Quick Answer:
If the drip starts when the water heater runs, thermal expansion or a failed expansion control is a common cause. Check whether other fixtures drip at the same time and look for an expansion tank or thermal expansion control on the hot water line. If you find no expansion device or the drip happens system-wide during heater cycles, a plumber should inspect the system.
Why This Happens
When water heats it expands. In a closed plumbing system (one with a check valve, backflow preventer, or a pressure-reducing valve), that expansion raises pressure. If the system has no working expansion tank or other means to absorb that extra volume, pressure can push water through the shower valve or out small leaks. A worn shower cartridge or loose trim can let that pressurized water escape as a drip when the heater runs.
Step-by-Step What to Do
1. Confirm the timing
- Listen for when the dripping starts. Does it begin shortly after the water heater cycles on or when hot water temperature rises?
- Note whether the drip stops when the heater is idle. That link in timing points toward thermal expansion rather than a steady leak.
2. Check other fixtures
- Turn on and off other hot-water fixtures quickly and watch: do any sinks, toilets (tank fill), or other showers show extra dripping or filling when the heater runs?
- If multiple fixtures show activity at the same time, the problem is likely system pressure rather than only the shower valve.
3. Look for an expansion tank or pressure controls
- Inspect the hot water line near the water heater for a small tank (usually mounted above the heater) or a labeled device. If present, tap it lightly — a working expansion tank will sound partially hollow (air cushion).
- Check the incoming water line for a pressure-reducing valve or backflow device that can create a closed system.
- If you can’t find anything nearby, the system may lack proper expansion control.
4. Check the shower valve area
- Look behind the trim for visible wetness. A loose cartridge seal or corroded valve can leak when pressure spikes.
- If the drip comes from the showerhead, it may be valve leakage or residual water; if it drips from the valve body or trim area, the cartridge or seal is suspect.
5. Test cautiously
- If you are comfortable, lower the water heater thermostat a few degrees to reduce cycling and see if the dripping diminishes. Alternatively, observe the system with the heater turned off (follow manufacturer instructions or call a pro if unsure).
- Document when the drip starts and which fixtures show it — this helps a plumber diagnose the cause faster.
6. Use related troubleshooting resources
- If the issue started after utility work, check notes on **Dripping after city outage** for patterns. If only an upstairs unit is affected, compare with **Drip only upstairs shower** for similar causes.
What Not to Do
- Don’t overlook thermal expansion—if dripping coincides with heater cycles, a plumber is appropriate to assess expansion control.
- Don’t ignore multiple fixtures dripping at once; that suggests a system pressure issue, not just a single valve.
- Don’t attempt major repairs on the water heater, its pressure relief valve, or gas connections unless you are trained and qualified.
- Don’t force or jam the cartridge or trim parts; that can make a small leak worse or damage components.
When to Call a Professional
- No expansion tank or it feels full of water (waterlogged) rather than air.
- Dripping happens at several fixtures when the heater runs or the T&P valve discharges.
- You can’t locate the source, the shower valve requires disassembly, or you’re uncomfortable working near the heater.
- Persistent high pressure in the system or repeated leaks after simple checks.
Safety Notes
- Hot water and steam can cause burns. Avoid placing your hands where scalding water may spray while testing.
- If you need to shut off the water heater, follow the manufacturer’s instructions. Don’t tamper with gas fittings or electrical connections if you are not trained.
- If a relief valve is discharging water, treat it as a sign of excess pressure; do not block the discharge and call a plumber promptly.
Common Homeowner Questions
- Why does the shower only drip when the heater runs? — Heating water raises pressure; without proper expansion control, that pressure can push water past a shower valve.
- Will adding an expansion tank fix it? — Often yes, if the system lacks one or the existing tank is waterlogged; a plumber can confirm sizing and installation needs.
- Can I replace the shower cartridge myself? — You can if you have basic plumbing skills, but if the dripping is from system pressure you’ll still need a professional to address expansion control.
For more related articles, see the Dripping Showerhead After Shutoff hub.