What is a thermal expansion tank?

A thermal expansion tank is a small metal tank connected to the incoming water supply near your home's water heater. When water is heated, it expands. Expansion tanks are designed to absorb the excess pressure caused by the expansion of heated water. Much like how steam can lift a lid off a boiling pot of water on the stove, the hot water heater in your home can create significant pressure in your plumbing system, sometimes 2-3 times your normal pressure! This excessive pressure can cause loud noises, unusual flow from faucets, and even burst pipes (flooding!). A properly functioning thermal expansion tank is a critical safety device that prevents these issues.
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Is a Thermal Expansion Tank Required?
In general, the Uniform Plumbing Code (UPC) requires a thermal expansion tank if any of the following are true:

• The building has a storage-type water heater.
• Incoming water pressure from the street exceeds 80 psi.
• The water system has a device that prevents excessive pressure from dissipating back into the main water supply, such as a check valve or backflow preventer.

How Does a Thermal Expansion Tank Work?
A thermal expansion tank consists of two main compartments separated by a rubber diaphragm:

• Water Chamber: This section connects directly to the water piping near the water heater and receives the excess water when the heater is heating.
• Air Chamber: This section is pre-charged with air and absorbs the pressure generated by the expanding water.

When the water heater turns on and begins to heat cold water, the water expands into the water chamber of the expansion tank. The rubber diaphragm flexes, compressing the air in the air chamber and absorbing the increased pressure. This process prevents the pressure from rising to dangerous levels within the system.

Why is a Thermal Expansion Tank Important?

• Protects the Plumbing System: By absorbing excess pressure, a thermal expansion tank prevents damage to your faucets, filtration systems, toilet and shower valves, appliances (such as dishwasher, refrigerator, clothes washer), pipes, water heater, and everything connected to your water system. Preventing excessive pressure cycling caused by thermal expansion extends the lifespan of your plumbing system and reduces the likelihood of leaks or bursts.
• Ensures Safety: High water pressure can create potentially hazardous situations, such as a ruptured water heater or water filter housing. A thermal expansion tank mitigates these risks by maintaining safe pressure levels.
• Prevents Water Hammer: Water hammer is a phenomenon where water flow stops suddenly, causing a shock wave in your plumbing system. It is often identified by loud, violent banging or knocking sounds coming from water piping. High pressure caused by thermal expansion magnifies this problem and can ultimately cause catastrophic failures such as blown-off angle-stop valves, cracked fittings, and burst plumbing connectors.
• Improves Efficiency: Consistent water pressure ensures that your water heater and other appliances operate more efficiently, potentially lowering energy costs over time.
  
  

Installation and Maintenance
Installing a thermal expansion tank is a straightforward process that can be done by a professional plumber. The tank is usually connected to the cold water supply line near the water heater and secured to the wall or ceiling with a bracket or strap. For the thermal expansion tank to function properly, the air chamber must be at the correct pressure before installation. Most expansion tank manufacturers specify that the air chamber be pressurized to the same or slightly higher than the incoming static water pressure.
As with all mechanical components, expansion tanks will eventually fail. Water quality, supply pressure, and hot water temperature all play a role in how long the expansion tank will last.
External corrosion or water leaks in the tank are clear signs of failure. An air chamber that no longer maintains pressure or releases water when checked also indicates the tank has failed. Sometimes, a defective thermal expansion tank may not be obvious. Spikes in water pressure that dissipate quickly often indicate a failed expansion tank. However, pressure spikes can be difficult to capture without continuous water pressure monitoring.
Many water heater failures, burst water lines, and floods have occurred due to failed thermal expansion tanks. In many cases, the homeowner was not aware that their thermal expansion tank had failed until it was too late.

Conclusion
A thermal expansion tank is a small but crucial component to protecting your home's plumbing system. It helps ensure the safety, efficiency, and longevity of your water heater, piping, fixtures, appliances, and overall plumbing system. Make sure your thermal expansion tank is working properly and keep an eye on your water pressure.
Don Alarm products continuously monitor your water system pressure and immediately sound an alarm when the pressure is too high. With Don Alarm, you can rest assured that your pressure is at a safe level and your thermal expansion tank is working properly.​

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