Light moisture or a small puddle the first time a new tank heats, in cold weather or after heavy use is often condensation and fades once the tank is hot. Water that keeps coming back, pools, or traces to one spot is a leak. A condensing gas heater also makes condensate that must drain freely.
When water around a water heater is condensation
A.O. Smith's help guide for its tank heaters says: "A puddle of water near a newly installed water heater is usually condensation and not a leak." It explains why. "Since the water heater is full of cold water, you may get some condensation (puddle of water near the water heater or drops of water hitting the burner and sizzling) the first time the water heater operates." And: "Condensation is normal when the water heater is newly installed and may also occur in very cold weather or during periods of heavy use." Then the useful test: "Condensation will go away when the tank of water is hot."
The mechanism is the same as a cold glass on a patio table. Cold water inside the tank chills the metal, and moisture in the surrounding air collects on the cold surface and runs down. A.O. Smith's other guide says the same for both fuel types: on an electric heater, it can happen when cold water cools the outer surface in a warm environment, "usually resolves as the heater warms up," and "will generally not be seen" because of the insulation around the tank. On a gas heater, it happens "when cold water enters the tank or when the burner fires after a period of inactivity" and is "typically temporary."
Rheem puts the line plainly: "Small amounts of condensation are normal, but pooling water indicates a problem."
How to tell condensation from a leak
Don't guess from one wet look. Use a short test.
- 1Dry everything. Wipe the tank, the pipes on top and the floor.
- 2Lay dry paper towels under the pipes, the T&P discharge pipe, the drain valve and around the base.
- 3Let the heater run through a full heating cycle, then check again after a few hours.
- 4Read the pattern.
| What you see | Most likely |
|---|---|
| Light moisture, gone after the tank is hot, only on the first run or a cold day | Condensation |
| Small beads on the cold supply pipe or tank jacket in humid weather | Pipe or jacket sweating, see why pipes sweat |
| Wet again after drying, one spot soaks a towel | Leak at a fitting, valve or tank |
| Rust streaks, mineral crust or a ring at a joint | Slow leak, check water heater leaks from the top |
| Water around the base that keeps coming back | Possible tank failure, see leaks from the bottom |
If water is clearly leaking, A.O. Smith says to turn off the dedicated shutoff valve on the cold water line to stop further leakage while you diagnose. Make sure the power or gas is off before you inspect.
A condensing gas heater makes water on purpose
High-efficiency condensing gas heaters pull so much heat out of the exhaust that the vapor turns to liquid. That liquid, condensate, is designed to flow out through a drain line. If you see a little water at the condensate drain or trap, that can be normal. If the drain line is blocked, kinked, raised or backing up, the water has nowhere to go.
This is where the page gets serious. A.O. Smith's condensing gas manual warns that "An improperly installed or malfunctioning condensate drain can cause the water heater to emit carbon monoxide." It says condensate lines "must be free and clear of debris and must not allow back flow through the hose," must not be crimped closed, and that you should not install an external condensate trap because the exhaust tee assembly has an internal one. It also says to "Install gas and carbon monoxide detectors in the vicinity of the water heater."
If the heater is a condensing model and the drain is wet, backed up or not running, shut the unit off and have a licensed plumber check it. Carbon monoxide is odorless, so a detector is the only warning you get. Our page on whether a water heater can cause carbon monoxide covers the signs and the response.
What a Phoenix homeowner should do next
- New heater, first run, tank still warming: wipe up, watch it for a day, and check that it stays dry once hot.
- Old heater with a recurring puddle: treat it as a leak. Run the towel test and find the source.
- Condensing model: check the drain line and confirm the CO detector works.
- Anything with rust, a rotted base or water that is not stopping: shut off the cold supply, turn off power or gas, and call.
For a corroded tank, A.O. Smith says it is usually more practical and cost-effective to replace than repair. See signs a water heater is about to fail for what to look at before it floods. HQ Plumbing and Air installs and repairs tank, tankless and condensing water heaters, with 24/7 emergency service and an upfront price before work starts. See our water heater services.