The Dundalk Eagle

Chim­ney fires: how they im­pact your home and how to pre­vent them

- By CON­TRIB­UTED BY ACE OF DI­A­MONDS Fires · Disasters · United States of America · Baltimore County · Baltimore · Rosedale, MD

Your chim­ney–and the flue that lines it– adds ar­chi­tec­tural in­ter­est to your home, but its’ real func­tion is to carry dan­ger­ous flue gases from your fire­place, wood stove or fur­nace safely out of your home.

As you re­lax in front of your fire­place or bask in the warmth of your wood stove, the last thing you are likely to be think­ing about is the con­di­tion of your chim­ney. How­ever, if you don’t give some thought to it be­fore you light those fall and win­ter fires, your en­joy­ment may be very short-lived.


Dirty chim­neys can cause chim­ney fires, which damage struc­tures, de­stroy homes and in­jure or kill peo­ple.

In­di­ca­tions of a chim­ney fire have been de­scribed as cre­at­ing loud crack­ing and pop­ping noise, a lot of dense smoke, and an in­tense, hot smell.

Chim­ney fires can burn ex­plo­sively – noisy and dra­matic enough to be de­tected by neigh­bors or peo­ple pass­ing by. Flames or dense smoke may shoot from the top of the chim­ney. Home­own­ers re­port be­ing star­tled by a low rum­bling sound that re­minds them of a freight train or a low fly­ing air­plane. How­ever, those are only the chim­ney fires you know about.

The Ma­jor­ity of Chim­ney Fires Go Un­de­tected

Slow-burn­ing chim­ney fires don’t get enough air or have fuel to be dra­matic or vis­i­ble and they of­ten go un­de­tected un­til a later chim­ney in­spec­tion, but, the tem­per­a­tures they reach are very high and can cause as much damage to the chim­ney struc­ture – and nearby com­bustible parts of the house – as their more spec­tac­u­lar cousins.

Cre­osote and Chim­ney Fires: What You Must Know

Fire­places and wood stoves are de­signed to safely con­tain wood-fuel fires while pro­vid­ing heat for a home. The chim­neys that serve them have the job of ex­pelling the byprod­ucts of com­bus­tion – the sub­stances pro­duced when wood burns.

Th­ese in­clude smoke, wa­ter va­por, gases, un­burned wood par­ti­cles, hy­dro­car­bon, tar fog and as­sorted min­er­als. As th­ese sub­stances exit the fire­place or wood stove and flow up into the rel­a­tively cooler chim­ney, con­den­sa­tion oc­curs.

The re­sult­ing residue that sticks to the in­ner walls of the chim­ney is called cre­osote. Cre­osote is a black or brown residue that can be crusty and flaky or tar-like, drippy and sticky or shiny and hard­ened. All forms are highly com­bustible. If it builds up in suf­fi­cient quan­ti­ties, and the in­ter­nal flue tem­per­a­ture is high enough, the re­sult could be a chim­ney fire.

Con­di­tions that en­cour­age the buildup of cre­osote in­clude re­stricted air sup­ply, un­sea­soned wood, and/or cooler than nor­mal chim­ney tem­per­a­tures.

Air sup­ply may be re­stricted by clos­ing the glass doors, by fail­ing to open the damper wide enough, and the lack of suf­fi­cient makeup air to move heated smoke up the chim­ney rapidly (the longer the smoke’s “res­i­dence time” in the flue, the more likely is it that cre­osote will form).

A wood stove’s air sup­ply can be lim­ited by clos­ing down the stove damper or air in­lets too soon or too much. Burn­ing un­sea­soned wood, because so much energy is used ini­tially just to drive off the wa­ter trapped in the cells of the logs, keeps the re­sult­ing smoke cooler, com­pared to if the sea­soned wood is used. In the case of wood stoves, over­load­ing the fire­box with wood in an at­tempt to get a longer burn time also con­trib­utes to cre­osote buildup.

The Ef­fect of a Chim­ney Fire on Your Chim­ney Ma­sonry Chim­neys

When a chim­ney fire oc­curs in a ma­sonry chim­ney, whether the flue is an older, un­lined type or tile lined to meet cur­rent safety codes, the high tem­per­a­tures at which they burn (around 2000°F) can “melt mor­tar, crack tiles, cause lin­ers to col­lapse and damage the outer ma­sonry ma­te­rial”.

Most of­ten, ther­mal shock oc­curs and tiles crack and mor­tar is dis­placed, which pro­vides a path­way for flames to reach the com­bustible wood frame of the house. This event is ex­tremely dan­ger­ous, call 911 im­me­di­ately.

fac­tory-built, metal

Pre­fab­ri­cated, chim­neys

To be in­stalled in most ju­ris­dic­tions in the United States, fac­tory built, metal chim­neys that are de­signed to vent wood burn­ing stoves or pre­fab­ri­cated metal fire­places must pass spe­cial tests. Most tests re­quire the chim­ney to with­stand flue tem­per­a­tures up to 2100°F – with­out sus­tain­ing damage. Un­der chim­ney fire con­di­tions, damage to th­ese sys­tems still may oc­cur. When pre­fab­ri­cated, fac­tory-built metal chim­neys are dam­aged by a chim­ney fire, they should no longer be used and must be re­placed.

Spe­cial Ef­fects on Wood Stoves

Wood stoves are made to con­tain hot fires. The con­nec­tor pipes that run from the stove to the chim­ney are an­other mat­ter. They can­not with­stand the high tem­per­a­tures pro­duced dur­ing a chim­ney fire and can warp, buckle and even sep­a­rate from the vi­bra­tions cre­ated by air tur­bu­lence dur­ing a fire. If dam­aged by a chim­ney fire, they must be re­placed.

Nine Signs that You’ve Had a Chim­ney Fire

Since a chim­ney, dam­aged by a chim­ney fire, can en­dan­ger a home and its’ oc­cu­pants and a chim­ney fire can oc­cur with­out any­one be­ing aware of them it’s im­por­tant to have your chim­ney reg­u­larly in­spected by a CSIA Cer­ti­fied Chim­ney Sweep. Here are the signs that a pro­fes­sional chim­ney sweep looks for to see if there has been a fire:

“Puffy” or “hon­ey­combed” cre­osote, warped metal of the damper, metal smoke cham­ber con­nec­tor pipe or fac­tory-built metal chim­ney, cracked or col­lapsed flue tiles, or tiles with large chunks miss­ing, dis­col­ored and/or dis­torted rain cap, heat-dam­aged TV an­tenna at­tached to the chim­ney, cre­osote flakes and pieces found on the roof or ground, roof­ing ma­te­rial dam­aged from hot cre­osote, cracks in ex­te­rior ma­sonry, ev­i­dence of smoke es­cap­ing through mor­tar joints of ma­sonry or tile lin­ers.

Bal­ti­more County res­i­dents who think they may have ex­pe­ri­enced a chim­ney fire, or are look­ing for other ser­vices re­lated to chim­neys or fire places can reach out to Ace of Di­a­monds— a lo­cal Rosedale busi­ness that can in­stall and main­tain wood and pel­let stoves as well as gas and wood fire­places and in­serts.

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