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Horizontal RMH for heating hot tub

 
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Hello there. I've been on a rocket stove kick for about a month now and have found this forum to be an incredible resource.

A close friend just bought 40 acres in the Sierra foothills, complete with gold mines and massive tilling piles of round river rock.

The plan is to build a river rock hot tub and heat it all with wood!

With a conventional RMH construction, I thought I'd have the radiator submerged in water, or nested into one side of the tub for heat exchange.

Is it possible to skip the riser and radiator all together and run a refractory tube through the floor or seat, and exhaust out the other end?


Here's a quick sketch I did on Fusion 360.

Any Help is much appreciated!

David
Mass_Water_Heater_bath_tub_2020-Dec-28_01-30-29AM-000_CustomizedView3640829052.png
Mass water heater bath tub idea
Mass water heater bath tub idea
 
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Location: California
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David Date wrote:

The plan is to build a river rock hot tub and heat it all with wood!

Is it possible to skip the riser and radiator all together and run a refractory tube through the floor or seat, and exhaust out the other end?



Nice project.  I think you will have problems getting that long a horizontal burn to draft.  Starting a cold rmh in the fall for the first burn is often difficult.  A small fire under the vertical chimney is often needed.  This is why a lot of rmh go out and back with the vertical chimney immediately behind the barrel radiator, easier cold starts.  
 I suggest a conventional rmh burn chamber and then cover the barrel almost completely with cob.  Run the bench under the hot tub.  This way you are using the heat from the normal heat storage part of the rmh.  Tom


 
pollinator
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David Date wrote:With a conventional RMH construction, I thought I'd have the radiator submerged in water, or nested into one side of the tub for heat exchange.

Is it possible to skip the riser and radiator all together and run a refractory tube through the floor or seat, and exhaust out the other end?



My understanding is a simple "No".  Sorry to be a bummer.

Although it would seem more efficient to not lose heat to risers and bells and such,  a key part of an RMH is that it achieves a high enough temperature in the riser and bell to completely burn all the combustion gases ... thus its clean and everything downstream of it is exposed to just hot air.  The riser is very much a pump that drives the RMH and its height needs to be in a very tight range ... too long is too powerful and the gases move too quickly, too short and it doesn't have enough power to create draw and push exhaust through the system.

The other problem you run into is a simple one of temperatures - the insulated riser is critical because it is is insulated and thus it doesn't rob the RMH of the heat it needs to get going.  If you just have a fire tube running through the water then you will probably never attain necessary temperatures as the water will soak up the heat.  A criticism of the Snorkel style wood heater for tubs is that b/c the whole fire is surrounded by water, the firebox never gets hot enough for a clean burn.  So you end up with smoke and a pipe that will accumulate creosote and slowly clog ... until the creosote catches fire!

So you need the riser.  I don't think the bell is necessary here... and maybe what you need is a stratification chamber under the water.  But it has to be absolutely water tight as any water that comes into contact with ~1000 degree air will flash to steam and bad stuff happens. And you need to have a pipe/chamber that transmits enough heat to the water  (assuming you are treating the water as the mass.  Might be interesting to consider a different mass that heats the water).

I'll second Tom's concerns about starting the thing.  You might consider some of the design elements in the "Season Extender" that Uncle Mud built at Wheaton labs - and which Paul discusses in a recent podcast (#519 - RMH in the Tipi).  I'm thinking of the boomerang path that has the final exhaust pipe adjacent to (even thermally connected to, or even inside) the bell, thus "priming" the pump and softening the cold air plug.
 
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I helped friends build a rocket fired hot tub some years ago. It was among a pile of granite boulders packed with (sandy) soil. We made enough space around the hot tub (old cast iron tub) for air to circulate, and built the J-tube at the base of the pile so that after being collected under the barrel, the hot gases could travel sideways and up-ish. We worked the dividers so that the gases flowed around and under the tub and then back next to the barrel and a short chimney pipe. I couldn't stay to see it finally tested, but heard that it worked effectively, with the only issue being that there was flow space up to near the top of the tub, and the top edges got too hot. I would advise not letting the flow path much above the lower half of the tub, definitely below the waterline.

We had a watertight container and made a mostly airtight flow path among the boulders around it, definitely not watertight. If you are planning to make the tub of mortared rocks, you can't count on that to be watertight enough to keep the airflow path dry, and will have to use metal duct buried in the rocks for airflow. Be aware that that option will mean a lot of mass to heat up before you start heating the water, and a delay of perhaps hours from starting the fire to a warm tub. The cast iron tub started warming the water within minutes of starting the fire.

And finally, I second everything Eliot says above.
 
Eliot Mason
pollinator
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Glenn - that sounds like an upgraded Cowboy Hot Tub!

That problem with air flow space above the water line is an interesting one and definitely something to watch for ... hard to get out of the tub without getting burned in that instance!

I was once eyeing a large stainless steel mixing tank ... cut it in half and have two 6' diameter x 4' deep shells.  Build a wood interior with lots of flow space (for water circulation), then plunk the thing down on a foundation with a stratification chamber.  The top could be wrapped in insulation (and wood). Alas the challenges of just moving a 6*8 cylinder were too great to start that project.
 
Tom Bolls
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This. I think this would work well.  

Eliot Mason wrote:I'm thinking of the boomerang path that has the final exhaust pipe adjacent to (even thermally connected to, or even inside) the bell, thus "priming" the pump and softening the cold air plug.



Force the draft with the chimney mounted right up against the barrel.  Tom
 
Doody calls. I would really rather that it didn't. Comfort me wise and sterile tiny ad:
Morgan Superwool Plus non-ceramic fiber blanket for Rocket Mass Heaters
https://permies.com/t/153507/Morgan-Superwool-ceramic-fiber-blanket
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