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Expansion Tanks: What Are They And How Do They Work?

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If you have steam heat, you are familiar with the slow whistling sound that comes from your registers when the heat is on. You also know the pain of having to check the water level in your furnace to make sure that it does not run too low. Have you ever wondered why the registers whistle and what that means for your heating system?

What Does It Mean?

When your furnace heats the water into steam, there is an increase in volume of the water in the heating system. Because the system is closed, when there is no more space in the pipes for the steam to expand to, the pressure increases. As the pressure gets higher, it finds the weak spots in your heating lines, which is at specific valves on the registers designed to vent pressure to prevent damage to the lines.

As these valves vent steam, they whistle and your system loses water, which is why you have to refill it. The problem is that high-pressure steam is also damaging to the system as a whole, even with the relief valves, eroding the inside of the pipe and damaging seals.

So, How Can an Expansion Tank Help?

This small tank is added to your heating system to safeguard it from high pressures. A typical tank is roughly two feet high and a foot and a half in diameter. It is split into two sides separated by a rubber diaphragm. One side is filled with pressurized air and the other is open to the heating system and filled with water.

As the water heats and begins to expand, it pushes against the diaphragm, absorbing the extra pressure. As it pushes against the air side, some of the air will leak out. There is a valve stem on the expansion tank that allows air to be pumped back in.

What if You Do Not Have Steam Heat?

Expansion tanks are not just for closed system steam heating. They are also used for hot water heaters to protect the system against a water hammer effect. A water hammer is caused when water is flowing through a system and then a valve is closed suddenly. The velocity of the water moving through the pipes is preserved through a conservation of energy and rebounds throughout the system as a pressure wave. This pressure wave is called hydraulic shock and can cause pipe collapse and damage to heating elements. The expansion tank gives the pressure wave somewhere to expand against before it reaches the heater.

Whether you have a closed steam heat system or just a hot water heater, an expansion tank is a worthwhile investment to protect your home. It can not only mitigate possible damage from everyday use, but also prevent long-term damage from increased pressure. Talk to heating services for more information.


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