Chimney Smells Like a Dead Animal: What to Do

How an animal ends up dead in a chimney

Birds, squirrels, bats and raccoons get in through a flue with no cap or with a cap whose mesh has rusted or torn. Clay flue tile is smooth and vertical, so a small animal that drops in often cannot climb back out. It lands on the damper, on the smoke shelf just above it, or at the very bottom of a heating flue, behind the cleanout door in the basement.

Young animals can die in a nest when the mother is trapped elsewhere, chased off or killed, and stops returning. Either way, the smell builds over a few days and is strongest in warm, humid weather, when the house tends to pull air down the chimney and into the room. Summer and early fall, after the nesting season, are when most of these calls come in.

Is it really a dead animal?

Chimney odors are easy to mix up, and the character of the smell narrows down the cause before anyone climbs a ladder. Trust your nose, then confirm with a flashlight once you are sure no fire or appliance is running in that flue.

Timing is a clue as well. An odor that appears suddenly and grows stronger over several days, often with flies near the fireplace screen, fits remains. One that comes and goes with the weather and has been there for years fits creosote or damp masonry. A smell that started after scratching noises stopped is the classic sign that an animal fell in and could not get out.

• Heavy, sweet and rotten, getting worse by the day: remains of an animal
• Sharp ammonia, like a litter box: urine and droppings from a raccoon or bat roost
• Old campfire or asphalt: creosote, often stronger on humid days
• Damp basement or wet cardboard: water soaking the masonry
• Rotten eggs near a gas appliance: a possible gas leak, so step outside and phone the gas company or 911 from there

Where the remains usually are

In a fireplace flue, the first place to look is the smoke shelf, the ledge behind and above the damper where falling debris collects. It cannot be seen from the room without opening the damper and using a light and a mirror. A closed damper can also hold an animal directly on top of its plate. In a boiler or furnace flue, remains drop straight to the base, so the cleanout door near the floor of the basement is where they turn up.

Multi-flue chimneys in brownstones and rowhouses add complications. An animal can fall into an unused flue that serves a sealed fireplace, and the smell then seeps through gaps between flues into the one you use. Sometimes the source is not in the flue at all but in a wall cavity or attic space beside the chimney, where an animal got in through a gap at the flashing or a soffit.

What not to do

A fire seems like a quick way to burn the smell off. It spreads the odor through the house instead, and a dry nest sitting on the smoke shelf can catch and send burning material up the flue. Remains lodged in the flue can block it, which pushes smoke back into the room. In a heating flue, a blockage can push exhaust and carbon monoxide into the basement.

Spraying air freshener up the flue covers the smell for a day at most. Reaching past the damper with a bare hand is a bad idea too: remains and droppings are unsanitary, and the smoke shelf is lined with sharp debris and soot. Leave the roof alone as well; chimney tops are high and slippery, and an animal still alive inside can bolt.

What you can do right now

Close the damper and any glass doors, and air out the room with a window. A box of baking soda or a shallow tray of activated charcoal in the firebox absorbs some of the odor. If the smell is near the flue of a boiler or water heater, make sure the carbon monoxide alarms on that level and near bedrooms work, and get the flue looked at promptly, since a blocked heating flue is a safety issue, not only an odor problem.

If the smell is coming from a wall or ceiling rather than the fireplace, note where it is strongest. That detail helps the technician decide whether to start in the flue, the attic or the space around the chimney.

How the smell gets cleared

The first job is finding the source. A technician checks from the fireplace with a light and mirror, from the chimney top, and through the cleanout door if it is a heating flue. Everything comes out: the animal, the nest, droppings and soaked debris. Then the flue gets swept, and the smoke shelf, damper and firebox are cleaned and treated with an odor treatment suited to masonry.

While they are in there, the crew looks for damage the animal left behind, such as chewed damper gaskets, torn cap mesh or nest material packed around a boiler connector. Brick and mortar absorb odor, so a faint smell can linger for a short time after removal, especially on humid days, before it fades.

When it smells like urine instead

A sharp urine smell usually means a living animal, or one that lived there for a while. Raccoons often use the smoke shelf as a latrine, and bat colonies leave droppings that pile up over a season. The odor soaks into the masonry, so it can linger after the animals are gone.

Listen before anything else. Scratching or chattering at dusk and overnight means the animal is still home, and it has to be removed humanely before the chimney is cleaned or capped. Capping a flue with a live animal or young inside traps them, which leads straight back to the original problem.

Keeping it from happening again

A stainless steel cap with mesh on all four sides, fastened securely to the flue or the crown, closes the door animals use. Every flue needs covering, including the boiler flue that homeowners tend to forget and any unused flue in a multi-flue stack. A single cap that covers the whole top of a multi-flue chimney is one way to do that.

Trimming branches that hang over the roof removes an easy route to the top. A glance at the cap from the ground each spring, with binoculars, catches torn mesh or a cap that has blown askew before the nesting season begins.