White Smoke Coming From a Water Heater
On this page (6 sections)
The Short Answer: Most white “smoke” at a water heater is steam: water hitting a hot surface, relief-valve discharge, or normal vapor from a high-efficiency vent outdoors. But vapor inside the house at a gas heater’s draft hood can be exhaust spilling, and haze with a burning-plastic smell from an electric heater can be overheating wiring. If it isn’t clearly harmless outdoor vent vapor, turn the heater off and find the source.
Read the cloud
| What you see and smell | Likely source | What to do |
|---|---|---|
| Thin white plume at an outdoor plastic or stainless vent on a cold day, no odor | Water vapor from a condensing or tankless heater | Normal |
| Steam from the relief-valve discharge pipe | Tank overheating or over-pressure | Turn the heater off; stay clear of the pipe |
| Steam or sizzling at a gas burner after heavy hot-water use | Condensation dripping on the burner | Usually normal if it stops as the tank warms |
| White haze at the draft hood on top of a gas tank, headache, soot, yellow flame | Exhaust spilling into the room | Turn the heater off, ventilate, get out if anyone feels ill |
| Gray or white haze with a hot-plastic or electrical smell from an electric heater | Overheating wiring or thermostat | Breaker off; do not open covers |
| Light smoke and paint or oil smell the first time a new heater runs | Manufacturing residue burning off | Ventilate; should stop quickly |
Steam from the relief valve
The temperature and pressure relief valve opens to release water when the tank gets too hot or the pressure gets too high. Steam or very hot water from its pipe means the heater’s temperature control may have failed. Turn off the gas control or the breaker, keep people away from the discharge, and call a plumber. Never cap the valve or its pipe. See water heater suddenly too hot and overflow keeps discharging.
Exhaust spilling from a gas heater
An atmospheric gas tank relies on hot exhaust rising up the vent. If the vent is blocked, disconnected, or overpowered by a fan or dryer pulling air from the room, exhaust can spill out at the draft hood. On a cold day this can look like white vapor. Soot on the top of the heater, a lazy yellow flame, or a carbon monoxide alarm all point to this.
What you can check safely: turn the gas control to OFF, open a window or door, and look at whether the vent connector on top of the heater is attached. Check the outside vent cap for blockage. Do not relight until a technician has checked draft and carbon monoxide. Related: vent pipe leaking and carbon monoxide leaks.
Smoke from an electric heater
Electric heaters have no flame, so real smoke means something electrical is overheating: a loose terminal, a burned thermostat, or water reaching the wiring. Turn off the breaker from a dry spot and leave it off. See what causes a thermostat to burn up.
What the technician will check
For steam, they test the thermostat or gas control, the relief valve, and house pressure. For a gas heater with spilling exhaust, they check draft, vent condition, combustion air, and carbon monoxide levels before the heater is used again. For an electric heater, they inspect the wiring, terminals, thermostats and elements.
Frequently Asked Questions
Q: Is white vapor from my tankless water heater’s outside vent normal? A: Yes, especially in cold weather. High-efficiency units produce water vapor that condenses into a visible plume outside. It should not appear indoors.
Q: Can I keep using the heater if the steam stopped? A: If the steam came from the relief valve, no. The valve opened because something was wrong, and the cause needs to be found first. If it was condensation sizzling on a burner after heavy use and the heater is otherwise normal, it can stay in service.
Q: Why does my brand-new heater smell and smoke a little? A: Oils and coatings from manufacturing can burn off during the first firing. It should clear within a short time. If it continues, or you smell gas, turn the heater off and call the installer.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- CPSC: carbon monoxide fact sheet (opens in a new tab)Carbon monoxide hazards and emergency response.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
- A. O. Smith: standard electric no-hot-water diagnostic chart (opens in a new tab)Professional diagnostic context for standard electric storage heaters; not instructions to open or test your appliance.
Editorial standards and corrections · Prepare a useful service request