How to Fix Common Water Heater Expansion Tank Problems

Posted by
John Woodard on May 05, 2023

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.

Learn more: What is a water heater expansion tank?

How to troubleshoot common water heater expansion tank symptoms

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

1. Leaking expansion tank

2. Tank pressure loss

3. Tank pressure too high

4. Air blockage

5. Condensation on outside of tank

6. Tank needs recharging

7. Relief valve triggers too often

Leaking expansion tank

Water can leak from an expansion tank at one of three places:

  • Pipe fittings. If the pipe fittings at the top of the tank are loose, water may leak out. When this happens, simply tightening the fittings will resolve the issue. Be careful not to overtighten the fitting, as this may cause problems down the road.
  • Drain valve. Like a pipe fitting leak, the drain valve may leak small amounts of water if the valve is too loose. Tightening the valve an appropriate amount should resolve the issue.
  • The tank itself. If water is leaking from the tank itself, then the diaphragm is likely at fault. A leaking diaphragm must be examined and diagnosed by a professional, who will determine whether the tank is repairable or not.

Tank pressure loss

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.

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.

How to check the pressure of an expansion tank

To check the pressure in a water heater expansion tank, you can use the same pressure gauge you use on your car’s tires.

  • Remove the cap protecting the Schrader valve at the top of the tank.
  • Use the pressure gauge on the Schrader valve the same way you would use it on a tire.
  • Ensure the pressure inside the tank matches the pressure the tank was initially charged with.

pressure gauge

Tank pressure too high

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.

Learn more: What is a pressure gauge and how does it work?

Air blockage

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.

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.

Condensation on outside of tank

Condensation on the outside of an expansion tank can signal one of three issues:

  • Water temperature is too high. When the water heater’s temperature is set too high, condensation can build up on the outside of the tank. If your expansion tank suffers from condensation, double-check the temperature setting of the water heater.
  • Tank is too small. If an expansion tank is too small for the heater it is connected to, it will fill with more water than is appropriate for the size of the tank. Condensation can build up when the tank is filled and emptied more frequently than it should be.
  • Tank is in a warm, humid climate. If an expansion tank is subjected to high temperatures and humidity, condensation can build up on the outside of the tank. Ensure that the tank is installed in a location that is free of too much humidity and heat. An expansion tank should also have sufficient breathing room to avoid condensation.

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.

water heater repair

Tank needs recharging

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.

How to recharge an expansion tank

  1. Check the tank for any weak points. These can be vulnerable when pressure increases in the tank during the recharging process. Ensure all fittings are adequately tight as well.
  2. Turn off the water heater’s switch and close the valve between the water heater and the expansion tank.
  3. Connect a hose to the tank. This can be a garden hose or the hose that comes with some expansion tank models.
  4. Open the drain valve and drain the tank into a container, lawn, or garden. You will need to loosen the drain valve to remove all water from the tank.
  5. Ensure the tank is completely empty of water. If the tank is not completely emptied, the recharge will not be effective.
  6. Reattach and open all connections to the expansion tank. You should begin hearing the tank fill.
  7. Once the expansion tank is filled, check the pressure inside the tank. The psi should match the pressure of the system.

Relief valve triggers too often

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.

Appropriate expansion tank size is determined by the capacity of the water heater and the pressure of the supply water.

Water heater capacity up to 60 gallons

  • Water pressure < 60 psi – 2-gallon expansion tank
  • Water pressure 60 psi or greater – 3.2-gallon expansion tank

Water heater capacity > 60 gallons

  • Water pressure < 50 psi – 2-gallon expansion tank
  • Water pressure 50-60 psi – 3.2-gallon expansion tank
  • Water pressure > 60 psi – 4.4-gallon expansion tank

What is the cost of replacing a water heater expansion tank?

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.

Learn more: How to fix 5 common water heater problems




If you have any additional questions, please do not hesitate to contact us.

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