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Under Pressure: Water Heater Expansion Tanks

Basic physics tell us that heating a fluid causes that fluid to expand – this is called thermal expansion.  That same principle applies to your water heater.  Every time your water heater fires up and heats the water inside, the water expands.  This expansion can increase the pressure on your plumbing if there is nowhere for that increased water volume to go.  Some homes in our area have open water systems, meaning the water can flow back into the city supply lines as it expands to reduce pressure.  Many of the homes in our area are closed plumbing systems.  This means there is either a check valve in the water meter or a pressure reducing valve installed near your home’s main valve that prevents water from flowing back into the city water supply lines.  So when the water heats up and expands, there needs to be somewhere for that increased volume and pressure to go.

Pressure With Nowhere To Go

When the pressure in your plumbing increases because of thermal expansion, it can damage the pipes, fittings, fixtures, and appliances throughout your home.  Here’s a few problems you might notice:

  • Banging sound from your pipes when running water
  • Dripping faucets
  • Toilets that run when they haven’t been flushed
  • Appliances that wear out faster than they should
  • Leaking from the temperature and pressure relief valve on your water heater
  • Water heater failure

The Solution: An Expansion Tank

An expansion tank is a smaller tank that is installed above your water heater on the cold water line.  It is divided inside by a rubber diaphragm or bladder, and one part holds compressed air and the other part allows water to flow in and out as needed as the water heater heats up.  The air in the tank can absorb the increased pressure during thermal expansion.   The expansion tank relieves the pressure on your plumbing by giving the expanded water somewhere to go.

How To Tell If Your Expansion Tank Has Failed

When the rubber bladder or diaphragm in an expansion tank tears, the expansion tank loses the compressed air inside that provides the cushion needed to accommodate the increased pressure.  The expansion tank then becomes water-logged, and must be replaced.  Most expansion tanks will last 5-10 years, but should be replaced immediately if there is a sign of failure.  There are two simple tests you can perform to check if your expansion tank might have failed:

  1. Knock or tap on your expansion tank – it should have a hollow, ringing sound.  If your knock sounds more like a dull thud, then it is most likely waterlogged.
  2. Briefly press the small valve on the end of the tank – you should hear a hissing sound as a small amount of the compressed air is released.  You should not see any water come out of the valve.

When In Doubt, Call A Licensed Plumber

The pipes, fixtures, and appliances that make up the plumbing in your home are designed to withstand only a certain amount of pressure.  If you are noticing issues related to high pressure or a failed expansion tank, diagnosis by a licensed plumber is not something that should be delayed.

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