Water is at maximum density at 34degF and when either heated or cooled it expands up to about 10% greater volume.

When water freezes in a line it is that 10% increase in volume that ruptures the pipe or vessel wall.

When water is heated it will also expand up to 10% until it reaches the boiling point. When water flashes to steam it expands by 12 to the third power or 1728 times its original volume.

However, while everyone assumes the boiling point of water to be 212degF, that is an incomplete statement. Water boils at 212degF at standard atmospheric pressure at sea level. As the pressure increases the boiling temperature of the water also increases. Given the maximum allowable pressure in a domestic potable water system and the maximum allowable working temperature of the water heaters the water will only expand up to about 5% by volume. As a safe rule of thumb they recommend that the expansion tank should be between 5% & 10% of the volume of the water heater, therefore if you have a 50gal. water heater an expansion tank that is 5gal or greater would be sufficient.

Watts & Amtrol are both old and well respected manufacturers of both water heater and boiler safety and control devices up to and including commercial power generating boilers so I would have no problem in relying upon either of those brands.

On the other hand, if the truth was told, probably there are only one or two companies actually making the expansion tanks and they are painted and labelled for other companies, so there really is not a hill of beans difference in any of them so long as they are ASTM certified.

kudzu9,

If you have the opportunity to upsize, a 4.4 Gallon Thermal Expansion Tank is a good choice for water heaters from 50 to 120 Gallons. A larger tank provides additional protection by further reducing the pressure increase during a heating cycle.

I work for Amtrol Inc., inventor and leading manufacturer of Thermal Expansion Tanks. I'd be glad to answer additional questions you may have.

The link below will assist you in sizing. Best of luck.

Ken Cerpovicz
Amtrol Inc.

Here is a link that might be useful: Thermal Expansion Tank Sizing

link does not work

Probably because the link is 12 years old as is the entire thread.