Why is the barrel not covered in a RMH?

I am curious as to why the barrel is not wrapped in thermal mass (i.e. 3-6" of cob) although that would slow the initial radient heating of the room, wouldn't that provide a longer, more even heating of the room?

Ease of removal. It is my understanding that "fly ash" (the light gray floating stuff) needs to be cleaned out periodically. Removing the barrel makes it much easier to vacuum out that section. Plus you can inspect for creosote.

BUT - I do not have a RMH, so I'm just regurgitating info I've read here.

- L. Daniels

Mine has about 2" of cob covering it. Boy, did it steam up the house the first time I fired it up and drove all the excess moisture out of the cob!

I did put a piece of iron pipe with a pipe cap down at the bottom though, so I would have a clean-out that I could access with the shop-vac.

James Fleming : Often what we call a barrel is actually a 55 gal capacity Drum with one end completely removable and held in place with a clamping band !

With this style Barrel/Drum an easily removable Top allows easy cleaning. Other times this style drum is dropped down and centered over the Heat Riser with the

open end down, and actually allows about the best access to totally clean the Transitional and Ash Pit areas ; extra care and the reapplication of cob to create a seal

at this location during cleaning makes this a pretty even trade-off! Y.M.M.V.

Whether we call it Science or Black magic, The prompt or initial heat energy radiated off of the Barrel Allows for the rapidly cooling, heavier Exhaust Gases to sink

Down the outside of the Heat Riser, simultaneously being replaced with hotter and lighter gases Rising within the Heat Risers Center section, this push-me pull-you

pumping action drives the flow of still hot Exhaust Gases 50+ feet horizontally through Your Rockets Thermal Mass !

As each RMH that is ever built is a custom job done by different Individuals, who bring individual skills to their Rockets creation -each working rocket will have their

own character and running attributes, Often the lower 1/3 to 1/2 of the barrel can be covered in cob with no noticeable effect on its Roar or operation !

I hope this is timely and Helps ! For the Good of the Craft ! Big AL

Late Note - see link below :

James Fleming wrote:I am curious as to why the barrel is not wrapped in thermal mass (i.e. 3-6" of cob) although that would slow the initial radient heating of the room, wouldn't that provide a longer, more even heating of the room?

Yes but what about instant heat when you're room is cold. In a traditional masonry stove it takes about an hour before the heat makes it through the mass and starts heating the room, the drum gives it straight away and the mass stores it for the long run, so you have the best of both.

The bare barrel serves two positive functions: quick radiant heat to balance the time it takes for the mass to start radiating, and the cooling effect of the radiation is said to add to the "pump" action cooling the superheated gases that rose through the heat riser so they become denser and more willing to fall to the base of the barrel and enter the mass ducting.

The inspection aspect is also important, though there could be other ways to achieve that.

Thanks all - it seems that the fact the bare metal loses heat more rapidly is the magic of it all. How hot does the outside of the drum get? Should one put a guard or screen of some type to prevent someone from touching it accidentally?

The sides of the barrel can get very hot, up to 5-600 degrees F or more depending on the system, lower near the base of the barrel. Usually systems are built with a base skirt of cob or masonry that keeps people from accidentally getting close enough to brush against the barrel (this is needed for enclosing and insulating the combustion zone anyway), and some systems are designed with the ducting returning to the vicinity of the barrel before rising to the chimney as a visual reminder that "this looks like a hot area", as well as picking up a bit of radiant heat from the barrel to improve the chimney draft.

James Fleming wrote:Thanks all - it seems that the fact the bare metal loses heat more rapidly is the magic of it all. How hot does the outside of the drum get? Should one put a guard or screen of some type to prevent someone from touching it accidentally?

My two inches of cob can get up to 140F when the RMH is roaring full blast, so consider that a lower bound. Metal is going to be quite a bit hotter, enough to give a nasty burn if touched accidentally.

- as compared to the surface areas of a Wood stove that can easily match the external temperatures ! Big AL !

John Elliott wrote:

James Fleming wrote:Thanks all - it seems that the fact the bare metal loses heat more rapidly is the magic of it all. How hot does the outside of the drum get? Should one put a guard or screen of some type to prevent someone from touching it accidentally?

My two inches of cob can get up to 140F when the RMH is roaring full blast, so consider that a lower bound. Metal is going to be quite a bit hotter, enough to give a nasty burn if touched accidentally.

Well, usualy, you're warned well in advance by the radiated heat!


And Allen, despite what is writen in the book, i realy can't believe in the pull effect in the barrel can be strong enough to be worth anything. For that instance, when you say, "when you put the barrel on, the chimney goes up". Even when an experimental has no horizontal pipe after, this is the case. So, there's no pull in the barrel.

For me, to have any type of pull, the gases would have to be cooler than room temperature, below feed tube's mouth height. Because the temperature differential, and hence the presure differential has to occur with the room the rocket is in. It's not because it's cooler than the gases in the heat riser, that the gases in the barrel will create a pull. They will ease the downdraft process by being less hot. But will never be heavier than room temp gases.

I think you're right about the barrel giving no actual push/pull, but even "easing the downdraft by being less hot" has value in a critical system where all the factors contribute to success.

Since the gas path after the riser needs to go downward in a typical RMH, it may be easier for that to happen if the gases in the barrel are distinctly cooler than in the riser. Relatively, the barrel gases are heavier than the riser gases and will tend to drop. If they were the same temperature there would be no inherent flow direction in that segment, and all the push/pull would have to be supplied by the chimney and possibly the feed/riser differential.