Cobb County Government



Stephen D. McCullers, P.E.
660 South Cobb Dr
Marietta, GA 30060
(770) 423-1000
(770) 419-6224 fax

Water System Website

Water / Sewer

Frequently Asked Questions

(Click on the question to see the answer).

1. What is Thermal Expansion?

Thermal expansion refers to the characteristic of water to expand when it is heated. Unlike air, which can be compressed, water grows in volume, and must be accommodated.

2. Why is Thermal Expansion occurring?

In a water heater, thermal expansion can create more pressure than the system can handle. When unchecked, this pressure can result in expensive leaks, as well as damage to the water heater, pipes, or other fixtures.

3.When is a Thermal Expansion device necessary?

Thermal expansion is necessary when any of the following occurs:
• A recent water meter replacement
• A hot water heater replacement
• The construction of a new home
• When a backflow preventer is installed on the water

(Water heater without an Expansion Tank)

4. How do I resolve this Thermal Expansion?

Thermal expansion can be solved by installing an expansion tank to your system adjacent to your water heater. This tank will accept overflow from your water heater, alleviating the pressure building up in the heating tank. Expansion tanks are inexpensive and the most conservation-friendly solution. There are also several other relief solutions available. Talk to your local plumbing or hardware vendor regarding these alternatives.

5. I have a Temperature and Pressure Relief (T&P), is that sufficient?

No. A T&P valve is not a thermal expansion device, because the constant dripping of water from the valve can result in a mineral deposit that can create a blockage, causing the T&P valve to become ineffective. Plumbing standards and codes require that thermal expansion be properly addressed.

(Water heater with an Expansion Tank)
The expansion tank’s size, color, and position may vary. Tanks are commonly installed in a low or high upright position.

6. Are there any building codes with requirements for resolving
thermal expansion problems?

Yes. Section 607.3.2 of the Standard Plumbing Code states that if a system with a water heater has a backflow prevention device installed and as a result thermal expansion causes an increase in pressure, a device must be fitted to limit the pressure to 80 pounds per square inch (psi) or less.

7. Where can I get this device?

Thermal expansion solutions are available at hardware stores or your local plumbing vendors.

8. If you have further questions regarding the thermal expansion
requirements and alternatives check the Cobb County Water
System website at: or contact:

Cobb County Water System
Backflow Prevention Supervisor
at (770) 528-3343 or (770) 528-8446

9. What is backflow-prevention?

It is the prevention of reverse flow in a water system from the normal or intended direction.

10. What can cause something like this to happen?

Backflow and/or backsiphonage can occur in a water system by a drop in supply pressure due to heavy demand or use such as fire fighting, or by repairs or breaks in the supply pipe, etc.

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