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Thermal Expansion

What is thermal expansion and why do I need a thermal expansion tank?
When water is heated, it becomes dense and expands. Heated water, unlike air, cannot be compressed the volume expands. In a standard 40-gallon tank, water heated to about 140˚F expands and volume in the tank increases from 40 to 40-1/2-gallons.
After a home inspection, the buyer asks to have a thermal expansion tank installed on the water heater. The home inspector, through the agents, tells the seller “a thermal expansion tank is a safety device that is a code requirement”.
The following is an excerpt from the Georgia Department of Community Affairs (GDCA) Standard Plumbing Code Amendments, 2001, 2002, 2003 and 2004 Amendments (PDF)

607.3.2 Backflow prevention device or check valve. Where a backflow prevention device,
check valve or other device is installed on a water supply system utilizing storage water heating
equipment such that thermal expansion causes an increase in pressure, a device for controlling
pressure shall be installed.  The device shall limit thermal expansion of the water being heated to
not more than 80 pounds per square inch (552 kPa) static pressure at any fixture on the system.
A potable water expansion tank, or auxiliary relief valve or other approved device that limits
pressure to 80 psi or less shall be acceptable.  It shall be installed in accordance with
manufacturer’s instructions. (Effective January 1, 2001)
A thermal expansion device is only required when a backflow prevention device is installed on the incoming water supply.
A backflow device prevents expanded water from flowing back into the main water supply. Backflow prevention devices are currently not on all water services, as some people might believe and if a backflow device is installed a thermal expansion device is required.
Most plumbing contractors tell consumers a thermal expansion tank is required because it is in the “code”. The code does refer to devices that limit thermal expansion and requires such device when a backflow prevention or check valve is present on the incoming water service.
Each county or municipality can impose its own rules, regulations and standards. Some counties and municipalities do not address backflow devices or thermal expansion devices. Some counties state it is necessary to install of an expansion device when certain plumbing repairs occur such as, water meter replacement, water heater replacement, new home construction, installation of a backflow prevention device or the installation of a pressure-reducing valve. Check with your local water department for clarification.
The recommendation to install a thermal expansion tank is not going to disappear. The issue started sometime between 2002 and 2004 when some plumbing contractors realized they had an opportunity to increase revenue so many of them refused to install new water heaters unless a thermal expansion tank was included. Sometime during 2005 or 2006, some counties adopted regulation addressing this issue when they required thermal expansion devices when certain repairs were completed, see above. Now advertising touts the devices as a most “conservation-friendly” solution. Keep in mind all this occurred after the GDCA amended the codes January 1, 2001.
I imagine eventually all water service lines will be equipped with backflow prevention device and all water heaters will be equipped with thermal expansion devices.
In the mean time, homeowners should be aware a thermal expansion tank is not the only solution; there are several other acceptable solutions. Talk with your local plumbing contractor or hardware store.