Zero-Clearance vs Masonry Fireplace: How to Tell

What zero clearance means

Zero clearance refers to the way these units are built. A factory-built fireplace has insulated or air-cooled metal walls, so its outer case can be installed with very little space to the framing around it, as long as the clearances in its installation instructions are followed. That makes it lighter and easier to install than a masonry fireplace, which needs a heavy foundation and noncombustible surroundings.

It is also called a prefab or factory-built fireplace. The unit and its chimney pipe are tested and listed together as a system, so the parts are meant to be used together. Outside, the pipe usually runs inside a framed chase covered with siding, topped with a metal chase cover and a cap.

Where you find each kind

Most older houses in our service area have masonry fireplaces, from coal-era fireboxes in brownstones to large brick fireplaces in colonials. Factory-built units are common in houses and additions from the later decades of the twentieth century onward, in condos and townhouse developments, in finished basements, and in renovations where a fireplace was added without building a masonry chimney.

Some houses have both: a masonry fireplace in the original living room and a factory-built unit in a family-room addition.

How to tell which one you have

A flashlight and a quick look inside the firebox, with the fire out and cold, usually answers it. The signs below usually agree with each other; if they do not, the rating plate settles it.

• Firebox walls: large flat panels, often with a printed brick pattern, point to factory-built; individual firebricks set in mortar point to masonry
• Metal edges: a metal frame around the opening and louvers or vent grilles above or below it suggest factory-built
• Flue: a round metal pipe above the damper means factory-built; a square or rectangular clay tile means masonry
• Outside: a chimney boxed in siding or stucco on a newer house is often a chase over a metal pipe
• A label: factory-built units usually carry a metal rating plate with the model, often inside the opening or behind the lower grille

Side by side

The two types differ in how they are built, how they are repaired and what can be added to them.

Why the difference matters for repairs

Repairs follow different rules. A masonry fireplace is repaired with masonry: firebrick, refractory mortar, a new damper, a liner. A factory-built fireplace is repaired with the manufacturer's listed parts, such as replacement panels, a matching damper, or a section of the same brand and model of chimney pipe. Mixing in parts that are not listed for that unit can void its listing and create a fire hazard, because the clearances to framing were set for the original system.

The two also wear out differently. Masonry fireplaces can last a very long time with upkeep. Factory-built units have metal parts that rust, warp and burn out, and when key parts are no longer made, the whole unit may need replacing rather than repairing.

Common problems with factory-built units

The most frequent issues are cracked refractory panels, which expose the metal firebox behind them; rust on the chase cover on top, which collects water and drips it down the pipe and into the firebox; separated or corroded pipe joints; and missing or damaged parts such as the damper or the glass doors listed for the unit. Fires that are too large for the firebox, or burning materials the manual prohibits, speed all of this up.

Water stains inside the firebox of a prefab unit very often trace back to the chase cover, not to a roof leak.

Another sign worth knowing: a factory-built firebox that is too hot to touch around the outer louvers, or scorch marks on the wall or mantel above, points to overfiring or blocked cooling air passages. Stop using it until it has been checked.

When a factory-built unit is worn out

A factory-built fireplace reaches the end of its life when the firebox metal is warped or burned through, panels can no longer be sourced, or the chimney pipe is corroded and its matching parts are out of production. At that point the safe options are to retire the unit, replace it with a new listed fireplace and chimney system, or, where the existing chimney system allows, fit a listed insert or gas appliance with its own liner.

Replacement means removing the old unit and its pipe, often opening the wall and chase, and installing the new system to its own instructions, including clearances and framing. It is a bigger job than a masonry repair, but it gives you a fireplace with a known listing and available parts.

Gas fireplaces are worth mentioning here too. Most direct-vent gas fireplaces are themselves factory-built units with their own listed venting, and they follow the same rule: repairs and parts come from the manufacturer's instructions, and the glass and vent seals are part of the safety system.

Inserts in a zero-clearance fireplace

Many people want to add a wood or gas insert to make the fireplace heat better. In a masonry fireplace, that is common. In a factory-built fireplace, an insert can only go in if the insert's manufacturer lists it for use in that type of unit, and installation has to follow both sets of instructions. Some factory-built fireplaces are not approved for inserts at all.

Read the rating plate and the manuals, or have a technician check them, before buying an insert for a zero-clearance unit.

Upkeep for both kinds

Both kinds need a yearly inspection and a sweep when deposits build up, as NFPA 211 advises. On a factory-built unit, the inspection pays close attention to the refractory panels, the metal firebox, the pipe joints and the chase cover and cap on top. On masonry, it covers the firebrick, the mortar, the damper, the liner and the chimney above the roof. Keep the manual for a factory-built unit with the house papers; it is the reference for every repair.