What You Need to Know About Inspecting Expansion Tanks

Two metal tanks with copper piping branching off them.

Expansion tanks relieve the pressure created by the boiler.

An expansion tank is an essential part of certain heating systems, and when you’re inspecting a home, you’ll want to make sure it’s in good working order. What should you look for when you’re inspecting an expansion tank?

Boilers operate by heating the water that heats a home. The heating process causes the water to expand and put pressure on the system, and an expansion tank relieves this pressure. Although boiler explosions are rare, when they happen, the results can be catastrophic, including loss of life, so the proper function of this system is critical.

Where to Find the Expansion Tank

When you’re doing an inspection on a home that has a boiler, always look for the expansion tank.

These tanks are usually attached to the boiler, but in an old house, it might be on the top floor or even in the attic. The idea was to locate them above the highest radiator. Some of these tanks have windows that allow you to check the water level inside. These open-system expansion tanks are equipped with an overflow pipe, which disposes of the water in another location, such as a drain or spill sink.

Once you locate the expansion tank, check it for leaks. The tank may be located in a dark area, so have a powerful flashlight handy. Sometimes a leak is apparent, but other times less so. Check all around the tank for signs of dampness, especially near pipe connections.

Waterlogged Expansion Tanks

An expansion tank is usually about a third full of water. When the boiler comes on, more water comes into the tank, compressing the air inside.

Traditional closed-system tanks — which are sometimes mounted in the ceiling — are made of metal, but the more modern ones are made of rubber. The rubber tanks have two sides — one for air and one for water. Traditional tanks only have one reservoir, which is more likely to allow the water to absorb the air, causing the tank to become waterlogged.

Water tank with two expansion tanks one red and one blue with copper piping extending from each of them.

Check the pressure and the valves on the boiler and the expansion tank.

Inspectapedia lists several signs of a waterlogged expansion tank, including water spilling through the pressure valve every time the boiler comes on. When this happens, the boiler will take in more water as it cools down. This not only wastes water, but it could result in clogged valves due to mineral deposit buildup.

Another sign of a waterlogged expansion tank is that it may seem heavy. Lightly rap on the tank and listen for a tinny sound. The tank may also move a bit when you tap on it. If it feels heavy and makes a dull sound when you tap on it, it might be waterlogged.

Check Boiler Pressure

Turn the boiler on and check the pressure readings. Safe pressure readings vary according to the size of the boiler. Check the tag on the boiler; this will tell you what the psi should be for the system. If it’s high, a waterlogged expansion tank might be the cause.

Also look for any signs of rust. Rust may or may not be a reason to replace a tank, but the potential homeowner should be made aware of it. They can decide whether to have a plumber come in and evaluate the tank’s integrity.

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