Wood-Burning Stoves
This is not complete, it is a work in progress...

 

Introduction
    People (especially of the female kind) love fireplaces but a fireplace is a terrible way to heat a house as most of the heat goes up the chimney. While the fire in the fireplace radiates some heat into the room, most of it goes up the flue with the burnt gases.  A chimney uses much more air than is strictly needed for combustion and all this hot air is heated and goes up the flue and is replaced by cold air that comes into the house to replace it.
    A stove can be built and used with the technology that was available to the Romans and yet for centuries the fireplace was the only means of heating a room from the humblest shack to the highest palace. Only in the late 19th century would stoves become widespread and they were (relatively) quickly replaced by central furnaces but the installation of a central heating system requires much more effort and expense so a simple stove remains a good way to heat a room.
    A stove is much more efficient and I intend to discuss here some basics about stoves as I have some experience with them.  Some people are discouraged when they experience difficulty in getting a stove to light or draw but with an understanding of some basic concepts a stove can be a safe and convenient source of heat.
    What the stove does is bring the fire into the room and provide much more heat exchange between the combustion gases and the room air. Also the amount of air the stove needs to draw from the room is much smaller than a fireplace would need. The increase in efficiency is enormous.
    This is a very simplified analysis comparing a fireplace and a stove: For the flue to draw well the combustion volume must be very enclosed, the more, the better. In a fireplace, the same walls that enclose the combustion, insulate against the transmission of heat so that the better the fireplace draws, the more heat it wastes.  What the stove does is it encloses the fire with metal walls that conduct the heat into the room but provide an enclosure for the system to draw well.
    There are many kinds of stoves but let us start describing the most basic.  In the Rastro, the flea market in Madrid, you can (or could when I first wrote this, years ago) buy a basic wood-burning stove for about $30 which is just a sheet metal vertical drum.  I have used one of these for several years and it has given me very good service.
    Figure 1 is a sketch showing how it is built.  The exhaust pipe is 10 cm (4") in diameter and I have drawn it with a 45 angle because that is how I would prefer it but actually the stoves that are sold in the flea market have either horizontal or vertical exhaust pipe connections.
    At the top there is a circular cover that allows the user to insert wood and on the side, at the bottom, there is an opening that allows ashes to be removed and also the stove to be lit.  These openings are covered during normal use and only opened when necessary.
    There is also a length of pipe (4 cm = 1.5" inside diameter) entering the stove from the top and this is where the combustion air enters.  This is a very important feature because it preheats the air and makes the combustion much more efficient.  I am surprised to see it in cheap stoves and not in the more expensive ones.  Many barbecue users have learnt the hot air blower trick and this works on the same principle that hot air burns better.  A small cover at the entrance to this pipe allows for the regulation of air intake.
    Three metal 10 cm legs keep the stove off the floor.  It is a good idea to place it on a metal sheet or tile floor but in my experience the surface directly under the stove does not get very hot.  One has to be more careful about surrounding objects and surfaces which can get very hot creating a substantial fire risk.  We shall come back to this later. A metal mesh surrounds the stove to prevent accidental burns.
    Thin sheet metal stovepipe flue will only last two or three seasons.  It gets very hot and with rain and creosote ends up rusting.  Creosote is a stuff like tar that results from incomplete combustion of wood and it is very corrosive.  The hot unburned gases condense when they cool and form creosote which is acidic and will corrode metal.  It can also ignite if it gets hot enough.  The best prevention for creosote is to assure a hot and complete combustion of the gases before they leave the stove to enter the pipe.
    Because the stove is quite light it can be easily tipped over.  To prevent this it can be secured by anchoring it to the floor but I have found that a few bricks inside make it stable enough.  You should always be careful though and it is not a good idea to use a stove to heat a room where children will remain unattended.
    This type of stove can be easily and cheaply built from scratch or using a metal drum.  The stovepipe is also cheap thin sheet metal.  For very little expense one can have an efficient wood-burning heating stove.  Of course this stove will not provide the decorative value of a large and heavy cast iron stove but those cost many hundreds of dollars.  This type of stove can be jury rigged for the winter and removed in summer.  The stove and exhaust pipe can easily be cleaned and/or replaced whereas a permanent installation is much more expensive, not only to install but also to maintain.  Flues require periodical cleaning to remove carbon deposits and creosote which create the risk of fires. 
    What makes the gases circulate is the vertical stovepipe where the hot combustion gases rise and draw fresh air into the stove.  The stovepipe is an essential part of the system and should be well designed.  A vertical difference of 2 - 3 m (7 to 10') should be enough to give a good draw in still air but we shall talk about wind later on.
    The main point to remember is that it is the hot gases in the stovepipe that provide the draw.  I used to fill the stove with wood and light it from below with some paper and it would just make smoke and refuse to draw.  The reason for this is that the small quantity of hot gases never fill the flue pipe but rather they cool as they go through the wood above and there is no draw.  Now I can light the stove getting a good draw from the very beginning.  This is my technique: I place at the bottom of the stove a small amount of paper and kindling.  Then I light a sheet of newspaper, introduce it in the stove through the top cover and put the cover in place.  This gets the circulation started as these gases go straight into the pipe.  After a second I light the kindling through the ash opening.  I continue to introduce paper through this opening for a little while and once the fire is burning some small stuff well I will gradually add some bigger chunks of wood.
    A hot, raging fire burns much more efficiently than a colder, smoldering fire.  In a smoldering fire a portion of the gases do not reach combustion temperature and exit the stove without burning.  Not only are they wasted but these gases pollute the atmosphere and are known carcinogens.  It is therefore better to have a small hot fire than a big smoldering fire.  The best way to achieve this is by constantly adding very small bits of wood that burn fast (the extreme example is a furnace that burns liquid or gas fuel) but this requires constant attention and also means a lot more work chopping wood.  I have developed a good technique where I will burn big chunks of wood by maintaining a high temperature with smaller bits.  This gives me the best result. 
    In any case and contrary to what many people think, burning wood for heat pollutes much more than burning gas or oil.  The advantage of wood is if one can get it for free (free meaning not paying cash because there is a lot of labor involved).  Because of the labor involved wood is so expensive that it is not economical to buy wood for fuel.
    Fuel (wood) preparation is very important.  As I mentioned, one should use a mixture of sizes so the stove burns and draws well.  At first the wood releases gases that need high temperature and a good supply of air to burn well.  It is essential to maintain high temperature in this phase to assure good combustion.  The gases burn with flames or, rather, the flames are the gases burning.  If one slows the combustion by restricting air intake during this phase what happens is the gasses go up the flue unburned.  Hot temperature is aided by turbulence because it could be that part of the gases are extremely hot while other pockets do not reach ignition temperature and go up the flue unburned. 
    Newer stoves have a device that helps better burning, a sort of "catalytic converter".  It is a grid placed at the exhaust of the stove.  This grid is at very high temperature and will ignite any pockets of unburned gases that pass through it.  In many parts of the USA environmental laws require stoves to have such a device and it is illegal to operate a stove that does not have it. 
    Wood should be cut and split to chunks of suitable size and let to dry and cure long enough before it is burnt.  Damp wood burns poorly and very especially green wood will burn badly and make a lot of creosote.  If you absolutely have to burn green wood it is best to burn it with dry, cured wood that will maintain a high temperature and burn all the products of the green wood.  After the wood has released all the gases then what is left is solid carbon (coal) which will burn with a red glow but no flame.  At this stage combustion can be regulated by restricting the air intake.  The combustion slows down but the unburned carbon stays there until it burns.  I would assume there would be an increase in the amount of carbon that burns into carbon monoxide instead of carbon dioxide but I have no figures or other information on this. 
    The gases burn at relatively low temperatures but once the carbon (coal) is burning it will get hot enough to make the sheet metal glow red and after enough times the metal will start to buckle and then develop cracks which will damage the stove.  In fact, the main difference between a wood-burning and a coal-burning stove is that the latter is built of thick cast iron capable of withstanding the higher temperatures. 
    When burning wood in a sheet metal stove it is therefore preferable to follow a burning technique that does not allow too much coal to form at once but rather to have a continuous process with a small quantity of coal burning in the bottom and some wood burning with flame on top of that. 
   

Carbon Monoxide
    A word about carbon monoxide, (CO) is in order.  When carbon burns completely and efficiently it combines with two atoms of oxygen to form carbon dioxide (CO2) which is gas normally found in the atmosphere in small quantities and also produced by our breathing and other chemical processes.  But when carbon burns incompletely it will combine with only one atom of oxygen and form carbon monoxide which is undetectable because it has no color or smell.  But carbon monoxide is extremely poisonous and many people die from it. 
    The hemoglobin in the blood, as it passes through the lungs, combines with oxygen which it carries to the muscles and other body organs.  This is merely a chemical process.  But hemoglobin has much greater affinity for carbon dioxide so what happens is that it will bind to carbon monoxide and lose its ability to transport oxygen around the body. 
    Carbon monoxide is impossible to detect with our senses and the symptoms of carbon monoxide poisoning can be difficult to detect as they can be mistaken for other things until it is too late.  Fresh air is the immediate remedy but it should be remembered that the cells already bound to carbon monoxide will stay that way and have lost their ability to transport oxygen even after the intake of carbon monoxide intake has ceased.  Fresh air may not be enough and it may require oxygen to save that victim.  Emergency help should be sought immediately. 
    Carbon monoxide is produced to a greater or lesser degree by all carbon combustion.  Gasoline motors produce a lot while diesels produce much less.  Fireplaces, stoves and furnaces produce amounts which depend mainly on the efficiency of the combustion.  To prevent carbon monoxide poisoning the best prevention is to:
    A- assure good combustion so the least possible amount of carbon monoxide is produced
    B- assure good draft and ventilation so what CO is produced is vented outside and
    C- install a CO detector. A detector is not foolproof though and it is essential to
          understand the issue and follow safety precautions at all times.
    Having said this it follows that a stove that draws well is a very safe way to heat a room because it creates a circulation of fresh air into the room and exhausts all gases to the outside.  The different covers of the stove are not airtight but during normal use the air should flow from the room into the stove.  This flow can easily be checked with smoke or a flame.
    

Installation
    The hot gases in the stove heat the metal walls of the stove from inside and this metal communicates heat to the room by convection and by radiation.
    Convection means the room air that is close to the stove raises as it is heated and is replaced by colder air. Air should circulate freely around the stove and, of course, this circulation can be aided with a fan.  Placing a new stove inside the old fireplace may be practical but it is a terrible idea from the viewpoint of heating efficiency because the air does not circulate well.  The best place for a stove is well inside the room where heat radiated and yielded by convection will stay inside the room.
    Radiation means heat which is radiated out to the room in the form of electromagnetic energy.  An object placed a couple of feet away from the stove will get very hot indeed and one has to be very careful about providing adequate clearances all around the stove and the stove flue pipe or there will be a substantial risk of fire. 
    A good understanding of heat radiation will help.  Radiated heat is energy in the form of electromagnetic waves in the infrared zone.  It will travel through vacuum, air and some glass but it is blocked by most opaque bodies.  Try this: place yourself near a stove or other very hot object and you will feel the radiated heat.  Now place a shield of any kind between the two and you will feel no heat. 
    Most commercial stoves have an inner combustion chamber that is surrounded by an outer shell which has openings to allow for convection but which stops most radiated heat.  While this means it can be installed with much less clearance, it also means the C is much decreased.
    The best installation is to place the stove in the center of the room with lots of clearance all around.  While this is the ideal it is often unfeasible and a compromise needs to be found.
    If the stove is too close to a wall or furniture, a heat shield can be installed.  A sheet of metal (aluminum flashing works well but even aluminum foil glued to cardboard or plywood will reflect heat well).  There should be enough space between the shield and the wall or furniture to allow for air circulation.  The key here is to experiment and see what works.  Monitor temperatures very closely at first and you will soon get an idea of what needs protecting and how much.
    A stove installed and operated properly is very safe and should not present a significant fire risk.  Nevertheless it is prudent to have a fire extinguisher at hand and devise a fire-fighting and escape plan.  This is good even without a stove. 
    At first you should observe and monitor the stove very closely until you get to know it well.  Tiny sparks can fly out through the air intake and surfaces can get very hot.  You should clear all the surrounding of any combustible materials.
    

Getting the stove to draw well
    Hot air and gases will rise through a vertical pipe because they weigh less than the surrounding air.  The longer the pipe, the more upwards pressure.  Of course, as the gases rise through the pipe, they are also cooling so after a certain amount, adding more pipe will help little.  In still air eight or ten feet of vertical pipe should provide enough draw and I would consider that a minimum.
    The situation is complicated by wind and here is where one hears all sorts of folk remedies.  At first I also tried a number of these remedies, mostly in the form of devices installed at the top of the flue pipe.  After I saw they did little or nothing I decided to study the problem analytically.  Here are my conclusions.
    The necessary and sufficient condition for a stove to draw is that the pressure at the air intake be higher than the pressure at the stovepipe.  In still air this condition is very easily achieved by the hot gasses in the pipe but wind can create some back pressure differential of its own.
    Let us call P(g) the pressure differential generated by the hot gases in the absence of wind and P(w) the pressure differential between the same two points created by the wind in the absence of hot gases.  These two pressures are quite independent from each other.  Now if P(w) is greater than P(g) and of opposite direction, then the circulation in the stovepipe will be reversed and the combustion gases will flow into the room filling it with smoke.
    It follows from this that increasing the length of the flue pipe will increase P(w) and help in preventing this from happening.  But strong winds can cause great pressure differentials which can be difficult to overcome only by increasing the length of the pipe.  We need to study the wind dynamics more closely.  Note that we can now forget about the hot gases and operation of the stove and concentrate on studying the counter-pressure created by the wind.
    Making the stovepipe as long and high as possible has two positive effects.  We have already mentioned that it increases the natural draw of the system but, by placing the top out and away from the building, it avoids any high pressure areas which may be caused by the wind going around the building.
    Another thing that helps is to have the top opening facing away from the wind.  This seems pretty obvious but unless the wind direction is pretty constant we would need to install a rotating top and this might not work so well so I have mostly just left the top opening horizontal.
    So, what do we do when a strong wind just makes it look like there is no way to have the stove draw properly?  Here's what I did. 
    This drawing illustrates roughly how I had my stove set up.  The quasi-vertical stovepipe was about 10' long and ended just above the roof gutter. 

stove

    This was not really a good setup.  The wind would create high pressure at the top of the pipe as it hit the slope of the roof and it would blow right into the pipe.  Because this was something I just rigged for the winter it was not worth installing a much longer pipe.  The wind was sometimes very strong and would just seem to blow into the fluepipe so strongly that it seemed hopeless to try to counter it.  After some thinking and some tests I got the wind itself to provide the solution. 
    I would close the door to the room and open the window as necessary so that the wind coming in the window would create high pressure in the room and blow into the stove and out the flue pipe.  It might seem at first that you are just letting the cold in but soon the stove heats the room enough and you only feel the cold if you are directly in front of the window.  By doing this, the stronger the wind the stronger the stove draws.
    

Getting the flue pipe through the wall
    The flue pipe gets extremely hot and will radiate enough heat to ignite things which are too close.  Extreme care must be taken where the flue pipe goes through a wall.  One way to do it is to install a metal pipe of greater diameter through the wall so that there is a cushion of air or heat-resistant insulating material.
   
   

 


I started this page in 1999.
Last major expansion was 2010-01-08.
Will continue when I get another spurt of inspiration, maybe in another ten years.