Fireplace Enclosure

What’s the “rule” with regard to enclosing the space around factory built fireplace systems (code references are appreciated)? My thinking is that this should be a completely enclosed chase.

I was able to access these back-to-back fireplaces by squeezing up between the framing from a basement of sorts.

The missing firestop and inadequate clearance were obvious, although inaccessible (flat roof, no attic). What about the spacing between the flues?

They both terminated at the same height from a shared chimney.




I don’t have a code reference for you but they both appear to be triple wall “zero clearance” type flues so what added risk would result for them touching?

I agree about the need for a fire stop however.

You’ve seen Tim’s reply on the other forum I’m posting for others but if this was chased you or I would never see these defects as constucted.
That being said I stll like it when I see them chased because I see more attempts to install incorectly when they aren’t. At least the chase provides some protection from flame spread.

Unless code supercedes: Follow the manufacture instructions

1001.10 Multiple flues.
When two or more flues are located in the same chimney, masonry wythes shall be built between adjacent flue linings. The masonry wythes shall be at least 4 inches (102 mm) thick and bonded into the walls of the chimney.
Exception: When venting only one appliance, two flues may adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered at least 4 inches (102 mm).
1001.15 Chimney clearances.
Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum air space clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum air space clearance of 1 inch (25.4 mm). The air space shall not be filled, except to provide fire blocking in accordance with Section R1001.16.

  1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777 and installed in accordance with the manufacturer’s installation instructions are permitted to have combustible material in contact with their exterior surfaces.
  2. When masonry chimneys are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete wall less than 12 inches (306 mm) from the inside surface of the nearest flue lining.
  3. Exposed combustible trim and the edges of sheathing materials, such as wood siding and flooring, shall be permitted to abut the masonry chimney side walls, in accordance with Figure R1001.15 provided such combustible trim or sheathing is a minimum of 12 inches (306 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25.4 mm).
    1001.16 Chimney fireblocking.
    All spaces between chimneys and floors and ceilings through which chimneys pass shall be fireblocked with noncombustible material securely fastened in place. The fireblocking of spaces between chimneys and wood joists, beams or headers shall be to a depth of 1 inch (25.4 mm) and shall only be placed on strips of metal or metal lath laid across the spaces between combustible material and the chimney.
    1002.4 Factory-built fireplaces.
    Chimneys for use with factory-built fireplaces shall comply with the requirements of UL 127.
    M1805.1 General.
    Masonry and factory-built chimneys shall be built and installed in accordance with Sections R1001 and R1002, respectively. Flue lining for masonry chimneys shall complywithSectionR1001.8.
    M1805.2 Masonry chimney connection.
    A chimney connector shall enter a masonry chimney not less than 6 inches (152 mm) above the bottom of the chimney. Where it is not possible to locate the connector entry at least 6 inches (152 mm) above the bottom of the chimney flue, a cleanout shall be provided by installing a capped tee in the connector next to the chimney. A connector entering a masonry chimney shall extend through, but not beyond the wall and shall be flush with the inner face of the liner. Connectors, or thimbles where used, shall be firmly cemented into the masonry.
    M1805.3 Size of chimney flues.
    The effective area of a natural draft chimney flue for one appliance shall be not less than the area of the connector to the appliance. Chimney flues connected to more than one appliance shall be not less than the area of the largest connector plus 50 percent of the areas of additional chimney connectors.
    Exception: Chimney flues serving oil-fired appliances sized in accordance with NFPA 31. M1805.3.1 Size of chimney flue for solid fuel appliance.
    Except where otherwise specified in the manufacturer’s installation instructions, the cross-sectional area of a flue connected to a solid-fuel-burning appliance shall be not less than the area of the flue collar or connector, and not larger than three times the area of the flue collar.

True, but irrelevant :wink: Any required chase could potentially cover construction defects.

I’ve never seen a manufactured fireplace where the entire backside was accessible. That’s really where I’m looking for the clarification. I will be contacting a manufacturer this week and will update this thread then.

Michael - based on what was visible from the top, it appeared that these were standard B-vents, but I can’t say for sure. The missing fire-stop made it easy for me to recommend repairs, so they can make the determination if additional clearance is needed as well.

Jeff, it may be a photo effect they just looked very fat like the triple wall zero clearace type. Was the outer diameter greater than 10"?

I believe they were 14". . .

I don’t remember ever seeing any 14" B bent. Where have you seen that size B used?

I’ll see if I can find the pictures, but I believe I’ve seen 12", 14" and 16" B-vents for manufactured FP’s. I’ll try and verify this as well.

Here they are. 3" to 36" B-vents and connectors. . .

Thanks Jeff. That’s good info.

Let us all know how it turns out. You have to wonder why someone thought it was ok to have no clearance to combustables and no firestops.:weird:

I spoke with a factory rep from Superior today, who confirmed that these should be enclosed by a complete chase, with firestops at each floor level and attic.


I have a single framed chase on the exterior of my home. The common space shares 2 distinct vents. Each floor penetration is firestopped. The vents are separated by air-space, with no hard/constructed/physical separation of any type.

The chase was built at the initial time of construction, but only contaned the flue from the heating system. 2nd flue was added sometime later. Manufactured fireplace installation for the 2nd flue.

On to the odd… Today, I saw manufactured wood-burning appliances, from differing condo units, share a single, large diameter metallic chimney. The configuration is inspected by the local municipality (fire marshall) every year, after being cleaned and inspected by a certified chimney sweep.

There is some funky stuff out there…

The rep said that the shared chase was okay, but I should not have been able to access it as I did. The chase should have been sealed. Apparently, combustion vents are allowed, but they’re supposed to be piped to the firebox.

The clearance and firestop were obvious blunders, but the terminations should have been separated by at least two feet - vertically or horizontally.

Do you feel comfortable calling out the need for a chase without a code to back it up? I’d hesitate to go on what one manufacturer says, because you won’t always know who the manufacturer is. I’ve just skimmed this thread, but I see no definitive code reference for a chase cited.

The “code” says that manufactured fireplace systems are to be installed according to the manufacturers instructions, so yes, I feel comfortable calling out the need for a chase/enclosure.

In this case, an “expert” was needed regardless of the enclosure. Based on the installation, there may be even more problems that I am unaware of.

Did the rep you spoke to specify the clearances to cumbustables and to the other vent?

I did not know the brand of the FP, so most of his comments were general in nature.

Clearance can be as little as 3/4" in some cases, but he was unaware of any “zero-clearance” allowances for vents. He also said there should be at least 1" between vents, however, their terminations should be separated by at least two feet.

Jeff and all who read this thread,

I have misused the term zero clearance.

That term should only refer to the clearance to cumbustibles of the prefabricated metal firebox assembly and not the flue vent. The triple wall flue I mentioned does have clearance to combustibles requirements of 2" and it also requires firestops between floors and insulation barriers if it passes through insulation.

Thanks for the information you passed along.:o

That is correct. Sides, back and bottom will often have a “zero” clearance allowance. The top is generally 16" or more.

My understanding is a chase (vertical fire-stop) has been mandated since 1992, and I alway call it out if it doesn’t have one. Dale Feb of D-Way fireplace Inspection Service is a recognized authority on prefacricated fireplaces.