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Thermal Expansion in Your Home

Thermal expansion is a rise in the water temperature that causes the water to expand, as a result you will experience increased pressure in your water system.

All houses now operate on a "Closed Water System". There is a meter and a check valve at the street as required by current codes. In the past, excessive pressure was able to go back into the system. Now it cannot, but it has to go somewhere.

As the water heater heats the water in the tank, it expands and creates additional volume in the piping system. Because we have a Closed System, the pressure is contained and continues to build. When the pressure reaches an excessive amount, it must be released. The extra volume of water will be discharged or absorbed.

There are two primary devices used to address this situation, one of which will be found in your house. A Thermal Expansion Relief Valve (TERV) will allow for intermittent release of water though a discharge pipe when the pressure in the system exceeds the set pressure of the TERV. This discharge could range from a cup to a gallon of water a day. The amount will vary depending on numerous factors found in each household. Water dripping from the discharge pipe indicates the TERV is working properly. If it never drips, there may be a problem with the valve.

The other device is the Thermal Expansion Tank (TET). The TET is installed on the cold water line usually near the water heater. It is a pre-pressurized tank with an air chamber isolated from the water system by a diaphragm and set at the system pressure in the house. As thermal expansion increases the pressure in the water system, the expanded water will apply pressure against the diaphragm allowing the TET to accept the extra volume in the system. When thermal expansion subsides as the water cools or is used, the extra volume is drawn back into the system.

The T&P Valve on your water heater operates as a third expansion device. However, if it is releasing water though the discharge pipe, it may be an indication of a possible problem. The actual thermal expansion device may not be operating properly or there may be a problem with the water heater. If this occurs, you must determine the problem and take action to correct it. Do not confuse the TERV discharge pipe with the T&P line.


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