Wood Burner Basics
A Primer for the Novice (and others).

There's more to the operation of a Wood Stove-Burner-Furnace-Boiler than throwing in the Firewood and taking out the Ashes. Whether you're starting from scratch by ordering Steel Plate and Welding Rods, or  buying a ready-to-go unit, there are many factors to understand and consider.

These factors are not related to Wood Burner size or shape, but generally apply to all Wood Burners.  This page assumes that Wood Burners are "air tight" with Draft controlled by the Operator.


ALWAYS WEAR SAFETY GLASSES AND LEATHER GLOVES
WHEN TENDING ANY WOOD BURNER.

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Starting at the bottom; the Wood Burner should be sitting on a panel of fireproof material to protect the flammable structure from the radiating heat.

Legs must be sturdy enough to support the weight of the loaded Wood Burner (In the case of a Boiler, this is a considerable amount), and long enough to keep it safely above a combustible floor. VERY long Legs will bring the Wood Burner up to a point where you may be able to tend it without bending over, but it will also require you to lift Firewood higher, increase the chance that it will tip over, and move it up to where the room air is warmer which will decrease its efficiency.  The shortest Legs that support the Wood Burner safely above a combustible Floor is common.

A word should be said here about Efficiency. Many Wood Burners are meant to heat the surrounding air. Some Wood Burners (Boilers) have incorporated water tanks that heat water which is then piped to remote areas where Radiators release the heat into the surrounding air. Therefore, a Boiler may be more efficient if it is well insulated, and does not allow heat to escape into the surrounding air.

Efficiency is a measure of how much heat is available for the intended use. Any heat that escapes up the Chimney or is otherwise lost decreases Efficiency. Wood Burner Efficiency of 100% is the goal but is not realistic. Efficiency of 80% is much more common. That means that for every 100 pounds of wood you burn, 20 pounds is lost to inefficiencies.

A Wood Burner in a cold room is more efficient than a Wood Burner in a hot room. The reason is that heat is more readily transferred to the cooler air. To demonstrate this, imagine a Glass Jar filled with 70 degree water. If we put this Jar in a Refrigerator set at 40 degrees, within 1 hour the water will be approaching  40 degrees. If instead, the Refrigerator is set to 55 degrees, it may take 2 or 3 hours for the Jar to approach 55 degrees. But if the Jar is put in a 70 degree room, the temperature will not drop.  So the heat in the Jar does not go into the surrounding air, and its heating efficiency is zero.


If Floor Flammability is a potential problem, you have the option of replacing the floor with a Tile or other fire-safe material. Or suspending a Steel or Sheet Metal Reflector just below the Wood Burner to absorb the radiated heat and safely dissipate it into the air. If the Wood Burner is used over a concrete or Earthen floor, then this suspended Steel Plate will add to the efficiency of the unit by absorbing and radiating  heat into the surrounding air, which would otherwise be absorbed by the floor.

Fire Bricks look pretty much like regular Bricks, except for color. But they are made especially to withstand the temperatures inside a Firebox and protect the Wood Burner from burnout which would make the Wood Burner unsafe. Although they reduce the size of the Firebox somewhat, Fire Bricks are a desirable feature found in many of the better Wood Burners on the market.

Ash can act somewhat as Fire Bricks in cases where there are no Fire Bricks. If 3 or 4 inches of ash are left in the Firebox, it acts as an insulator and helps to protect the Wood Burner from burnout. Approximately  1 quart of Ash is produced for every 200 pounds of Firewood burned, depending on the species.

The Firebox Door  is where you add Firewood and also remove Ashes on some models. It is used a LOT and must have husky hinges and locking mechanism. A monthly application of Graphite dry lubricant (made for Lock mechanisms) will extend Hinge life. Some models have the Draft Control or Blower mounted on this Door. There should be an air-tight Channel/Baffle leading to the lowest part of the Door to prevent Sparks from shooting out and also to minimize the escape of  smoke during Chimney  down-drafts created by the wind. The Door usually has a rope-type Gasket to seal the Door and prevent uncontrolled Draft from giving an uncontrolled fire. These Gaskets can be checked periodically for sealing by the same method as a Refrigerator; a single thickness of newspaper inserted behind the Gasket should pull out with noticeable resistance, or tear in the attempt. A good Door will have an adjustment to make it snug on all four sides.to prevent unwanted Draft.

A Grate is a metal grid that suspends burning Firewood above the Ashes, allowing Oxygen to get to the flames better than if the Firewood is surrounded by smothering Ash. Grates are usually very easy to remove, since they are a replaceable item with a lifetime of several years, depending on how  hard the Wood Burner is fired. 

Some Wood Burners have a Fresh Air Tube to bring Oxygen to the Stove Pipe area. Its purpose is to burn gases which are sometimes given off by the fire. Certain combinations of maximum heat and minimal Draft cause gases to be formed, in the absence of sufficient Oxygen to burn them. This tube provides heated fresh air to allow these gases to burn and add to the Chimney Draft.

If you ever open  a Wood burner Firebox Door and are greeted by a "Poof", you have allowed Oxygen to enter when these gases were present. It is best to always open the Firebox Door . . . S-l-o-w-l-y.

The Draft Control is usually located in the Ash Cleanout Door or the  Firebox Door. It is a sliding or rotating device that allows the Draft Inlet to be opened or closed, and any adjustment in between. The more it is opened, the more air is allowed to enter the Firebox to feed the fire, the quicker the wood will be burned, and the quicker its heat will be released.

Its low location minimizes the tendency of smoke and gases from escaping during a Down Draft, when wind blows down the Chimney with greater force than the hot smoke trying to get up the Chimney. This Draft opening usually has a Baffle behind it to prevent Sparks from shooting out of the opening.

The maximum Draft opening is calculated to limit the amount of air entering the Firebox to an amount that will not allow the fire to burn out of control. In that case the Wood Burner or its Stove Pipe might get red hot. This creates a dangerous situation which will limit the life of the Wood Burner or Stove Pipe, and may start a Chimney fire !

Draft is a simple yet very complicated subject. It works on the principle that hot air rises. If it did not, smoke would not go up the Chimney, and Hot-air Balloons would not rise. Hot-air Balloons have a Propane Burner aboard which heats the air inside the Balloon to make it warmer than the air outside the Balloon. 

The underlying reason for this is that hot air weighs less than cold air. So, like a Cork in water, it is forced up and displaced by the  heavier medium. To prove this, fasten a Balloon tightly to a 1 liter Soda Bottle as shown here. Hold the Bottle under hot tap water. Notice that the Balloon gets bigger. Some of  the expanding air has been forced out of the Bottle. So the air in the Bottle is now less heavy. If the air in the Bottle weighed 2 ounces, and heating it forced half of it into the Balloon, then the half of the air remaining in the Bottle would weigh 1 ounce. So the 1 liter of air  is lighter when heated. Most materials expand when heated.

With no Draft, the fire will die; with too much, the Wood burner will overheat. Draft is influenced by the Draft Control, by how hot the Firebox is, the location, thickness, diameter and hight of the Chimney, and also the Weather. When the Draft Control is open the fire is allowed to burn hotter with minimal creation of smoke and Creosote. It also allows a greater volume of hotter air to go up the Chimney and keep the Chimney walls hotter instead of having a cold Chimney cool the ascending air and decrease its tendency to rise.

A Chimney that's built inside a heated or insulated structure or on the South side of a building will naturally be warmer than an external Chimney on the cold, windy North side of a structure. Chimney opening diameter also effects the temperature of the rising air. Imagine if the opening were ten feet in diameter, there would be very little heating of the Chimney walls by hot air from the Wood Burner. A high Chimney will be cooler than a shorter one, and will cool the hot air on its way to the top. Imagine a Chimney one mile high; the hot air would have no chance of heating so tall a Chimney and the result would be a cold Chimney that cools the hot air.

A Combustion Blower can greatly improve the efficiency, safety, and functionality of a Wood Burner. It can be controlled by a Thermostat that is on the wall of a heated  room, so when the room gets cool, the Blower will quickly raise the temperature of the Wood Burner and the room. The Blower can also be controlled by an Aquastat that is in contact with the water in a Boiler, which will quickly increase the temperature of the Boiler water available to the associated remote Radiators.
Blowers should be sized to the Wood Burner and an optional adjustable inlet allows you to fine tune the incoming air velocity for your particular conditions. Blowers are often mounted on the Firebox Door, but they can be mounted to the top or side with a sheet Metal channel bringing the forced air to the Door where it helps keep the Door cool.

The Blower creates intense Flames so quickly that there is much less Smoke created by smoldering Firewood, which equates to less Creosote. Blowers also maintain a more even heat with less fluctuation than if the Homeowner adjusts the Draft Control to adjust the room temperature.


This is especially true if your Firewood is not of uniform quality; if you burn different species like quick, hot-burning Yellow Pine and Slower, cooler-burning Maple with different degrees of seasoning. Then the time required to get the Firewood burning well is much less noticeable. Especially in the morning when you may have to get some heat in a hurry from minimal glowing embers.

A more constant heat can also be maintained if you are able to add a piece of Firewood every 15 minutes or so, instead of allowing the fire to die down and then adding a fresh batch which will take considerable time to catch fire and produce heat, while acting like a shield to keep the existing fire from radiating heat to the Wood Burner for release into the heated room.

In addition, some Combustion Blowers have an automatic Draft Control incorporated in their design. The Blower opens a Gate which allows air to enter from the Blower, but when the Blower is shut down by the Thermostat or Aquastat, then the Gate closes which minimizes hot air escaping up the Chimney because there is no incoming air. This design can increase the efficiency of the unit.

You can also control the Blower with a Timer or make a Set-back Thermostat using a Timer and 2 wall Thermostats.. The Blue Thermostat is set for night time temperature. The Red one is set for Day time temperature. When the temperature reaches 60F. the Blue Thermostat turns the Blower off , Unless the time is between 5 am. and 9 pm., then the Red Thermostat will allow it to run until the temperature reaches 70F. This design is integrated into manufactured Set-back Thermostats. The Aquastat will only allow the blower to run if the  water temperature drops below the Aquastat's  setting.
A Baffle may be located between the Firebox and the Stovepipe. Its purpose is to keep the hot Flames from going straight out the Chimney. Then on its lengthened trip to the Chimney, it will lose some heat making the Wood Burner more efficient.

A Boiler may encompass the whole Wood Burner including the Fire Box Door.  Some Wood Burnes may even have pipes filled with water inside the Fire box.. This is an attempt to capture all the heat possible which increases the efficiency factor of the Wood Burner. These internal Pipes need frequent cleaning to remove the Soot and Creosote which act like insulators.

A domestic Hot Water Coil may be found In the hot water of the Boiler. It is a Heat Exchanger which removes heat from the boiler and transfers it to the water in the Coils. This "domestic" hot water is then used throughout a home for bathing, laundry washing, etc.

A Circulator is used to move hot water from the Boiler to remote Radiators to heat that remote area. They are controlled by Thermostats in the remote area. Placement of Circulators is normally at the lowest point of the system to prevent a loss of water from running them dry, which can cause damage.  Shown top-mounted here for clarity only.

A Damper is a manual control that fits inside the Stovepipe. It acts to restrict the heat going up the Chimney. They are usually used at night to slow the burn and preserve hot Coals for morning rekindling. It is manufactured with holes in it and clearance around its perimeter to prevent the  Stovepipe from being closed completely, which might cause smoke and gases to enter the room housing the Wood Burner.

A Stovepipe Automatic Draft Regulator automatically compensates for the varying effects of Wind and minimal Draft. It has an adjustment to fine tune it to your particular circumstances. On a cold Winter morning, you may notice that it's closed. The reason is that it is not allowing air to be sucked from the room, which would decrease the Draft. Instead, all the draft is being pulled through the Wood Burner. But midday when a hot fire is burning, the Automatic Draft Regulator will be seen merrily swinging  up and down in cadence with the Wind to keep a constant Draft in your Firebox.

Spark Arrestors are usually constructed of wire mesh which impedes the travel of sparks out of the chimney. However, unless you are able to have a hot smokeless fire burning most of the time, then the Soot and Creosote buildup on the wire mesh may be more of a nuisance than it's worth. Having a chimney Sweep climb your roof to clean a Spark Arrestor is not inexpensive; especially if it occurs on a regular basis.

Chimney Helmets will help keep rain out of your Chimney and Firebox . Some are made to rotate by a vane on top to keep them turned away from the wind and lessen the effects of blown Rain and Down-drafts.

Creosote is that sticky black residue that attaches itself to cooler surfaces. Dry Firewood makes less Creosote than wet Firewood. It is a product of the particles in smoke; the particles which make smoke visible. A hot fire burns these tiny particles, but a fire which is retarded by a closed Draft Control produces these tiny particles which go out the Chimney and land on nearby objects as "Fly Ash".  The bottom line is that a small hot fire is better than a large smoldering one which produces the same amount of heat.

An accumulation of Creosote inside a  Wood Burner or its Chimney can cause problems with mechanical devices. It is also the culprit in many Chimney fires. It is the reason why Chimney Sweeps climb roofs with long Brushes. An occasional hot fire may help to minimize this build up.

Down drafts are caused by wind gusts which are stronger than the air trying to go up the Chimney. When they collide, the smoke and gases are pushed back down the Chimney. If the down Draft is strong enough, it may be able to force  smoke out any openings including an opened Draft Control.

Rain can cool the Chimney decreasing the Draft, in sufficient amounts, it can also dampen the fire itself. And in rain storms, it may be able to reach the Ashes and create a rust-causing goo that may rust holes in the Wood Stove causing its premature demise.

Repairs can be done satisfactorily by a good Welding Shop if the Wood Burner is in generally good condition and the damage is not too extensive. The cost of replacement must be balanced against repair costs and state of the art efficiencies to determine which action to take.



A weekly handful of common Table Salt thrown on a hot fire,
is said to turn Creosote into harmless dry flakes.

The bottom line is; Burn a safe HOT fire at all times,
and your Wood burning experience will be enhanced.

Always follow the Manufacturer's instructions
on every aspect of  heating your home, and if anything seems unusual,
their Dealer is as close as your Cellphone.









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