Rather than go to all the trouble of building an RMH, why wouldn't it be possible to do the following?:

1.  Set up an ordinary wood stove at one end of the room or area you want to heat.
2.  Run stove pipe horizontally through the entire room.
3.  Surround the horizontal stove pipe with mass; e.g., cinder blocks or bricks.
4.  Run the stove pipe out of the house.

This would be a lot easier than building an RMH and recycle commonly available pre-existing stoves.  (Or, you could make a 30- or 55-gallon steel drum stove with an inexpensive retrofit kit, which is still easier than building an RMH.)  Also, instead of the heat going out of the chimney, it heats the area increasing the efficiency simply by the length of stove pipe.  Sure, it's still probably not as efficient as an RMH, but you could get away with a smaller wood stove and therefore use less wood, to to bring about savings that way.  Perfection shouldn't be the enemy of the good.

Your idea is an improvement over a wood stove located next to an outside wall, and many people have done just that in sheds and work buildings.  However there are a couple of other features of the RMS you are over looking - not that I'm promoting the RMS, just want to make sure all the details are covered.

The re-burning of the hot gas/smoke is very important both for squeezing more heat out (of the smoke and not out of the fuel) and for greatly eliminating the pollution going into the air (can you say asthma).  I would say this feature (heat riser - re-burner) is the most important feature of the RM stove, second would be the long collection of heat in the mass.

As for myself ~ the RMS are just to-ugly.  I like the rumford stove (except for the price)  for beauty and efficiency.

The problem is that you are relying on the stove pipe and the heat in it to draw efficiently. If you bend the pipe sideways then recover the heat with a thermal mass you take away the stack affect and you will need to replace that with a blower or fan, which adds to the difficulty.

I think there are things you can do to improve on existing stoves, but I think it is important to not think of it as rocket-i-fying. 

For one thing, a rocket mass heater works on the idea that the "chimney" is able to be shortened via insulation and then works to push exhaust.  It must be vertical to work.  Plus, the level of draw must be wide open - and a certain amount.  And ... and ...  well .... there's lots of stuff.

I think it is possible to construct something that would have the rocket part without the mass, so the focus would be purely on instant heat. 

If you want to get more efficiency out of an existing wood stove, I think the smartest thing is to add mass.  I once read an article where I guy cut his wood needs by more than 50% just by hauling in lots and lots of rocks and stacking them around his wood stove. 

What I don't understand is that in the book and videos, what is called the "burn tunnel" of the rocket mass heater (which essentially is horizontal stove pipe), is often long and sometimes extends through a room and even wraps around corners and the like.  Then it becomes vertical and is called a "heat riser" (in essence a chimney). 

So my question (poorly stated the first time) is, as long as the final portion of the stove pipe is vertical, why exactly wouldn't a long horizontal burn tunnel work on an ordinary wood stove?  Would it really require an exhaust fan and/or a larger air intake to work?

No, I think you've misunderstood a bit. The burn tunnel and heat riser are both part of the stove, they do not go around a room or anything like that, the burn tunnel and heat riser for most of the J tube, on which the barrel rests. That generates most of the draft needed to keep the stove working.

OK, let's try again.  I'm looking both at Paul's richsoil article (http://www.richsoil.com/rocket-stove-mass-heater.jsp) and the Rocket Mass Heaters book by Ianto Evans and Leslie Jackson.

There are horizontal ducts that carry the exhaust through the bench mass before turning up and out.  They can be long and winding. 

Whatever they are called, could you modify a wood stove with such a duct system and if so would it be advantageous?  Would it burn more efficiently?  Would it create a stronger draft?  Would it heat a larger area?

The stove would run less efficiently with a weaker draft.

From an article, Chimneys for Wood Stoves

"A warm chimney will provide greater draft than a cool chimney because a warm chimney does not cool the rising smoke. This is why stoves generally perform better after they have run long enough to warm the chimney than they do when they are first started. For the same reason, chimneys that are located within the house have better draft than those located completely outside."

It would seem to me, therefore, that thermal mass surrounding the stove pipe of a wood stove would create the warmth needed for better, not worse, performance.  Whether that can compensate for the horizontal pipe, I dunno.  wood stove guides recommend no more than 8-10 ft. of horizontal pipe before turning vertical, but they only consider naked pipe.  I'd like to think it through as much as possible before trying it out, though.

Must be vertical. The horizontal run is the one that hurts the draft and the efficiency (which isn't great without a catalytic converter anyways).

There are horizontal ducts that carry the exhaust through the bench mass before turning up and out.  They can be long and winding. 

What you are describing is not a rocket stove, but a Masonry Stove, which are also very efficient.  They transfer all of the heat from the exhaust before venting it.  Well built masonry stoves may be fired only once a day, with the thermal mass slowly radiating the stored heat for the rest of the day.

Well, no, you need a vertical run of hot air to develop the draft force that pulls air through the system. If you try to run your exhaust gasses horizontally you will remove all the energy from them before you develop the draft, with out airflow you won't have air and with out air you wont have complete combustion.

So -

Because the RMS offers a narrow short rise (in the heat riser) right off the bat the draft is established first - Then the smoke is detained a bit running down the outside of the riser, inside the barrel to continue the burning of it - And then drawn out through a horizontal cob mass before being sent outside..... correct?

Okay, then can someone theoretically allow the smoke to run up a section of stove pipe from their stove before turning it horizontal and covering it in a cob mass?  Eliminating the barrel of course.

That would probably work even better than a rocket stove, although it may not burn as cleanly, depending on how turbid the flow of the gas at the top of the heat riser gets.

not really the short narrow draft in a RMH is the chimney the heat riser is around 1200 to 2200 degrees at the top. from the top of the heat riser to the end of either the vertical or horizontal exhaust the system pushes. this is why we can run a Rocket stove exhaust down hill or stick the end of the exhaust into a dry well. a vertical exhaust is actually a bit of a problem since it requires at least 90 deg F to get rise in the exhaust duct. 

to the original poster;
give your idea a try out doors and see how it works.

Thanks for jumping in Ernie and explaining 

paul wheaton wrote:... hauling in lots and lots of rocks and stacking them around his wood stove. 

I do that with my tent stoves if I'm going to be in one place long enough to justify the labor, like during deer season. (I pitch my tent in the same place every year, for a week or two.) The rocks will stay warm for a couple hours after the stove goes out. Plus, I don't have to worry quite so much about my sleeping bag, pad, etc. touching the stove.

I'd like to add thermal mass around my Jotul woodstove to store some of the heat and re-radiate it. My concern is that the stove was installed in a corner with the minimum allowable clearances to the two side walls - this is the required distance between the stove and any flammable surfaces. If I add stones or bricks around the stove, I will be bringing heated surfaces closer to the walls. Is this a fire hazard? Do the stones actually get that hot? Thanks.

http://www.handprintpress.com/featured/greenhouse-heater/

Your welcome

This must have been covered somewhere in the forum.. but.. are there any problems with just surrounding a regular woodburning stove with cob or lime plaster etc, i'm not planning on changing the flue etc, just gaining extra thermal mass for longer heat output.

Thanks, Matt @ Henbant

It will make the stove interior hotter since heat will not radiate away as fast from the stove surfaces, which will increase the combustion efficiency. However, if it is too effective, it will let the metal internal parts get so hot that they corrode quickly or even start to melt. A little of this treatment is good, a lot can destroy your stove.

There would be no such thing as too much thermal mas placed around the stove at a slight distance, so that the stove itself would not overheat.