How to tell a chimney is due for sweeping, from the firebox up

When a chimney needs sweeping: signs at the damper plate

Learning how to tell if your chimney needs cleaning starts at the damper. With the fire out and the ash cold, open it and put a flashlight on the top face of the plate and the ledge behind it. Wear glasses, since things fall. What sits there is a fair sample of what is higher up, because deposit that loosens in the flue lands on those surfaces first.

Draw a fingernail or the tip of a key across the deposit. A dusting that wipes away is normal after fires. A layer thick enough to hold a clear groove is buildup worth removing. A black coating that looks shiny, blistered or like dried tar is a different material, glazed creosote, and it calls for a professional look before another fire.

Some fireplaces have no damper plate at the throat because a damper was fitted at the top of the chimney instead. There, check the smoke shelf behind the throat with the same flashlight, and leave the top of the chimney to whoever sweeps it.

A draw that has changed

A flue lined with deposit is a narrower, rougher flue. Smoke that used to rise cleanly may hang at the top of the opening while the fire is lit, or roll out when a log shifts. If you now crack a window or a stove door to get a fire going where you did not have to before, the passage may be clogging.

Draw has other causes, so this sign needs company. A damper that is partly closed, a cold flue on the first fire, exhaust fans pulling air down the chimney, and a chimney too short for the roof around it all produce the same symptom. Buildup is the likely cause when the change came on gradually over a run of fires.

A smell with the fireplace cold

Creosote has a sharp, sooty smell, something like an old ashtray or hot asphalt. It is strongest when the fireplace is not in use and air flows down the chimney into the room, which happens when exhaust fans run or when a closed-up house is being cooled. A smell that shows up that way and fades with the fans off points to deposit in the flue.

Not every fireplace odor is creosote. A musty smell suggests water in the masonry, and a rotten or animal smell suggests something in the flue. Those need a look too, but a sweep will not cure them.

What lands in the firebox, and what it means

Keep a note of what you find and when. It helps whoever sweeps the chimney decide where to look first.

• Black flakes or crumbs on the grate or hearth: creosote loosening in the flue. A cleaning sign.
• Fine black powder: soot. Normal in small amounts after fires.
• Sand, grit or chips of orange or tan tile: mortar or flue tile coming apart. A repair sign, not a cleaning one.
• Twigs, leaves or nesting material: something is building above. The flue may be partly blocked.

How the fire and the stove glass behave

A fire that is slow to catch, smolders and gives off more smoke than flame can be a wood problem before it is a chimney problem. Wet or green wood spends its heat boiling off water, so the smoke stays cool and leaves more deposit behind. EPA Burn Wise advice is to burn wood that is dry, split and seasoned, and to test pieces with a moisture meter before they go on the fire.

On a stove or insert, the door glass tells you something too. Glass that blackens soon after being cleaned points to smoky, cool fires, and cool fires are the ones that build creosote in the flue above them.

Stoves and inserts show it at the connector

On a wood stove, buildup shows first where the gases leave the firebox. With the stove cold, a light through a clean-out cap or into the flue collar shows whether the connector pipe is coated. A crust that narrows the opening, or flakes that fall when the pipe is tapped, means the flue above is collecting the same material.

The cap screen tells the same story from the other end. A screen furred with black deposit restricts the draft and marks a flue that needs brushing. Binoculars from the yard can often show it.

Signs that mean stop, not schedule

Some signs call for action now. A loud rushing or roaring sound from the chimney, popping and cracking, dense smoke or sparks from the top, or a wall beside the chimney that is hot to the touch can mean a chimney fire. Clear everyone out of the house, then call 911, and stay out until the fire department says it is safe.

After any chimney fire, the flue needs a full examination before it is used again, and NFPA 211 lists a chimney fire among the events that call for a deeper inspection. A glazed, shiny deposit is the other stop sign. Brushes cannot lift it, it is dense fuel, and removal calls for methods beyond a sweep.

What you cannot see from the room

Deposit collects heaviest in the smoke chamber and toward the top of the flue, both beyond the reach of a flashlight from below. Nests, leaves and a damaged cap are invisible from the firebox. A chimney can show none of the signs above and still need attention. NFPA 211 therefore puts the inspection on a yearly schedule instead of waiting for symptoms. The signs in this guide tell you when to call sooner. The yearly check covers the rest, including the parts of the flue no homeowner can see.

Signs that point to something other than dirt

A few symptoms look like a dirty chimney and are not. Water in the firebox after rain, rust on the damper and white deposits on the brick point to water entry, which a sweep will not fix. Smoke that spills only when a certain fan or appliance runs points to air pressure in the house. A flue that draws badly from the very first fire in a new house may be the wrong size.

When a sign does not fit the cleaning pattern, say so when you book. A visit meant to find the cause is set up differently from one meant to clean.