A wood burning stove needs a chimney to allow the smoke and gasses to go up and exit the house in a safe and approved manner. Most wood burning stoves use what is called black chimney pipe from the stove to the inside ceiling or wall of the house. From there, special double or triple wall chimney pipe topped with a chimney cap is used.
In this article we will look at:
- Single wall and double wall black stove pipe
- The chimney pipe
- How to determine the chimney pipe height above the roof
- What to look for when looking at a house with a wood stove
- Why a chimney in snow country may need diverters to protect it
- How to prevent chimney fires
This is a companion article to “Wood Stoves, Valuable Buyer’s Guide (Choose The Right Size).”
Single Wall And Double Wall Black Stove Pipe
Black stove pipe is what goes from the top or back of the wood stove up to the ceiling or out the wall above the stove. It is called black stove pipe because it is black in color.
This black stove pipe comes in two different styles; single wall and double wall. Each style has its own benefits and its own draw backs.
Single wall stove pipe needs to be at least 18 inches from combustibles. Because its only one thickness of metal, it will radiate more heat out into the room than double wall stove pipe. Single wall stove pipe costs less than double wall and it can be cut to any length needed.
The metal of single wall stove pipe is 24 gauge metal or thicker. (A lower gauge number equals thicker metal.)
There is a Magic Heat Reclaimer (link to read reviews on Amazon) that can fit in the single wall chimney above the wood burning stove. It’s made in sizes to fit 6″, 7″ and 8″ diameter chimneys. It reclaims up to 30% of the heat that escapes up a chimney. It has a small electrical blower to circulate the heat.
Double wall stove pipe can be as close as 6 inches to wall combustibles and 8 inches from ceiling combustibles. Double wall stove pipe makes for more uniform updrafts – smoke goes up the chimney pipe better than single wall stove pipe. This means there is less chance of smoke coming out into the room when the stove door is opened.
Double wall stove pipe typically has less creosote accumulation in it than single wall stove pipe. The less creosote accumulation the less the chance for a chimney fire. A stove pipe that is hotter than 250 degrees F is too hot for creosote to condense inside the stove pipe.
So one way to keep down creosote in a single wall stove pipe is to have a fire hot enough that the temperature in the stove pipe is 250 degrees F.
Black stove pipe, whether single or double walled has a crimped end and a smooth end on each length of pipe. When installing the black stove pipe the crimped ends always point toward the stove. This way any creosote formed goes back down into the stove instead of flowing to the outside of the black stove pipe.
To know what size black stove pipe is needed for a given wood stove, you measure the inside diameter of the flange on top of the wood stove. The most common diameters are 6 inches and 8 inches.
At the ceiling level, there is either a black stove pipe box or a round black stove pipe thimble that fits between the ceiling rafters. The black stove pipe from the stove fits into this box or thimble. The metal chimney pipe fits down through the roof and connects onto the box or thimble. There are special thimbles for when stove pipes enter a wall.
If the wood stove’s black stove pipe is going to connect into a masonry chimney there is usually a stainless steel flue liner that the black stove pipe connects too. This flue liner runs up through the masonry chimney. A good quality stainless steel flue liner will last for 15 – 20 years. A low grade flue liner may only last 2 or 3 years.
The Chimney Pipe
Chimney pipe is usually double insulated, chromed steel pipe. This pipe is made to last for many years exposed to the elements in a variety of climates. It is not uncommon for this chimney pipe to last a lifetime if properly installed.
Chimneys need to be cleaned out before the start of each wood burning season. This can be done by a chimney sweep service or as a do it yourself project. If you choose to do it, you can get chimney brushes and connecting rods from most building supply or hardware stores. The inside of the chimney is swept out using an up and down with a twisting motion over all the chimney pipes insides.
The purpose of this is to prevent a chimney fire. If the creosote build up on the chimney walls is as little as 1/8 inch to 1/4 inch it can catch fire. If the smoke and gasses in the chimney is at 451 degrees F or hotter the creosote can catch fire and you have a chimney fire. A chimney fire may end up burning down the entire house! Chimney fires need to be avoided.
The smoke coming out of a chimney can tell you a lot about the condition of the fire in the wood stove. White smoke is made up primarily of water vapor. This white smoke is normal when starting a fire or adding wood to an existing fire. But if white smoke lasts all throughout the fire’s burning, it means you are burning very wet wood.
If the smoke is blue, that can be a bad sign. It means the wood in the stove is not burning completely. This adds more creosote to the inside of the stove pipes as well as wastes energy by the incomplete burning of the wood.
How To Determine Chimney Pipe Height Above The Roof
How high does a chimney need to be so it will draw the smoke and gases up from the stove and out into the air above the roof? There are two factors to consider when installing chimney pipe. Both factors must be taken into consideration if you want a satisfactory chimney.
Factor number one; the chimney has to extend at least 3 feet above the highest point where it passes through the roof. This means the 3 foot height must be measured from the roof on the upper, (uphill so to speak), side of the roof at the base of the chimney.
Factor number two; the top of the chimney pipe must be at least 2 feet higher than any part of the roof or building that is within a horizontal distance of 10 feet from the top of the chimney pipe.
Both factors must be taken into consideration if the chimney is to draw satisfactorily.
In addition to the two factors above, you need a chimney cap on top of the chimney. A chimney cap prevents animals, particularly birds from entering the wood stove and getting into the house when there’s not a fire burning and the stove door is opened. This happened to us a few times one summer while waiting for the landlord of a rental house to get a new chimney cap.
A chimney cap also keeps out moisture – rain – out of the stove pipe. This means the stove pipe will last longer. It won’t rust on the inside. A chimney cap also protects the roof, if the roof is flammable.
Without a chimney cap on top of the chimney, burning embers might escape which could catch any nearby dry grass or bushes on fire.
What To Look For When Looking At A House With A Wood Stove
The first thing to look for is where in the house is the wood stove located. The stove’s location is a good predictor of how well or how much of the house is heated. The closer to the center of the home, the better the wood stove will heat the home.
The next thing to consider is the wood stove’s size. Unless you are familiar with that brand of wood stoves and how big each model is, you will need to see the manual that came with the stove. If the wood stove is the main or only source of heat for the house, is it big enough to reasonably heat the house.
Remember, the stove may heat the house OK, but that is only half of what you need to know. The other half, and just as important, is can the stove hold a fire all night long? (For at least 8 hours?) If not, you will not be happy when you have to wake up around 3 a.m. each morning to put more wood into the stove!
If possible, see how much insulation is in the attic and what should be the insulation in the walls. If there is a crawl space, how much insulation, if any is under the floor? For the attic and crawl space, you can usually look and see for yourself.
The walls are a different matter. One way is to take the cover off a light switch or electrical plug trying to put a thin ruler between the plastic or metal box holding the switch or plug to find out how deep the wall is. A simpler method is to look at how thick the wall is when looking out a window.
A wall framed with 2″ X 4″ studs will be about 5″ to 5 1/2″ thick, thicker if the house has brick siding. A wall framed with 2″ X 6″ studs will be about 7″ to 71/2″ thick, thicker if it has brick siding. If you do not have anything to measure with, use a dollar bill, it is about 6″ long.
Check to be sure the wood stove has a non-flammable hearth or platform extending out at least 18 inches from the front of the stove. This is needed to catch the hot coals that will occasionally fall out of the stove when the door is opened.
If there is not a fire in the wood stove, open the door and look at the fire brick lining the stove. Are there any cracked or broken fire bricks? Are any of them deeply pitted? If so they will need replaced, a fairly simple DIY project.
Rap on the stove pipe above the stove. Careful if there is a fire in the stove, the pipe can be hot! If the rap sounds muffled, it can mean there is a buildup of creosote inside the stove pipe that needs to be cleaned out.
Look at where the stove pipe enters the box or thimble at the ceiling of where it enters the wall. Any discoloration around the box or thimble? Be particularly leery if there is discoloration from dampness by the box or thimble at the ceiling. Something has happened to cause water to leak down around the chimney pipe.
Lastly, look at the chimney from the ground. Does it look straight? Are the braces, if the chimney is tall or well down from the roof’s peak, straight? More on the chimney and braces in the next section.
Does the chimney have a chimney cap on it? Can you tell if the chimney cap is plugged with creosote, lots of black fluffy looking stuff around or hanging out of the chimney cap? A small pair of binoculars can help you check out the chimney cap from the safety of the ground!
Why A Chimney In Snow Country May Need Diverters To Protect It
A chimney in snow country needs to be carefully looked at if it is midway or farther down the roof from the peak. This is because the weight of snow sliding off the roof can easily take a chimney with it.
Since many roofs in snow country are metal roofs, this helps protect the roof from catching fire in wildfire season and keeps hot embers from a wood stove from catching the roof on fire. It also makes it easier for snow to slide off the roof in winter time. Snow sliding off the roof, if the snow is deep enough and slides far enough can take the chimney with it.
If the chimney is tall, there will be braces connected to the chimney and also to the roof. Look at the braces. Are they straight or bowed? Is one brace bowed more than the other one? There is a good chance that there is a leak around the chimney on the side opposite the side that has the most bowed brace. Look for a water mark on the ceiling inside the house by the black stove pipe.
We had this happen in a rental house. The chimney was located about 24 feet down a very steep metal roof. Initially there was a series of metal inverted V’s placed up roof of the chimney to hold the snow from smashing into the chimney.
The roof on that side of the house was replaced and the inverted V’s were not put back on. Why they were not put back on was asked and the roofer said they weren’t needed since the chimney had braces on it. The picture above proves that roofer was wrong!
When the chimney sweep came out to sweep the chimney before the next winter he asked about putting diverters on the roof to protect the chimney. The cost would have been about $250. The owner of the rental house decided the diverters were not needed.
It did not take long for the snow to smash down and bow the braces and bend the flashing out of shape on the up roof side of the chimney as seen in the picture. Sure enough, there soon appeared a water mark on the ceiling inside corresponding to the smashed flashing and on the side opposite the bowed brace.
Then, you guessed it! More snow smashed down against the chimney and knocked it over – see the picture. We heard the snow smash against the chimney and rip one of the braces off the roof. A quick trip outside confirmed our worst fears.
Look at the picture of what it looked like from inside. Notice that the box at the ceiling had moved and the black stove pipe was no longer squarely against the flange on the black box. We had just placed more fire wood in the wood stove and had turned down the air vent after it had started burning good.
If the black stove pipe had come loose from the black ceiling box it would have thrown hot stove pipe, burning embers, hot smoke and gases onto the rug in the room where the stove was.
We have no doubt that the house would have completely burned down. All this because the owner was too stingy to spend $250 for diverters to protect the chimney from snow knocking it off!
We were without the use of the wood stove for almost 2 weeks. A patch was placed over the hole in the roof to keep snow and water out of the house. Fortunately the house had another source of heat that kept us from freezing along with the water pipes until the chimney could be replaced.
Here is what it looked like with the new chimney and the diverters placed to protect the chimney. The total cost it took to replace everything damaged was over $1,500! I was reminded of the old saying; “Don’t be penny wise and pound foolish!”
The right kind of diverters hold snow, proving that they work. If you are looking at a house in snow country and it has a wood stove with a chimney half-way or further down from the peak of the roof, be sure it has some means for keeping the snow from knocking the chimney off the roof!
Likewise, if you build or buy a house in snow country and install a wood burning stove in a location where the chimney is half way or further down from the peak of the roof, do it right the first time. Install diverters or a similar method for protection above the chimney. The house you save may be your own!
How To Prevent Chimney Fires
Chimney fires do happen, much more than they should. There are ways to prevent a chimney fire from ever burning your house down!
One way to prevent a chimney fire is to be sure to have the chimney inspected and cleaned out by a competent chimney sweep before each wood burning season. Or you can learn how to do it yourself, its really not that hard to do.
Be sure the wood you burn is dry, seasoned wood. This lessens the chance of having creosote building up on the inside walls of the stove pipe and catching fire.
Make it a habit to have a wood stove thermometer (link to read reviews on Amazon) on the top of the wood stove or a chimney thermometer (link to read reviews on Amazon) on the stove pipe about 2 inches above the top of the wood stove. Then at least once a day during wood burning season, be sure the fire gets hot enough that the thermometer reads 250 degrees F for a short period of time, about an hour or two.
This keeps the creosote from sticking to the inside of the stove pipes. If there is not creosote sticking to the inside of the stove pipes, there is no creosote to catch fire and burn causing a chimney fire.
When a chimney fire starts, there is a loud sound often described as a jet airplane taking off. It is a loud, roaring sound accompanied by loud cracking and popping noises. The stove pipe can glow red hot and it may shake or vibrate. Outside you will see very dense, dark smoke from the chimney and a plume of flames and sparks sometimes 10′ high.
When this happens, one quick method for stopping the chimney fire is to use a Chimfex stick (link to read reviews on Amazon), activating it according to the simple directions, and placing it in the wood stove beside the fire, not in the fire.
A Chimfex stick saved a friend’s house from burning down last winter when they experienced a chimney fire. In my opinion, its a good idea to keep a couple of these on hand for stopping a chimney fire. I know we sure do!
It is commonly believed that a chimney fire in black stove pipe and the insulated chimney pipe burns out the creosote and the chimney is safe to use again. This is not the case! The high temperature of the chimney fire and the shaking and vibration can damage the stove pipe and insulated chimney pipe.
Before using the wood stove after a chimney fire the entire chimney, from the wood stove all the way up and out must be inspected by a professional and all damaged sections replaced. It is not uncommon for there to be 10 to 30 gallons of creosote still in the chimney after a chimney fire and for a large portion of the stove pipe and chimney pipe needing to be replaced.
I understand wood heat is dry heat. Is there a way of adding moisture to wood heat? Yes, there is. It is actually very simple. Many people place a cast iron tea kettle with water on the top of the wood stove. This adds moisture to the air. Just remember to check the water level each day. An empty tea kettle does not add any moisture to the air.
If the wood burning stove does not have a blower on it, is there a way to help the heat radiate out from the stove? Yes, there is. It is a simple, usually 2 bladed fan, that sits on the top of the stove. When the stove warms up the blades start to spin.
The hotter the stove, the faster the blades spin. They push an amazing amount of air for the size of the fan. These fans are powered by just the warmth of the stove.
Is is a good idea to have a fire extinguisher handy when using a wood stove? Yes, it is like the old saying; “An ounce of prevention is worth a pound of cure!” It has never happened to us, but it is comforting to know that we have a fire extinguisher handy if a hot coal was to fall out of the stove and roll onto the carpet and start a fire.
You want a Fire Extinguisher that is labeled for Class A Fires, fires from things like wood and paper. A multi-class Fire Extinguisher such as a Class A, B or a Class A, B, C Fire Extinguisher would work well also. Class B is for flammable liquids such as grease, gasoline and oil. Class C is for electrical fires.