Water Dripping From the Water Heater Exhaust
On this page (6 sections)
The Short Answer: Water at a gas water heater’s exhaust usually comes from one of four sources: condensation from the flue gas, rain entering the vent cap or chimney, condensate that cannot drain and backs up into the vent, or water leaking inside the heater. A small amount of cool water on a condensing unit can be normal. Hot water, a steady flow, or water that continues with the heater off needs service.
Which kind of heater do you have?
The answer depends on the vent.
- Condensing tankless or high-efficiency tank (PVC or polypropylene vent): these heaters cool the exhaust so much that water condenses from it. They have a condensate drain line. The condensate is mildly acidic, so it should leave through that drain, not the vent.
- Non-condensing tankless (stainless vent): normally little or no condensate, though some can form in cold weather or on long vent runs.
- Standard atmospheric tank (metal vent with a draft hood): some condensation forms inside the flue on startup, especially after a lot of cold water has entered the tank. The vent itself should not drip steadily.
- Power-vent tank (fan on top, plastic vent): can collect condensate in the vent if the vent was not sloped as the manual requires.
An electric heater has no exhaust. If you have an electric heater and see water near the top, check water coming from the top of the water heater.
Reading the drip
| What you see | Likely source | Urgency |
|---|---|---|
| A little cool water after a long run, condensing unit | Normal condensate in the vent | Watch it; check the drain line |
| Drip only after wind-driven rain | Rain entering the cap, chimney, or termination | Have the termination checked |
| Steady cool drip, condensate line dry or frozen | Blocked trap, line, or condensate pump | Prompt; heater may lock out |
| Water at a plastic vent joint indoors | Joint not sealed or vent not sloped back to the heater | Service; flue gas can escape at the joint |
| Warm or hot water, or dripping with the heater idle | Leak in the heat exchanger or tank into the combustion area | Turn it off and call for service |
| White mineral crust at the flue collar or on the burner | Long-term water leak into the combustion area | Service |
| Soot, a burnt smell, or headaches near the heater | Vent not drafting; possible CO | Leave, ventilate, call from outside if a CO alarm sounds |
What you can check safely
- Feel the drip (carefully) and note whether it is cool or warm, and whether it happens only while the heater is running.
- Follow the condensate line from the heater to where it drains. Look for kinks, a section that rises, a frozen outdoor run, or a condensate pump that is full or not running.
- From the ground, look at the outdoor vent and intake terminations for a missing cap, snow, leaves, or nests.
- Notice whether the drip only appears after rain.
- On a tankless heater, check the display for an error code and write it down.
- Do not seal vent joints with caulk or tape. That hides the problem and can let flue gas into the room.
What the technician will check
A technician will work out whether the water is condensate, rain, or supply water. On a tankless unit, a common test is to close the service valves and watch whether the drip stops. If it does, pressurized water was leaking inside the heater. They will clear or reroute the condensate drain, check the slope and joints of the vent pipe and the termination, and make sure the vent drafts properly. If the heat exchanger or tank is leaking into the combustion area, they will discuss replacing it.
Repair or replace
Fixing a condensate drain, rain cap, termination, or vent joint is a repair. A cracked tankless heat exchanger may be replaceable on some models, depending on the age of the unit and warranty coverage. A tank that leaks into its own flue or combustion chamber cannot be repaired.
For brand-specific cases, see Rinnai leaking by the exhaust when turned off and Navien condensate drain leaking.
Frequently Asked Questions
Q: Is it normal for a condensing water heater vent to drip? A: A small amount of cool water inside the vent is normal, and most of it should run back into the heater’s condensate drain. Water dripping from the outdoor termination is usually a sign the vent was not sloped as the manual requires, or that the drain is backed up.
Q: Why does my water heater exhaust drip only in winter? A: Cold vent pipes condense more moisture, and outdoor condensate lines can freeze. Both are worse on long vent runs or vents through unheated spaces. A technician can check the slope, insulation, and drain routing.
Q: Is the white cloud from my outdoor vent water? A: On a cold day, a white plume from a condensing heater’s vent is usually water vapor. White smoke coming from a water heater explains when it is something else.
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.
- A. O. Smith technical bulletin 15: condensation (opens in a new tab)Why combustion condensate can resemble a tank leak; not a diagnosis of another manufacturer’s model.
- 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.
Editorial standards and corrections · Prepare a useful service request