Water heater expansion tanks regulate water pressure in a hot water system to prevent damage to plumbing and appliances in a home. Steel expansion tanks were used until the mid-to-late twentieth century, while newer diaphragm tanks occupy modern water heater installations. Since 2006, all original and replacement water heater systems have been required to include an expansion tank. Because they are relatively new to many hot water systems, expansion tanks pose problems that many homeowners have never faced. Consequently, troubleshooting an expansion tank can be tricky when something goes wrong. Below you can find common problems with water heater expansion tanks and how to resolve them.

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If the temperature and pressure release valve in an expansion tank activates regularly, that is a sign that the tank is working properly. However, it also indicates that the expansion tank is too small for your water heater. If you are in doubt about what size expansion tank you need, always opt for the larger size. An expansion tank that is too large will not negatively impact the performance of your hot water system, but an expansion tank that is too small can cause relief valve discharge and eventually failure of the tank itself.

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If the pressure inside the expansion tank is too high, it can be corrected by opening the Schrader valve at the top of the tank. Allow a small amount of air to exit the Schrader valve and check the pressure again. Continue this process until the pressure is appropriate for your specific tank. If you accidentally cause the pressure to drop too low, you can use a tire pump to raise it to the appropriate level.

An expansion tank can lose pressure if air leaks from the valve or the diaphragm itself. If the leak comes from the valve, more air can be added to the tank using an air compressor pump or a manual tire pump. A manual pump is preferable because it is less likely to rupture the tank’s bladder due to high pressure. The air pressure within an expansion tank should match the system pressure. Most expansion tanks come calibrated at around 40 psi. Before adding air to your expansion tank, ensure you know the correct air pressure for your specific tank.

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Steel expansion tanks were common until modern diaphragm tanks took their place. Unlike diaphragm tanks, steel expansion tanks do not separate the water and air inside. This means water inside the tank can absorb air, causing the water inside to expand. When water fills up too much space inside the tank, the pressure inside fluctuates, and the tank fails to operate correctly. When this occurs, the tank must be recharged before it can operate normally. This involves emptying the tank and refilling it with the amount of water that will achieve the appropriate pressure. You can recharge an expansion tank yourself, or it can be done by a professional. A diaphragm expansion tank never needs to be recharged because the water is separated from the air, meaning the water cannot absorb air inside the tank.

If the leak is coming from somewhere other than the valve, the diaphragm of the tank has likely failed. When this happens, a professional must look at the tank to determine whether the tank is fixable or must be replaced.

With the cost of the expansion tank, professional labor, and all relevant supplies included, you can expect to pay around $300 on average for an expansion tank installation. While expansion tank installation can be less expensive as a DIY project, professional installation is recommended to avoid errors from damaging your hot water system.

When condensation exists on the outside of an expansion tank, it poses safety concerns with electrical wiring, floor damage, mold growth, and potential harm to the tank itself. If your expansion tank is coated with condensation, allow a professional to inspect your tank for damage.

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A properly functioning water heater expansion tank prevents airlocks from occurring. If you have an expansion tank installed and still experience an airlock, the tank is failing to regulate pressure within the system. While an airlock can sometimes be removed by connecting a hot-water fixture to a cold-water fixture with a hose, the expansion tank must be fixed or replaced to address the source of the problem. If your system experiences an airlock, contact a professional to inspect your expansion tank.

When problems arise in water heater expansion tanks, they are often caused by leaks, pressure changes, or air blockages.

An air blockage occurs in an expansion tank when the air inside the tank becomes trapped. The role of an expansion tank is to regulate pressure within your hot water system, but, when an expansion tank fails, the air inside can block water from flowing. This occurrence is referred to as hydronic airlock. If the cold water in your home is running at a normal flow rate, but hot water is trickling, sputtering, or not running at all, an airlock is likely the cause.