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.
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.
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.