Most Wood Furnaces Are 30% Efficient

Marvin Pirila

With the right stove, the hardest wood, and firewood properly seasoned, you can have an efficient fuel source.

Did You Know That Almost All Wood Furnaces Are 30% Efficient?  Probably not, because unscrupulous dealers don’t want you to know…the good news is that there is a wood furnace that is far more efficient, and better yet, a multi-fuel/combination.

Regular Wood Furnaces lose 40% of its Energy in Unburned Gases 

Most solid fuel furnaces and stoves are grossly inefficient compared to oil or gas appliances for various reasons:
  -Wood burns at different temperatures because of the different amounts of water in it.
  -Wet wood burns at a lower temperature and is therefore less efficient because of incomplete combustion.

Forty percent (40%) of the energy in wood is unburned gas released as wood burns. This gas fails to ignite and release its energy because it starves for oxygen. To burn these gases, an injection of a controlled amount of air just above the flame is necessary. This action increases the efficiency of the appliance.

A lot of smoke out the chimney indicates that wood energy is being lost and loss of efficiency.

Some unscrupulous wood furnace and wood stove manufacturers advertise their products as having very high efficiencies. In comparisons to newer oil or gas furnaces, tested with certified and accepted equipment, these statements prove to be false. These tests show that their true efficiencies are 30% or less, very different from those of newer oil or gas furnaces. 

The American Gas Association (AGA) and Underwriters Laboratories (UL) use the one accepted test method that measures the CO2 and temperature of the stack. The good news is that there is a wood furnace that burns wood with much greater efficiency. The added bonus is that offers central heating using multi-fuel or combination wood/gas, wood/oil or wood/electric furnace The homeowner can burn wood as a primary fuel, using the gas or oil burner to ignite. 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. These furnace components make this furnace extremely efficient and use a fraction of the amount of wood of an outdoor boiler, other wood furnaces, stoves, or indoor boilers without these features. The Yukon-Eagle Multi-fuel wood is UL Listed (approved) to heat your home with wood or coal without electricity. Other benefits of multi-fuel and combination fuel central heating furnaces are optional air conditioning, electronic air cleaning, and/or humidifier. 

Facts about solid fuels

Wood and all solid fuels such as coal, corn, and grain have the same 12,000 BTU’s per pound of energy. Other facts include:

  -Hardwood (oak, maple, etc.) is a better burning choice than softwood (pine, conifer, poplar, etc.) because it is denser (heavier per square foot)
  -A fresh cut tree has about 50% moisture content
  -Split wood, air-dried for a year, has about 20% water content. This lower water content is one reason why air-dried wood burns better and hotter than freshly cut wood, reducing creosote formation
  -After deducting water content, heat values of woods have about 8,000 Btu’s of usable energy per pound.

Wood stoves come with some downsides

All wood stoves produce heat but there are some downsides. The room they are in gets most of the heat while rooms farther away are cooler or cold. From a heating efficiency standpoint, you should 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. Most wood stoves are what they call “air tights”. A better word may be “creosote generators” because once 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 failure, 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 are expensive to replace. In contrast to a wood stove, a wood-burning appliance has secondary air to burn the creosote and a circulating air blower to blow the heat away from the firebox. 

Wood fireplaces are grossly inefficient

A wood fireplace is extremely poor for heat efficiency. The EPA sponsored a meeting in Portland several years ago where 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”. While fireplaces are great to watch wood burn, don’t buy one to save on your heating bill. That is, unless it has a massive heat exchanger and blower above the fireplace. Note: A plenum is a separate space provided for air circulation for heating, ventilation, and air-conditioning and typically provided in the space between the structural ceiling and a drop-down ceiling. A plenum may also be under a raised floor.

Add-on wood furnace 

Add-on wood furnaces, also known as a sidekick or furnace helper, are simply furnaces used in conjunction with your primary gas, oil, or electric furnace. The warm air sheet metal plenum at the top of the wood furnace connects to the warm air plenum of their primary furnace. A hole in the lower part of the wood furnace casing connects to the return air duct on their primary furnace. Sometimes, back draft dampers are used so heated air in the wood furnace does not re-circulate back through the warm air plenum on the primary furnace. An add-on wood furnace is usually in your basement next to an oil, gas, or electric furnace. It usually has its own thermostat and optional circulating air blower. Add-on wood furnaces without blowers utilize the blower from the primary furnace to circulate its heat. Add-On used as a Wood Stove Add-ons work as stand-alone wood stoves when not connected to any ductwork. Stand-alone add-ons are not as efficient as add-ons connected to a furnace with a blower. Heated air stays close to the wood stove without the presence of a blower. 

Central Heating using a wood furnace 

A central heating wood furnace is either a stand-alone wood furnace that has its 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, with its own duct system, is common used in homes with electric or hot water baseboard systems. The stand-alone system allows you to add whole house air conditioning, electronic air cleaning or humidification. 

Outdoor Wood Boiler 

Outdoor wood boilers are also known as outdoor furnaces. Usually these systems work in conjunction with an indoor gas, oil, or electric furnace. These systems heat water that is stored between the firebox and an outer casing. A water pump transfers 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. These are “open systems” where water is subject to air. Often, this destroys the firebox in just 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. In comparisons to indoor furnaces or wood stoves, these systems use huge amounts of wood. 

Disadvantages of an Outdoor Wood Boiler 

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. These systems burn wood with a low fire, so even 180-degree water in the water storage system is very cold to a wood fire. When the temperature of the firebox cools, creosote forms on it, acting as an insulator that restricts heat from the firebox to the water. This adds to the boilers inefficiency. In fact, State Farm Insurance Company requires wood boilers be 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/boilerstories for more information. 

Indoors Wood Boiler 

Generally, indoors wood boilers accompany a gas or oil boiler that uses the same “closed” water system. This ensures that air cannot get into the water system, which can cause corrosion and failure of the primary boiler along with the wood boiler. The water in the wood boiler, being only 180 degrees or cooler, is susceptible to the same creosote problem as the outdoor boiler. Additionally, at 212 degrees, steam forms, and the safety valve will blow the steam off into your basement drain. This tends to be a frequent problem with indoor wood boilers. 

Wood Pellet Stove or Furnace

Wood pellet stoves and furnaces, in comparison to oil and gas, 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.

Courtesy of www.yukon-eagle.com, the world’s leader in wood furnace technology.