Is the Smoke Chamber a Smoking Gun?

Originally published in SWEEPING magazine and MASONRY CONSTRUCTION magazine.

By Marge Padgitt and Gene Padgitt, C.F.I.

Smoke chambers are a hot issue-literally as well as figuratively. As a Certified State Fire Investigator, Gene sees quite a few house fires caused by problems in the smoke chamber area. You should be aware of these problems. Most problems occur due to improper construction of the facial wall without correct clearances to combustibles, but many chimney fires start in this area as well.

To address the problem of improper construction, let’s discuss a typical fireplace. The finished firebox, chamber, chimney, flue and facial wall have been built and look fine from the outside and from inside the chimney. Sometimes a block wall has been built between the chamber and the stud wall. Sometimes a block wall is also the front wall of the smoke chamber. Is it filled in with mortar for a solid masonry wall? What is the clearance to the combustible stud wall? We don't know because we can't see this area during a normal Level I or II inspection. In front of the smoke chamber, the wood header and studs are installed and ready for the finishing work. This is where many problems arise. Many builders do NOT follow clearance requirements and place the combustible wood wall right next to the bricks in front of the smoke chamber! And what else might there be in this area? Plywood, pressboard, or insulation? Insulation acts to keep heat in, and will help to keep the heat in this area even better, which is what we don't want!

What about pyrolization of combustible framing or finish work around the chimney? Pyrolization is the chemical change of wood due to exposure of heat over time. Wood may then ignite at 200-225° instead of the normal ignition temperature of 450-500°. It is important to note here that only HEAT is required to ignite the wood, not FLAME. Have you ever placed your hand on a facial wall after the fireplace has been in use for while? Hot, isn't it? And by the way, without X-ray vision or a tiny camera, no one can see behind the brick facial wall that is built in front of the stud wall. It may not be constructed of solid masonry, as many would have you believe. Yes, there are many competent masons and builders who build this area properly, but there are also many who do not. Some older masons have told me, "I've been doing this for 50 years and don't plan to change now." I wonder how many of their 20, 30, and 50-year old houses are in danger of a potentially destructive fire.

I spoke with Genevieve Bures, a State Fire Investigator and operator of Bures Consultants in Rocky River, Ohio, to discuss smoke chambers. She said the smoke chamber is the most vulnerable area of the chimney. Gene and I agree. The chamber must be examined at every inspection and after any damage has occurred to the chimney. In fact, it is our opinion that this is the most important part of the chimney to examine. If you suspect that the chimney has suffered a chimney fire, (note that I said Chimney fire, not Flue fire, because a chimney fire does not always contain itself to the flue) clean the smoke chamber and take a good hard look at it. Is it parged with high-temperature refractory mortar as is required by NFPA 211 and IRC Code? Probably not. How many smoke chambers do you see that are parged? Are there any holes or cracks? There may even be tiny cracks that you can't see. Rembember, even though a masonry chimney looks solid, it is never completely solid.

During a chimney fire, the smoke chamber is a hot oven. This is the area where most of the creosote accumulates, and logically, where fires most often start. So, it burns the hottest. And you can bet that if there is a stove "slammed" into the firebox without a direct connection to the first flue tile or a properly sized liner, there was a lot of highly flammable glazed creosote in this area that burned first, and set the creosote in the flue on fire second. How hot does it get? Testing in our industry shows that chimney fires can reach well over 2100 degrees, and may get as hot as 3000 degrees, as in a test the Midwest Chimney Safety Council did several years ago. And after the fire is out, the chimney continues to heat up!

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 smoke chamber next to the facial wall or just heat transferring to wood on the otherside without proper clearance. from normal use of the fireplace. After pyrolization, the wood may ignite and can smolder for hours or even days until it can get 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. The homeowner may not be aware that there is a problem until it is too late. Smoke alarms may not go off because the smoke is kept inside the walls. We performed an investigation recently where the fire department was called out three times over a 24-hour period because the homeowner smelled smoke, but the source could not be located. Finally, the roof ignited and burned their $300,000 home to the ground.

So, what should you write on the inspection report or insurance evaluation in these situations? Here is a sample: "The smoke chamber may have been damaged by the intense heat of the chimney fire, and should be cleaned and parged with high-temperature refractory mortar to seal it. There may be a hidden combustible wall in front of this area, so it is important to repair the smoke chamber." That is a sample only-you will have to word your findings according to what you discover or suspect. However, I ALWAYS recommend parging in order to seal the chamber (we use Chamber Tech 2000) as required by NFPA 211. But if the chamber was not constructed correctly, or is over-sized, under-sized, or not corbelled correctly (take a look at NFPA for the requirements), or if it is badly damaged, we recommend complete tear down and rebuilding of the chimney. That is the only way to correct the problem in some cases. If you don't build chimneys, get together with a good mason and refer jobs to each other or hire him/her out as a subcontractor.

This may require some re-thinking when bidding on a reline due to a chimney fire. It may also require some education of your local insurance adjusters, who may believe you are just trying to get a few more bucks for extra work. Start using the term "Chimney fire" instead of "Flue fire" if you don't already, make sure the homeowner 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 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 will help keep heat inside the chamber and provide a smooth passage for smoke and flue gasses

Crack in a smoke chamber

Partially parged chamber

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Marge and Gene Padgitt own HearthMasters, Inc. dba Padgitt Chimney & Fireplace in Independence,
Missouri. both are CSIA Certified®, NFI Certified and have several other certifications. Gene is a State Certified Fire Investigator. You may contact them at 816-461-3665 or email mpadgitt@comcast.net