Unlined flue on an older chimney

You often can’t tell whether a flue is lined just by looking up from the fireplace. An inspection with a camera or light is how most people find out, often during a home purchase or after changing an appliance.

1Why an unlined flue is a problem

• Heat transfer. Without a liner, heat from a fire passes more easily through the masonry toward nearby wood framing.
• Gas leakage. Mortar joints crack and erode over time. Combustion gases, including carbon monoxide, can escape through them into the house.
• Corrosion. Flue gases contain moisture and acids that eat away at mortar from the inside, especially with gas appliances.
• Wrong size. An unlined flue is often far larger than a modern appliance needs, which hurts the draft.

2Signs to watch for

1. Ask what the inspection foundA camera inspection shows whether there’s a liner and what condition it’s in. Ask for the photos.
2. Look in the fireboxCrumbling mortar, bits of masonry or sand falling down can mean the flue is deteriorating.
3. Notice smoke or odorsSmoke in the room, or smells when the appliance runs, mean it isn’t venting properly.
4. Check your CO alarmsMake sure you have working carbon monoxide alarms, and test them.

3Fixing it

4What drives the cost

Relining costs depend on the chimney’s height, the liner material, the appliance it serves, whether the flue needs to be enlarged or cleaned out first, and roof access. Ask for the liner type, size and any insulation in writing, and why they suit your appliance.

Chimney cost

6Related chimney problems

• No cap on the flue
• Staining on the ceiling around the chimney
• Cracked crown at the top
• Creosote build-up in a wood-burning flue
• Carbon monoxide alarm sounding
• Spalling brick on the exterior stack
• Gas appliance venting into an old flue

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