What Does an Expansion Tank Do

expansion tank

Some of the most dangerous issues for property owners are those that involve water and fire hazards. The water leaks caused by problematic plumbing systems constitute the major risk for them, especially systems that don’t have an automated shut-off valve on a water heater. To protect those storage tank heaters and the property, the thermal expansion tanks were introduced.

What is an expansion tank?

A thermal expansion tank is a smaller tank that prevents significant pressure increases caused by thermal expansion of heated water. This tank is split inside into two parts: one part is filled with the air and another one – with the water. The tank is mounted near the water heater (on the pipe with the cold water) and is connected to the waterline of the plumbing system. Expansion tank’s capacity is usually relatively small: about 2 gallons or 5 liters. Some are 5 gallons or more.

thermal expansion tank Amtrol ST-5 for water heaters

What Does an Expansion Tank Do?

The quick answer: water heater expansion tanks are designed to absorb high pressure. They are designed to fight the negative impact of changing water pressure. Fluctuating water pressure is very bad for the water heater and your entire plumbing setup.

Lets first talk about the effect of thermal expansion. When hot water expands, it increases the pressure inside of the storage tank. Before, when the water pressure in the tank built up it would simply pass a little of the water back through a water pipe and out of the house, alleviating the extra pressure. This method currently doesn’t function everywhere because many public water systems have check shutoffs that do not allow water to go back. This avoids backflow catastrophes in within a home but puts an additional strain on a water heater’s tank. That is why many plumbers are suggesting for a water heater expansion tank to be installed with storage tank heaters (if not with any heater).

The first line of defense against the pressure surge is a pressure regulator. The second one is a thermal expansion tank. As we know, water, like many other materials, expands when it gets heated. For example, 50 gallons of cold water can easily expand to 51-52 gallons after heating. The expanded water creates high pressure in the whole system. High pressure is bad for your water heater and the plumbing system in general. It can cause your appliances and pipes to malfunction or to break prematurely, which will result in water leaks, water damage, and other problems. If the pressure regulator is not working properly, the expansion tank will not work properly either. In case of any trouble, it is better to change them together to ensure that they don’t cancel each other’s work.

What is a Pressure Regulator?

Pressure reducing valve, Water pressure regulator

A Pressure Regulator is a very important part of the plumbing system. A pressure regulator (aka Pressure Reducing Valve) is placed in-between your home system and the city water coming from the main. A pressure regulator controls the water pressure that can enter your house. For example, the city water pressure can reach 110 pounds per square inch (PSI). The pipes and appliances in your home are designed to operate at 65 PSI. If your home did not have a regulator that can absorb city water spikes, your appliances would be damaged. High pressure can also lead to pipe bursts in the walls and other problems. It is very important to test your pressure regulator and make sure that it is functioning properly.

How does the expansion tank work?

After we hook it up to the plumbing system, the water goes to the water portion of the expansion tank. There is a rubber membrane inside that moves within the tank as more water gets into the tank. The other (air) side of the tank has a bicycle-type valve and we pump that section up to match it to the specific pressure that we are going to set at the pressure regulator.
While the water heater will be getting hotter, the cold water will heat up and expand. Part of the expended water will get into the “water” part of the expansion tank, which will cause to squeeze the “air” part of the expansion tank. That will effectively absorb the extra pressure, eliminating the problem.

expansion tank

Do I Need an Expansion Tank?

Why don’t we always see expansion tanks near water heaters? Because the expansion tank is optional in some cases. The water heater manufacturers often recommend installing them, especially when installing tank water heaters. Installing them is not always necessary, but in certain applications, they can help you resolve many issues if your plumbing system is expected to deal with high water pressure situations. For example, if you changed the failed pressure regulator and you are still getting spikes in pressure, installing an expansion tank will most likely solve your problem. That’s why installing expansion tanks is now necessary for tank heaters in many parts of the United States. Some manufacturers can even invalidate the service warranty on a heater if the expansion tank not installed. So, overall, it is a great safety tool to have on any water heater and one that can extend both the longevity and warranty of your equipment.


So, what are the benefits of expansion tanks?
They extend the life of your water heater as well as the lifespan of all other appliances and plumbing fixtures in your home. An expansion tank can curb the water hammer and banging sounds from pipes. When used in conjunction with a pressure regulator, it will prevent leaky faucets, toilets that fill without flushing, damaged dishwashers, and other plumbing problems.

You can also read this post about other Problems of High Water Pressure written by Peter Hemp.

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