J-tube Rocket Heater Designs

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 (with  or without ovens)
• hybrid stoves or heaters such as the rocket mass heater

Depending on your application, each stove type has its pros and cons, Dragon Rocket Heaters can be configured as any one of these three types. Some materials can gradually absorb heat and just as gradually release it. This attribute is achieved with thermal mass. The opposite 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 / Rocket Heater Barrel Build Kits

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 are:

• quick to install
• small footprint
• often have space on top for cooking
• 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 a hunting cabin. Since there is no heat storage, the space can get too hot when the fire is lit and too cold when it is not. The Rocket Heater Barrel Build falls into this category.

Comments On the Barrel Build

6" versions work with 50 gallon steel barrels, the 4" with a 30 gallon barrel. It is more efficient than a traditional wood stove plus it has options for adding thermal mass to prevent rapid cooling.

Thermal mass options for the 6" 

• a cast refractory barrel cap 
• a 2nd barrel with bricks inside
• stacked bricks surrounding the barrel 
• a small bench attached to the exhaust

Cons

• They are more dangerous due to higher surface temperatures of the stove or barrel (300-500F)
• The barrel cosmetics are not great
• Thermal mass for longer heat retention options are more limited 
• Not available for the 8" models

Masonry Heater / Castle Build 

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 (such as steel) which will release heat quickly. Instead, the exhaust is routed through bells or channels made of heat absorbing materials, until most of the heat has been absorbed where it exits out the chimney. These designs are usually heavy, permanent, and occupy a large area within a building.

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.

The castle build can be compared to a masonry heater which takes a while to warm up. By using chimney flue liners as a structure, a castle build can be constructed without the complex bricklaying required for a masonry heater, making them a less expensive option. The more efficient combustion core insures you will get more heat from the same amount of wood. 

The heat in the castle build starts from the inside of the masonry structure and penetrates into the room after an hour or two. The advantage is it will continue to warm the room after the fire has gone out.  The amount of heat stored can be adjusted for different climates and firing rates. The rate at which heat is accumulated into the system is dependent on the combustion system size. Depending on the size of your implentation it will take from 1-3 hours for the heat to migrate out to surface. However the heat will continue to migrate for many many hours, unlike a cast iron stove. 

This design approach of using large bells vs long chanels for heat capture facilitates a very strong draft, since there is no friction from flues. It provides for smaller foot print designs since the heat capture is vertical rather than horizontal and it is inherently more efficient than flues, meaning you need less mass to capture the same number of BTUs. 

The castle build also allows for some immediately radiated heat from the steel exhaust pipe.

Pros

• Can be covered in a variety of materials to fit with most decores
• Stores most of the heat in thermal mass for extended warming
• Cooler surface temperatures less than 200°F most of the time. This is the temperature that hydrondic radiators utilize. 
• Heats like a massonry wood heater but is less expensive
• Can accomodate an Oven
• Takes less floor space than a rocket mass heater
• Available in 4", 6", and 8" models

Cons

• Takes longer and requires more tools and strength to build than a barrel build
• Take more time for heat to begin radiating into the space
• Thermal mass for longer heat retention options are more limited 
• Not available for the 8" models
• It is more expensive due to the the amount of thermal mass

Hybrid Heater / Rocket Mass Heater 

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 uses inexpensive materials. 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 two-thirds 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, straw, and sand) 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 don't mind the heater taking quite a bit of floor space. By offering a 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.

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