How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
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What is the little tank on top of the water heater for?
Saturday, August 11, 2018
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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