FIREBOXES, SMOKE CHAMBERS & CHIMNEY CONSTRUCTION

Inspecting Smoke Chambers
About 60% of house fires that occur around chimneys originate in this area.

Source: MASONRY CONSTRUCTION MAGAZINE
Publication date: July 1, 2007
By Marge Padgitt, CCS, NFI

A Level II chimney inspection is performed at the time of any cleaning, for real estate or insurance purposes, or as an annual check requested by a homeowner. A fire investigation is completed after a structural house fire has occurred, and is usually requested by an insurance or forensic company. This inspection could be considered Level III, but goes much further than that and is called a C&O (cause and origin) by the insurance and forensic companies.

My husband, Gene, is a State Certified Fire Investigator and does the onsite investigation. I write all the reports, so we both get to see what happens in the real world after there has been a house fire. Some fires only cause monetary losses to the structure, but others result in injury or death to the occupants.

Reasons for fire
Most house fires related to chimneys are due to an installation error, or problems not discovered by someone doing an inspection, maintenance, or repair work. Rest assured, the last person doing anything to the chimney is the first person the insurance company goes after. Then everyone else from the builder, to the supplier, to the manufacturer, might be involved. In our experience, about 60% of house fires that occur around chimneys happen at the smoke chamber area.

A hole in a chamber wall (noted by white tape on a wire pushed through from the other side) shows where heat escaped and ignited a header on the back side. The smoke chamber is the transition area in a masonry chimney that starts just above the damper and continues to where the first flue tile is installed. It serves as the support for a clay tile flue liner. Chambers are usually corbeled brick or block work, starting wide at the base and narrowing down to accommodate the flue liner. Most chambers are 3 ft - 4 ft in height, but can be shorter or taller and out of proportion.

Most chamber problems occur due to improper construction behind the facial wall and in front of the chamber. The finished firebox, chamber, chimney, flue, and facial wall may look fine from both the outside and inside the chimney. The facial wall is not a structural part of the chimney, but a decorative front that is independent of the chimney (if it was built correctly).

The area behind the facial wall cannot be seen during a normal inspection. A wood header and studs may be installed incorrectly, with inadequate clearance to the chimney. Some builders do not follow clearance requirements, and place the combustible wood framing right next to the smoke chamber. Other combustibles that may be found in this area are plywood, paper-covered sheet rock, or insulation.

Pyrolization is the chemical change that occurs to combustibles as they are exposed to heat over time. Any wood in the vicinity of the chimney may be exposed to heat. Pyrolized wood can ignite at 200° F - 225° F (or possibly lower) instead of its normal ignition temperature of 450° F - 500° F.

It is important to note that only heat is required to ignite wood or other combustibles, not a flame. We have seen cases where the wood headers ignited when exposed to heat from gaps in this area, even though they were more than 2 in. from the chamber.

There could be voids in the smoke chamber that are unseen by the human eye, but that can allow smoke and heat to transfer through the masonry. Also, as the masonry warms up during a high-heat event, cracks can open even further. It is usually safe to assume the worst, even if you can't see it.

Chamber inspection
The smoke chamber may need to be swept or cleaned to remove creosote before being inspected. Any glazed creosote must be removed by power cleaning. Use a chimney inspection camera because the lighting will be better and you can see angles not visible from below the chamber with the eye. The chamber is probably not parge coated smooth, as required by NFPA 211 standards. I hardly ever see parged smoke chambers in original construction.

Older NFPA 211 standards stated (under 8-2.1.5, 2000 Edition): “…The inner surfaces of the smoke chamber shall be smooth...” which was rather vague and confusing and cause for a lot of debate on the subject. A change to the 2006 addition now reads (under section 11.2.1.13): “The inner surfaces of the smoke chamber shall be parge coated smooth, with an insulating refractory mortar…” In the widely used 2003 International Residential Code, it states (under section R1003.8): “…When the inside surface of the smoke chamber is formed by corbeled masonry, the inside surface shall be parged smooth.”

Note that even though a code requirement, if the building inspector passed the construction of the house, the homeowner may have no legal recourse against the builder if it was not parged.

The reason for parging is to fill any gaps in the masonry and provide a smooth transition for smoke. Open corbeled brick are a great place for tar droplets to get caught. Look for holes, cracks, and mortar bond breaks where heat and smoke can escape.

Like an oven
The smoke chamber is a hot oven where much of the creosote accumulates and many chimney fires start. If inspecting after a chimney fire, you can bet that if there is a stove “slammed” into the firebox without (at least) a direct connection to the first flue tile or a properly sized liner, there may have been a lot of highly flammable glazed creosote in this area, which probably burned first and then set the creosote in the flue on fire after that.

Testing shows that chimney fires can reach well over 2100° F. After the fire is out, the chimney continues to heat up. Expansion due to excessive heat causes cracks and mortar bond breaks, surface spalling of the brick, and sometimes in severe fires, cracks occur through the brick or block. Most of this type of cracking in the chase occurs in the chamber area and at the top of the chimney.

A house fire originating in this area may not have anything to do with a chimney fire, but may be the result of heat escaping through small holes or cracks in the chamber walls from normal use of the fireplace. After pyrolization, the wood may ignite and can smolder for hours, or even days, until it gets enough oxygen to burn. As soon as the area burns through to the attic or an upper floor where oxygen is plentiful, it then becomes a free-burning fire.

Inspection reports
What should be written on the inspection report if you find visible gaps or unparged brick? Here is a sample: “The smoke chamber may have been damaged by the intense heat of a chimney fire, and should be cleaned and parged with high-temperature insulating mortar to seal it,” or “There may be a hidden combustible wall in front of this area, so it is important to parge coat the smoke chamber.” Word the findings according to what you discover or suspect.

I always recommend parging if the chamber is unparged or only partially parged.

A complete tear down and rebuilding of the chimney is recommended if the chamber was not constructed correctly, and is over-sized, under-sized, or not corbeled correctly (check NFPA for the requirements), has restrictions, or is badly damaged. Rebuilding is the only way to correct the problem in some cases.

On the report, mention NFPA 211 standards and I-Code requirements, even if they are not a part of the local codes. In court, you may be asked: “What is the standard followed in your industry?” Even if it is not a local code requirement, the chimney professional will have to answer that it is the NFPA 211. It is prudent to make recommendations based on this standard.

Make sure the homeowner or buyer understands the severity of the problem, and list parging as a repair item that needs to be addressed when evaluating chimneys (even if no chimney fire or other damage has occurred). Parging offers more heat protection and is a lot less expensive than tearing down the facial wall, rebuilding the interior, and rebuilding the facial wall. It is not the answer to all problems, but parging helps keep heat inside the chamber and may avoid a house fire.

It is better to make strong statements about your opinion on the condition of the chimney than worry about what someone may say about ou. Should a problem ever occur, you have evidence to back up the statements.

Look for obvious damages due to movement of the chimney from piering or settling, lightning strike, chimney fire, or explosion. Check for soft, blown out, or missing pieces of mortar, and gaps between the brick, block, or stones. If the chamber is parged, look for cracks in the masonry of the back wall of the exterior chimney, or the front facial wall at the level of the smoke chamber area. This area indicates expansion, probably due to excessive sudden heat, and is a good indication that there is smoke chamber damage as well.

Check for smoke chamber size requirements. A chamber too small may cause drafting problems. A chamber too large takes up space where the protection of a flue liner should be. Anything outside of the norm should be mentioned in the report.

Size requirements, according to the I-Codes are: “The inside height of the smoke chamber from the fireplace throat to the beginning of the flue shall not be greater than the inside width of the fireplace opening.” The chamber cannot be inclined more than 45 deg from vertical, according to the NFPA 211, and not more than 30 deg from vertical, according to the I-Codes if corbeled masonry is used.

The smoke chamber thickness requirement is 8 in. of solid masonry on the front, back, and side walls, unless the chamber is lined with 5/8-in. thick vitrified clay lining or 2-in. thick firebrick. The area is difficult to access unless there is an open spot where you can view the thickness. The height cannot be greater than the inside width of the fireplace opening, and the depth should not be greater than the depth of the firebox. Any deviation on size may not be cause for alarm or reason for reconstruction, unless the fireplace smokes.

If the chimney has been relined, check to see if the liner is sealed to the smoke chamber at the base with a bottom plate and/or insulating refractory mortar. If the flue liner extends into the chamber, it should be taken out or cut out. Check to be sure the liner has not fallen down into the chamber first, which happens when it is not supported correctly.

If a wood stove insert has been installed in the fireplace, there should always be a stainless steel transition piece connecting the stove to a flue liner that goes all the way from the stove to the top of the chimney (positive connection), or at least to the bottom of the flue liner (direct connection). NFPA and I-Codes still say it is ok to put the transition piece from the stove to just where the flue liner starts.

I hope this situation changes soon to require full relining to allow for better drafting and performance of the appliance. If there is no transition piece, flue gasses have a tendency to accumulate in the smoke chamber area and cause an excessive amount of third-stage glazed creosote to form in the chamber, which may drip down into the firebox and on top of the stove. With this type of installation, the risk of a chimney fire is greater. Glazed creosote can be identified by its black, hard, shiny, tar-like appearance. It is extremely flammable.

See if there is any burnt creosote in the smoke chamber or flue. Honeycomb or ash creosote is very lightweight, has a honeycomb appearance, and is grey or black in color. It results when glazed creosote is burned, so only occurs during a chimney fire and is proof positive that the chimney has suffered a fire, so you should be on the lookout for smoke chamber damages.

Also look for nearby flues serving other appliances that may be improperly attached to the smoke chamber. Each appliance must have its own flue and cannot share a chamber or flue serving another appliance. This requirement by NFPA 211 Standards and the International Residential Code keeps toxic flue gasses contained so they don't enter the living space.

During a normal Level I visual or Level II inspection of a chimney, demolition work will not be required in order to see what is behind the facial wall or other walls around a chimney. Since you will not know what is behind the facial wall, assume the worst. Protect yourself by putting a clause in the inspection report that states what you did not have access to and did not inspect which may not save you from litigation, but could help your case if a problem arises.

There are some occasions when chimney problems may be out of your level of expertise. If you suspect structural or foundation problems, refer the client to a structural or consulting engineer for further evaluation. This may be required if the chimney has been exposed to an explosion, has been moved by piering, or experienced ground settling. Just state that you recommend further evaluation by someone else in your report.

PHOTOS

Looking up inside a smoke chamber which is parged with mortar. The Clay Flue starts above the smoke chamber.

Clay Flue set on a re-enforced poured solid concrete smoke chamber.

Re-enforced concerete smoke chamber. Clay Flue lined construction.

Firebox and Smoke Chamber built out of fire brick. This firebox was built for the Kenya Hotel in Japan.
The fireplace has openings on both sides. Each opening is 5' X 5'.

Firebrick firebox using clay re-factory brick. Fireclay is used to bond bricks together re-factory mortar joint are very tight approximately 1/16 of a inch. There are many different shapes and sizes depending on what the customer wants

A existing firebox has been removed and is being rebuilt. It is very important to have a strong firebox if the brick are loose the backwall could collapse and would be a fire hazard.

Chimney stack showing 2 clay flues divided by a brick wall to prevent smoke from going from one flue to the other.

Installed stainless steel liner in chimney flue. Size was too big causing sever drown drafting, blowing soot and gas fumes through out the residence. By installing liner pilot light was able to change direction of venting, venting fumes out the chimney. Two 4inch fresh air vents were also brought into the flue beside the firebox underneath the firebox floor bringing fresh air from out side into the firebox.

Below...
Existing fireplace has been torn out and removed the old bricks are cleaned and reconstruction of the fireplace is under way. An aluminium gas liner is installed to the furnace and hot water tank and runs up through the roof. The fireplace opening is made without a firebox. A fireplace insert is being installed with a 6 inch stainless steel liner hooked up to the unit.