Water Heater Vent Pipe Leaking Water
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The Short Answer: Water on a gas water heater’s vent pipe usually comes from one of three places: exhaust condensing inside a vent that runs too cold or too long, rain getting in at the roof or wall termination, or condensate from a high-efficiency unit that is not draining properly. Any of these can corrode the vent and let exhaust escape, so a leaking vent should be inspected, not taped over.
Which vent do you have?
- Metal vent (usually double-wall Type B) from a draft hood on top of the tank: standard atmospheric gas tank heaters.
- Plastic pipe (PVC, CPVC or polypropylene) with a fan on the heater: power-vent tanks and condensing tank or tankless heaters.
- Stainless steel vent with a fan: many non-condensing tankless heaters and some power-vent models.
The vent type determines whether water in it is expected and where it should go.
What the drip is telling you
| Where and when | Likely cause | Urgency |
|---|---|---|
| Metal vent, drips mainly when the burner starts in cold weather, clear water | Exhaust condensing in a cold or oversized vent | Get it inspected; repeated condensation rusts the vent |
| Metal vent, rusty water, rust holes, or joints separated | Vent corroded or disconnected | Turn the heater off; exhaust can leak |
| Water at the draft hood with soot, or yellow flame | Exhaust spilling instead of venting | Turn the heater off; carbon monoxide risk |
| Drips after rain or snowmelt only | Water entering at the cap, flashing, or roof jack | Repair the termination |
| Plastic vent, slightly acidic water at a joint while running | Condensate escaping a joint or not draining back to the heater’s drain | Have the vent slope and drain checked |
| Plastic vent drips at the outdoor termination in cold weather | Normal on many condensing units | Usually fine if the area drains and does not ice up |
On condensing heaters, water in the vent is normal, but it is supposed to run back to the heater’s condensate drain, not out of a joint. See Navien condensate drain leaking and Rinnai leaking by the exhaust. A. O. Smith explains why combustion condensate can look like a tank leak in its condensation bulletin (opens in a new tab).
Why condensation in a metal vent is a problem
Atmospheric heaters are designed so exhaust stays warm enough to rise and leave the house before it condenses. A long horizontal run, an outdoor chimney, a vent that is larger than it should be, or a cold attic can cool the exhaust too much. The water that forms is mildly acidic and rusts the vent from the inside. Eventually joints leak exhaust as well as water. Sealing the joints with tape or caulk does not fix the cause and can hide a hazard.
What you can check safely
- With the heater off and cool, look along the vent for rust, holes, gaps at joints, and a connector that has slipped off the draft hood.
- Check the vent cap or wall termination from outside for damage, missing parts or debris.
- Note when the dripping happens: only at startup, during rain, or all the time.
- Confirm your carbon monoxide alarms work.
Do not take the vent apart or seal it with tape, silicone or caulk.
What the technician will check
They will check whether the heater vents properly with the burner running (draft at the draft hood on atmospheric units), inspect vent size, slope, length and material against the heater’s installation manual, check for carbon monoxide, and inspect the termination and roof flashing. For condensing units they also check vent slope back to the heater and the condensate trap and drain.
Repair or replace
Most vent problems are fixed by repairing or rerouting the vent, adding or fixing a cap, or correcting the condensate drain. If water from a vent problem has been dripping onto the heater, they will also check the burner and the top of the tank for corrosion.
Frequently Asked Questions
Q: Is some condensation on a metal vent normal? A: A little moisture for a minute or two right after a cold start can happen. Regular dripping, rust, or water running back onto the heater is not normal and means the vent needs attention.
Q: Can I seal the vent joints with foil tape? A: Not as a repair. Vent pipes must be assembled with their listed connections. Tape over a rusted or separated joint hides a leak that may let exhaust into the home.
Q: Why does my tankless heater’s vent drip outside? A: Condensing tankless units produce water vapor that can condense at the termination, especially in cold weather. That is normal if it is not icing up a walkway or dripping back inside.
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.
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