Water Heater Smells Like a Dead Animal?
On this page (5 sections)
The Short Answer: First decide where the smell is. If the room around a gas heater smells rotten, treat it as a gas leak: leave and call the gas utility from outside. If only the hot water smells, it is usually bacteria reacting with the tank’s magnesium anode. A true carcass smell near the burner or vent can mean an animal got into the combustion area or flue.
Where is the smell?
| Where you notice it | Most likely cause | How urgent |
|---|---|---|
| In the room, closet or garage, gas heater | Gas leak (odorant in the gas) | Emergency: leave, call from outside |
| Only from hot taps; cold water and room air are fine | Hydrogen sulfide from bacteria and the magnesium anode | Unpleasant, not dangerous; schedule a fix |
| Hot and cold water both smell | Water supply (well or municipal), not the heater | Test the water |
| Near the burner door, draft hood, or vent; feathers, droppings, nesting material | Animal in the combustion area, vent, or vent cap | Turn the heater off; blocked vent can cause CO |
| Near the base, jacket or drain pan, especially with damp insulation | Rodent in the insulation or pan, or wet insulation going sour | Not an emergency; find the source |
| At a floor drain next to the heater | Dry drain trap letting sewer gas in | Pour water into the drain |
Sulfur smell in the hot water only
This is the most common “dead animal” complaint, covered in full in hot water smells like rotten eggs. Sulfate-reducing bacteria in the water react with the magnesium anode rod inside the tank and produce hydrogen sulfide. It tends to be worse after the heater sits unused, on well water, and with water softeners.
The fix is in the tank, not the pipes. A plumber can replace the magnesium anode with an aluminum-zinc or powered anode and disinfect the tank. Removing the anode entirely stops the smell but shortens the tank’s life, so it is not a good long-term answer. Bleach or other chemicals should never be poured into the relief valve. Electric tanks get this smell too; a related page covers sulfur smell in tankless heaters.
An animal in the burner area or vent
Mice, birds and squirrels can get into the vent cap, the draft hood, or the space under a gas heater. On newer gas tanks, the flame arrestor screen at the base can also collect nesting material and lint.
What you can check safely: With the heater’s gas control turned to OFF, look with a flashlight at the vent cap from outside, at the draft hood, and around the base for droppings or nesting. Keep the heater off if you find any. Do not reach into the burner area or remove the inner combustion door.
What the technician will check: the vent from cap to heater for blockage, whether exhaust is spilling at the draft hood, the burner and flame arrestor, and the combustion seal. A blocked vent can push carbon monoxide into the house, so this needs to be cleared and tested before relighting. More on that in white smoke from a water heater and the vent pipe page.
Smell from wet insulation or the drain pan
A slow leak can soak the foam insulation under the jacket. Combined with a dead rodent or mold, it smells strongly. Look for rust or water at the base. If water is coming from the tank itself, the heater needs replacing; see leaking from the bottom.
Frequently Asked Questions
Q: Can an animal get inside the water tank? A: No. The tank is sealed and pressurized. Animals get into the vent, the burner compartment, the insulation, or the drain pan. A rotten smell from the faucet is a water-chemistry problem, not an animal in the tank.
Q: Why does my brand-new water heater make the hot water smell? A: A new tank comes with a fresh magnesium anode, which reacts most strongly when it is new. If your water has sulfate-reducing bacteria, the smell may appear within weeks. An aluminum-zinc or powered anode usually solves it.
Q: Is the sulfur smell in the water a gas leak? A: Not if the smell is only in the hot water and the room air is clean. Gas odorant smells similar, which is why you should always check the air near the heater first.
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.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
- CPSC: carbon monoxide fact sheet (opens in a new tab)Carbon monoxide hazards and emergency response.
Editorial standards and corrections · Prepare a useful service request