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The following article has been printed in a National magazine and may only be used for individual or association newsletters - not other National publications. .
The Smoke Chamber—A Smoking Gun?
By Marge Padgitt, CCS, and Gene Padgitt, CCS, 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 that you should be aware of. Most problems occur due to improper construction of the facial wall without correct clearances to combustibles.
To address the problem of improper construction, lets take a look at the typical diagram of a fireplace. In diagram #1, the finished firebox, chamber, chimney, flue and facial wall have been built and look fine from the outside and from inside the chimney. In diagram #2, a block wall has been built in 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 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 competant 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 his 20, 30, and 50-year old houses are going to burn down now.
I called Genevieve Bures recently to discuss smoke chambers. She said that 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 look at. 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? Probably not. How many smoke chambers do you see that are parged? Are there any holes or cracks? There may be tiny cracks that you can’t see.
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 start most often. 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 in 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. After the fire is out, the chimney continues to
A house fire originating in this area may not have anything to do with a chimney fire (see photo #1), but may be the result of heat escaping through small holes or cracks in the facial wall 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 did 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? 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, and 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. Tell them that you have new information that addresses the smoke chamber. And tell them about NFPA 211 2000 section 8-2.1.5 …”The smoke chamber height shall not be greater than the inside width of the fireplace room opening. The smoke chamber depth shall not be greater than the depth of the fireplace fire chamber. The inner surfaces of the smoke chamber shall be smooth…” (this means parged with a high-temperature refractory material). Check NFPA 211 for other building requirements. 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.
Marge and Gene Padgitt own Padgitt Chimney Services in Independence, Missouri. They have 19 years experience in the industry, and both are CSIA Certified and have several other certifications and degrees. The Padgitt’s also serve on the board of directors for the Midwest Chimney Safety Council. You may contact them at 816-461-3665 or E-mail email@example.com.
Some items to remember about smoke chambers: