When I first came to look at the chimneys on This Old House TV's Charlestown project, series producer Bruce Irving told me that there were four fireplaces with beautiful mantels, but was unsure of their condition. Little did Bruce or the homeowners know that these fireplaces were actually fire hazards. Whenever I look at a job, I examine not only the chimney but the entire chimney system. This means checking the distance between combustibles—such as wood framing or mantels—and wood or coal stoves, boilers furnaces, fireplaces and smoke pipes in addition to inspecting the actual chimney for proper height, size, lining and clearance from combustibles. I also check the sides, rear and underside of the firebox as well as any hearth extensions for proper size and clearances. A certified chimney professional should both clean and evaluate your whole chimney system once a year. In most of the United States, no license or certification is required, so homeowners should ask their sweep for an up-to-date certificate from the Chimney Safety Institute of America, which requires sweeps to pass a written examination based on fire codes, clearances and standards for chimney construction and maintenance. In addition you should call references and ask for proof that the chimney sweep is fully insured. Many municipalities actually have codes that are more stringent than the national fire code, so it is important that your sweep also have knowledge of local codes. Today the predominant lining of masonry chimneys is terra cotta tile. Prior to the 1920s, chimneys were typically unlined, consisting of brick or stone and soft mortars, with their wood framing in direct contact with the chimney. The most commonly burned fuel was coal, which created an acid rich environment within the chimney that actually ate away at the unprotected brick and mortar. This combination of acids, exposed wood framing and lack of lining is recipe for a house fire. In 1994, 32,500 chimney fires in the United States resulted in 70 deaths, and $181.8 million worth of damage. Virtually all of these fires were preventable with proper yearly inspections. Burning wood produces a sticky, tar-like flammable substance called creosote that can ignite and send flames up the flue, raising temperatures as high as 2,300 degrees—hot enough to melt mortar, crack flue tiles and char or burn nearby studs and rafters. Creosote comes in three different strengths: first-degree creosote is a fluffy gray dust; second-degree is denser and looks like blackened popcorn; and third degree creosote is crystallized and hard like rock candy. Most chimney professionals now have a closed circuit TV camera that can be inserted up the chimney for a closer look at creosote buildup. Although this service costs a little more, it usually reveals otherwise hidden problems and also allows homeowners a first-hand look at the culprit. The four fireplaces in the Charlestown house figured prominently in the renovation plans. If they could get them working, the homeowners could add over $20,000 in value to their home. But this was not as simple as it sounded. The chimneys were unlined, originally used to feed hot air from the basement furnace into the rest of the house. In the first floor dining room, the tiled hearth and gas log were added around the turn of the century. In the second floor dining room, the fireplace was gradually built up around an outlet for a wood stove pipe. In the second floor living room, currently outfitted with a gas-fired log, I found charred wood in the framing around the chimney from the heat that the logs threw off. Because these fireplaces weren't originally used for wood burning, they would be complicated—and expensive—to get fired up. I recommended that the homeowners focus on the two living room fireplaces: relining the chimneys, building masonry fireboxes and moving the wood a safe distance from the hearths. All this rehabilitation would cost nearly $20,000—the very cost they could recoup whenever they sell the house. This news was more than they had bargained for; most of the hazards were hidden within the walls behind the decorative fireplaces they fell in love with. But these chimneys were a classic case of what you see isn't always what you get.