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.