The situation
Illustrative example. Not a real job or customer.
A Brea homeowner finds a small puddle every morning at the end of the water heater's relief discharge pipe. During the day it stays dry. A toilet in the house also refills on its own now and then. An expansion tank was installed a few years ago with the current heater.
- The owner logs when the puddle appears: always overnight, after evening showers and the dishwasher.
- A licensed plumber puts a gauge with a peak indicator on a hose bib overnight.
- The gauge shows normal pressure when cold but a large spike after the heater runs.
- Tapping the expansion tank sounds solid all the way up, and water comes out of its air valve: the diaphragm has failed.
The plumber replaces the expansion tank, sets its air charge to the house pressure and supports it properly. The relief valve stops dripping, so it does not need replacing.
Why it happened at night
In this example the house is a closed system because of a pressure regulator at the main. After evening hot water use, the heater refills and reheats a full tank. The water expands, and with a failed expansion tank it has nowhere to go, so pressure climbs until the relief valve lets a little out.
Replacing the relief valve alone would have hidden the problem for a short while at most. The overnight log and a peak-reading gauge found the real cause.
What the tests showed
| Test | Result in this example | Meaning |
|---|---|---|
| Static pressure, cold | Normal range | Regulator working |
| Peak pressure overnight | High spike | Thermal expansion not absorbed |
| Expansion tank tap test | Solid, waterlogged | Diaphragm failed |
| Air valve check | Water instead of air | Confirms failure |
Lessons from the example
Questions to ask in a similar situation
- Can you measure peak pressure over a heating cycle?
- Is my expansion tank still holding its air charge?
- Is my pressure regulator holding a normal pressure?
- Is the relief valve itself sound, or does it need replacing too?
Common questions
Is this a real case?
No. It is an invented example to explain a common cause of relief valve drips.
Can an expansion tank fail without leaking?
Yes. When its internal diaphragm fails it fills with water and stops absorbing expansion, often without any visible leak.
Why did the toilet refill on its own?
Pressure spikes can push past a toilet's fill valve. Once pressure was controlled, that symptom would be expected to ease too.
General information for Brea, not a quote or professional advice. In an emergency, call 911.