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How Wood Stove Efficiency is Measured & Improved

Quality Wood Burning StovesWhen searching for the best wood stove, stop by a UFS Member Store showroom and view a number of beautiful wood burning stove models to help narrow down your selection. Our network of privately owned hearth stores offer a great selection of wood stoves at great values.

The point is often made that wood stoves are more efficient than traditional fireplaces and can, therefore, help to cut energy costs. Open fireplaces are usually measured at only 10% while wood stoves have efficiency ratings between 30% and 90%. If you have ever wondered how heating efficiency is measured, here is the answer.

Wood stove efficiency is: The amount of chemical energy in firewood that ends up in the room as opposed to up the chimney. Measuring it requires conditions that are carefully controlled, and it is done in laboratories especially designed for environmental testing.

How Wood Stove Efficiency is Measured

Figuring out the amount of chemical energy stored in firewood is not difficult because BTU (British Thermal Unit) ratings per cord for most types of firewood have already been rated. Various tree species have slight variations in BTU ratings, but there is practically the same amount of energy in every kilo of seasoned (dried) firewood. Measuring the amount of energy going into a wood stove involves merely weighing the wood before it is placed in the stove.

The fluid dynamics taken into consideration in the complex process of calculating the amount of heat transferred from a wood stove into a room include measuring both:

  • Air speed of the heat being moved into the room, and
  • The air temperature in different areas of the room.

Comparing the percentage of energy that is transferred into the room to the total amount of wood burned determines wood stove efficiency.

This calculation arrived at by researchers is actually impossible to measure in the home; but that’s not a problem, since manufacturers provide the efficiency rating for each wood stove sold.

Whatever the rating of your wood stove, there are steps you can take to improve efficiency. Factors which affect the efficiency of wood-burning stoves are divided into these two categories:

  • Those that affect wood-burning (combustion) efficiency and
  • Those that impact the transfer of heat from the stove into the room.

Ways to Improve Combustion Efficiency

Combustion efficiency is about keeping as much energy from the firewood as possible inside the wood stove. Fires with combustion that is more complete are hotter and also more efficient.

Firewood’s chemical structure breaks down into small molecules as it burns; the hotter the temperature, the smaller the molecules. You want the fire hot enough to burn the very smallest molecules, which are carbon monoxide and hydrogen. In cooler stoves, the wood doesn’t break down as efficiently, the larger molecules don’t burn properly, and the result is smoke.

Cheap Enerzone Wood StoveBut the most efficient burning requires more than high temperatures. Extra oxygen is a secondary component. Modern, high-efficiency wood stoves include a feature in which a secondary burn takes place. Preheated air is added to the firebox in a separate area, where combustion gases are burned (the secondary burn). Air inlets are included in the design of the top wood stoves, and the turbulent high velocity air jets result in a very clean burn.

How Heat Transfer is Improved

The way a stove is built determines how well it heats a room. Purchase a stove with the highest efficiency rating in order to have more heat conversion. Features of the best stoves include heat exchange components in the design. Fireboxes are well-insulated in the top designs. Even the weight of a wood stove is an indication of heat transfer capabilities; the heavier, the better. Wood stove design and insulation both affect how much heat is kept in the main firebox, which, in turn, affects how much heat is released into the room.

It helps to have a better idea of what heating efficiency means when you shop for wood stoves and see that some stoves have higher ratings than others. A more expensive high-efficiency stove, if used often, pays for itself quickly — since you use significantly less firewood to heat your home. Stoves with the highest efficiency ratings continue to cut energy costs for many years.

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