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Help! My hot water heater is getting bigger!


It is called bulging and it can be a potentially serious problem. Although the fix is quite simple, it may be too late to save your current water heater. This article will explain what causes water heater bulging and how to prevent it.

If your water heater looks like one of the pictures here, then it is suffering

from bulging. In order to understand why this happens, we need to consider a scientific principle called thermal expansion. Essentially, science tells us that matter can change shape, area and volume in response to a change in temperature. We all know that ice (a solid) turns to water (a liquid) and eventually steam (a vapor) when it is heated and that when most matter is heated, it will expand. Thermal expansion occurs consistently in a hot water heater. As either a flame or an electric element heats the water that is held in a water heater that water will expand in volume. As hot water is being used or consumed through a faucet, that extra volume simply passes through the open faucet. The problem comes into effect when the faucets are not open and water isn’t being consumed.

A hot water heater has a thermostat that is used to regulate the temperature of the water in the tank that is eventually delivered to your faucets. After extended periods of time, and in as little as a few hours depending on your setup, the water in the tank will begin to lose temperature. This certainly happens if you are away for a more than a day. When the thermostat senses that the temperature of the water in the tank is lower than its setting, it will turn on and begin to heat the water until it reaches its target temperature. This is completely normal- the idea is to have hot water when you need it. If hot water is not demanded however, this same cycle will repeat: the water in the tank will cool; the heater will heat the water, and so on.

Here is where the problem comes in… remembering the principle of thermal expansion, when the water is heated and increases its volume it now needs more space. If all of the faucets and taps in the house are closed (as they always will be unless someone is using them), the water system in your home is considered a closed system- essentially a loop. Since all pipes in your home are continuously filled with water, there is no place for this extra volume to go when the water is heated. Most water departments have something called a back-flow-preventer attached to the water connection that enters your home. Essentially this is a trap that only allows water to flow in one direction to prevent back-flow into the water mains. So, if every tap is closed and water in your pipes isn’t allowed to exit your home, this newly created energy will look to go somewhere. If there is a loose fitting in your home, it could blow out and cause a flood. More likely, it will want to push outwards wherever it currently is… your hot water heater! There is no guarantee that your tank will wind up looking like one of the tanks displayed in this article, but then again, there is no guarantee that it won’t… unless there is plan in place to absorb that extra volume created by the heating of the water.

The expansion tank is at the heart of that plan. If you are heating your home with a boiler, then you might already be familiar with expansion tanks, or at least what they look like. The only purpose of these devices is to absorb the extra volume that is periodically created when the temperature of water is raised. An expansion tank allows water to partially fill the tank in normal conditions. The remaining area inside of this tank is filled with air and separated by a rubber bladder. When hot water is created and expands in volume, the expansion tank pressurizes, or compresses the air, which creates additional space for the water. As the water cools, and inevitably reduces its volume, the air pressure stabilizes and takes back that additional space in the system.

Expansion tanks for hot water heaters (slightly different than those for heating systems since the water

in them needs to be potable) are relatively inexpensive (usually around $50) and are not too difficult to install. What they can provide in peace of mind against a potential flood in your home is invaluable. The installation of an expansion tank is best left to a professional who has performed these types of installations before—though with some research, an advanced DIY-ER should be able to complete this project. Part of the installation involves testing pressure in your system and charging the tank with the appropriate amount of air.

Handy Andy is always happy to take a look at your individual situation and make recommendations to improve the quality of systems in your home.


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