It’s a water heater expansion tank, designed to reduce the changes in water pressure due to the expansion and contraction of the volume of water as it heats and cools in a closed system. This can create dangerously high fluctuations in pressure that will fatigue a home’s water supply piping over time and lead to failure. An internal rubber bladder is filled with compressed air that cushions water pressure changes. Most tanks are rated at a maximum working pressure of 150 psi. 

  Although the tank can be mounted vertically or horizontally, we have only seen vertical mounting in our area. A horizontally mounted tank would require additional support and should be rated by the manufacturer for a sideways mount.

   Expansion tanks rust as they age, and we look at both the tank and fittings for signs of corrosion. The tank shown at the top of this blog, on closer examination, also has galvanic-type corrosion—a white, powdery buildup caused by an electrolytic reaction between the adjacent galvanized steel and copper piping.

    Also, see our blog posts Are water heater expansion tanks required by code? and Can a water heater thermal expansion tank be installed sideways/horizontally? and Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises?

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Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code? 

• Can you use a light switch for a water heater disconnect? 

• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

• Why should a tankless water heater have an isolator/service valve kit installed?

• When was a gas water heater first required to be elevated 18 inches above a garage floor?  

• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home? 

• What is the purpose of a thermostatic mixing valve above a water heater? 

• Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

• What is the difference between a manufactured/mobile home water heater and a regular water heater? 

• Does a tankless water heater require a pressure relief valve?

• When is a water heater drain pan required?

• Why is there water in my water heater drain pan?

• What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

• Where do I find the water heater in a mobile home?

• Does a tankless water heater in an attic require a drain pan? 

• Does an electric water heater require a disconnect? 

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.