Hi all! My apologies if this has been discussed here before, but I have not come across it anywhere:
I am in a position to build another
rocket stove (for a neighbor this time) and this has gotten the wheels turning in my head again. I see two major sources of inefficiency in the RMHs built by myself and others, both related to draft: 1) most systems rely on buoyancy of hot (warm) exhaust gasses at the end of the system for reliable draft. This obviously wastes heat. 2) In real-world practice, the stove rarely gets shut down at the appropriate time, and because of the buoyant draft at the end of the system, will continue to extract heat from the system (and house) and dump it outside.
This all has me thinking that a system relying on the heat-pump effect will have much better real-world efficiency. The system exhausts horizontally out the wall, and when the fire burns out, the draft grinds to a halt. I know that horizontal-venting systems are recommended against because any little wind will tend to cause a backdraft. However, there's a more fundamental design problem that causes this condition:
If we figure our core temps are around 2000F and room temp is around 70F, the core gasses will be thermally expanded 4-5X compared to room gasses. As the gas travels through the system, it cools back down to room temp before being ejected from the house. This means that to maintain gas velocity through the system, the system exhaust must have 1/4 the cross-section of the intake (conservatively). For an 8" system, this means the exhaust
should be 4". An exhaust with small cross-section and high draft will be far less susceptible to wind-induced backdraft. This arrangement *will* increase load on the system, but this is easily overcome by appropriate scaling of the heat pump (riser and barrel combo).
I have other ideas, including a passive thermal-bypass bell, and that a bell does *not* stratify heat but rater sets up a convection current, but I'll save them for another time.
If there is one thing my 38 years have taught me, it's that my bright ideas are rarely as bright as I think, and so I submit them to you all for picking-apart. Do you think a system as described could be started without the assistance of a bypass valve or fan, etc.?
Thanks for your input,
--Phil
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