Expansion tanks are one of the least-understood components of a water heater installation — yet they are required by code in the majority of modern American homes and play an important role in protecting both your water heater and your plumbing system. Here is everything you need to know.
When water is heated, it expands. A 50-gallon tank of cold water at 50°F expands to about 50.9 gallons when heated to 120°F. That extra 0.9 gallons of water has to go somewhere. In an open plumbing system — where water can flow back into the municipal supply — it flows back harmlessly. In a closed system, it has nowhere to go, which creates a pressure spike every time your water heater fires.
An expansion tank provides a small buffer volume where this expanded water can flow without increasing system pressure. Inside the tank, a rubber bladder separates a small air charge (pre-pressurized to your incoming water pressure) from the water side. When pressure rises, the bladder compresses slightly, absorbing the expansion and keeping system pressure stable.
Your system is closed if it has any of the following on the main water supply line entering your home:
Most homes built in the last 20 years have a PRV. If you're not sure, a licensed plumber can confirm it in minutes. Many municipalities also require backflow preventers on all residential service connections, making closed systems the norm in most jurisdictions.
Code note: The International Plumbing Code (IPC) and Uniform Plumbing Code (UPC) both require thermal expansion control (typically an expansion tank) whenever a check valve, backflow preventer, or pressure reducing valve is installed on the water supply. In most jurisdictions, a building inspector will require an expansion tank as part of a permitted water heater installation if these devices are present.
Expansion tanks come in several sizes — typically 2 gallon, 4.4 gallon, and larger. The correct size depends on your water heater tank capacity and your incoming water pressure. For most 40–50 gallon residential water heaters at typical municipal pressures (60–80 PSI), a 2-gallon expansion tank is sufficient. Larger tanks (65–80 gallons) typically require a 4.4-gallon expansion tank.
The expansion tank is installed on the cold water supply line feeding the water heater, and must be pre-charged to the incoming water pressure before installation. WaterHeaterMan includes expansion tanks in all installation packages where code requires them, at no additional charge.
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