ensure that you receive this important information regarding the potential dangers of thermal expansion in a closed system. 

Thermal Expansion 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 & 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&P valve, a water tank may explode.  Please read the attached brochure for additional information.

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.




Neither the Town of Friday Harbor nor any agency, officer, or employee of the Town warrants the accuracy, reliability or timeliness of any information presented here, and shall not be held liable for any losses caused by such reliance on the accuracy, reliability or timeliness of such information.
Portions of such information may be incorrect or not current. Any person or entity who relies on any information obtained from the system, does so at his or her own risk.

Copyright 2017 Town of Friday Harbor Washington.  All rights reserved.