Expansion Tanks and Hot Water Boiler Systems

By Mark J. Donovan

Have you ever wondered what the heck that large green, grey, or blue bulbous thing is hanging off the side of your boiler system is?

Well it’s a hot water expansion tank and it plays an important role in your hot water heating system and domestic hot water system.

An expansion tank is used in closed hot water heating systems and domestic hot water systems. It is used to relieve the pressure that might build up in the hot water plumbing pipes so that they do not become damaged and begin to leak.

Excessive pressure can occur due to thermal expansion or by water hammer. Water hammer is when you turn on and off the water and would otherwise hear a bang if it was not for the expansion tank.

The expansion tank contains two chambers that are separated by a flexible rubber diaphragm. One side of the expansion tank contains air (pressurized to 12 PSI), and the other side is open to the hot water system and contains water. When the pressure in the hot water system rises the water pushes down on the diaphragm and gently squeezes the air. As a result, the pressure in the hot water system is maintained at a constant pressure as the water thermally expands or contracts, thus protecting the hot water pipes from being damaged.

If the diagraph fails the entire tank will fill up. When this situation occurs the hot water piping system is at risk of damage. Fortunately expansion tanks have a backup system, in the form of a pressure relief value, that sits just above the expansion tank. If the tank fails and the pressure gets too high in the hot water pipes, the pressure relief valve will go off. Hot water will be sprayed out of the system and may make a little mess, however it’s a much smaller problem than a cracked solder joint somewhere in the piping system.

To determine if your hot water expansion tank is working properly you can tap it and listen to hear the difference in sounds between the top portion and the bottom portion. One half should sound hollow and the second half, with the water in it, should sound like a dead thud when you tap it.

If it sounds like a dead thought both on the top and bottom chances are the diaphragm in the expansion tank has failed and it should be replaced.

You can also feel the tank as well to determine if it is working correctly. If it is warm on the upper half, and cool on the bottom half it is most likely working properly. If it is warm throughout the entire tank, chances are the tank is filled with water and needs to be checked by a plumber.

So that’s what the big green, grey, or blue tank hanging of your hot water heating system is all about.