The Problem with Fireplace Inserts
Do any of these look familiar?
Do you have something that looks like this in your fireplace?
If you do, the chances of it being installed to the proper standards are slim! Most inserts are simply pushed into the fireplace opening. The stove then depends on the fireplace flue to vent the by-products of combustion (smoke) to the outside atmosphere. The only protection you have to keep deadly gases from entering your home is the stove surround. Needless to say, this is not a safe thing!
In this industry we call them "slammers" and they are very hazardous!
Back in the late 70's and early 80's, when we were going through yet another "energy crisis", where oil/gas prices were up!
Everyone, including me, thought, "put an insert into the fireplace" and eureka! "Cheap heat!"
It was not until the number of chimney fires that had inserts installed started to rise dramatically that the practice of "slamming" an insert into a fireplace was more closely investigated. It was determined by such entities as The National Fire Protection Association (NFPA), The Wood Heating Alliance and The National Chimney Sweep Guild that there was a very big problem with having such a setup.
What is the problem?
In a nutshell, the flue of the fireplace was to big to vent a stove properly! What! How can that be, bigger is better, right? Wrong! When it comes to venting technology, bigger is not better, in fact it can be deadly!
A wood stove insert is designed to vent through an exhaust 6'' (typical) which is 28 square inches.
A fireplace is designed with a flue that, at a minimum, is 13" x 13" OD (outer diameter).
A 13" x 13" OD is actually 11" x 11" id (inside diameter). 11" x 11" = 121 square inches. This is 4 times (appx.) larger than what the stove was designed to vent through.
This sizing difference leads to several issues:
1)Temperature differential. The "hot" exhaust gases, normally 600-800 degrees, venting through a "cold" chimney and flue. The chimney is going to be whatever the temperature is outside and it must be cold or you wouldn't be building a fire!
First, lets talk about creosote. Creosote is a natural by-product of burning wood. It can be defined as a combustible deposit that originates as condensed wood smoke, including tar, vapors and other organic compounds. Tar droplets, called "tar fog", is the major component of smoke. Creosote accumulates on the walls of the flue as the smoke is making its way to the outside atmosphere. Smoke, as is natures law, travels in a spiral. The flue (normally) is square or rectangular in shape, therefore smoke collides with the walls of the flue on its journey upward. This is called "resistance to flow". Anywhere that the hot smoke touches the cold flue it will condense and stick to the surface. It usually condenses into a gooey, gummy, dripping MESS. This gooey, gummy, dripping mess is the fuel of a chimney fire. Depending on conditions of use, you can get into a hazardous situation in as little as 5 weeks. This is why it is important to not just have the correctly sized vent, but the vent should be insulated! When a exhaust pipe is insulated, there is little temperature variation from the stove to the outside atmosphere. When there is little temperature variation, the creosote accumulation is greatly reduced.
2) Smoke Residence Time. This is how long the smoke is in the flue. Ideally, The shortest amount of time the smoke spends in the flue is best. This can only be accomplished by using the properly sized and insulated vent pipe that the stove was designed, built and tested for. When using the correctly sized and insulated pipe, the residence time is far shorter than when using the original fireplace system.
Are You Covered By Insurance?
A major consideration is your home owners insurance coverage. I am not aware of any insurance company that will cover a fireplace insert that is not installed to NFPA 211 Standards. If you have an insert already installed, I would encourage you to talk to your agent or check your policy! Do not get burned, literally!
For more information go to:
A fireplace insert will have a properly sized and insulated liner with a "direct connection" from the stove, through the chimney and exhausting to the outside atmosphere.