Masonry vs. Factory-Built Chimneys

Two different ways to build a chimney

Masonry. A mason builds the fireplace and chimney in place from brick, block or stone on its own foundation. The firebox is lined with firebrick, and the flue is usually lined with clay tile. Many older New Jersey homes, from Victorian twins to mid-century colonials, have masonry chimneys.

Factory-built. Also called prefab, manufactured or zero-clearance. The firebox is a metal box with cast refractory panels, and the chimney is a metal pipe with two or three walls. The whole system is designed, tested and listed by the manufacturer. It is often enclosed in a wood-framed chase on the outside of the house, finished with siding, stucco or a brick-look veneer. You will find them in many newer homes and additions.

Factory-built chimneys for heating appliances are tested to UL 103, which covers metal chimneys that provide a vertical path for flue gases and are installed according to NFPA 211 and building codes. Factory-built wood fireplaces are tested to UL 127. For example, one current wood-burning fireplace manual lists its unit to UL 127 and requires a specific brand of listed chimney pipe.

How to tell which one you have

Start in the living room with the fireplace cold.

1. Look at the firebox wallsMasonry fireboxes have individual firebricks with mortar joints. Factory-built units usually have large molded panels, often with a printed brick pattern, set in a metal frame.
2. Look for metalMetal edges around the opening, a metal screen track, vent louvers above or below the opening, or a metal top inside the firebox suggest factory-built.
3. Find the labelFactory-built fireplaces carry a metal data plate, often inside the firebox, on the smoke shelf area, behind a louver or on the side of the opening. It lists the maker, model and listing. Masonry fireplaces have no label.
4. Check the outsideA chimney with siding or stucco over a wood frame, topped by a flat metal panel with a round pipe and a metal cap, is almost always a factory-built chase. Solid brick or stone that continues down to the ground or foundation usually means masonry. A brick-veneer chase is possible, so this step alone is not conclusive.
5. Look at the topA round metal flue pipe with a manufactured cap points to factory-built. Square or rectangular clay tiles point to masonry.

If the clues conflict, for example a brick exterior with a metal pipe on top, the chimney may be a relined masonry chimney or a factory-built system in a masonry-veneer chase. An inspection sorts that out.

How they differ in practice

How masonry chimneys tend to fail

• Water. Cracked crowns, failed flashing and porous brick let water in. Freeze-thaw cycles then break mortar and brick.
• Mortar erosion. Joints wear out faster than brick and need repointing.
• Liner damage. Clay tiles crack from heat shock, settling or chimney fires, and joints open up.
• Firebox wear. Firebrick cracks and refractory joints erode.
• Structural movement. Settlement can cause leaning or separation from the house.

Most of these can be repaired in place. When damage is widespread, sections are rebuilt. See Repair or rebuild a chimney?

How factory-built systems tend to fail

• Rusting chase covers. The flat metal pan on top of a chase can rust, sag or hold water. Once it leaks, water runs down inside the chase onto the chimney pipe and the fireplace box. See chase covers.
• Corroded metal. A trade article by a working chimney mason describes factory-built fireboxes and flues corroded badly enough to puncture with a brush. Water from a failed chase top is a common cause.
• Cracked refractory panels. Panels crack with heat and age. The manual sets the limit on how much cracking is acceptable before replacement.
• Damage after a chimney fire. Testing by the National Bureau of Standards (now NIST) found that some factory-built chimneys buckled or developed holes after creosote fires, while masonry chimneys cracked inside but held heat in better.
• Mismatched or modified parts. Pipe from a different brand, a non-listed cap, an added insert the manual does not allow, or a removed part can all break the listing.

Why factory-built systems follow the manual

A factory-built fireplace was tested as a complete system with specific pipe, cap, clearances and accessories. The installation manual for that model is effectively its rulebook. The manual we reviewed for one current wood fireplace states that the unit and its components must not be altered or modified, that only the listed chimney may be used, that the chimney must extend a minimum distance above the roof, and that changes may void the listings.

That same manual asks owners to inspect the chimney frequently when the unit is new, at least twice a year during the heating season after that, and not to let creosote build past a thin layer. CSIA's homeowner FAQ similarly advises sweeping factory-built systems whenever any appreciable buildup appears, because acidic deposits can corrode metal parts.

For this reason we start any factory-built job by identifying the make and model, finding the data plate and checking what that system allows. If the label is missing or the unit is no longer supported, repair options may be limited.

Repair, replace or convert

Masonry. Usually repaired part by part: crown, flashing, mortar, liner, firebox, damper. Full or partial rebuilds are an option when the structure itself is failing.

Factory-built. Small, listed parts such as caps, chase covers and some panels can be replaced. When the firebox or chimney pipe is badly corroded or damaged, the realistic options are replacing the unit with a compatible listed system or converting to a different appliance that is approved for that opening. A factory-built fireplace cannot be relined like a masonry chimney unless the manufacturer allows it.

Gas conversions, inserts and replacements are decided by what the specific units are listed for. See Factory-built fireplaces: care and limits for more.

Questions homeowners ask

Sources

1. UL Standards & Engagement: UL 103, Factory-Built Chimneys (scope summary)
2. Innovative Hearth Products: Montecito wood-burning fireplace installation and operation manual (PDF)
3. Journal of Light Construction: Replacing factory-built fireplaces
4. National Bureau of Standards (NIST), NBSIR 83-2771: Intensity and duration of chimney fires in several chimneys (PDF)
5. CSIA: Homeowner FAQs

This guide is general information. It can't diagnose a specific chimney; an inspection can.