Expansion Tank
Jan 03

Expansion tank. Why do I need one?

Expansion Tank

Expansion tank located on the cold water inlet pipe.


With the rising costs, premature failure rates and insurance claims on the rise a change was due for the water heater industry. That change took place in the form of an expansion tank.  If you reference the picture above, you will see the expansion tank mounted on the cold-water inlet pipe. Expansion tanks are now required by many jurisdictions and manufacturers to be added during the installation of new water heaters. Expansion tanks have a minimal increase on costs of material and labor when compared to their added value of an increased lifespan of the water heater. Homeowners insurance providers were very big proponents of requiring the addition of the expansion tank due to the increased number in claims surrounding water heater failure. Historically over the last 20 years the average lifespan of a water heater was 8-10 years, with it not being uncommon at all to come across the occasional water heater which was 15-20 years old. Somewhere in the time surrounding 2011 it was clear that the average lifespan was dropping to 6-8 years.

The Cause- Its impossible to pinpoint one single cause contributing to the lessened lifespan of the water heaters. It’s easy however to identify multiple. Raw materials rising in cost, the availability of more “economical” products, increased water pressure in city mains, back-flow prevention and water quality.

The rise in availability of products at the big box stores drove manufacturers to compete on price more than ever. The big box stores were all driving toward being the lowest price point and attracting customers based on just that. The obvious problem with that is there must be sacrifice somewhere when costs are cut. These cuts came in the form of less raw materials being used during manufacturing. Tanks became thinner. Even the more established and professional grade water heater manufacturers, which were not doing business in the big box space, were forced to thin their tanks due to the rise in cost of raw materials. Thinning tanks seemed to be the final variable in the equation.

Thin tanks + harsh water + increased pressure + closed systems = Tank failure

The Effect- The variables in this equation basically all represent one thing, an excessive amount of strain on the steel tank of the water heater. The steel tank will flex as water is heated and pressure fluctuates. Inside the steel tank is a vitreous glass lining that is designed to protect the tank from direct exposure to the harsh water. This glass lining is about as thin as a light-bulb and with the tank flexing more than it should this lining cracks quickly. With the tank being the literal weak link and the flexing of this tank exposing that weakness even further we see the addition of the expansion tank.

“What does an expansion tank do?” Great question. At it’s most basic, which is exactly what it is, it is a tank for handling expanding water. When water is heated it will expand and physically need to take up more space. This causes a major issue because water itself cannot be compressed, so this heated water which now needs to take up more space will look for places to go. The places it usually frequents would be the welded seems of a water heater tank, the spring and seat on a temperature and pressure relief valve or a fragile faucet. Most immediately on the welded seems of the water heater. The expansion tank gives the swelling water somewhere to go.

Expansion Tank

Diagram of expansion tank

Expansion Tank Design- Expansion tanks are built using a steel tank with a butyl rubber bladder. Inside of the steel tank along with the rubber bladder is a pressurized cushion of air. The design of the tank allows the expanding water to push its way into the bladder and expand into the cushion of air. As the space surrounding the air in the tank gets smaller the air is compressed and increases in pressure. When the water cools down and the swelling is relieved the pressurized air pushes the water back into the system. All the while the water heater tank doesn’t even realize this is happening, instead of the water causing the tank to flex it causes the air in the expansion tank to compress.

Simply put, expansion tanks are helping to relieve wear of the water heater tanks due to increasing pressure and expansion, to prolong the life.

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