Do You Need a Water Heater Expansion Tank? What It Does and When It Is Required
When water heats up it expands. An expansion tank absorbs that pressure so your plumbing does not.
Safety Disclaimer
Water heater maintenance involves working with pressurized systems, scalding hot water, and potentially hazardous electrical or gas connections. Always shut off power (electric heaters) or gas supply (gas heaters) and allow water to cool to a safe temperature before beginning any maintenance. Wear appropriate safety equipment including gloves and eye protection. If you're uncomfortable with any step, contact a licensed plumber.
The Physics
Water expands by approximately 2% when heated from 50°F to 120°F. In a 50-gallon tank, that is roughly 1 gallon of extra volume. That expanded water has to go somewhere.
Open vs. Closed Plumbing Systems
Open System (No Backflow Preventer)
In an open system, the expanded water pushes back into the municipal water supply through the cold water line. No expansion tank needed. The city’s water main absorbs the pressure.
For more on this topic, see our guide on Water Heater Anode Rods Explained: The Part That Saves Your Tank.
Closed System (With Backflow Preventer or Check Valve)
Many municipalities now require backflow prevention devices on residential water lines to prevent contaminated water from flowing back into the public supply. These devices create a closed system. The expanded water has nowhere to go, and pressure builds inside the plumbing.
For more on this topic, see our guide on What Size Water Heater Do You Need? A Sizing Guide by Household.
Without an expansion tank in a closed system:
- Pressure can reach 150+ PSI (normal is 40-80 PSI)
- The T&P valve may discharge repeatedly
- Fittings, valves, and appliance connections can fail prematurely
- Toilet fill valves may drip continuously
- The water heater tank itself can sustain damage
How an Expansion Tank Works
An expansion tank is a small (2-5 gallon) tank installed on the cold water line above the water heater. Inside, a rubber diaphragm separates an air chamber from the water side. When water expands, it pushes into the tank, compressing the air cushion. When hot water is used and pressure drops, the air pushes the water back into the system.
Is It Required?
Expansion tanks are required by code (IRC/UPC) in any closed plumbing system. If you have a pressure-reducing valve, check valve, or backflow preventer on your main water line, you need an expansion tank.
Installation
- Mount vertically on the cold water line above the water heater
- Pre-charge the air side to match your home’s water pressure (typically 40-60 PSI)
- Use a tee fitting and support bracket
- Cost: $40-$100 for the tank, $150-$300 installed by a plumber