J-tube rocket vs Batchbox heater for open boiler

I have plans for a utility building for our...utilities. Heat, water, water filtration, power, gray water filtration, etc.. In the building, I will place (2) 5,000 gallon poly tanks for a total of 10,000 gallons of heat holding water. These tanks will not be under pressure and open to the atmosphere. The two tanks will have two connecting pipes. One at the bottom for cooler water leaving and the top for the return from the boiler. The system will have either a rocket stove or a batch-type stove to heat the water. Through the stove, after the riser, will be a thermosiphoning coil made of copper or stainless steel to retrieve cooler water from the bottom of the heat tanks and send it back to the top pipe connecting the two tanks for warm water. I have no plans for water above 130F which is below the upper limit of 140F of the poly tanks. I should fear no explosions from the system as long as I can ensure the thermosyphon coil is of proper size, incline and length to be able to keep the water moving even under the highest heat to  avoid boiling. Inside the tanks will be poly, copper or stainless coils for conducting heat from the water to heating pipes to transfer the heat to the house's radiant tubing and return to the lower part of the tanks for replenishing the closed circuit's heat. Our hot water will also be obtained by a coil through the tank to the house from the fresh water cistern (also a 5,000 gallon poly tank). I expect to keep the water between 100F and 130F year 'round for use in our home and greenhouse. Summer sun will also warm our water tanks through solar water heaters on the greenhouse roof. This should allow me to store 2,500,000 BTU in insulated room inside a well insulated building for a 2-3 day burn to reheat perhaps once or twice per month.

Does anyone have experience with boilers on a larger scale?

Which type of burning method and size (rocket stove or batch burn) might be more efficient at heating with a thermosyphon? I have experience with a 4" batchbox attached to a large bell. I loved the concept for our camp and it worked very well, but in my case, the riser needed to be lengthened to be more efficient of burn and my box needed to be insulated to allow the fire to heat quicker as the small box could get too cold after a day of rest. I limited the length of the riser since we needed a comfortable cooking height.

I own everything I need for this project except for the coils inside the tanks and the thermosyphon coils. Cost to finish the water heat project should be minimal.

It is ambitious, but I have things I like to do other than continually feeding a fire daily. It will be nice to be able to visit family out of state for an extended period of time and be comforted knowing we have enough heat to keep our greehouse and house from freezing.

I would build the biggest batch box you can.

J tubes are less efficient and take more tending.
They also need a longer heat riser to be effective, and just as much insulation.
Most advocates for the J will say it takes less skill to use,  but they also recommend building them in a  location that is in the center of family life.

I would not build a coil,  rather I would use a directly heated tank that was connected to my giant storage tanks.
The steel tank from a gas water heater can be had for free.
I would use this instead of a coil so I could avoid finessing the size /safety factor and to save money.
It would be unpressurized of course.

William Bronson wrote: I would build the biggest batch box you can.

J tubes are less efficient and take more tending.
They also need a longer heat riser to be effective, and just as much insulation.
Most advocates for the J will say it takes less skill to use,  but they also recommend building them in a  location that is in the center of family life.

This is originally my thoughts as well. I was not sure of the size, though. I have a spreadsheet of dimensions for batchbox designs, but I do not have the BTU rates for such heaters. While I know the type of wood and it's moisture content is relavant, it would be nice to have a starting point to work with.

Also, this boiler will not be in living quarters, but in a utility building and in the same insulated room next to or between my heated water tanks to take advantage of any heat radiating from the heater.

William Bronson wrote:I would not build a coil,  rather I would use a directly heated tank that was connected to my giant storage tanks.
The steel tank from a gas water heater can be had for free.
I would use this instead of a coil so I could avoid finessing the size /safety factor and to save money.
It would be unpressurized of course.

I made an experiment with a 275 gallon fuel tank full of water. I laid it on it's side over a 6" rocket stove riser on one end. Only the bottom was heated. The remainder had 6" of rock wool laid on it. I had a burn for about 6 hours in which the water had warmed only a few degrees so I was discouraged over a flat steel heating surface. The gas water heater would be a rolled steel plate thicker than the tank I used and would take longer to transfer heat through. While a gas water heater tank can be had for free, I want to be as efficient as i can afford since we will be using this long into our retirement years and the less amount of work we have to put into heating the water, the longer we can live in our home in comfort. Looking at   Heat Transfer Properties of Metals at engineeringtoolbox.com, Aluminum and copper are much preferred to steel. Aluminum is cheaper, so when the time comes, I will have to determine the amount I will be willing to spend and the shape it needs to be. I may be incorrect in my assessment and will admit error. Anyhow, thank you for your assistance.

Wow 275 is huge,  I was thinking 40 to 80 gallons.
I think you probably lost a lot of heat to the air, even small batch box stores are known to boil gallons of water  rather quickly.
Peter van den Berg  calculates the btus per hour of a 6" batch box to be about 77323 in the following thread:

https://permies.com/t/57202/BTU-hr-Batch-Box-Rocket#:~:text=Lower%20heating%20value%20efficiency%20is,yield%20about%2077323%20btu%2Fhr.

That was 2016,he might have changed those calculations since then.

If you enclosed the heat exchanger, be it a coil or a tank, inside of a bell,  you will increase the time the exhaust is in contact with it and that should improve heat transfer.

Most wood boilers seem to use a water jacketed firebox,  but that's not good for rocket stoves.
A water jacketed barrel enclosing the riser would be great,  but could require advanced fabrication.
I have successfully created a water jacketed feed tube with a steel inner cylinder ,  terra cotta outer cylinder with cement and silicon caulk in between.
Since tempatures never rise above 212F,  regular silicon caulk and cement work fine.
Scaling up, a  55 gallon barrel could serve as an outer cylinder, a 30 gallon drum(or 40 gallon water heater)  as the inner.

Stainless steel cooking gear is relatively cheap, heat resistant and has good heat transferring properties.
Steam table pans,  giant mixing bowls, stock pots and bain maries all have potential as parts of a heat exchanger.

William Bronson wrote: Wow 275 is huge

Yes, that was . It was what I had at the time and I figured if I was trying to heat 10,000 gallons at one burning, I might have to go big! remembering back, it got to 119F under the rock wool at the very top as it was stratifying. The water may have started at 50F. It would take a lot of figuring at different levels of stratified temperatures to figure the BTU captured. I cannot take the time for that right now. I'm sure it's going to do the same for me in the big tanks. I believe I'm going to have to use pumps to circulate the water to mix it to de-stratify the water since I cannot have water above 140F in any one part of the tank as the poly tank can become weak and may leak. If mixed, I don't believe it will stratify. I have not found any evidence to the contrary. If there is, I will have to reconsider my options as I am off grid and do not want to operate pumps full time to keep the water mixed.

As far as the heating tank or coils, I don't want to go too awfully big as it will be expensive for the materials, and too much heat will be lost in the process of heating the water. I may be wrong. I'm thinking I could make the entire room that the tanks would be a part of would be a "bell". Not only would the boiler be heating water directly, but the exhaust could envelop the tanks to heat them from the outside. Having over 800 sq ft of heating area of the outside of the tanks would definitely help in warming the water. Then I get back to the poly not being being able to be exposed to temperatures above 140F. Alas, I have crazy thoughts and should not entertain them.

A 6" batch stove at 77,000 BTU would take 32.5 hours at 90% efficiency to create 2,500,000 BTU. Now, that would be three 10 hr days of burning which I'm fine with. But, how efficiently can I transfer 2,500,000 BTU to water? If I am only 70% efficient that means I have burn another 14 hrs (1.5 days) to save enough of the 3,600,000 BTU. If I can cut losses to 15% instead of 30%, I can save 1/2 a day burning.

I know it sounds like a complicated process to be able to cut burn time to a minimum and I could do it with a smaller tank system, but I'm looking to heat a 1500 sq ft house, possibly two houses, a 750 sq ft utility building and a 1600 sq ft greenhouse for fresh winter product for 2 families.

As I see it, i need to now determine boiler size (BTU), material and type of heat exchange and if the water will stay de-stratified once mixed. Perhaps, the most important part of this will be the latter. If the water will not stay mixed, I cannot operate mixing pumps full-time...mooting the first two questions of BTU output and heat exchange.

Lunch is over. Time to go back to work. Thank you for making me think.

The temperature limit on on your tanks do complicate things but the price for them is right.
Having them in an insulated building heated by the same rocket stove that is heating the water will at least help keep them warm.

If you put a pump on the loop that comes from the rocket stove it will give you more control than  thermosiphoning.
With that control, you could potentially prevent flash boiling in the coil and stratification in the tanks, with just one pump.

To lower the time spent on tending fire  you could build two rockets and run them in parallel.
It would mean more infra structure but also built in redundancy.
They probably have separate chimneys.
The 8" batch box rocket is a known and tested design that produces even more BTUs/hour.
Building two 8" rockets and firing them at the same time could severely shorten the time spent heating the giant reservoirs.

For a different approach Martijn Macaopinodescribes his indoor TLUD stove in this post:
Indoor biochar producing TLUD gasifier

Its not as efficient as a rocket in that the carbon in the wood is left unburned, but it is clean and can be set up and  allowed to burn untended.
He uses pellets and agriculture  waste like nut shells, but there are 55 gallon sized TLUDs that burn chunks of log.
Capturing the heat from one of these  bigger TLUDs would go a long way to warming the thermal mass.

William, thank you for the comments and suggestions. I had not thought of having more than one heater! Doh! Many minds make for lighter work?

My tanks are each 8' diameter x 12' tall. They are the same as those big yellow tanks you see at DOT spots along the highway in colder climates where they may store liquid salts in them for use in tanks on their trucks to spray on the roads when their own truck tires are slipping and need traction. We obtained them for this purpose at auction where I could not turn down the price. Many gallons of water hold a lot of heat and I don't mind not being able to store near boiling heat like I could in a steel tank because it forces me to work with lower temperatures for the water. There is less chance of getting injured, plus, with plastic I'm not going to try to pressurize it.

I'm trying striving for simplicity and efficiency for everything I do. If I can be more efficient and still do it safely, I don't need to spend more in time, material, labor, money later. If I pass before my wife, I want it simple (not that she is anything but a genius) so she can go on with her life and not have to worry about anything but daily chores. I would hope she would find someone after me who would be able to understand what bells and whistles are, but I cannot rely on that.

Here's my thinking:
Clean burning
Able to load about an hour worth of wood (who doesn't need a break to go burn something every hour)
Insulated tank storage room
Inside the heat storage room should be an outside air intake pre-heater for combustion in the heater
Fuel feed door at a comfortable height (no bending over)
Fuel feed door outside the insulated room where the tanks are stored because who wants to be in 100F+ heat
Vacuum system for moving ash to a location in the greenhouse for use there or for distributing elsewhere
Two paths to the chimney (one for direct upward flow to aid in starting and the other for for "work" flow)
Temperature sensors. room, three on each tank top, middle and bottom. Pre-heated air temp, combustion temp, pre heat exchange, post heat exchange and exhaust for efficiency calculations and adjustments
Electric motors atop each tank with shaft down into water with a propeller to mix water on a timer or by automatic switch triggered by sensor differences
I was hoping for thermosyphon piping, but that may not fit our purpose. May have to use a pump to be most efficient(Looking for thermosyphon data on pipe material, sizing and water flow)
Pressure blow-off valves near the heat exchanger just in case even though it is an open system, there is plenty of water above the heat exchanger to create lots of pressure in a "flash" scenario

I know there are other ideas i can incorporate, but I have other projects awaiting me now and cannot keep my mind in this post.

As far as determining the size of the heater, the determining factor is size and efficiency of the heat exchanger...and of course, our budget!

I have thought about TLUGs before. We get plenty of material we can put through our chipper and have wanted to make a source of fuel for a generator to recharge our batteries. Perhaps that could be incorporated sometime in the future. Right now, i have too many projects going. But thank you for reminding me of that possibility. I'm sure it will be going through my head as I am modifying our utility trailer today.

I forgot I had these heat exchangers. They were used in an atrium in a university. The actual exchanger in each is 144" x 30". I think I should be able to use these. While very tall, I need at lease that height for my tanks as they are 144" tall as well, but these exchangers will most assuredly require pumps to be sure the water is circulated at a high enough volume to not boil/flash to steam.