The Firebox

What to look at

Firebrick condition: cracked, spalled, missing or loose brick, which is a repair rather than a cosmetic matter.

The mortar joints between the firebrick, which should be tight — open joints let heat through to the structure behind.

The back wall and the throat, which take the most heat.

The damper: does it open fully, close properly, and is it rusted or jammed.

The smoke shelf behind the damper, for debris, mortar and nesting material.

And the smoke chamber above, which should be smooth-sided and frequently is not.

Open joints are not cosmetic

The firebox is the only thing between a fire and the combustible structure behind it. Open mortar joints and missing firebrick let heat and flame reach that structure. It is a repair rather than a tidy-up, and a sweep who flags it is not upselling.

What the damage tells you

Water staining on the firebox back or floor: water coming down the flue, which means the crown or the cap.

A rusted damper: the same, over a longer period.

Debris in the firebox: tiles or mortar failing above, which is a liner question.

White efflorescence inside the firebox: water moving through the masonry.

Heavy soot on the firebox face above the opening: smoke spilling into the room, which is a draft problem.

And cracked or heat-damaged firebrick with a history of very hot fires, which also suggests looking at the flue for creosote and for heat damage.

What cleaning involves and does not

Ash removed, with the understanding that a thin layer of ash in a wood fireplace is useful rather than a problem.

The firebox brushed down and vacuumed, including the throat and the smoke shelf.

Glass doors cleaned, and the gasket checked on a sealed appliance.

The damper cleaned and operated.

Not pressure washing or wet cleaning the firebox, which drives water into the masonry.

And not painting or coating the firebox interior with anything not rated for it, which is a thing people do and should not.