Wood burning stoves can be roughly divided into 3 categories based on how much of the heat from burning wood is stored vs. immediately released into the surroundings. The 3 types are;
- cast iron or steel stoves
- masonry heaters/ovens
- hybrid stoves or heaters such as the rocket mass heater
Depending on your application, each stove type has its pros and cons, Dragon Heaters can be configured as any one of these three types depending on your requirements.
Some materials can gradually absorb heat and just as gradually release it. We have another blog which lists the various candidate materials for this purpose and how well each of them does. The antithesis of heat storage is something which gets hot fast, radiates its heat, and cools off fast. An example of this is steel or copper.
Cast Iron / Steel Stoves
This first category will heat up its immediate space in a hurry. All the heat from the fire is radiated by the metal into the space. The advantages of this type of stove are:
- quick to install
- small footprint
- moderately priced
- easy to obtain
- heats space quickly
- relatively lightweight, so more easily transported
This is ideal for a space which is not continuously occupied and needs to heat up quickly. For example a workshop or perhaps a cabin not frequently used.
The disadvantages of this type of stoves are that they go cold quickly and, consequently, have to be fired frequently.
In the prototype of a Dragon Heater shown to the right, a steel barrel is heated by the exhaust from the fire. A second barrel can be added for even more radiated heat. When the gases cool off, they fall down and are removed from the building through the chimney pipe.
The opposite approach to a steel box is a traditional masonry heater. These are very common in Europe, and Russia. They are designed to collect all of the heat via a massive thermal store and have no provision for highly conductive materials which will release heat quickly. Instead, the exhaust is routed through bells or large flues made of heat absorbing materials, until most of the heat has been absorbed, and it exits out the chimney. These designs are usually heavy, permanent, and occupy a large area within a building (even during the summer).
There are some masonry heater kits, which help reduce cost, but in most cases, a highly skilled designer and builder are required, making masonry heaters expensive ($20,000+). They take a long period of “firing” to warm up and a correspondingly long time to cool down. Because the heat is stored in the masonry material, it is even and comfortable; no one has to get up in the night to light a fire to keep the room warm.
For more pictures of some modern masonry heaters, check out the Masonry Heater Association website:
In between these two extremes is the Rocket Mass Heater as described by Ianto Evans and Leslie Jackson in their book, Rocket Mass Heaters. Their design is inexpensive to build. It consists of a combustion system of feed tube, burn tunnel and heat riser. The heat riser is covered with a steel drum which absorbs and re-radiates a lot of the heat from the fire. In that way, it is similar to a cast iron or steel stove. In our tests, about 2/3 rds of the heat is radiated into the space from the barrel or drum, and 1/3 is available for thermal storage.
After leaving the steel drum, the exhaust is routed through a horizontal flue buried in cob (clay) which is made into a bench similar to a masonry heater. The cob absorbs the heat and releases it slowly. Thus, the rocket mass heater, in theory, yields the same advantage of not having to get up in the middle of the night to light the fire as the masonry heater at a lot less expense.
Traditional rocket heaters are labor intensive to build, are tricky to build correctly, and are problematic aesthetically unless you have an adobe or rustic style house and have a large space requirement. By offering pre-engineered shippable rocket heater cores, we hope to solve many of these drawbacks.
Dragon Heaters can be built to operate as any one of these style heaters depending on which one best suits your application.