Can fireplaces start a house fire?
Yes, fireplaces are capable of starting a house fire! Many people don't realize the possible dangers fireplaces pose. These dangers can be caused by such things as lack of maintenance or incorrect installation. According to a 2008 study by the National Fire Protection Agency, 24,300 fireplace fires in the US caused $246,000,000 in damage. Of those, only 23% were caused by creosote buildup in the chimneys which means most of the fires in the 2008 study started from other causes.
Fireplace types vary and include wood burning fireplaces, gas burning fireplaces, and pellet stoves. Most fireplaces installed today have metal inserts that attach to a metal chimney. The metal remains behind the nice brick or stone facade and the outside chimney chase. Older fireplaces have a clay liner.
Chimneys vary by the type of fireplace. Gas fireplaces use different venting than wood burning units. Wood fireplaces burn much hotter than gas units, reaching 2,000 degrees. This level of heat can ignite other combustible material located near the fireplace.
How do fireplaces fail?
Creosote is a black, tar-like material that collects in the chimney flue. This buildup is highly combustible and can be ignited, causing a chimney fire. This condition can be prevented by having your chimney professionally and regularly cleaned.
Clay flue liners are susceptible to cracks. When cracks occur, hot gases can escape into the fireplace chase or into the home, sometimes causing carbon monoxide to enter, as well. These gases also may cause nearby framing members to ignite. Gas entry and ignition can be prevented by having your fireplace inspected and cleaned by a certified inspector. The Consumer Product Safety Commission website provides excellent education on chimney failures.
In addition to cleaning the unit, proper fireplace maintenance is also mandatory. Hot gases must be able to travel up and out. Gaps in a system allow hot gases to get into the chase or the home which can cause carbon monoxide entry or fire. If a fireplace insert is available, the metal box is meant to fit up against the brick or stone fascia and hearth. The connection point should also contain refractory cement which prevents heat from getting into the space between the insert and the chase. The same cement material is used for wood burning fireplaces with gas igniters. Over time, this cement can crack and may even fall out. Cracking and gapping issues would be discovered in regular inspections.
Many substantial fires (and fires to brand new homes) are caused because of incorrect installation, incorrect clearances, improper venting, incorrect rough in of the surround chase, and insulation. The National Fire Protection Association, the manufacturer of the fireplace, and the venting manufacturer all have certain guidelines for proper installation and all of them must be followed. For instance, improper installation can occur if the chimney chase is left open in the attic and the insulators blow in cellulose insulation. The insulation then travels down and the chimney chase traps the heat around the fireplace insert. The trapped heat near the insert causes the wood and other combustible material to start a fire.
Negative Pressure and Carbon Monoxide
Many people aren't aware of the dangers caused by negative pressure and carbon monoxide. Unfortunately, they can be life threatening.
Many homeowners seem to want to keep their homes as airtight as possible; they believe doing so will reduce their heating bills. But oxygen is needed by more than just the people living in the home. Cooking devices, fireplaces, water heaters, and furnaces, just to name a few, also need oxygen. When a home is airtight, all of these devices fight for oxygen. Adding to this issue are bathroom fans that remove 60 square feet of air per minute from the home. Many homes have more than one bathroom fan, and the strongest fans take out the air, pulling air even from the weaker fans and other devices. If a stronger bathroom fan begins to pull oxygen from the fireplace, carbon monoxide can also be pulled into the living area. If the fireplace is pulling the oxygen, carbon monoxide can be pulled from the water heater. This "pulling" is negative pressure and it's dangerous because carbon monoxide is odorless, colorless, and deadly.
Negative pressure can be fixed by installing an air exchanger to equalize the pressure. Contact a qualified heating contractor to review pressure issues in your home and to give you more information about preventing a potentially-deadly situation.
So have your fireplace checked each year and be sure to have it cleaned each year if you burn sappy wood. We want you to stay safe this winter by keeping fires where they should be: In the fireplace!