Wood Heat Storage – Flues vs. Bells

Users often want to capture excess heat from burning wood and then have it gradually released later, overnight for example, when no one is tending the fire. One of the simplest ways to do this is by heating a significant amount of thermal mass (water, clay or “cob”, brick or stone), from the exhaust after combustion. There are a number of schemes for this and how much heat you can store is dependent on both the materials used and how the heat from the exhaust is transferred. I am going to leave the materials issue for a different article.

When using solid forms of thermal mass such as clay, brick or stone, there are two basic approaches to the passive capture and storage of heat from wood burning exhaust. One approach uses flues, another chambers or bells.


The most common approach for heat capture is to use flues. Using flues, the hot exhaust from combustion is given a circuitous route through some form of thermal mass (clay, stone, or brick). The tricky part is that the path must be long enough to allow sufficient time for the hot gases to transfer their heat to the surrounding mass, but not so long it loses too much heat and velocity, causing the stove to stall. Many masonry heater designs rely on this approach, as do most “rocket mass heaters”.

In the case of masonry heaters, the exhaust is routed through masonry lined flues. Often these flues or channels are larger than the exhaust chimney to allow additional time for capture of their heat, but they are still considered “flue” designs since all the gases move together.

In the case of rocket mass heaters, the exhaust is routed through steel pipe that is matched in size to the chimney exhaust and is typically covered with “cob” a clay based building material. This is the heat capture technique developed and outlined in the book “Rocket Mass Heaters” by Ianto Evans and Leslie Jackson.  The gases heat the pipe which, in turn, transfers the heat to the cob where it is radiated back into the room.

Rocket Heater


An alternative to the flue approach is the use of chambers or bells. A specific version of this approach is called “Free Gas Movement”. A lot of the basic research was done by V. E. Grum-Grzhimailo (1864-1928) in Russia in the early 20th century. Subsequently, Igor Kuznetsov has been developing and implementing masonry heaters using chambers also in Russia. He has also written about the physics of gas movement to maximize heat extraction and put much of his findings in the public domain.

In a bell system, the exhaust is routed into large chambers where the gases are allowed to collect. They will then, by process of physics stratify by temperature, with the hottest gases being at the top and the coldest at the bottom.  The exit point for the chamber is then always positioned at the bottom so that only the coldest gases are removed and the hottest gases remain. If two or more chambers are put in series, the hottest and coldest gases for each chamber will be successively cooler.

Double Bell

This approach has a number of important advantages.

Hot Gases are not swept out with cold gases

In a flue based system, both the hot and cold gases are intermixed and carried at equal speed to the exit. By allowing the gases to stratify, only the colder gases are being evacuated and the hotter ones are trapped and remain in contact with the thermal mass until they have cooled.

Prevents damper induced rapid stove cool off 

Because flues sweep all the gases together, if the damper is not closed “in time” the remaining hot gases are swept away along with in residue heat in the flue. With bells, the hot wood gases collect and cannot escape until they have cooled, preventing rapid stove cool off from a damper left open too long.

Gas velocity losses reduced

As gases move through flues, they develop drag. Each turn creates even more resistance reducing the chimney’s ability to pull the gases out. Too many turns or flue runs which are too long can result in a stalled and failed heater.  Conditions are not always uniform, so when designing a flue system a “draft reserve” is needed to insure proper stove operation. The problem is that providing for additional draft margin, often means compromising on heat extraction capacity.

When heat extraction is done via bells, the travel distances and directional re-routing of gases is minimized, allowing heat extraction to take place without large frictional losses. Gravity separates the hot and cold gases without introducing any form of drag on the chimney’s draw.

Improved performance during prolonged firing

In a flue approach, the longer the stove is run the hotter the flue walls become, decreasing their ability to absorb heat. However, a second chamber (or bell) will always be cooler (than the first one) and thus allow better heat extraction.

Faster removal of ballast gases

Exhaust gases from burning wood are comprised of those gases which were part of the combustion process and those that were merely heated by proximity to the combustion. Gases that do not directly participate in the combustion are called “ballast gases”. For example, nitrogen, which comprises approximately 80% of atmospheric air, is a ballast gas. Ballast gases are not as hot and cool off quicker. In a bell system where gravity naturally separates the temperatures this allows the ballast gases to be removed 1st, providing more time for the higher temperature gases to transfer their heat to the thermal mass while not slowing down the overall gas velocity. If all gases are expelled at an equal rate, as in a flue system, this is not possible.

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  1. Thanks Smathieu for your explanation of the bells and flues. I’m contemplating the purchase, this fall, of a 6 ” dragon heater. Are there instructions included with the purchase for assembly…for those of us that are not masons?
    My Best to You,
    Eureka, Ca

    • SMathieu says:

      Thanks for your comment. Have you watched the video of the 4″ build. Its pretty easy. There are complete sketchup drawing and detailed instructions included with the plans. We have several different plans and more coming so it depends on which style you decide you want to go with, you would get that particular plan.

      Thanks again

  2. With the rocket stoves it is said, that the exhaust can be horizontaly, through the outer wall, so no vertical chimney is needed.
    The low exhaust temperatures of 100+ Fahrenheit would, to my knowledge, not produce any upward draw in the chimney anyway.
    Since the dragon heater exhaust temperatures from the second bell are also low and the rocket also produces a pushing power to the exhaust gases, is there a vertical chimney needed at all?
    Building a vertical exhaust increases the costs immensly if there is none already.
    The natural gas- and oil-burners with the very low exhaust temperatures work only in a over-the-roof-system with a fan pulling/pushing the exhaust gases out.
    Thank you for your efforts,

    • We recommend a chimney. Even with rocket heaters the notion of no chimney often does not work and people have bad smoke back. We ran it just fine at the Mother Earth News fair with only a 4ft chimney in the rain, so it might work, but you may have more tendency to smoke back from outside conditions.

  3. Any chance of seeing a 6″ batchbox in the bell system, instead of the J tube? Great lookin’ products, Peter Van den Berg really dialed it in!
    Thanks for your response,

    • SMathieu says:

      The Batchbox in the 3 various iterations we tried were not stable enough to want to resell them. They need more work. We will get back to them later, but for now it is on the back burner.

  4. Awesome info Thanks a lot!

  5. Douglas Smith says:

    I generally like the design and presentation, but this sentence seems totally bogus:

    “Improved performance during prolonged firing
    In a flue approach, the longer the stove is run the hotter the flue walls become, decreasing their ability to absorb heat. However, a second chamber (or bell) will always be cooler (than the first one) and thus allow better heat extraction.”

    Sooo, then the walls of the bell do *not* heat up, decreasing their ability to absorb heat? In any case, in a flue, the pipes nearest will combustion will up first, and the flue farthest will still be cool, and able to absorb more heat. The goal would be to burn until the flue farthest from the combustion has reached it’s optimal heat storage capacity.

    • While the flue approach does have heating differentials between the beginning and end, the extra friction introduced from very long runs make it more problematic to provide for more heat storage. There is minimal drag introduced with additional bells compared to an extra 20-40 ft of flues. This problem of heat capture vs. increased drag has been a great challenge in designing traditional masonry heaters. Bells allow for easy additional heat storage, because they do not introduce significant additional friction.

  6. Hi! I am interested in the concept of ISA and its variations especially for materials such as metal and masonry could expand the concept or recommend literature or web sites?

    • The best starting point is the blogs on the physical properties of insulating vs thermal mass materials. From there you can do so rough calculations based on how much heat you want to store. This website http://donkey32.proboards.com/ does some on going discussion of the whole topic. But frankly it depends on so many different factors that rules of thumb even are hard. You kind of just have to build it, log it and see where you are. Unfortuanately, no many people log their results and post the data into the clear.

    • ISA is a manufacturer of ceramic fiber boards.

  7. I like your explanation of the bells, but in a flue system encapsulated in a thermal mass, heat is transferred into the space many hours after the burn. So even if a bell is more efficient wouldn’t the heat disapate quicker than a flue setup? Can a thermal mass be built around a bell system? Regards

    • Our designs incorporate thermal mass into the bell(s). Bell is probably a misnomer because it has nothing to do with metal or making a noise. Bells in this context are chambers which allow the gases to stratify. In a flue setup, all the gases move together. That is the only difference.
      How fast the heat dissipates under specific environmental conditions depends on the arrangement and material characteristics of the thermal mass. The conductive refractory material used to make dense fireclay bricks holds more heat and is more conductive than the clay used in cob. Soapstone is even better than fireclay bricks.
      We have a chart about this elsewhere on this blog.

  8. Do you have any examples of people using your cores in flue-type rocket mass heaters? I saw a comment on another site that, while your core is very nice and convenient, this person had found that secondary air in the burn chamber tends to cause backdrafting (smoke coming out of the wood feed). I can see where the lower pressure requirements of a bell system might not be so finicky in this respect. I am very interested in your core and barrel build accessories, but I want to put the thermal mass under the floor in an existing, above-ground, structure. This means that the whole mass will be lower than the burn chamber. I have seen where folks have successfully done this with custom-built RMH’s, but was wondering if you are aware of any installations like this using your cores?

    • Putting the thermal mass below the burn chamber results in poor drafting. We tried it. You can look at our design under 6″ Dragon Heater Bell Design – Build (parts 1, 2, & 3).

      >> this person had found that secondary air in the burn chamber tends to cause backdrafting (smoke coming out of the wood feed).

      This is the result of “this person” not following the design instructions which come with the unit. They have constrained the flow in some way.

  9. Do you have any examples of a system using your cores with water as thermal mass? I cannot put a cob bench in my building, too much weight.

    • We have not worked out the details of using a Dragon Heater to heat water. We did some exploration using an 8″ combustion system, but did not finish working it out.

  10. Roger Jones says:

    Planning a self-build timber frame house and just starting research on inclusion of some type of masonry heater. Seems your concept offers some alternatives and perhaps cost savings over other designs. Anxious to learn more, especially for use in a two story “classic” timber frame.

  11. What kind of burn time can we get. How often must it be fed to maximize the heat storage. Are we looking at only using the radiant mode of harvesting the heat, or might there be an active method. I had a Solar heating system that used Wash River rock as a storage medium, It used air as a way of moving the heat in/ and then out as needed from the rocks. Possibly with the flu system, and the wash river rock around the exhaust tube.
    Probably would need a much larger volume of rock than the Bell chamber method.

    • Burn time increases with the size of the burn tunnel. The 4″ is about 10 min. The 8″, 30-40. What do you mean by an active method of harvesting the heat? All our tested designs are on the website. They incorporate dense refractory in the form of fireclay bricks to store the heat in the bell chamber and release it when the fire is out. These designs work as we have reported them on this blog.
      If you can find the data on wash river rock, you can see where it fits on the thermal storage chart. Then, you will know how much you need.
      If you want a document on building the castle build, use the contact us form on the dragonheaters.com website.
      Thank you for your interest.

      • By active I was referring to adding a fan that would strip off the heat from the outside and allowing more heat to be absorbed. Possible being able to move the heat to remote parts of the home. A simple box fan blowing on the outside would be an active method and low tec way of accomplishing this too. The idea of the wash river rock if a method of storing and retrieving heat using forced air rather than having to deal with the problems of using water as a medium of storage. there are trade offs both ways I’m sure. It does require a lot more room (volume ) of rocks than water. But if you are building new, putting a storage area under the house would be doable. Just need an insulated box! a Large insulated box.

  12. I have bought the chimney tiles required to build the castle masonry heater and just need to cast the caps and base . Thinking about casting the base (floor plate as one piece on our concrete floor. My question is : have you folks tried running a copper coil in the second tower to use in heating water ? Would it lower the temp of the exhaust to a point that it may not draw ? Would varying the length ie. making a shorter run of copper tubing eliminate any problem ? I desire to use this water to use for domestic use or augmenting our in floor heating. I think your design is really well thought out, is further experimentation on going ? I notice that updates in designs are rather few. Sorry that sounded like a criticism, but that is not how it is intended. Just very excited about the potential of your design.

  13. According to what I’ve read about traditional masonry heaters, because they use an outer stone or brick envelope the surface temperature only reaches about 150 degrees & slows down the release of heat into the surrounding area so that they only need a burn as seldom as every 12 to 48 hours and making them safe for people to touch. How do your units compare in this regard?

    • SMathieu says:

      Dragon Heater castle builds operate on the same principle of storing the heat in the masonry material and the fact that the exterior of the build stays relatively cool. The rest of the results are a matter of BTU’s. How many BTU’s worth of wood are consumed by the fire and stored in the masonry material? How many BTU’s does the masonry material radiate into the space of your dwelling? The answer to the last question depends on factors such as the weather outside and the details about your house. The burn chamber of a Dragon Heater does a better job of extracting the BTU’s from the wood you are burning than a traditional masonry heater. Usually, traditional masonry heaters have very thick walls which must be heated up. The thicker walls would make it both longer to feel the heat when you start burning wood and longer for all that material to cool down.

      We recommend that the Castle Build be built with fireclay splits which are only 1-1/2″ thick. This approach reuslts in a less expensive unit which has to have loads of wood more often than a traditional masonry heater.

  14. Each of these methods of transmission and heat of the preservation is effective in its own way. The detailed instructions will help to understand the principles of their work.

  15. Like your reasoning!
    If flue gas temperatures are reduced (extracting heat better) is there an increased creosote build-up?….or chimney fire danger?
    Or is the efficiency of the initial burn chamber enough to make that a non-issue?

    • Creosote and other build-ups in the chimney come from the initial combustion releasing volatiles from the wood and then the fire not being hot enough to consume those volatiles. In Peter van den Berg’s design, the heat riser is a second burn chamber where the volatiles are consumed. So, creosote is not a danger even if you wanted to burn woods with a lot of sap.

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