Identifying the Proper Masonry Heater Size
Efficiency claims in literature regarding masonry heaters vary, and little information exists on how researchers acquired the figures. The following discussion provides basic information on how we achieve the figures. Keep in mind there are a number of variables that affect any specific circumstance, such as weather, location, etc. Sometimes the ultimate decision on the size of a heater is as much an art as a craft or science.
Net Energy Yield of Wood
While energy content of wood can use volume or weight measurements, split wood is not of uniform size. Therefore, weight proves more practical for laboratory tests. Rather surprisingly, the differences in the weight-based energy yield between diverse tree species are not very large. However, pound for pound dense hardwoods, such as oak or hickory, are only about half the size of pine, spruce, fir or cedar.
Reducing the fuel’s moisture content is the only thing you can do to improve the above figures. You must multiply the figure by the overall thermal efficiency of the stove, heater or fireplace. The difference can be huge and can range from -10% to +90% depending on the unit used.
Masonry Heater Radiant Heat
A number of components determines the amount of the heat actually transferred into the room, as follows:
- Combustion efficiency. The efficiency of all new technology stoves is in the 90 per cent range. Older stoves and heaters will vary.
- Heat transfer (thermal) efficiency. The firing strategy of the operator and the thermal mass of the heater determines how much of the heat generated in the fire box escapes through the chimney into the atmosphere. Manufacturers design newer metal wood stoves and fireplace inserts with fire brick lining to burn cleanly. Nevertheless, because they lack the thermal mass which acts as a heat exchanger, they still have poor thermal efficiency. Although the overall efficiency equals the combustion efficiency multiplied by the heat transfer efficiency, overall efficiency does not fully describe the true efficiency of a heater. Consider the following.
- Continuity of output. If a stove or heater has little or no thermal mass in which to store heat, it transfers all the heat into the room during the burn time. If you allow the fire to burn rapidly, the room becomes overheated. After the fire dies down, the room becomes underheated and uncomfortable as it cools down quickly. Because of the higher temperature differential between the room air and the outdoor air, an overheated room loses more heat to the environment. With a masonry heater, this uncomfortable and inefficient cycle does not exist. The fire transfers heat into the masonry (thermal mass), which radiates it slowly into the room. This creates a comfortable, safe and healthy environment.
- Partial charge efficiency. Because non-smoldering combustion requires high temperatures, many stoves do not handle small fuel charges efficiently. Soapstone masonry heaters can handle all reasonable fuel charges with equal efficiency. A small fire on milder days of the cooler seasons is possible and efficient. The heater still stores and distributes heat the same way as on colder days, which is an important feature.
Soapstone masonry heaters combine all four of the above categories!