rocket bath?

Sue Rine wrote:Nice work!

Thanks, Sue ... I was crumbling in my hands today the black clay that I had unearthed last week. It's been in the sun and as it dries more,more of the exterior becomes flaky... lol... no need to "heat test" it... but, maybe I could dump a bunch of it in a hole and then slather the top of that with the durable red clay to create a nice heat sink. Maybe... anybody have experience on crumbly black clay? Will it crumble and sink with heat? Ideas on structural integrity... wrap it around rocks and put the rocks in the hole and cover with red clay? The desired effect is to have a warm bath available in a comfortable space any time of year, outside. And, to create a heat sink close to the house.

My guess on your "black clay" is that it has a lot of fine organic matter and a lot of silt in it, given that it is sitting in a level flood-prone area. I might even expect it to be somewhat insulating, if it tends to get powdery when dried out, which would not be much good for thermal mass. If you can keep it dry, though, it might be useful under the rocks and red clay to insulate that from the ground.

yeppers, I think your characterization is correct. silty, organic, in the low zones of a flood plain two blocks away from the base of a hilly park with a lot of Eucalyptus (little diversity and the understory does not protect the soil in heavy rains). The street gets a lot of runoff. Before the houses were built here, I'm sure the hill added a lot of matter to this area. Thinking about how I might keep that material dry in large scale, outdoor settings.... seems a lot of effort would have to go into that engineering. Might be better for other, protected space, insulation needs....hmmmm... I'll test it's insulating qualities, Thanks, Glenn!

Hi, I'm the guy who built the baths on https://rocketstovestuff.wordpress.com/


It's been nearly 3 years since I started experimenting with this concept and have come to the same conclusion as Erica, that the primary heat source ideally should be solar or better yet a compost hot water heater, like the one developed by Jean Pan. The rocket is there for a booster to maintain the desired temperature as this varies amongst people.

I got the idea from Ianto Evans book, just that little diagram and one paragraph near the back. I also worked with Tim Barker in New Zealand for a few weeks on and off as he was coming back and forth from PRI in NSW whilst I was doing a 6 month internship in building and appropriate technology. That's where I learnt how to build the stoves and I'd say I learnt the most from Tim.

In reply to a few comments previous, I felt the experience of bathing under the stars was a one up on inside (personal preference), so that was the whole point in the 'outside' bath. Also I appreciate the idea of having a bell under the bath but it would make it a bit tricky to load the feed chamber when your sitting in it, especially if the bath is built up on four walls. I have built them on slopes which is great as the stove sits downhill of the bath but on flat sites I just build a metal frame and lift the whole tub up by 4-500 mm. This is a good height to still be able to drop more wood in the feed chamber. To get around burning your bum I simply cut a piece of wood to fit the inside of the bath, it's appropriate, easy and very comfortable, especially if you put a yoga mat on top of that as well! I place a yoga mat on top of the water whilst heating also. The first two I made were cobbed and this definitely slowed down the heating. Now I don't use cob, but a light earth mix (insulation) of straw and clay slip to about 3-4 inches thick all around the bath. Now they heat up in 30 - 40 minutes. Marion is right in saying that we just want hot bath water, not scalding for cooking. And yes you could just dig a hole in a bank but I felt like making something that looked nice as well as providing basic function.

If it's not appropriate to your environment and personal context, find out what is and do just that. I've had a lot of fun experimenting and hope to keep building beautiful baths =)

Thanks for all the interest, advice and support.

Remember to find a bathing buddy, it saves water

In the summer time the water of my bath tub gets green and worms grow in it after a few days. Since the amount of water is large and I don't want to change it all the time, does anyone have an idea of what could I use instead of Chlorine to keep the water clear ? I heard something about hydrogen peroxide but I am not sure? Thank you.

I had not time to explain myself better before, but I do believe that the best way to heat up water with wood is using a rocket pocket! You make the rocket pocket with a metal barrel, you immerse the whole system in the water, add wood to the stove and there you go. There is no heat loss to the surroundings, to the earth nor to the air. All heat created by the stove is directly transmitted to the water (Except for the chimeney!). You actually contain your whole fire inside the water! What can beat this??? A rocket pocket works on the principles of the rocket stove which makes it very efficient, but has no unnecessary mass to heat up for this case. I have tried it and it works fantastic. If you make your bath big enough you can have your stove in it, or if not, you can immerse the rocket pocket inside a bigger barrel filled with water and then circulate the water from the big barrel to the tub. The only complication is to make your rocket pocket water tight, but you can use a barrel with a water tight lid and weld your two tubes to it. Not so difficult.
The same principle can be applied to warm up shower water using wood. I also agree that preheating the water with a solar system is a great and cheap solution.

Christos Alexiou wrote:I had not time to explain myself better before, but I do believe that the best way to heat up water with wood is using a rocket pocket! You make the rocket pocket with a metal barrel, you immerse the whole system in the water, add wood to the stove and there you go. There is no heat loss to the surroundings, to the earth nor to the air. All heat created by the stove is directly transmitted to the water (Except for the chimeney!). You actually contain your whole fire inside the water! What can beat this??? A rocket pocket works on the principles of the rocket stove which makes it very efficient, but has no unnecessary mass to heat up for this case. I have tried it and it works fantastic. If you make your bath big enough you can have your stove in it, or if not, you can immerse the rocket pocket inside a bigger barrel filled with water and then circulate the water from the big barrel to the tub. The only complication is to make your rocket pocket water tight, but you can use a barrel with a water tight lid and weld your two tubes to it. Not so difficult.
The same principle can be applied to warm up shower water using wood. I also agree that preheating the water with a solar system is a great and cheap solution.

Christos Alexiou wrote:I use a rocket pocket entirely immersed into the water. It works great. It is like having a fire inside the water. Zero heat lost through the walls of the barrel, but all the heat transfered into the water.

Sue Rine wrote:Is a rmh for an outdoor bath perhaps overkill? We simply dug a hole in the ground on a small bank. The bsath fitted snugly in the hole. From the lower side of the bank we then dug under the middle of the bath, a pit to build a fire in and an escape route for the smoke at the back of the bath. I suppose you could say that that formed a more or less j shaped firebox with the bath as the top surface of the firebox. It heats the water to bath temperature pretty quickly. It's best to let the fire die down to embers before getting in the bath and essential to sit on a towel! It's lovely to have a bath that doesn't cool down while you're soaking. The embers are enough to keep the water at a good temperature.

The trouble with these solutions is that water boils at 212 F (100 C), and that means the metal water-jacket or tub in the flame path will not get much hotter than boiling water.
To be a clean, efficient fire, the firebox surfaces have to stay much hotter - 1000 to 2000 F (550 to 1100 C).
Water directly in contact with a metal firebox, or flame in contact with a water-filled pot or tub, results in dirty fire. The flames can be too cold to burn completely, resulting in smoke and creosote deposits on the relatively cool metal surfaces. (Remember, even actively boiling water is much cooler than the 350 F / 200 C minimum temperature threshold to prevent creosote condensation in chimneys.)

One solution as I believe James has done is to have a firebox big enough and well-insulated enough to completely burn the smoke. It's the exhaust, not the active flames, that is heating the tub's thermal mass. This is an appropriate, efficient, and clean solution, and can be run on yard debris as Marion pointed out. (No tree-cutting required!)

An option I was thinking along the lines of Christos' project, but a little cleaner, would be to put the pocket rocket into a second, larger barrel with a layer of bricks and clay between them. Not so insulating as to prevent good heat transfer, but just enough insulating effect to let the fire to stay hotter on the inside surface, while the heat is being soaked up by water on the outside surface. A painted outer barrel should theoretically not even lose its paint. It's possible (even likely) that the interior barrel would get hot enough to warp, melt, or burn out over time, so it would be smart to make the brick-and-clay liner relatively well-built so it could take over the job of being the firebox if needed. The outer barrel would be responsible for water-tightness and protecting the firebox from leaks.

I like James' yoga-mat suggestion as a cheap way to prevent evaporative heat loss.

In fact, James, kudos in general, and thanks for sharing your project.

Christos Alexiou wrote:In the summer time the water of my bath tub gets green and worms grow in it after a few days. Since the amount of water is large and I don't want to change it all the time, does anyone have an idea of what could I use instead of Chlorine to keep the water clear ? I heard something about hydrogen peroxide but I am not sure? Thank you.

Warm, stagnant water, exposed to sunlight and surface pollination, is going to grow life. That's what it does. It's a life-giving resource.

I would suggest locating the hot tub where the cooled water can be used promptly for any on-site irrigation, flush toilets, or other useful purposes.

You can't responsibly do that if it's chlorinated or salted to the point where algae won't grow - most of what stops algae would also poison other green plants.
The less-toxic methods besides chlorine include salt water and a bromine salt option. But I believe all three are toxic to plants.
Also, bromine has been accused of being stinky in higher concentrations, and corrosive to metal parts, which may not matter in an all-plastic hot tub but is not a great thought if you're sitting in a cast-iron pot above a stove.
http://www.clarityforspas.com/bromine-vs-your-health/

The premise of this older research paper suggests the same thing: bacteria in water are largely derived from soil, so your anti-guck treatments would more or less measure their effectiveness by their ability to sterilize soil. Not what we want on a permaculture plantation: sterile soils are dead, and salt-sterilized soils won't grow most plants even with chemical fertilizers. www.jstor.org/discover/10.2307/41226136?sid=21106406591093&uid=4&uid=3739256&uid=2&uid=2129&uid=70&uid=3739648

You can use small amounts of hydrogen peroxide instead of bleach if you're trying to scrub algae stains off the walls after leaving the water for a while, because it does break down to water and oxygen. But it takes living things with it in the process- in larger quantities it would sterilize a certain area of soil before it breaks down.

You can also use small amounts of alcohol or iodine to inhibit bacterial growth in some situations, but I wouldn't want to use the quanties necesary for a hot tub. Stains, waste of good hooch, possible reverse-breathalyzer complications, toxic to other plants too, evaporates quickly so you'd be breathing it and then replacing it.

I'd probably use a biodegradable soap or detergent for scrubbing, and just cycle the water more often.

Boiling the water first (as can be done in Gaviotas-style solar condensers) or just holding up around 180 F where most bacteria can't survive is a highly effective way to sterilize water. And it doesn't render it toxic once it cools.

I think the proper, respectful way to treat water as a precious resource is not to poison it so you use less of it (you will still need to change it out eventually, at least once a year),
but to maintain and honor its life-giving properties and find other uses for it on the land.

You can irrigate with the glucky water, plants love extra nutrients as long as it's not too salty.
Filter it first, as you would for pond or lake irrigation water, if you have fine-perforation irrigation systems. Then when it's time for the next bath, refill with your clean water source (rainwater, well water, boiled creek water, whatever). Even algae-glucky water is nice to have around in droughts - you can dip out of it with a watering can for several days if you don't need that much water all at once.
If your house uses flow toilets and septic, you can also turn off the toilet tank refill valve and hand-dip bucketfuls into the toilet tank. Or if you have the plumbing experience for greywater systems, you could install a T or Y-valve with backflow preventers to let you switch from potable water to the hot-tub-drain-water, and be able to run this water directly into the toilet tank. Again, if it's getting really glucky, use a filter before it gets to the small-diameter parts of the toilet plumbing.

If you are looking at frequent baths for a big family, or weekly parties after band practice, then it might be worth insulating well and trying to keep the water largely too hot to encourage bacterial growth.
Bringing the water to a full boil every few days, then covering it in between with a relatively light-proof and airtight seal, could extend the use of the same tank of water for several sessions. You'd basically have a giant self-canning water jar.
The danger is that the water would spend a lot of time in that lukewarm "danger zone," so boiling before bathing would be a safety precaution.
(Legionairre's disease, one of the few bacteria that thrives above about 120 F, is not very dangerous for soaking, it causes pneumonia if inhaled as aerosols (very fine particles) such as taking a shower.)

Aeration also helps reduce the bacteria and some types of algae problems in stagnant water. Small pond and "waterfall" pumps can provide aeration, solar-powered or even foot-powered (a bladder bag under a walkway or driveway can run more water through the system every time it's activated.
Aeration alone may not be enough to prevent algae growth, but I believe it favors less-toxic forms of algae and bacteria than anaerobic conditions.

If you are so water-efficient that you don't need this much water for anything, and the tub would be a total luxury, then consider other luxuries that you could stack with it.
The tub could be kept full, like a pond, for fire suppression, and just boiled and/or filtered before hot-tubbing. You might draw from it for "dirtier" jobs like doing a pre-rinse on picnic dishes, and refill it with cleaner water when it's time to bathe again.
Or for a full change, the tub could be drained into a greywater filtration bed (sand, wood chips) for eventual use by trees; do a willow coppice, alder or fruit or nuts, clumping bamboo, or any water-thirsty plants that are too delicate for your region without added water. It could be done in stages, a third or a half at a time, to stretch the irrigation benefits out over a week or month.

If your region is so water-rich that draining this much water out of a tub on your property would bring your groundwater levels up past the foundations, or saturate and de-stabilize your slippery clay soils, then definitely stop self-flaggelating about "wasting" this much water. Use harvested rainwater, and have the hot tub be your way of slowing and sinking that excess water during times that it's not as rainy (such as the summer drought for us West Coast types).
You might do a shallow pond with a lot of filtration (wood chip, aquatic plants) for this hot tub to be drained into once it cools, kind of a "me first, plants second, fish third" approach. The pond's surface area would in turn raise the humidity of surrounding areas, and you could position raised beds or tree plantings where their roots could benefit from any overflow.

Decorative pond technology includes solar-powered pumps that can be used to filter and aerate the water. You'll have to clean the filter pretty often, or consider using a coarser filter to avoid burning out the pump.

Some of these ideas are starting to remind me of baptismal fonts.

I definitely think the simplest is to irrigate with the bath water after it's cooled, or put it to other uses.
If you're feeling water-guilt just thinking about it, plan the size of the tub so you can use up the bathwater inside a week.
Count up the amount of greywater you could use (things you now do with potable water, like water plants, flush toilets, hose down the muddy dog, maybe even pre-wash laundry cycles if your hot tub hygeine is good. etc).
The hot tub can just be a pre-use of this water before you re-use it for things you already do.

I very much favor solar water heating, or at least pre-heating, using opaque hoses. You can do a fancier process if you like - but just hooking up a solar-powered pump to cycle the tub water through a long dark-colored hose laid out in the sun and back to the tub for a while, before you ever light a fire, will get you pretty close with minimal infrastructure. Ideally, aerate the water a bit with the same pump, it should cut down on the stinkies.

. . .

Likewise, it's worth calculating the BTUs of heat you'd need. Is wood the best way to get this heat? How much wood are we talking about?
James could probably answer this better from experience; I'll try to calculate it from theory.

From this PDF: click here
" First, let’s look at how much energy it takes to heat water. One British
thermal unit (Btu) is the energy required to raise one pound of water by one
degree Fahrenheit. A U.S. gallon of water weighs 8.3 pounds. So, to raise
one gallon of 55°F water up to 120°F would require (100 [sic*] × 8.3 × (120 − 55))
= 540 Btu.
Using this information and an
energy rate of $1.00/therm
and $0.10/kWh, the following
table illustrates how much it
would cost to heat 100 gallons
of water from 55°F water up to
120°F using different types of
water heaters.

Energy & cost to heat 100 gallons of water
Water Heater Type . . EF . Btu . Cost
Standard Gas . . . 0.65 . 83,000 . $0.83
Condensing Gas . . 0.95 . 56,789 . $0.57
Electric Resistance 0.92 . 58,641 . $1.72
Heat Pump. . . . . . 2.25 . 23,978 . $0.70
Gas Demand . . . . 0.95 . 56,789 . $0.57
At these utility rates, it is more than twice as expensive to heat water
with electric resistance heating than with a standard gas WH. Also note
that although the electric water heater has a higher efficiency, the rating
does not account for the fact that electricity is produced by a power
plant, requiring 3.4 Btus of source energy to generate 1 Btu of electricity.
Electric water heaters typically have the lowest first-cost making them
common in many buildings. "

*[if you care to do the math, you'll see he got ahead of himself. The 540 figure is for 1 gallon, not 100 gallons; so that first equation should be
"(1 gal × 8.3 lb/gal × (120 − 55)°F) = 540 Btu."

In some parts of the US and Canada, incoming water might be anywhere from 45 to 70 degrees F. Heating from 45°F to 100°F would be about 450 BTUs by the above math.
So let's just use that number 500 BTU as a per-gallon heating estimate.

One pound of wood contains about 8000 BTUs if burned bone-dry and super clean, but we only get about 6000 BTUs if we burn it under typical conditions in a woodstove. Some of the difference goes to drying 15% to 20% of water weight out of the wood (this adds to the original weight, but isn't just neutral in the fire - the heat lost to evaporating that water definitely subtracts from the energy available). And more heat is lost shepherding the exhaust up the chimney - no matter how fancy your stove, the exhaust coming out will likely be warmer than the air that went in. (from the Sweep's Library, onlinechimneysweep.com)

Let's figure on a pound of reasonbly dry firewood, 15% moisture content (you're storing it under a shed roof, on raised rails or a pallette floor, but it's still a little humid near the bathhouse.)
And let's figure we are not perfect, so we get about 6000 BTU/lb.

So 6000 BTU/lb of wood / 500 BTU/gallon: one pound of wood could heat about 12 gallons of water.
For a 55-gallon tank you'd be looking at about 5 lbs of wood - if you use it efficiently. That's less than an armload.
Two to three times that if you burn it smoky and put a lot of unburned fuel (smoke) and waste heat out with the exhaust.

I think it's pretty reasonable to think that if you have a spare 100 gallons of water available from time to time throughout the year, it would be OK to heat it up and enjoy it, and water a coppiced wood lot below the tub.
I would imagine an extra 100 G of water at the right time could put more than 9 lbs of growth on a stand of trees.
Might encourage enough growth to use the good wood for fences and building projects and beanpoles, and feed the scraps to the fire, and still be building biomass overall. Heating the water doesn't give the trees much benefit, but at least you know that overall you're giving more to the stand than you take. (The exception, where heat might benefit your trees, would be when it's exquisitely timed for frost prevention - if you have to babysit an orchard during spring frosts, which usually occur on clear starry nights, a hot tub uphill this could be an absolutely magical way to do it).

Of course if you have access to pipes and glass and so on, solar collectors would not use any wood at all.
But they would take up space and sunlight, and introduce non-biodegradable elements into your landscape.
If you can't grow trees on your water supply at all (it's too alkaline or salty) then definitely go solar - the shade may be more help to plant growth than the sunlight, and flushing with larger amounts of the cleanest available water a couple times a year could help de-saltify the soils (compared to more efficient irrigation that just balances evaporative losses).

If you can grow trees like gangbusters with a little more water, wood heat might be reasonable.
All depends on how often you plan to use the bath, change the water, and whether the nearby spaces and resources are better suited for a solar collector, or a downstream orchard or wood lot.

Final thought:
It's fun to speculate about the absolutely perfect way to do something - the lightest footprint, the most stacked functions, the most efficient way to use water or energy or whatever.
But as they say, perfect is the enemy of good.
Don't let a focus on that abstract ideal of perfection blind you to a sense of proportion.

Super-efficient water recycling that requires toxic water treatments ... why would you do that? Are you really "saving" the water for anything useful? Or just rendering it useless? Sterility is appropriate to surgical theaters, but it's a questionable tactic for everyday life.
If the goal is to keep the water from getting icky, then share it. Cycle the water. Don't hoard it, and then try to find a way to make stagnant water stop breeding "icky" life.

Denying yourself the luxury of a hot bath because it wastes fuel, when you use fuel for other things like transport, keeping your house warm, or running power tools?
If you can do gravity-fed hot water using only wood, clay, and durable parts that are already on your land, such as bricks or an old cast-iron tub, that has its own integrity.
If you have no wood on your land and would have to drive around to get it, then driving around to get parts for a solar heater might have more integrity, and more long-term benefit with less cost.
If you are already dealing with wood as a waste product of orchard or wildfire mitigation maintenance, then this seems like about the most good clean fun you could get from it. Mulching it takes fuel if you use a machine. Instead, you can let the goats eat the bark first, and you are left with quick-drying stick fuels that are just about perfect for this use.

Think about the project in terms of what allows you to be your most complete human-being self, not trying to take over the cycle, but taking in benefit from your landscape and exuding your wastes into it in ways that benefit other things in turn.

In a salty desert, running a solar still with the clean water outfall into your cistern/covered tub could be a service to the landscape, and if the occasional hot bath is your "operator fee," it seems a fair price. The only accusing glances you'll get are when you disrupt their participation at "their" artificial hot springs.

One of my favorite Bible quotes is "you shall not muzzle the ox when he treadeth out the corn." (threshes the grain). If you are doing genuine service to your landscape as its steward, you get to benefit too.

The goal, after all, is not to break even or "save" or "limit" resource consumption - resources are not consumed, they are cycled and shared.
The goal is to build toward greater abundance and diversity of life wherever you touch.

Will the ability to take warm baths after a long day of loving land-labor increase your stamina and dedication to the project of reclamation?
Will it attract more help and understanding?
Will it be a healing resource for both people and the land? (A shower or bath is a safety feature, also useful for emergency care in case of heat-related injuries or allergic reactions.)

Would this potentially make my orchard-spraying, burn-pile-smoldering, synthetic-happy neighbors jealous enough to try things my way?
Oh, sorry, it wouldn't be safe for you to do this with all your pesticide burden, but you can borrow some organic tree trimmings from us.
(It would need a pretty good privacy screen to avoid violating their taboos, I'm sure.)

If you are going to cut a path through virgin rainforest and set up a glamping destination with teak-fired rocket hot tubs, which open wound in the forest canopy is colonized within 5 years by destructive campesinos.... well then, you might want to reconsider.
But I think there's a lot of ways to do this completely guilt-free and productively.

How does June sound?

-Erica W
[edited to include PDF citation]
edited by moderator to fix link

I'm building an outdoor bath heated with branches and other assorted crap ( possibly turn some horse poo into firelogs) question is. The draw isn't very good. I'd like to extend my riser but bath is already on the limit of height that the kids can get into. Has anyone built a core with an angled riser or split flow riser or possibly even angle the entire core. My thoughts are run the riser up one or both sides of the bath. After exiting the riser the hot goodness will hit a brick wall and head down the side of the bath travel the length of the bath and leave a flue pipe at with end. The bath will have rockwool insulation between it and the ground and dry river rocks and gravel broken bricks etc as thermal mass. Exterior between bath top and ground will be completely filled in with red clay brick reclaimed from my the chimney that I pulled my coal range out of. Thoughts suggestions criticism sought after looking forward to some advice.

Cheers Kim and Russ
Kaponga Taranaki uncoiling nutters.

I would advise you to pull the rocket core out from under the tub and let it have a taller riser. Then channel the exhaust from that down and around the tub, to exit on the far side. I helped build a rocket-fired hot tub last year which did this. I set up the hot airflow to go all around and under the tub and then go up a chimney for good draft. The workers after I had to leave finished the tub enclosure with cobbed rocks and a channel all the way to the top edge of the tub, and they found that it heated too much at the top. So I would say to make the enclosure go only halfway up the sides of the tub to get best heat distribution. You definitely want to allow hot airflow under the tub, but the riser directly under it will give a dangerous hot spot.

Yeah thanks for that I was thinking that too I'm thinking bout mounting it across the end of the tub putting a brick bell over it for safety with the kids using it and passing the hot gasses directly under the middle to a flue dead centre at the other end ( maybe 2 4 inch flues cause that's what I have lying around. Using bricks and mortar down the sides. Would 6 ft of flue height give enough draught?

It looks like your feed is around 6" x 8" more or less, so the chimney should be no less in cross section. Two 4" diameter flues would give about half of that, and stifle the draft. If the 4" sections can come apart, you could recombine them to make a single length of 8" duct.
I don't recall the length of the chimney duct on the one I worked on, but I think it may have been only 2' and drafted well enough. It was certainly not more than 5'.

My feed and tunnel will end up being 6 inch I think twin 4 inch flues should do the job?? Are you in NZ

Or I'll just split them down the sidewall and add in some more stainless steel welded in and make em oval or another random shape. As long as I keep the surface area the same it should work??? Also thinking about putting a section of spiral flat bar on the top 6 inches or so to help induce more draught during wind to prevent blow back gets very windy here

Two 4" flues make a bit less area than one 6", not to mention the surface drag effects that don't scale down with a smaller duct. This means that two 4" give decidedly less capacity than one 6". If you can split the 4" ducts and put them together to make a 6" to 8" duct, you will get better results.

As long as the top of the chimney is high enough that winds blow straight across it, they will not cause blowback in most circumstances. If anything, wind is likely to increase the draft.

Cheers for that I'll do that I think I'm a welder so I'll make my 4 inch ones fit or just use the outer sleeve. Which I think is just over 6 inch. If I was to put the riser inside a bell is it ok to put it hard on one wall of the bell insulated of course. I have access to calcium board insulation at 1 inch thick if I used that for the riser is insulation still required?

Do you know what temperature the calcium board insulation is rated for? If it cannot take constant 2000F it is likely to fail fairly soon.

You can offset the riser in the barrel; doing so will increase the heat flow on the wider side, making that side of the barrel hotter than the tight side. If this works for you, go ahead.

The calcium board is used as refractory replacement in our gas fired boiler firing temperature upwards of 1600degc it should be fine I have just scored some calcium aluminate cement so I think I'll cast one on top of a peter berg batch box

So here's my new heating unit 4 inch peterberg batch box. Next thing anyone know how much duct I can run off it or would it be better to utilise the area under the bath as a bell are under there would be 900 x 500 x 200-400mm high. The riser will be insulated with superwool and a brick bell user the top and then down under the bath also thinking about putting some copper pipe in the first bell to thermosyphon heat the water in the bath..

Nice! I would definitely treat the whole bottom of the tub as an enclosed bell - less work to do as well as more efficient than running ducts under it.

I'm with Glenn on making the underneath of the bath a bell.

Making a thermosiphon, why not, if you know what you are doing.

There's one thing i have to say, your riser is too short.

Glenn Herbert wrote:Nice! I would definitely treat the whole bottom of the tub as an enclosed bell - less work to do as well as more efficient than running ducts under it.

I thought so too
Would you suggest having the bottom of the bath the roof of said bell and a part wall down the middle to force the hot air to return back to the same end of the bath the batch box is sited.
I was thinking 3 courses of brick high 900 mm long and 500 mm wide with a partial wall down the middle say 600 to 650 mm long.

Satamax Antone
Post Today 19:37:48     Subject: rocket bath?
I'm with Glenn on making the underneath of the bath a bell.

Making a thermosiphon, why not, if you know what you are doing.

There's one thing i have to say, your riser is too short.

I'm a maintenance engineer by trade adwork on boilers and Heat recovery steam generators on Gas turbine generators, If I can't sort the thermosyphon Ill be amazingly pissed off  
Also Satamax what makes you say the riser is too short Its just under 650mm from the bottom OK thats 70mm lower than peters spreadsheet but the riser is cast refractory cement with 5-8 mm NZ volcanic pumice and volcanic pumice sand as the aggregate When it had dried properly I couldn't believehow light it was wrapped around that is 2 inches of soperwool blanket pieces I scrounged up out of a bag of offcuts from work. Its burning real well no smokeback even with our horrible winds we have here at the moment.

One more thing I want to build the first bell so the flue gasses only come down one side of the riser on an angle to feed into the underbath bell. Can anyone see a problem with this.

OOPS Satamax Just read my previous reply

SORRY bro I didnt mean it to sound like you didnt know what you were talking about. Been trawling here for years and some of your work has inspired me to crazy obsessed tinkering. Not to mention Glens stuff Peters Amazing batch boxes and all the other legends o here.
Gimme 6 to 12 months and Ill be ding some experimenting of my own there is always some way of making something more efficient.

Russel, 70mm short, is still tooo short!

Your batchbox seems real big compared to the height of the heat riser, tho, i can't see the inside dimensions.  Hence the comment.

Bath wise, i've been saying it for years,  Build four brick walls, dump the bath in there. Adapt a ladder, set of stairs or whatever. And chuck a batch box directly bellow. Make it tall enough. Or you put the  top of the heat riser where the taps would be normally. Insulate the top where the gases hit  directly. And let the bell effect work.

I had a chance to gather one steel bath. But couldn't be arsed. Too many projects on the back burner!

There's a front view

Russell Dinning wrote:
Also Satamax what makes you say the riser is too short Its just under 650mm from the bottom OK thats 70mm lower than peters spreadsheet.

To my eye, I'm inclined to think Satamax is right. Are you aware that the spreadsheet gives internal dimensions only? This way, the riser height is measured from the firebox' floor.

I measured the riser from firebox floor again last night I'm just shy of 660 mm not hard to cast a little bit more on the top how much would you recommend as minimum height. My riser was cast around a piece of plastic down pipe 110mm outside diameter.( just measured it my eyecrometer must need calibrated)

Another thought I had was to cast a large radius (150mm minimum radius) that will direct the flow on a down angle of about 45 degrees. Could anyone see this causing issues (I have a steel bend I can use for a trial)

I added 70 mm to the riser no smoke within 4 mins reloaded with split wood( macarocarpa)  40 mins later look what I got ok ill listen to the pros now lol. Going to trial the aforementioned bend on top of this see what happens

Ok finally got a chance to build the bell and get the bath on top my mortar skills are shite I know know this lol. Here's a picture half way through I'll try another picture of the finished (sort of) unit just got to mount the flue properly and seal up any gaps and shitty mortar jobs. Thinking bout cobbing the outside is it possible to add perlite to a cob mix for insulation? I know it's not a normal thing to insulate the mass but I want as much heat as possible to soak into the cast iron bath as possible.

I also stuffed rockwool between the outside of the bell wall and the fence and between the bath and the fence too. Should help it hold its heat better

In the overall view picture above I had put a mini bell with an elbow out of the top of the riser into it feeding the main bell under the bath. Now I still need to get my eye calibrated because I got some old vitrified clay pipe out of my paddock.  Used an elbow for the transfer into the mini bell and clay pipe for the exit from the main bell. Problem and the reason I need calibrated is the clay pipe looked like 4 inch but was 3 and 3/4 inch ( insert cursing here) smoked stalled and even had a small woodgas explosion in the bell dislodged 2 bricks. Lesson learnt find and use tape measure.

The question I have is what are peoples thoughts on transition from top of riser to bell I want to loose as little heat as possible before the bell. I have plenty of 115 id stainless steel steam pipe.i could weld up a nice you bend down into bell. Or could I cast some form of insulating transition manifold or cut up some light refractory board I have lying around and make a transition box. Would matt walkers riserless set-up work the batch itself is three pieces fire box in heavy fire brick rear wall separate to riser stuck together with some clay sand mix. Thoughts anyone???

Russell, use 6 or 8 inches, or even better, a plenum. Making convecting gases turn is difficult, there's friction, over-presure and all.  I tend to say, you need at the very least 150% of heat riser's CSA just for a smooth 90 elbow. In this case, may be even 200% would be good.

Cheers max I was thinking that I might just incorporate the riser into the bell like a barrel but with insulating or insulated brick update in a week as to how I get on

Hey Russell, you know English isn't my native language, and i might have skipped some stuff in your text.

If you have a bell above the heat riser, which feeds into the bell under the bath, that's perfect. I can't remember what is your system size, but, if six inches, leave as much, as a top gap.

My system size is 4 inch and yeah starting to think bell I have a pressed steel radiator in the garage might make that one wall and insulate it and use it as a heat exchanger for thermosyphon. And yeah your English is awesome for a frenchie lol

Max by plenum what do you mean could you do me a quick sketch or link. My system size is not quite 4 1/2 inch. Supposed to be 4 but didn't have 100mm of pipe for riser mold so used what I had 112 mm ish.

Russell Dinning wrote:Max by plenum what do you mean could you do me a quick sketch or link. My system size is not quite 4 1/2 inch. Supposed to be 4 but didn't have 100mm of pipe for riser mold so used what I had 112 mm ish.

Well, i mean a volume, big enough, where the gases can turn freely without friction.

https://permies.com/t/38889/a/44150/yomegastove2.skp?download_attachment=true

Imagine, you open the door of the oven, and the bath bell is attached to the oven door's hole.

By the way, will you raise the bricks to the bath's lip, or you just want to heat the bottom?

From experience reported from one I helped build, you don't want the bell enclosure to go all the way up the sides of the tub. The people who finished the enclosure on that one leaned rocks with cob plaster up against the very top of the tub, and found that the top sides got too hot for comfort, as heated air rises. I think having the bell enclosure come up about halfway would be ideal.

Yeah that's the plan Glen bout 3 more courses of bricks should do it then insulate the rest with some rockwool that's past its use as pipe insulation here at work then hit the whole thing with cob. Thinking bout nice smooth insulated sheetmetal plenum from top of riser 12 inch square down to 6 or 8 inch into the bell ( refractory coating on the inside with insulation between it and the sheet metal) hold the heat until it gets into the bell. And form a plenum inside the bell out of brick for the exhaust. I may have to shorten the internal wall also I have a feeling I have too much surface area under there considering I'm trying to heat water through cast iron and she's only a 4 inch batch. Hoping I don't time will tell.

Russell, plan a bypass, to get the chimney warm, and get things moving. Just in case. Peter usually says, make the bypass at mid height in the bell.