Why heated water needs somewhere to go
Water expands as it heats. In an open system, that small extra volume pushes back toward the street main and nothing happens. Many homes, though, have a pressure-reducing valve or check valve that stops water flowing backward, which makes the house a closed system.
In a closed system, every heating cycle raises pressure throughout the pipes until someone opens a faucet. The relief valve may drip to let that pressure off, and fixtures and joints feel the spikes. An expansion tank gives the extra volume a place to go.
Key parts
- T&P relief valve
- The valve on top or side of a tank heater that opens if pressure or temperature gets too high.
- Discharge line
- The pipe from the relief valve that carries released water to a safe location.
- Expansion tank
- A small tank with an air bladder, usually on the cold line near the heater, that absorbs expanded water.
- Pre-charge
- The air pressure inside an expansion tank, set to match house water pressure.
- Closed system
- A plumbing system where a valve prevents water from flowing back to the street.
Matching the drip to a cause
| What you see | Possible cause | Next step |
|---|---|---|
| Drip after each heating cycle | Thermal expansion in a closed system | Check for an expansion tank and its charge |
| Expansion tank present but drip continues | Tank waterlogged or bladder failed | Have the tank tested and replaced if needed |
| Constant drip regardless of use | Worn relief valve or high supply pressure | Test house pressure and inspect the valve |
| Hot water discharging heavily | Overheating or control failure | Turn the heater off and call a plumber |
Checking an expansion tank
Find it
Look on the cold supply near the water heater for a small tank, often blue, gray or white.
Tap test
The top half should sound hollow and the bottom half duller. A tank that sounds full all the way may be waterlogged.
Leave the air valve alone
Pressing the air valve can let air out. If water comes out of it, the bladder has failed.
Report it
Tell the plumber what you found along with any pressure gauge readings.
What affects the repair
- Tank or valve
- Replacing an expansion tank is a different job from replacing a relief valve.
- Supply pressure
- If the PRV is also failing, both should be addressed.
- Discharge routing
- A discharge line that ends in the wrong place may need correcting.
- Heater condition
- An older heater with control problems may be better replaced.
Questions to ask
- Is my home a closed system, and do I need thermal expansion control?
- Is the expansion tank charged to match my water pressure?
- Is the relief valve worn, or is it responding correctly to high pressure?
- Does the discharge line end where it should?
Common questions
Do tankless heaters need expansion tanks?
Tankless units hold very little water, so expansion is less of an issue, but a closed system may still need control. Ask a plumber about your setup.
Can I just replace the relief valve?
Relief valve work involves a pressurized, hot appliance. A licensed plumber should do it and find out why it was opening.
How long do expansion tanks last?
It varies with water quality and pressure. Hard water and high pressure tend to shorten their life, so check yours periodically.
General information for Amerige Heights, not a quote or professional advice. In an emergency, call 911.