maximizing exhaust

Hi there,

I am thinking of building a rocket mass heater as an outdoor warmish sofa for colder autumn and winter days. As such, it will be built in de garden, and will only occasionally be heated. The first and most important feature is to heat the thermal mass. I will insulate the bottom and sides, to maximize heating through the top.  I'm looking at a reasonable sized rocket mass so it does not take over the garden entirely, and am thinking about the following dimensions (of the excellent book "The rocket Mass Heater: builder's guide":

consistant cross-sectional area (csa) of 6''

feed of 12''

burn tunnel of 18''

heat riser of 40'''

However, I want to maximize the heat exchange of heat from the exhaust to the thermal mass. Therefore, make the exhaust as long as possible. Longer exhaust requires more push from the barrel, so there are two solutions I am thinking of:

'regulate' heat exchange of the barrel: since I'm talking about winter, I should not worry too much

create an exhaust that I can vary in length.

As you sometimes see a bypass towards the chimney to start up more easily, one can imagine one or more bypasses in the exhaust itself to lengthen or shorten the exhaust. If I think that the exhaust can take it, I could open a valve to an additional second exhaust tube (or even third). If I notice that it does not work (for whatever reason), I could close it as such. Advantage is that this does not have to be airtight valves.

Would that make sense or am I missing on some details or basic understanding of the RMH.

You could do that, though I think you would be better served by a half-barrel bell instead of a long pipe. No friction to speak of, and you can have the chimney right next to the combustion core to allow a bypass for starting. With a good chimney (maybe 10'/3m tall), you can get a fire drawing easily and then shut the bypass to put the heat through the bench. You can also have a solid masonry layer around the heat riser barrel so you don't have too much heat going out to the air but keep most of it for the bench.

I would probably make the core dimensions a bit larger (still 6" diameter), say 16" feed, 20" burn tunnel, and 48" riser, so you can use longer wood and have to feed less often.

One thing you will have to work out is how to protect the core and bench from rain and snow. Using all brick instead of cob might be enough, or you might need some sort of roof.


I would make the chimney as a plunger tube into the top of the half-barrel space, so the bottom of the tube can be raised or lowered to whatever gives the best balance of heating and drafting.

How long do you intend to use it at a time? What materials for the mass? I think cob takes about an hour per inch to heat up (someone corrent me if I'm wrong). I used cinder blocks, and without mass over them, it took about an hour to warm up. If it is for relatively short spans of use, I think I would go with a rather thin bell of some sort. Bricks or rock would heat up relatively quickly.

Don't forget about Paul's idea. (I just hear about this in the podcasts, I think it's in the Fisher-Price House.) The exhaust runs far enough to give up most of it's heat to the bench, but then passes through a bit of exposed vertical pipe, right next to the barrel. This heats that section of pipe, giving it a good draw. This way the system doesn't just push from the inlet end, it also pulls from the exhaust end. He (and Ernie and Erica) say it really helps the flow, which aparently helps the heating more than it's hurt by the bit of heat that escapes through the chimney because of it.

That is a brilliant idea! Indeed, the increase the pull, simply heat up the chimney. Simply smart.