What is an expansion tank?
An expansion tank is a small pressure vessel that absorbs excess water volume created when water heats and expands in a closed plumbing system.
When water in a closed heating system gets hot, it expands. In older open-vented systems, that extra volume could spill back into the main supply. Modern closed systems trap the water, and without a way to handle the expansion, pressure builds dangerously inside the tank, pipes, and fixtures.
An expansion tank sits in the supply line, usually near the water heater. It contains a rubber diaphragm or bladder that separates air on one side from water on the other. As heated water expands, it pushes into the tank and compresses the air chamber. This absorbs the pressure spike instead of letting it damage the water heater, relief valve, or plumbing connections.
Most expansion tanks are small, cylindrical steel or stainless steel vessels ranging from 2 to 20 gallons, depending on your system size. They have a fill valve at the top (similar to a tire valve) and connect directly to the supply line. Over time, the rubber bladder can wear out or air can leak from the vessel, reducing its ability to absorb expansion. When this happens, pressure relief valves begin to leak or cycle repeatedly, signaling that the tank needs replacement.
In Ontario, water heater service providers can inspect and replace expansion tanks as part of routine maintenance or water heater installation.