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Serving Orange County

- SINCE 1981 -

Serving Orange County

- SINCE 1981 -


Hours: Monday-Friday 7:30-5:30PM
Saturday: 8:00-1:00PM


Saddleback Plumbing Heating & Air Blog

Explaining the Thermal Expansion Tank on Your Water Heater


How much do you know about your water heater? It’s okay if you answer “Not that much.” Homeowners can’t be expected to have expert knowledge of every major appliance in their house. It’s the reason you have professionals to handle jobs like repairing plumbing in Mission Viejo, CA.

We’re going to shed some light on an important part of your water heater today, the thermal expansion tank, often just called the expansion tank. Understanding the purpose of this component will give you a better sense of the workings of the water heater as well as when you may need to call for repairs.

Pressure inside a water heater

Some basic science: when liquids heat up, they expand. In This is the source of steam power. As water heats up, it creates increased pressure inside a closed system. Now imagine your water heater. The water tank probably holds between 40 to 50 gallons of water. When that water heats up, the expansion inside the tank is tremendous. If the tank were filled all the way to the top with water, that pressure would cause the tank to burst when the water was heated.

Clearly, that’s not happening. Your water heater does its job without blowing up. (Yes, water heaters have exploded, but this is extremely rare and usually only happen in old steam water heaters.) So how is the increased pressure getting absorbed? 

In older water heaters, an air cushion inside the tank absorbed the pressure. In other words, the tank wasn’t filled fully, leaving air at the top to allow for thermal expansion. But there was a problem with this: allowing oxygen into the tank made it much easier for the tank to rust. Rust requires the presence of oxygen. To make water heaters more resistant to rust, all oxygen had to be removed. This is where the thermal expansion tank enters the picture.

The thermal expansion tank at work

The expansion tank is located above the main water tank. It’s connected to the tank and provides an air cushion that doesn’t allow air to come into contact with the water. The way it does this is with a watertight membrane in the expansion tank that holds a pocket of pressurized air. As the water expands in the tank, this membrane serves as the air cushion to absorb the pressure increase and maintain safe pressure inside the tank. 

Thermal expansion tank repairs

Like any component in a water heater, a thermal expansion tank can malfunction. It could have the membrane rupture and lose the water cushion, for example. If you notice water leaks from the water heater or hear it making a rumbling sound (which means high pressure), we recommend calling our technicians right away. Your water heater isn’t in danger of exploding, as we mentioned above, but this high pressure will damage it, cause leaks, and may result in you having to replace the whole unit early. 

When you have any doubts about the performance of your water heater, reach out to us. We’re the local experts you can trust.

Saddleback Plumbing Heating & Air has served Orange County since 1981. Schedule water heater service with us today.

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