Any Gotchas with My Sketch?

1 sq = 3 inches

Core walls and insulation is 1:1 mix of clay and perlite.

Thermal mass is cob.  

Since it would be heating a hot tub outside, I think I would need to insulate base from ground and outside of thermal mass for safety, eliminate potential fire hazard.  

Obviously the flue would extend through the thermal mass to the chimney, even though I didn’t draw that far.  

Insulation would be 2-3 inches thick around riser and again around cooling area.  Would 9 inches be high enough for smoke to cool or would I need to have more headroom between the top of the riser and the top of the cooling area before exhaust drops through the cooling chamber and through the thermal mass to the chimney?

Has anyone tried to make a 1:12 scale model of a RMH and fire it up?

Hi Chris;
Well you got me... confused that is! I can't make heads or tails of your drawing.

As far as questions, Yes definitely insulate under your hot tub.
I would insulate the outside of your tub, not for fire hazard but to keep the water warm.
I don't follow your other questions.
Maybe you could draw a clearer picture for us?
Have you built a RMH before?
Do you have a copy of the RMH builders guide?

thomas rubino wrote:Hi Chris;
Well you got me... confused that is! I can't make heads or tails of your drawing.

As far as questions, Yes definitely insulate under your hot tub.
I would insulate the outside of your tub, not for fire hazard but to keep the water warm.
I don't follow your other questions.
Maybe you could draw a clearer picture for us?
Have you built a RMH before?
Do you have a copy of the RMH builders guide?

I have not built one before and I don’t know when I will find the funds for the book.  I am watching videos and reading articles in an effort to understand.  

I am sketching and thinking of building the area for the gases to cool to the side instead of around the heat riser.  The area for cooling gases is on the side of the heat riser rather than surrounding it.   I know, my blue print drawing skills are horrible.

Metal is typically used to cool these gases, but I don’t have a drum or scrap sheet metal.  I don’t know how long it will be before I can afford it, so I am trying to find a way to make it from perlite and clay insulation or cob.  No metal at all.  No brick or firebrick either.  

Insulating underneath and sides definitely makes sense, I want most of the heat in the water, not in the air or ground.  If the combustion chamber, heat riser or thermal mass should get hot Enough to cause problems with drought or winter dry grass, that is covered as well.  

My design would have one “u” comprised of the combustion chamber, the side tunnel and the heat riser.  Then a second one between the heat riser and the cooler side going through the thermal mass.  Insulation around the core of combustion chamber, tunnel and heat riser.  

It is more or less making sense to me, but I need to either make a 1 inch to 1 foot scale model or know it will work safely before I build.  Which means more research.

Hi Chris - I don't know your whereabouts, but if you need a steel drum you can get these for free in many places. Especially jiffy lube locations. I get a couple every now and then from the local auto mechanic. All they have in them is a little bit of clean motor oil, which I carefully pour off to use as general purpose lube and rustproofing.

If you've never built an RMH before, it really pays to stick to the basics and try the simplest things first before you get into variations and experimenting. Barrel radiators are proven to be the easiest way to shed heat from the system and provide draft  via densification. You'll give yourself a real advantage by keeping it simple the first time, and then getting creative once you've had a round of hands-on experience.

Phil Stevens wrote:Hi Chris - I don't know your whereabouts, but if you need a steel drum you can get these for free in many places. Especially jiffy lube locations. I get a couple every now and then from the local auto mechanic. All they have in them is a little bit of clean motor oil, which I carefully pour off to use as general purpose lube and rustproofing.

If you've never built an RMH before, it really pays to stick to the basics and try the simplest things first before you get into variations and experimenting. Barrel radiators are proven to be the easiest way to shed heat from the system and provide draft  via densification. You'll give yourself a real advantage by keeping it simple the first time, and then getting creative once you've had a round of hands-on experience.

I don't have a pickup, but I might be able to find someone.  I didn't know that auto shops gave away drums!

Would it be better, if I can get ahold of one, to leave the drum exposed?  Why?  It seems like some heat is lost before the thermal mass where the drum is exposed.  That might be good in an enclosed area, but it seems counter productive outside.  

Just trying to wrap my head around this concept and the rules and why's behind those rules.

Hi Chris;
Much better drawings this time.  I see what you have in mind (well sort of).  
You have 9" down as your feed tube & riser size?   Most are 6" or 8",  A 6" is 5.5" square the 8" is 7.5" square.
To make a proper rocket you need exact dimensions from the feed tube thru the riser.
You can certainly make 9" work but you will need 9" horizontal pipes to vent it. 8" is much easier to find.
I don't see where your hot tub is in these drawings?  How are you thinking the water will heat ?
Your "cooling area "?  What are your thoughts about it? What is its purpose?

As Phil suggested.  Any equipment/vehicle  shop will have barrels.  School buses, county road crew, even farmers buy bulk oil in 55 gal. drums.  Oil change shops are probably the most accessible.

The draft in a RMH is produced by the hot combustion gases losing heat immediately after the heat riser. A barrel is a simple way to do this, but a masonry bell is performing the same function. Cooling gases get denser and sink. This enhances the intake draft and pushes things along in the flue.

On my 4" system in the glasshouse, typical temperature readings are like this:

Top surface of drum: 275 C
Side, top: 190 C
Side, middle: 160 C
Side, bottom: 130 C
Flue between exit from cob mass and exterior wall: 40 C

thomas rubino wrote:Hi Chris;
Much better drawings this time.  I see what you have in mind (well sort of).  
You have 9" down as your feed tube & riser size?   Most are 6" or 8",  A 6" is 5.5" square the 8" is 7.5" square.
To make a proper rocket you need exact dimensions from the feed tube thru the riser.
You can certainly make 9" work but you will need 9" horizontal pipes to vent it. 8" is much easier to find.
I don't see where your hot tub is in these drawings?  How are you thinking the water will heat ?
Your "cooling area "?  What are your thoughts about it? What is its purpose?

As Phil suggested.  Any equipment/vehicle  shop will have barrels.  School buses, county road crew, even farmers buy bulk oil in 55 gal. drums.  Oil change shops are probably the most accessible.

The draft in a RMH is produced by the hot combustion gases losing heat immediately after the heat riser. A barrel is a simple way to do this, but a masonry bell is performing the same function. Cooling gases get denser and sink. This enhances the intake draft and pushes things along in the flue.

I am in the very beginning of the planning stages, nothing is finalized yet.  I could easily change the dimensions.  I think that the idea I am groping toward is a masonry bell, without the bricks.  

Heat always travels from warmer to cooler until it reaches equilibrium.  Since a RMH is adding heat in the fuel burning chamber that is drawn into the heat riser, that warmer hot air is going to naturally move toward the cooler end of the system!  Thank you, it's been a couple decades since physics classes and thermodynamics in college.  So, an bell above the heat riser to allow the gases to cool, get denser and "drop" down through the system combined with heat traveling in an attempt to reach equilibrium means I need a bell rather than a cooling area for the exhaust gases.

I am thinking of having the thermal mass wrapping around half to two thirds of the hot tub.  It would be either in the bench, the backrest or both.  

For the six inch metal, do you use air duct?  The stove pipe I found is insulated, which is not good for heat transfer.  Does the metal have a function other than transferring heat from gases to the thermal mass?  Most metals are excellent at both gaining and losing heat.

Hi Chris;
Lets see, about your question)  6" metal Are you referring to the pipes thru the mass ?  The function of those pipes is to provide a path for the heat to travel thru. Commonly hvac pipe is used,away from the core.   Very thin wall pipe and it is ultimately expected to fail, leaving behind a 6" clay tunnel.

Now lets talk about hot tubs.  rmh's have been used to heat them. Check for posts here at Permies.
From your description it sounds like you want to partially surround a standard hot tub with cob ? Your mass pipes buried in the cob ?
I may be wrong here but I think it will not get up to the desired  95 F - 104 F water temp.  Was this in a insulated room ?
Do some research here at Permies and see how others have heated water for their hot tub. I'm sure there is a way to get what you desire.

Hey Chris;  Maybe an old claw foot tub , drain pipe included . Could be completely buried in cob. You would need to hand fill it but so what!

Sounds like your on a limited budget, but your willing to work around that. We can help.
So your cooling area already is a small bell, it could be made larger.
Or bells can be made from many materials, even cob. Large metal tanks (old fuel oil tanks) are commonly used.  Bricks or cob can be added to metal tanks as available.
Or it would be massive cob work but the entire rmh could be built with clay and perlite and some scrap piping and something to form your cob into a bell.

Tell us how your hoping to build your hot tub and we may have more ideas for you to ponder.

Its possible a local library may have a copy of the RMH builders guide you could borrow or might be wiling to acquire one to have in their collection.

thomas rubino wrote:Sounds like your on a limited budget, but your willing to work around that. We can help.
So your cooling area already is a small bell, it could be made larger.
Or bells can be made from many materials, even cob. Large metal tanks (old fuel oil tanks) are commonly used.  Bricks or cob can be added to metal tanks as available.
Or it would be massive cob work but the entire rmh could be built with clay and perlite and some scrap piping and something to form your cob into a bell.

Tell us how your hoping to build your hot tub and we may have more ideas for you to ponder.

Its possible a local library may have a copy of the RMH builders guide you could borrow or might be wiling to acquire one to have in their collection.

Thank you so much for your time and input.

Are there any rules about bell volume to heat riser diameter?  Are there any rules about heat riser height to fuel burning chamber height?

Planning to have the hot tub outdoors.  So, either a RMH or a wood burning cob oven.  It would seem like a cob oven with chimney would lose more heat to the environment, even if I insulate the fire box and chimney.  Another advantage of a RMH is little to no smoke smell.  

Insulation between ground or air and cob thermal mass.  No insulation between cob and water.

Lime plaster to waterproof, both water in hot tub and from rain.  Lime paint to decorate sculpt.  

Steps down into hot tub, sink about 3/4th or more below grade/ground level.  This will help moderate temperature fluctuations over the course of the year, the thermal mass of the bulk will also help.  Yes, heat will escape from the water surface.

Hot tub floor with flat tumbled pebble mosaic to minimize slips.  Benches to sit on while soaking.  Sculpted backrest that contains the RMH and channels most of the heat into the water.  

I am not even close to having enough aggregate to start testing cob mixes.  Don’t have straw yet, either.  Still need to source lime.  So, early planning.  

I am harvesting soil that keeps getting fallen leaves and other organic materials in it.  Still need to sift or strain that out.  I am working on separating clay from remaining aggregate in my soil.  I think I am going to either wind up with a lot of clay and some aggregate or end up buying building sand.

Eventually, as funds become available, plant a nice screen of tall plants around the hot tub.

Hi Chris;  Yes there are parameters for sizes.
Sounds like you need to check out Peter Bergs batch rocket site,  here is the link http://batchrocket.eu/en/  
Peter is a master builder and freely offers all his information about bells and bell size.
Another good site is Kirk Mobert's (Donkey) here is his link http://donkey32.proboards.com/

And oh boy , I know nothing about finishing cob but... Waterproof? For a bath tub???   Hmm I'm not to sure your tub will hold up to bathing in.
What about a cob basin that you put a pond liner in?  Have to figure out a way to drain it but... at least it is waterproof.

Buying building sand is the way to go.  Lot to be said for #50 sacks of fireclay as well.

thomas rubino wrote:Hi Chris;  Yes there are parameters for sizes.
Sounds like you need to check out Peter Bergs batch rocket site,  here is the link http://batchrocket.eu/en/  
Peter is a master builder and freely offers all his information about bells and bell size.
Another good site is Kirk Mobert's (Donkey) here is his link http://donkey32.proboards.com/

And oh boy , I know nothing about finishing cob but... Waterproof? For a bath tub???   Hmm I'm not to sure your tub will hold up to bathing in.
What about a cob basin that you put a pond liner in?  Have to figure out a way to drain it but... at least it is waterproof.

Buying building sand is the way to go.  Lot to be said for #50 sacks of fireclay as well.

There are areas where people have used rubbed lime plasters for thousands of years to sculpt bathtubs.  It does need to be finished, I think with Castile soap, and cured.  

I barely have the funds to buy a $5 bag of sand, perlite or what have you every month or two.  I definitely do not have the $100+ needed for a pond liner.

Anything I can do to have free inputs will help.  Cob bells instead of fire brick.  Perlite and clay instead of refractory cement, minimizing fire clay, or eliminating it, as long as it is still safe.  

Minimizing the amount of straw, if I can substitute free straw-like materials for straw I have to pay $5 a bale for last I checked, even better.  

Reducing how much sand I have to buy by harvesting clay from my soil, I get clay to play with and eventually I will get some sand and small gravel, I can feel the grit in there.

Anything I can get, make or scrounge free helps.

I built a rocket fired hot tub several years ago, at a friend's place in Massachusetts. It was based on an old cast iron bathtub, with 6" J-tube, 100% cob aside from a couple of pieces of cheap stovepipe for chimney.

He had lots of boulders, so we excavated between them for a bell chamber under and around the tub, and built the combustion core among other boulders a bit lower next to the bell. I had to leave before the project was fully operational. He said that it worked well, except that the bell space surrounding the entire tub made the upper sides too hot for comfort. It would work better to keep the heated cavity 6-8" below the top.

You would get best results from having the tub top about level with the top of the riser, so hot gases can travel more or less horizontally to a bell under the tub. The more the flow has to go down, the trickier it will be to start the draft. A sunken tub with aboveground J-tube is probably never going to work reliably, unless you have a good hillside to work with.

On the subject of an outdoor cob structure, all the ones I have heard about have failed within a few years, no matter the coating, unless they were in a very dry and probably non-freezing environment. If you have lush deciduous trees like your photo shows, I believe you will need a roof to keep rain off, and to build all cob above grade and with ventilation under so it stays dry all year. I did build a totally cob outdoor rocket stove on a base of concrete blocks with open cores topped with flat stones, and just a piece of plywood on top to keep rain off when not in use, and that is undamaged several years later in the upstate New York climate.

The deciduous trees are over where I am harvesting soil and clay, the hot tub is on the other side of the yard with grass under nothing but sky.  

If your climate favors deciduous trees, you have enough rain that you need a roof to keep the cob dry. Plaster of any kind will not be sufficient, and you need to keep the cob away from ground damp.