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September 20, 2021

Wood Fire in wood burning trivia



Wood furnaces, wood stoves, wood pellet stoves and furnaces, Multi-fuel gas/oil wood furnaces, add-on wood furnaces, coal stoves/furnaces, corn stoves/furnaces.


You want to save as much money on fuel bills as possible, but then you ask yourself “what do I buy”. “ I don’t know a lot about burning wood or what my choices are”. Here are a few answers.  Not only have we been making wood burning stoves, add-on wood furnaces and multi-fuel wood furnaces for 30 years, we talk to people that have every kind of stove, furnace and indoor/outdoor boiler you can think of.  We hear a lot of stories, both good and bad.

Below is a list of many of the alternative energy choices people consider.

Keep in mind when you read these comments or ask questions from someone who has one of these products, some people realize they have not made the right buying decision and are embarrassed to admit it.  You have to ask a lot of questions and check with folks who have at least a few years experience with the product they bought.

We refer to the term BTU in some of these comparisons.  BTU means British thermal units, which is a common measurement of heat used for any kind of heating device. It is the amount of heat it takes to raise the temperature of one pound of water 1 degree.

A term you may hear is “heat loss”. This is the amount of BTU’s that is required to heat the structure after deducting the heat that escapes through your doors, windows and roof.

Another term you may hear is BTU input and BTU output.  Number 2 fuel oil has about 138,500 BTU’s in a gallon. LP gas has 95,000 BTU’s in a gallon.  A gas or oil furnace less than 15 years old that is connected to a chimney is about 80% efficient. That means that 20% of the energy produced by the furnace goes out the chimney. It is necessary for a gas or oil furnace to produce this amount of heat to take the bi-products of combustion out of the home.  A condensing gas furnace with a forced draft fan is about 90% efficient. That’s the one that vents with a 2 or 3 inch plastic pipe.  These effcencies are certified by either the AGA (American Gas Association) or UL (Underwriters Laboratories) or other accredited testing facilities accepted by GAMA (Gas Appliance Manufacturers Association) and DOE (Department of Energy)

Wood furnaces and wood stoves are not rated for efficiency by any of these organizations.  Most solid fuel furnaces and stoves are grossly inefficient compared to oil or gas appliances.  There are several reasons for this.  Wood with different amounts of water in it because of drying time burns at different temperatures.  Wet wood burns at a lower temperature and is therefore less efficient because of incomplete combustion. Forty per cent of the energy in wood is in the form of an unburned gas as it leaves the flame because it starves for oxygen as it leaves the flame. If a controlled amount of air is introduced right above the flame to burn these gases, the efficiency of the appliance increases. A heating appliance that produces a lot of smoke out the chimney is very inefficient because the wood energy is not burned.

Some wood furnace and wood stove manufacturers advertise their products as very high efficiency, comparing them to newer oil or gas furnaces when in reality they are as low as 30% or less when tested with certified and accepted equipment.  There is one accepted test method, which is one used by UL and AGA. A measurement of the CO2 and a temperature reading of the stack are taken. A chart provided by Bacharach Instrument Company analyzes both numbers and the result is the efficiency of the fuel being used.

Wood and all solid fuels such as coal, corn, and grain have the same 12,000 per pound of energy.  Oak is a better burning choice than pine or conifer only because Oak or other hardwood is denser, in other words, heavier per square foot.   A fresh cut tree has about 50% moisture content. In other words, a tree is 50% water.  Wood that is split and air-dried for a year has about 20% water content.  Water does not burn.  That is one reason why air-dried wood burns better than freshly cut wood.  Another reason is the wood will burn hotter with less water in it, reducing creosote formation.  Coal has virtually no air or water in it.  But coal will not burn unless you provide air to the fire chamber on a continuous basis. If you shut the air off to a coal, the fire will go out.

When figuring out heat values of different woods or grains, they all have about 8,000 Btu’s of usable energy per pound after deducting water content, which does not burn.  Coal remains at 12000 Btu’s per pound because it contains no water.

Wood stove 

A wood stove can be the least expensive appliance if you choose one made in Asia. The fancier they get, the more expensive they are, but not necessarily more efficient. They all produce heat but there are a few downsides. The room they are in gets most of the heat. The rooms farther away from a wood stove are cooler or even cold. Looking at a wood stove from a heating efficiency standpoint only put the wood stove in the basement, because all heat rises.  A wood stove will supply all the heat it produces   to the floors above the basement. But most wood stoves are what they call “air tights”.  A better word may be “creosote generators” because once you get the fire going and you shut off the air supply off so that the wood smolders, the temperature of the fire falls.  There is usually enough oxygen in air-dried wood to keep the fire burning without air to the firebox.  The downside is the fire burns at a low temperature, creating creosote buildup in the stove, smoke pipe and chimney. If the wood burning flame ignites the creosote, it can burn at temperatures in excess of 2,500 degrees, possibly causing flue pipe or chimney failure or worse. A catalytic converter provided by some wood stove manufacturers will burn some of the creosote and smoke.  Many of them fail prematurely and they are expensive to replace.  A wood stove is not the best choice as opposed to a wood-burning appliance that has secondary air to burn the creosote and a circulating air blower to blow the heat away from the firebox.

Wood fireplace

A good choice if you want to see the pretty flame and don’t care about heating efficiency.  A poor choice if you are buying it for heat efficiency. At a meeting sponsored by the EPA in Portland several years ago, it was determined that fireplaces were about 1% efficient.   One of the attendees spoke up and said, “But you can double the efficiency of a fireplace by adding glass doors”. The speaker said,” that is true, that makes them about 2% efficient”.  Fireplaces are great to watch the wood burn.  Don’t buy one to save on your heating bill unless it has a massive heat exchanger and blower above the fireplace.

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Add-on wood furnace

Sometimes referred to as a sidekick or furnace helper, all meaning it is a furnace used in conjunction with your primary gas, oil or electric furnace. 

An add-on wood furnace is usually placed in your basement next to an oil, gas or electric furnace.  It usually has its own thermostat.  You can buy one that has its own circulating air blower or one that has no blower.   With the latter system without its own blower, the blower from the primary furnace is used to circulate the heat produced by the wood add-on.   Many homeowners place this furnace near their up flow gas, oil or electric furnace.  The warm air sheet metal plenum at the top of the wood furnace is connected to the warm air plenum of their primary furnace. A hole is cut into the lower part of the wood furnace casing and connected to the return air duct on their primary furnace.  Back draft dampers are sometimes needed to insure that the heated air in the wood furnace does not re-circulate back through the warm air plenum on the primary furnace. 

*CAUTION: Check your local codes for chimney requirements and clearances to combustible surfaces for ducts.  All warm air ducts on any solid fuel-heating appliance must be made of metal. (No fiberglass-no plastic) A separate chimney must be provided for a wood-burning furnace if your primary furnace is gas unless the wood-burning appliance is UL Listed for two fuels in one chimney.  These codes are included in (National Fire Protection Association) NFPA 54 and NFPA 90, which are also state and local building codes.

Add-On used as a Wood Stove 

Add ons are sometimes used as a wood stove in the basement or even on a first floor.  When placed in a basement and not connected to any ductwork, it will work just like a wood stove.  The unit must be at least 18 inches, back and sides, from any combustible surface and have a clearance of 48 inches on the front for loading wood.  The top of the add-on or wood stove must be at least 18 inches from a combustible wall or ceiling as well as the barometric chimney damper, if used unless Listed by a recognized testing agency for lesser clearances.   If installed on a wood floor, a Recognized heat protection pad must be installed beneath the wood stove or wood add-on.   It will not be as efficient as a wood add-on connected to a furnace with a blower because heated air will not be forced away from the wood stove.

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Central Heating 
Wood Furnace

A central heating wood furnace is either a stand-alone wood furnace that has it’s own duct and circulating blower or an add-on that uses the same duct system as an oil, gas or electric furnace. The stand alone type with it’s own duct system is usually used in homes with electric or hot water baseboard systems that do not have ductwork. An advantage of the stand-alone system is whole house air conditioning, electronic air cleaning or humidification is easily added. (Note: Wood furnace ducts must be made entirely of sheet metal.)

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Multi-fuel or Combination wood/gas, wood/oil or wood/electric Furnace

This is a whole house, central heating furnace in which air conditioning; electronic air cleaning or a humidifier can be easily added.  It is operated by two side by side thermostats.  The advantages of this type furnace are the homeowner can burn wood as a primary fuel without having to start a wood fire. The gas or oil burner ignites the wood. When the wood burns down, the gas or oil burner takes over to keep the home comfortable.  The Yukon-Eagle brand has a massive heat exchanger (firebox), a secondary air system that burns the smoke and unburned gases and a large circulating blower that together makes this furnace extremely efficient, using a fraction of the amount of wood of an outdoor boiler or many other wood furnaces, stoves or indoor boilers that don’t offer these features.  The Yukon-Eagle Multi-fuel wood is UL Listed (approved) to heat your home with wood or coal without electricity.
* If an air conditioning coil is used with any wood burning forced air furnace, the ducts must be made entirely of metal, including the AC condensing water pan.

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Outdoor Wood Boiler

Sometimes referred to as an outdoor furnace. There are some advantages to this system.  These systems heat water that is stored between the firebox and an outer casing. Usually these systems work in conjunction with an indoor gas, oil or electric furnace.  A water pump is used to transfer the heated water to a water coil located at the top of your primary furnace plenum. When the thermostat calls for heat, the pump transfers the heated water to the coil.  The furnace blower distributes the heat to the rooms.  That’s the good news.  The bad news is these systems are referred to as “open systems” which the water is subjected to air.  This often times will destroy the firebox. Many or them last for only a few years.  Because of the high cost of these systems, it takes away any cost savings you hope for by burning wood.  Some manufacturers now use stainless steel, which is a deterrent, but not a guaranteed fix all.  These systems also use huge amounts of wood, as compared to an indoor furnace or wood stove. A disadvantage is if you let the water cool from not keeping enough wood in the firebox, it can take a long time to heat the water, leaving you with inadequate heat for a long time. Another disadvantage is these systems are designed to burn the wood with a low fire.  Even 180-degree water in the water storage system is very cold to a wood fire. When the temperature of firebox that cools, creosote forms on it, which is an insulator that restricts heat from the firebox to the water making the boiler even more inefficient.  In fact, State Farm Insurance Company requires that a wood boiler be placed at least 75 feet from any building because of the creosote fire hazard.  In addition, most of these systems emit so much smoke that many States and local communities have banned them. See www.woodheat.org/technology/outboiler.htm for more information.
If a water pump is necessary to pump the heated water, the boiler will not provide any heat during a power outage or electric pump failure.

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Indoors Wood Boiler 

They are not popular for a few reasons.  They are generally used along with a gas or oil boiler that uses the same “closed” water system.  This insures that air cannot get into the water system, which would cause corrosion and failure of the primary boiler as well as the wood boiler.  The water in the wood boiler, being only 180 degrees or cooler, is subjected to the same creosote problem as the outdoor boiler.  In addition, if the wood fire to more than 212 degrees, which causes steam, heats the water a safety valve will blow the steam off into your basement drain.  This tends to be a frequent unexcitable problem with indoor wood boilers.
A wood boiler that requires a water pump will not provide heat during an electric power failure or water pump failure.

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Wood Pellet Stove or Furnace 

Either one will save on your gas or oil fuel bill.  However, it may not save much on your overall heating costs.  Wood pellets are generally very expensive compared to a wood log.  Remember, there are about 8,000 Btu’s per pound of usable energy in either one.
A wood pellet stove or furnace that requires electricity will not heat your home in the event of an electric power failure.

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Corn Stove or Furnace 

Corn has the same btu’s per pound as wood. Dried corn with 12% moisture weighs about 56 pounds per bushel.  It has about 8,000 usable btu’s per pound. A bushel of corn will provide about 44,800 btu’s.  If a corn furnace is 70% efficient, it will deliver about 31,360 btu’s of heat into your home.  The heat loss in an average 3-4 bedroom home with reasonable insulation is about 25-30 thousand btu’s per hour on a winter day.  You can check your heat loss with a heating professional or your utility. It would take about 6 bushels a day to provide the same amount of heat as a gas or oil furnace that would put out an average of the same amount of heat.


Another consideration for homeowners opting for a corn furnace.  You must burn clean corn without fines (dust).  You must have a connection with a corn farmer that is willing to sell several sacks or a pickup load when you require it.  It is usually difficult to start a corn fire burning. Many require a lot of maintenance.  Corn produces a clinker harder than coal that can be difficult to remove.

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Electric Power Supply 

A wood stove, add-on wood furnace or a multi-fuel wood furnace will provide heat during power outages because they become gravity furnaces, just like our grandparents had. Heat rises, cold air falls. Outdoor furnace/boilers that require electricity to operate the pump will not provide heat.