How to Check Your Water Heater Expansion Tank

Why it matters

A water heater expansion tank is a safety device designed to handle thermal expansion. When water is heated, it expands in volume. In older "open" plumbing systems, this excess pressure could push back into the city water main. However, modern homes use "closed" systems with backflow preventers, meaning that expanded water has nowhere to go.

Without a functional expansion tank, this pressure builds up inside your water heater and pipes. This leads to:

• Preventing Tank Rupture: Repeated pressure spikes weaken the internal glass lining of your water heater, eventually causing the steel tank to corrode or burst.
• Protecting Fixtures: Chronic high pressure wears out faucet washers, toilet fill valves, and dishwasher solenoids, leading to constant leaks.
• Stopping T&P Valve Drips: If your Temperature and Pressure (T&P) relief valve is constantly dripping, it’s often because the expansion tank has failed, forcing the valve to work overtime to bleed off pressure.

How often

You should check your expansion tank once every 12 months. It is most efficient to do this at the same time you flush your water heater or check the anode rod. If you notice your T&P valve dripping or hear "water hammer" (banging pipes) when the heater is running, check the tank immediately.

What you need

• Tire Pressure Gauge: A high-quality digital or dial gauge is more accurate than a cheap "pencil" style gauge.
• Bicycle Pump or Air Compressor: To add air if the pressure is low.
• Screwdriver or Wrench: (Optional) To remove the protective cap from the air valve.
• Step Ladder: If your expansion tank is mounted high above the water heater.

Step-by-step

Follow these steps to ensure your expansion tank is pressurized correctly and the internal bladder is intact.

1. Identify the Tank: Locate the small, often blue or white tank connected to the cold water inlet pipe above your water heater.
2. The "Tap Test": Use a knuckle or a screwdriver handle to gently tap the top and bottom of the tank.

• The top half (connected to the pipe) should sound "thuddy" or solid because it contains water.
• The bottom half (where the air valve is) should sound hollow or "ringy" because it should be filled with air. If the whole tank sounds solid, the internal bladder has likely ruptured and the tank is "waterlogged."

1. Locate the Schrader Valve: Remove the plastic or metal cap on the end of the expansion tank. This is a standard valve, exactly like the one on a car or bicycle tire.
2. Check for Water: Briefly depress the center pin of the valve with your fingernail or a small tool.

• If air comes out: Move to the next step.
• If water squirts out: The internal bladder is torn. The tank is failed and must be replaced.

1. Measure the Pressure: Press your tire gauge onto the valve. Note the reading.
2. Determine Your Target Pressure: The air pressure in the tank must match your home’s incoming water pressure. You can check your home's water pressure by attaching a pressure gauge to an outdoor hose bib. Most homes are regulated between 40 and 60 PSI.
3. Adjust the Pressure:

• If the reading is lower than your home's water pressure, use a bike pump to add air until it matches.
• If the reading is significantly higher (and the tank was recently installed), you may need to bleed a little air out to match your home's PSI.

1. Re-Check in 24 Hours: If you had to add air, check the valve again the next day. If the pressure has dropped significantly, the valve or the bladder is leaking.

When to call a pro

While checking the pressure is a simple DIY task, you should call a licensed plumber if:

• Water leaks from the air valve: This confirms the tank is dead.
• The tank is heavy or sagging: If the mounting brackets look strained or the tank feels incredibly heavy (indicating it's full of water), it needs immediate replacement.
• You don't have an expansion tank: If you have a closed system (common in homes built after the 1990s or those with a pressure reducing valve) but no expansion tank, a pro should install one to protect your plumbing.
• Replacement is needed: Swapping a tank involves more advanced plumbing skills, including sealing threaded joints and potentially supporting the weight of a water-filled vessel.

Safety

• Weight Hazard: A failed expansion tank can weigh 40–60 pounds because it is full of water. Do not attempt to unscrew a tank unless you are prepared to support its full weight.
• Hot Pipes: The pipes near the water heater can be extremely hot. Wear gloves and long sleeves to avoid contact burns.
• Fall Risk: If your tank is mounted horizontally or high up, ensure your ladder is stable before reaching for the air valve.

Cost

• DIY Inspection: $0 (assuming you own a tire gauge).
• New Pressure Gauge: $10–$20.
• Professional Inspection: $75–$150 (usually bundled with an annual HVAC or plumbing tune-up).
• Replacement Tank: $40–$100 for the part.
• Professional Replacement: $250–$500 including labor and materials.