rocket mass heater: water as a thermal mass

What temperatures are commonly going through the ductwork inside a thermal mass?

Open vented addition to water mass.

As the water remains in the system the only corrosion is initial use and a small amount for evaporation replacement. This can be overcome by central heating corrosion inhibitor.
It would be a good idea to add a pressure release valve to the vent pipe especially if the expansion tank could be subject to freezing conditions.

If a tapping was taken from the top and bottom of the barrel and run through the coil of a hot water cylinder it could supply household hot water. Well that's the theory.

The only problem would be getting the exhaust heat through the barrels with enough surface area to heat the mass.

I first heard about rocket mass heaters a couple of days of ago and the first thing I thought was I don't want cob inside my house. What's cleaner and has more heat capacity? WATER! I think it's a great idea. It would also be advantageous to use your water thermal mass as a hot water source / potable water reservoir. Rust was also something I thought about and stainless steel is the obvious solution. Stainless steel 55 gallon barrels are very expensive (> $700). As an alternative what about stainless steel beer kegs (~$150 for 15.5 gallons). It would probably be easier to find used kegs as they are way more common that 55 gallon stainless steel drums. Beer kegs are designed to be under a certain amount of pressure. You could have a check value to release anything over your desired water pressure rating keeping enough pressure for hot water outlet. Also have a manual pressure release valve inside for humidifier. You may consider a manual pump as well to have water output if you aren't heating much during the summer.

Interesting. Some random thoughts:

I've been contemplating an outdoor boiler with lots of mass to supplement the heat in my house in winter. I have surplus wood galore, but the (dumbass) insurance cost of having the fire indoors is a real barrier. An outdoor "furnace" could be a good workaround.

Water is a lot easier to move from hot to cold places, which is enormously practical. I would have to mitigate the freeze/expansion risk though. I think that water in a boiler system isn't great stuff to make tea with. In small volume installations, managing legionella is also a concern.

Does water really have more heat capacity than cob? I always thought it was a density per volume thing. Clay and sand are ground-up rock, and sink in water.

Is it necessary to have a pressurized system? I understand the gains in efficiency, and on an industrial scale they are enormous. As a homestead install, I see a lot of tricky issues. Would beer kegs lose integrity if the pH/chemistry of the water wasn't compatible? I don't know.

Like I said, random thoughts. These things are worth exploring.

This is the idea I had too! But I don’t know enough about RMH yet to figure out how to make this work.. My friend used a humongous furnace that she has to be feeding so much wood to in order to heat her radiating floor system. There has to be a better way.

Emerson White wrote:It also occurs to me that this could be pumped through a radiant floor system in order to provide radiant heating in another part of a house. Have your rocket boiler on the first floor and use it to heat bedrooms on the second, etc. Does anyone know off the top of their heads what temperature water is run through radiant flooring?

I just learned how to quote people, so my comment in this threat was a reply to this comment.. Has anyone attempted to hook a RMH to a radiant flooring?

Just throwing this out there, but I've been told that rust in metal pipes comes about because the free oxygen in piped water supplies is always being renewed, whereas free oxygen in a closed system will oxidize what it can and then stop.  I was researching what kind of pipe to use in my drainback solar water heater system and came upon this fact.  So rust in your closed water barrel might not be a problem.

To keep water at same level across all barrels, each one would need to be connected to the next barrel, one after the other and
be located at the bottom. You could fill the whole system by just putting water into one barrel and it would flow from one barrel
to the next. And the water would rise mostly evenly and level out across all of them. (as long as flow is not too restricted between
any one of them)  

Same principle across the top (on the lid venting the head space of each barrel from one to the other) Any one of those barrels
could have a vent on it to release any over pressure to outside of the sytem but they all must be connected together otherwise.
You could just vent it outside to prevent excess humidity, or inside to increase humidity... whichever you may need at the time.

Having the barrels all conected together at the bottom allows for self leveling of your liquid mass. And being all connected together
at the top headspace allows for equalized venting pressure across the system with the a single vent for pressure exhaust.

Also, all of the barrels need to/should be at the same level at the top. Your liquid mass will overflow the top of the barrel whos top
is lower than the others. So if the top is same on all, then all can only fill that high before overflowing.

Lastly, for the sake of uniformity, it would be easiest for all those containers to be the same size. That is not an absolute neccessity.
BUT! the important thing is that the top level of each container be at the same level as all of the rest. Tall and shorter barrels still
need to be connected at the bottom to make use of the whole volume of each container. This all assumes the whole system is
being heated at the bottoms of all containers. (with some uniformity perhaps) For thermosyphon to work, there would also need to
be another set of connections like the fill and vent lines near the top but below the full level of your containers. This will allow the
heated liquid to flow across the top layer from barrel to barrel. (better stratification that way) And of course a line from the bottom
of the lowest reaching container for the cooler liquid to flow to the heating part of the system.

Maybe I'm wrong, but it seems to me that pressure relief valves would only be a
neccessary thing if you are heating water to the boiling point or higher in a closed
system.

For trying to store heat higher than 212° (F) = 100° (C), it may be worth considering
a different type of mass for the storage of that heat other than plain water; (rocks,
sand, cob, soap stone or soil maybe ?)

There are other things  that may be considered, but some reach into more exotic
things like brine, glycol, oils and maybe other things I've not mentioned... some
are caustic, costly and/or are a fire or explosion risk.

It may take more of a thing (other than water... or even the "exotics") to get and
hold heat at temperatures above the boiling point of water because of its lower
heat holding capability and efficiency. But generally speaking, I think they are less
expensive, less risk and easier to be managed by the average DIY builder. On the
downside, there is the cost of sweat equity and the space taken by the volume of
material needed to do the job.