If you've replaced a water heater recently, you may have noticed a small tank mounted on the pipe above it. That's a thermal expansion tank, and it plays an important role.
Why expansion tanks exist
Water expands when it's heated. In an open system, that extra volume pushes back into the city main. But many homes have a check valve, backflow preventer or pressure-reducing valve that makes the system "closed," so the expanding water has nowhere to go. Pressure spikes each time the heater runs.
What high pressure can do
- Cause the water heater's relief valve to drip or discharge
- Stress the tank and shorten its life
- Strain supply lines, valves and fixtures
- Make toilets run or faucets drip
How an expansion tank works
Inside the tank, a rubber diaphragm separates water from a cushion of air. As heated water expands, it pushes into the tank and compresses the air, absorbing the extra volume and keeping pressure stable.
Is one required?
Code requirements vary by jurisdiction, but in closed systems an expansion tank is commonly required when a water heater is installed or replaced. Your plumber will know the local requirement.
Signs your expansion tank has failed
- Tap the tank: the top half should sound hollow. If it sounds full all over, the diaphragm may have failed.
- Water comes out of the air valve when you press it.
- The relief valve on the water heater drips frequently.
Expansion tanks are inexpensive and quick to replace, and they protect a much more expensive water heater.
Need help?
Our water heater repair & installation team serves Davie and all of Broward County. Call (954) 870-5492.