Horizontal bench versus vertical bell

I apologize if this has already been talked to death, but in studying both rocket mass heater and masonry heaters, I am seeing these similar solutions converge into the RMH with various types of bells.  That has me wondering about the advantages or differences between these two design options.

It seems to me that the RMH with exposed barrel, connected to a low horizontal bench, not only provides more immediate heat from the radiator (barrel) but also heats up a mass that is lower to the floor, keeping more of that heat at "living" levels within the room.  The batch box designs and traditional masonry heaters frequently port the hot gases they create directly into a large, vertical bell or mass.  In contrast to the RMH, it seems like these would not provide immediate heat and the large vertical mass might not heat a living space as effectively?  I understand that both store and radiate heat, and that the idea is to limit convection, which is a less effective means of heating, although it does tend to distribute heat more evenly.

Is it safe to say that masonry heaters have always included all of the attributes of a well-made rocket mass heater, sans the immediate heat provided by the barrel?  By that, I'm asking if masonry heaters are as efficient in how well they convert wood to BTU's, and how well they store that heat energy to warm living spaces?  Are the batch fed RMH systems that flow directly into a vertical mass (bell) better labeled as a masonry heater?

Maybe this is a long-winded way of asking:  What are the differences between a rocket mass heater and a masonry stove, and is one superior to the other?  I know it is much more expensive to build the masonry heater!

Hi Jason;
Good question!
I guess the biggest difference would be that a masonry stove is instantly acceptable to your insurance company and a RMH although very similar is not instantly accepted.

A masonry stove, if built by an experianced mason will be beautiful, warm , safe and last for years. It will also cost over $10,000. But the insurance man will have no problems.
A RMH either a J tube or a batchbox with either a bench or a bell , could cost you $500-1000 at the most. They can be beautiful, warm,safe and last for years. The insurance man most likely will have issues...

In my opinion there is no difference between the two style stoves. They both heat a large mass with a super hot clean burn and then slowly release that heat back to your home.

A bell or stratifacation chamber can be built as a J tube or a batch box. Both can use a barrel if instant heat is desired.  They can be built low as a bench or as a taller containment.
A traditional bench with pipes filled with rock and cob, will hold heat longer than a bell.

Hi Jason

I agree, the two are very similar in their outputs, except for the expense you mentioned.

I would add another dimension and  insert the kachel ofen. An older European type stove that uses the principle of small sticks, open oxygen supply, short burn, and baffles leading the 1000 F exhaust through  a mass maze before letting go of the greatly cooled gases.  It then gives off a steady heat from the masonry of the stove,

This stove appears pretty easy to build, and the principles may be more or less identical to masonry stoves, but I'm not quite sure what the differences might be.

There are lots of plans about for this type of stove, and it can take many forms, including  a wall style mass that can be used to heat two rooms simultaneously. I wonder if it is enough like a masonry stove to appease insurance people

The biggest difference between the two is. By US law  a true masonry stove must be built with 2 skins making it nearly impossible for exhaust to vent indoors.  
Have brick bell rmh's been called a masonry stove ? Yes but by legal description they are not.

Is that a national law,  an insurance definition,a stove builder definition or a widely accepted state/ county definition?

This is what I found with a quick search.
If you are building a masonry heater, you typically will have to deal with your building inspector and also with your insurance company. The insurance company will usually want to know that your are installing a “listed appliance”, and\or that you have a building permit.
A listed appliance carries a tag from a recognized testing laboratory stating that it has been safety tested for clearances to combustibles in accordance with the applicable U.L. (Underwriters’ Laboratories) standards. The clearances will be spelled out on the tag.
Listing is possible with factory-made heaters, but not practical for site-built units. These fall under the building code, which carries provisions for clearances to combustibles for masonry fireplaces and chimneys. The building code allows the local authority to recognize a masonry heater as “equivalent to” a masonry fireplace.
An increasing number of code authorities now recognize masonry heaters specifically. The new International Residential Code (I.R.C.) is currently effective statewide in 27 U.S. states, and used at a local level in another 11 states. Eventually, this code will replace the various existing codes in all 50 states. The IRC extends the masonry fireplace and chimney sections of the codes with a masonry heater section. It
carries some seismic provisions, and then references the provisions of ASTM 1602 E - 01 “Standard Guide for Construction of Solid Fuel Burning Masonry Heaters”.
Code provisions for masonry fireplaces and masonry heaters typically specify the following:
Clearances to combustibles from: • The firebox opening • Cleanouts • The masonry itself Materials and minimum thicknesses for: • The firebox • A non-combustible hearth extension • Other surfaces If a prefabricated non-masonry chimney will be used, a U.L. listed “anchor plate” is required to make the connection from masonry to metal.
In 1983, a group of heater masons and other interested parties formed a trade association, the Masonry Heater Association of North America (MHA). One of the main reasons for forming the association was to address the building code question. This is being done as a task group under the auspices of ASTM, which is the world’s largest consesus standards organisation.
ASTM E 1602 - 1 “Standard Guide for the Construction of Solid Fuel Burning Masonry Heaters” builds on the code requirements for masonry fireplace and tightens them up for application to masonry heaters. For example, clearances to combustibles are doubled from 2 inches to 4 inches.
The Masonry Heater Association also conducts ongoing training for members, and has developed a heater mason certification program that adheres to a strict set of published standards.

wow, great to know, thanks

The essential difference between masonry heaters and rocket mass heaters is that the rocket heater has a specific kind of super-efficient combustion zone.  All the other variations can be incorporated in either category (The RMH is actually a subset of masonry heater), though masonry heaters typically do not go long and low.

A double-skinned bell can easily have as much mass and heat retention as a ducted bench and hold heat as long. The low-lying heat source has an advantage in function, which has to be balanced with all the other considerations. A bell can easily and profitably have an access panel built in for riser inspection and maintenance, which will give some instant radiator effect.

The principle of a kachelofen is identical to other masonry heaters; many European countries have their own traditional styles, with specific characteristics. A kachelofen is particularly decorative, with glazed hollow tiles making up the surface and filled to become part of the mass.

Aside from the double skin requirement, a legal masonry heater must be built or supervised by a recognized professional; one could supervise an RMH build which complied with applicable code provisions, and most likely be acceptable to code enforcement and insurance. The expense of a professional mason is one of the major differences between a common RMH and a "masonry heater". Either can be built with all new, fancy materials or scrounged parts.

Thanks Glenn, you confirmed what I suspected,, that the highly insulated heat riser is the main difference between both. I have thought a lot about the wall style mass heater/kachelofen, and with horizontal "risers" no longer being laughed at, it seems like making a highly insulated tunnel (horizontal or vertical) as the burn chamber, the secondary mass could take any form it wants and preserve the RMH efficiency.

Without a vertical riser, good combustion and draft would depend completely on a good chimney. Also, the sharp angle from burn tunnel to riser is part of the mixing and turbulence that makes for full combustion. Matt Walker's designs use a port for turbulence, not just a long tunnel.

Another consideration was expressed in a different thread:  The overall mass and its footprint/distribution in your floor plan can be quite different between a horizontal bench and a vertical bell.

To get as much mass from a vertical structure, it likely will need to have a secondary layer added to the outside, like a conventional masonry stove.  Presuming both have the same overall mass and weight, the support structure under the masonry stove would need to be a lot stronger.  The bench spreads the mass out over a larger area, so it's possible the floor or ground under it will not need to be reinforced quite as much.  I know the largest masonry heaters require a very large and heavy base.  It is quite common to actually design at least a portion of the house around that singular structure, due to its size.

I have pictured building a new home with two stories, where the RMH or masonry stove is erected on a large concrete pad on the lower level, dividing that room entirely in half.  It would extend upward into a large bell that divides the upper level in half with a stone half-wall as the top of the bell.  I'm sure a structure that size would weigh several tons, but would effectively heat the main rooms of both levels.  It would probably take a full day or more to completely warm up, but should then serve to moderate the temperature very effectively for quite some time.

Oh yeah, I forget that aspect.  I'm using an assisted draft, a small 10 watt fan mounted at the end of the exhaust pipe where it is cool enough the fan doesn't have to be special high temp or anything, and it gets pretty easy to start smokeless fires.

I had to install that fan when I started doing batch box stuff last year. I later found the 1 inch CF liner I had put in my 8" riser (left over from the j tube) had swollen and was restricting the flow to about 4-5 inches dia instead of 6. Thinking about it, that restriction may have been the main reason I had to assist the draft, I had been thinking that was part of the batch box design limitations.

Jason, that design sounds very ambitious, and indeed that amount of mass would require some good engineering skill just to figure tonnage and foundation--soil structure analysis, width and depth of foundation, - talking about a mountain of stone. Not something I would want to work on, but I'd sure like to see it if you build it