Water Heater Expansion Tanks & Relief Valves
A dripping relief valve usually means high pressure or a missing or failed expansion tank. We find the cause and fix it.
Updated October 2026 · By the Hot Water Heater Factory team
What does a water heater expansion tank do?
An expansion tank is a small tank, usually mounted above the water heater, with an air cushion inside. As the water heats and expands, the extra water pushes into the tank instead of raising the pressure in your pipes. That protects the water heater, the relief valve, faucets and appliance hoses.
Do I need an expansion tank?
Most homes with a pressure regulator, a check valve or a backflow preventer need one. These devices keep water from flowing back to the city main, which makes your plumbing a closed system where expanding water has nowhere to go. Many Inland Empire homes have a pressure regulator. If yours does and there’s no expansion tank, it’s worth adding one.
Why is my water heater relief valve dripping?
| Cause | How to tell | Fix |
|---|---|---|
| Water pressure too high | Pressure above about 80 psi on a gauge | Adjust or replace the pressure regulator |
| No expansion tank | Drips mostly while the water heater is heating | Add an expansion tank |
| Failed expansion tank | The tank sounds solid when tapped, or water comes out of its air valve | Replace the expansion tank |
| Water too hot | Very hot water at the taps | Check the thermostat or gas control |
| Worn relief valve | Drips even with normal pressure and temperature | Replace the relief valve |
Most homes do best with water pressure between about 50 and 70 psi.
How do I know if an expansion tank has failed?
- Tap the tank. The top half should sound hollow. If it sounds solid all the way up, it’s full of water.
- Water comes out of the small air valve on top of the tank.
- The relief valve has started dripping again.
- Rust or leaks on the tank or its connection.
Expansion tanks often last about 5 to 10 years.
Why does the expansion tank’s air charge matter?
An expansion tank comes from the factory with a standard air charge, often around 40 psi. If your water pressure is higher than that, the water squeezes the air cushion before the water heater even starts heating, and the tank has little room left to absorb expansion. The air charge has to be set to match your water pressure before the tank is filled.
It’s a step many installers skip. We check the water pressure and set the air charge on every expansion tank we install, at homes and commercial buildings alike.
What is a temperature and pressure relief valve?
It’s the safety valve on the side or top of every tank water heater. It opens if the water gets too hot or the pressure gets too high, usually at 150 psi or 210°F. Never cap or plug its discharge pipe. If it drips or opens, it’s telling you something is wrong.
See the relief valve entry on our water heater problems page, or tank water heater repair and replacement.
Related
- Common water heater problems
- Tank water heater repair and replacement
- Scald safety and water temperature
Expansion tank and relief valve service near you:
Questions we hear a lot
Is a dripping relief valve dangerous?
The drip itself isn’t dangerous, but it’s a warning. The valve is releasing pressure or heat for a reason, and if it’s ignored the valve can wear out or the cause can get worse. Don’t cap the pipe; have it checked.
How much does it cost to run a home without an expansion tank?
There’s no running cost, but the pressure spikes wear out the water heater, relief valve, faucet cartridges and appliance hoses faster.
Do you set the air charge on a new expansion tank?
Yes, every time. We check the water pressure and set the tank’s air charge to match before it’s filled. A tank left at the factory setting often can’t do its job.
Can a relief valve be replaced without replacing the water heater?
Yes. If the water heater is otherwise in good shape, the relief valve is a simple part to replace.
What water pressure is too high for a house?
Above about 80 psi is too high. Most homes do best between about 50 and 70 psi, set by a pressure regulator.
Dripping relief valve? Text us now.
Send a photo of your water heater’s label and tell us what’s happening. A real person texts you back.