Wood burning stoves are an excellent source of heat, either as a primary or as a secondary source. A wood stove emits a warm, dry heat that no other source of heat can produce.
I have burnt wood for almost 40 years and can personally attest to the quality of the heat and the savings wood heat provides.
Can I operate a wood burning stove more efficiently? Yes, there are many ways to improve the efficiency of a wood burning stove.
There are three areas we can look at regarding the efficiency of a stove. They are:
- Type of stove.
- Fuel (Types of wood and variations)
- Operation
Let’s look at each area individually beginning with a history of wood burning.
Wood Stove History
Burning wood to keep warm has been the best or really the only way to heat a home for thousands of years. Finding a way to bring the campfire into the home was a natural progression in the advancement of house construction. The early fireplaces and chimney served to draw the smoke out of the room and above the house and to the outdoors.
This advancement brought heat indoors and drew the smoke to the outdoors. The problem was how much heat was going up the chimney. It required large quantities of wood to provide a small amount of heat in a small area. Larger homes required fireplaces in multiple rooms to warm the whole, or at best portions of the home.
Insulation had not been invented yet so windy days would even be colder as the frigid air would pass easily through the walls. Supplies of wood were quickly consumed in urban areas for construction first, and then heat. Conservation was not a priority (survival was more persistent). Wood became scarce. Coal began to surpass wood as a heat source as early as the 1800s
Early wood stoves were produced from cast iron in the mid-1600. These were crude, but by setting them out in a room, and using a metal chimney pipe, the radiated heat improved the efficiency by almost 20%. This was quite a jump when the average fireplace with an open hearth loses 90% of the heat up the chimney!
In 1740 Benjamin Franklin designed the first Franklin stove, and another jump ahead in technology was achieved. His stove allowed some adjustments to the air flow into the stove and had a small cooking area on the top also.
Jump Ahead
It was not until the mid-1970s and the “Energy Crisis” (Never miss the opportunity of a good crisis) that several ingenious inventors came up with versions of an “airtight” stove.
This was just a box with legs and a door that sealed the air from entering the stove or at least dramatically reduced the draft of air from rushing up the chimney. Air vents controlled by a closing gear or pipe value could actually cut off the flow of air into the stove.
This allowed wood burning stoves to burn dramatically less wood and at the same time produces more heat for a home. Home built versions were popping up across the US. Several manufacturers began building and selling stoves, and by the early 1980s, the wood stove industry was in full swing.
The operation of a wood burning stove is a simple process. A fire needs three elements to burn. The three elements are heat, fuel, and air. If you remove any one element, the fire goes out. Reduce anyone, and the fire burns lower. The goal is to control the heat by using good fuel (wood) and limit the amount of air introduced into the firebox.
The stove firebox needs to be a sealed box (welded), and the doors need to seal to limit the airflow into the stove. Fiberglass rope seals infused with graphite have been developed and are placed between the stove wall and the door to seal the doors safely.
Air vents have been added to the stoves that allow minute adjustments in air flow to dial the temperature up or down just by opening or closing the air flow to the fire.
The air vents were typically on the door or along the side of the stove near the front. They were controlled manually (and still are for most stoves) by turning a valve or sliding a lever. The precise air control is what made these new stoves operate more efficiently.
Some of the stoves included glass in the doors to enjoy the fire. You could actually watch the fire burn hotter as the vents were opened and vice versa.
Efficiency
New Stoves
With this history in mind, how can a wood burning stove operate more efficiently? The newest wood burners are capable of being up to 70% efficient. This means only 30% of the heat is traveling up the chimney.
That is a pretty big jump from 90% of the heat escaping up the chimney and being wasted. How can they do that?
With a little help from the EPA, stove manufacturers have been required to reduce the PM (Particulate Matter) they generate and expel up the chimney. The newest stoves reburn the smoke either in a catalytic converter (similar to an automobile) or over a baffle by adding heated air to the smoke and burning off the PM. This process increased the efficiency of the stove while reducing emissions.
Wood
The type of wood you burn has a significant effect on the efficiency of the stove. Wood, like any fuel, has a BTU heat output value. The BTU (British Thermal Unit) value of wood is measured in thousands of BTU’s per cord of firewood. Hardwoods have a much higher value (BTU output per cord) than softwoods. Typically, a hardwood will burn longer but more importantly, the hardwood, with the higher BTU output, will burn hotter. This will provide more heat for a longer period of time! More efficient!
This quickly becomes obvious to the person cutting, splitting, hauling, and stacking firewood!
This chart lists common hardwoods and the BTU output per cord. Choose your firewood wisely, and you will burn more efficiently.
Hardwoods and Their Statistics
| Species | Green Weight/cord | Dry Weight/Cord | Millions BTU’s/Cord | Ease of Splitting |
| Apple | 4850 | 3890 | 27.0 | Medium |
| Ash, Green | 4180 | 2880 | 20.0 | Easy |
| Ash, White | 3950 | 3470 | 24.2 | Easy |
| Beech | 3760 | 27.5 | Easy | |
| Cherry | 3690 | 2930 | 20.4 | Easy |
| Dogwood | 4230 | 27.0 | Difficult | |
| Hackberry | 3980 | 3048 | 21.2 | Easy |
| Black Locust | 4620 | 4020 | 27.9 | Difficult |
| Maple | 4680 | 3680 | 25.5 | Easy |
| Sugar Maple | 3900 | 2750 | 19.0 | Medium |
| Mullberry | 4710 | 3712 | 25.8 | Easy |
| Red Oak | 4890 | 3528 | 24.6 | Medium |
| White Oak | 5570 | 4200 | 29.1 | Medium |
| Osage Orange | 5120 | 4728 | 32.9 | Easy |
| Black Walnut | 4584 | 3190 | 22.2 | Easy |
| Hickory | 5400 | 4650 | 29.2 | Medium |
Season Wood
Drying Wood
Wood for burning should be dried or seasoned to 15% to 20% moisture content. As you can see by some of the wood in the chart, this can reduce the weight of the wood by 1/3 (most hardwoods) to as much as 50% in some softwoods. Allowing the wood to dry and lose its moisture is necessary for the wood to burn properly. Wood that is too “wet” will waste heat as the heat created is used in drying out the wood.
Wood should be cut and split at least six months and preferably 12 months ahead, so it has sufficient time to dry. It should then be stacked in single rows, off the ground to let air move through the wood to dry it thoroughly.
Wood that has been dried to the proper moisture content will provide the maximum BTU’s per cord and will leave the least amount of ashes to be removed from the wood burner.
Properly seasoned wood will also burn “safer” as it will produce less PM or Particulate Matter. These are the contaminants that exit the chimney and stick to the walls of the chimney, causing creosote buildup.
Properly seasoning your firewood is an efficiency issue as well as a safety issue. Plan ahead to have your wood supply delivered, cut, and split well in advance of the burning season.
Chimney Care
Another item that affects the efficiency of a wood stove system is the chimney. Creating the proper draft and maintaining a clean, air moving system is essential to operating a safe and effective wood burning system.
First, the chimney system must be sized properly. Warm or hot air, in this case, naturally rises. The proper size chimney pipe will allow it to rise upward and exit the home at the desired speed and temperature. The NFPA211 document defines the proper installation and size requirement for the chimney pipe. Following these guidelines will ensure a safe chimney installation in your home.
Next, is cleaning the chimney. Burning wood and restricting the airflow into the stove will create a product known as creosote — a black tar-like substance that will coat the inside walls of the stove and chimney pipe.
If the creosote is not cleaned on a regular basis, the chimney will fill and constrict the air flow. Eventually, it could restrict the airflow enough to cause smoke to back up into the home. This is not a pleasant experience.
Inspecting and cleaning the chimney regularly (once a month) during the burning season is recommended.
An additional reason for cleaning the chimney is the possibility of the creosote catching fire from a spark or starting in the stove and working up to the chimney. A chimney fire may be dangerous and can cause additional damage outside the chimney if allowed to burn too long.
Cleaning Ashes
Another means to boost efficiency from a wood stove is to keep the stove clean. Removing ashes on a regular basis allows heat from the fire to reach its intended source. Ashes piled on the floor and walls will tend to absorb the heat and deflect it from the air or water chamber it should be heating.
Cleaning the ashes usually takes 30 – 60 minutes and can increase efficiency by 3% – 5%.
Burning hardwood that produces less ash is an efficient way to clean the stove less and allow the wood stove to heat at its maximum proficiency.
I have found oak pallets cut off ends, with no bark allow a three – four week period between cleaning ashes. When burning cut hardwood logs, I would typically clean the ashes out of my stove at a one – two-week interval.
Dry Wood
Keeping your wood supply dry is another simple trick to operating an efficient wood stove. This is not the same as seasoning wood for 9-12 months. That is of the utmost importance when burning wood. What I am writing about here is keeping your seasoned firewood out of the snow and rain for a day or two before you burn it.
This can be accomplished by storing the wood indoors just before use. This is sometimes undesirable as bugs, bees, and ants tend to come back to life when warmed.
Another way to keep the wood dry is to store it under a tarp or roofed area after seasoning. This will keep unwanted moisture (snow, ice, rain) off the wood. Even a well-seasoned piece of wood will have to be dried if it is covered in rain or snow. This “drying” time in the wood burner wastes heat on drying the wood as BTU’s are spent “drying wood” and not “heating” your home!
Door Gasket
The door gasket on a stove can easily be overlooked. They can be easily replaced and avoid allowing too much air into the stove.
The gasket is an integral part of the system that seals the stove and restricts the amount of air that the fire receives. Accurately and consistently controlling the airflow is what makes the stove work so well.
Over time, the creosote will run down the door and make the soft gasket hard. Due to the hardened gasket, the door will not seal as well and will allow additional air to enter the stove, reducing efficiency.
Gasket kits are available that come in a variety of shapes (flat, round), and sizes, and are infused with graphite to keep the gasket lubricated. These kits may also come with the glue required to attach the gasket to the stove or door face.
Replacing the gasket is an inexpensive and simple solution to what could be a major problem for some stoves. Changing the gasket once a season should typically resolve any air leak issues around a wood stove door.
Alternatives
There are several alternative methods that can be instituted to make a wood stove system more efficient. Some are alterations that require planning and installation, and some are simple items that can be purchased and installed in minutes.
Thermal Mass
A thermal mass sounds large because it generally is. A wood stove is usually not a small item, but a thermal mass can take up a large portion of the room around the stove or be part of the floor, walls, basement, or crawl space.
A thermal wall is typically a stove or brick wall, and floor system that once warmed up will hold the heat for a period of time. This mass can just hold the heat in the area or may have ducts or water pipes running through it to move the heat to the other areas of the home.
The advantage of a thermal mass is it will hold the heat for a long period of time. The disadvantage is how long it takes to bring the mass up to temperature.
Having a system that efficiently stores and distributes heat from a wood burning system requires a well-engineered system. The radiated output of the stove and the heat collection and storage properties of the mass media must be calculated. With a project and installation this large, guessing is not acceptable.
Done properly, a heat mass system can provide long term, consistent heat for an entire home.
Direct Heat
Most wood burning systems provide direct heat to a room or rooms. This is the heat radiated from the outer walls of the stove and into the room where it is located. Most other forms of home heating system (natural gas/LPG/ forced air furnace or boiler systems) are placed in areas away from living spaces. These systems are designed to keep the heat within the furnace and heat the air (forced air system) or (boiler) water that is being passed through the unit.
A typical wood burning stove will have a blower that moves air around the sides and/or top of the stove heating the air before it moves the air away from the stove. Some stoves have the ability to be connected to an existing forced air system to move air throughout the home.
The heat radiated from the outside of the stove is a bonus provided by a wood burning system. Since we all enjoy watching a fire burn (who doesn’t) and the pleasant looks of the new wood burning stoves make them a great centerpiece in a family room.
Outside Air
Air is one of the three requirements for a fire to burn. The air that feeds the fire is typically drawn from inside the room where the stove is located. If the air for the fire is fed from a single tube, we can draw that air from outside the home. This reduces the warm air that is drawn out of the home and up the chimney.
A stove that can accomplish this is a much more efficient system. The natural gas systems that are 90%+ efficient draw the air they use to burn their fuel from the outside. Every percentage point that can be gained in efficiency is a little less wood that is cut, split, hauled, stacked, and burnt. My back is feeling better already.
Monitoring the Temperature of the Stove
As was mentioned earlier in the Thermal Mass section of this article, monitoring the temperature of the stove while burning wood is critical to an efficient system. My stove, like 99% of the wood stoves in the world, has had a small, round magnetic thermometer stuck to the face of it for the past 40 years. This thermometer (I have probably only had 2 or 3 of them) has served its’ purpose and provided the temperature of the stove whenever I was near the stove.
What about the times when you lit the stove, left the door or vents open to get the stove hot quickly, and left the room? You returned to a blazing fire, smoke rolling out of the chimney and maybe even a chimney fire!
There is a temperature sensor available today that will send you an alarm, anywhere, anytime when the stove reaches a pre-set temperature. Not only will it send an alarm, but you can also view the temperature anytime on the free App on your smartphone.
In addition, it is accurate to .1 of a degree so you can know instantly if your stove temperature is rising or dropping. This is critical if you experience a blazing fire as described above. After shutting the doors or vents, knowing if the temperature is still rising or falling can mean the difference between a chimney fire and a quick burn of the inside of the stove.
When it comes to typical burning efficiency, this device will also help you know when the stove cools off too much. A low alarm can also be set, so you are notified when the temperature drops below a pre-set value. This provides you with a consistent temperature rather than the high and low of letting the fire burn out and restarting the stove from cold.
To see the only product of it’s kind, go to Tempsure.net to view the product in action.
Pellet Burners
If you would like the heat of a wood burning system without the work of cutting, splitting wood, then a pellet burning stove may be for you. Pellet burners boast the highest efficiency rating of any wood burning system. Many pellet burners are more than 70% efficient. The chimney required is typically similar to a 70-80% efficient furnace as the temperature of the output is 300 degrees F or less.
Since the pellets burnt in these stoves are factory produced, the moisture content is controlled and kept to the ideal 15%. The content of the pellets is a compacted hardwood and will burn with very little ash remaining. The pellets are available at most home improvement centers and the price is typically consistent throughout the winter.
Conclusion
Burning wood is an excellent alternative source of primary heat for a home. When the system is installed with thought given to the items mentioned in this article, long term efficiency can be achieved. Using methods such as a thermal mass system and drawing outside air for the firebox can increase efficiency even more.
Enjoying time around the wood burner on a cold winter evening can make the investment in a stove even more gratifying.