Overview
An expansion tank is a small tank installed near your water heater. When water is heated, it expands. In a closed system (one with a PRV or check valve), the expanded water has nowhere to go — pressure spikes. The expansion tank absorbs that extra volume, protecting your water heater, pipes, and fixtures from pressure damage.
How It Works
When the water heater heats water, the water expands. In a closed system, this extra volume has nowhere to go — without an expansion tank, pressure would spike, potentially tripping the T&P valve or damaging pipes. The expansion tank's rubber diaphragm flexes to absorb the expanded water, keeping pressure stable.
Key Components
Tank
A small steel tank, usually 2–5 gallons, mounted near the water heater.
Rubber Diaphragm
Inside the tank. One side has air, the other has water. Flexes to absorb expansion.
Air Charge
Pre-charged to match the house water pressure. Should be checked periodically.
Schrader Valve
A tire-style air valve. Used to check and adjust the air charge.
Common Issues
T&P valve dripping
No expansion tank, or a failed tank with no air charge.
Tank feels heavy / waterlogged
Diaphragm failed — tank is full of water instead of air. Needs replacement.
High pressure symptoms
Expansion tank not absorbing pressure spikes.
Maintenance Tips
- Tap the tank — it should sound hollow (air). If it sounds full (water), the diaphragm failed.
- Check the air charge annually with a tire gauge — it should match your house water pressure.
- Expansion tanks typically last 5–10 years. A waterlogged tank needs replacement.
- If your T&P valve drips and you have no expansion tank, installing one often solves it.