Bulging water heater and leaking

What is happening when there is an excessive pressure in the tank-type heater?

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The bulging water heater article explains how to check is your heater leaking due to an increased pressure of hot water inside the tank. Explore the symptoms and causes of bulging and how to prevent it.


What causes the bulging water heater?

An excessive pressure inside the tank can cause bulging water heater, which results in permanent deformation, leaking, and dislocated flue pipe and pipe fittings.

Common symptoms that are seen on the heater exposed to an excessive pressure are cockeyed nipples of the top of the heater and bulged or reversed bottom of the head.

Tank-type heaters are designed and tested on maximum internal pressures of 300 psi, without any distortion. If there is a deformation of the tank, like the bulged bottom, the odds are your heater was subjected to pressures above 300 psi. Maximum working pressure for which the heater was designed is 150 psi.

When the water inside the heater is cold there is only a static pressure of water in the tank, which is the result of the its weight. From the moment when you turn the heater ON, and the heating process starts, the temperature increases, the volume increases along with the pressure.

How to prevent an excessive pressure increases in water heaters

If your heating system is an open system, and if there are no obstructions to reverse the flow, the pressure in the tank will always be closed to the cold water supply. The hot water will expand back into the cold water supply and there will be no damage to the plumbing system. In this case, the chances for bulging water heater are minimal.

If there are obstructions, like with the checking valves, pressure reducing valves, shut-off valve, in the cold water line or water softeners, the system becomes closed system. This will result in the rapid pressure increase of the heated water until something fails or ruptures. This is why the expansion tank is a must to use, so it can accept the additional pressure. If the expansion tank was not installed, the pressure may become great enough for water heater bulging or even tank rupture.

The correct size of the expansion tank should be used on every new installation so it can maintain a consistent and safe working pressure. Keep in mind that the warranty will be voided, so whatever happens to the unit, it will be your expense.

Water is a non-compressible liquid and expands when heated. For each 10°F increase in temperature, water expands 0.2%. As the temperature increases, the maximum designed pressure can be exceeded and cause bulging water heater.

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How to test hot water pressure inside the heaters

One of the reasons why your heater leaks might be the pressure buildup or significant drop off inside the heater's tank (bulging water heater). This is a simple DIY project where the only tool you need is the pressure gauge. When a water heater leaks, it creates the puddle of water on the floor, as there is a crack or hole on the tank, or dislocated plumbing connections or unions. Follow the guide below to check and test hot water pressure:

  • In order to get the pressure reading inside the heater's tank, install a pressure gauge on the drain valve outlet and open the drain valve.

  • Close the main shut-off valve that is supplying the unit.

  • Make sure that all hot water taps are turned off. When the heater is working and since hot taps are closed, pressure will build up in the tank.

  • Run the heater by turning the thermostat up until the main burner or heating element comes on. Make sure to constantly check the pressure gauge as the pressure in the tank will increase rapidly.

  • When hot water pressure starts to rise, shut the heater off by turning the thermostat down. Watch pressure gauge to see if it holds pressure or drops off significantly.

  • If there are no leaks around fittings when the pressure drops, then the tank is leaking.

  • If the pressure inside the heater cannot rise, either hot water faucet is open in the system or there is a hole or crack in the tank, which doesn't let the pressure to buildup.

  • If the pressure holds for an extended period of time (10 – 15 min.), then the tank is not leaking.


The typical design of the tank is the convex top heads and concave bottoms. Due to the excessive pressure inside the tank, concave bottom might flatten, or changes to a convex or bulged shape. This is especially dangerous on gas models, where the movement of the bottom results in deformation of the head of the tank as they are connected by the rigid flue. This can further cause the restriction of the products of combustion from venting through the flue passage and chimney.

By simply looking into the combustion chamber you can check is there any distortion of the tank's bottom. Bulging water heater tanks must be replaced with the cause of the problem identified and corrected. The heaters with the bulging condition are not covered by warranty due to the improper installation.



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