Rocket Mass water heater using recycled gas HW heaters

Disclaimer - I have zero hands-on experience.

I'm looking at the idea of using water as the thermal mass in an RMH for a couple of reasons:
A) Water is a great thermal mass, and can be pumped to where it is needed.
B) I am a renter at the moment, and I cannot imagine a landlord that would say "You want to build a heavy experimental wood-fired furnace inside the rental home? No problem!". An outdoor water heater can be built a safe distance from the home, with hydronic radiators in the home.
C) I am experimenting with aquaponics. Hot water can be pumped through plastic pex tubing in the fish tanks to transfer heat safely to the fish water.

I propose -
1) Use four recycled gas water heaters with their burners and flue baffles removed. Gas water heaters have a 3 inch flue vent running up through their center through which the RMH flue gas could be piped to heat the water. The cross section of a 3 inch diameter circle is 1/4 the area of the 6 inch riser pipe, so the flue pipe should be split to the four tanks to keep from restricting the flow of exhaust gas.
2) The design does not use a barrel, just the 6 inch (Internal diameter) metal sandwich riser (6 inch metal pipe inside 10 inch metal pipe with perlite insulating between pipes) from the fire box up to a manifold built from 6" stove pipe tees and reducers to the top of the water heater gas vents. The hot exhaust gas enters the top of the water heater vents and flows down the internal vents, heating the water as it flows. The hottest gas heats the water at the top of the tank, making the most efficient use of the gas flow. Cooled exhaust vents from the bottom of the water heaters.The riser "sandwich" and stove pipe is to be covered with high temperature blanket insulation.
3) For safety, this will be a non-pressurized system with a reservoir mounted above the height of the water heaters. The entire system is filled with a non-toxic anti-freeze solution.
4) A hot water recirculating pump (with a thermostat) is used to pump the hot water through radiators located in the area to be heated.

As far as I can find, this idea has never been tried, so I would like to see what potential problems you might think of, as well as ideas for minimal cost.

Tim Williams : Welcome to Permies.com, our sister site, richsoil.com, and a Big Welcome to the Rocket and Wood Stoves Forum Threads, with 22,000 fellow
members world wide, you should be able to come here 24 / 7 and find someone who wants to talk about what you want to talk about !

Location, look at your Name space to the left of your thread and L@@K at mine. If you go to the Permies Toolbox at the top right of this page, you can click on
the "My Profile'' Button where you will be helped to post information to your profile and preferences on a new page ! This will help us give you answers to your
questions more specific to your general location, and put you out there to find near neighbors with Rocket and Hydronic' and Aquaponics experience in your area !

Unless you are truly lucky to have a remote hideaway, your project will create its own high degree of visibility, even having a remote Car Shed you can tuck this
all into It will be about 8 feet tall or more before being re-insulated, with a foot print close to 8' by 12', a shed to get you out of the weather will be needed on the
long run! Don't forget to add in the amount of floor space for indoor storage for at least several days worth of Wood !

The shape of the manifold into which your hot exhaust gases must pass before flowing into your individual water heater tanks will depend on whether you want
to plumb your tanks in Parallel, or in Series, if one tank is warmer than the others it WILL flow faster and move more of the hotter exhaust gases, this is in part
due to a condition called Laminar-flow, balancing the flow with dampers would require being able to read the gas flow temps at the tanks surface the center of the
flowing gases, and temperature of the tanks in real time. Were you thinking about 'charging' one tank with heat at a time? I wonder if this can be run by
hand !

With Its remote location, and the high degree of hands-on I vision for this project, you may not spend much time in your rental property, were you planning to
work from home ?

Then again, this may be easier than I think, probably you could try building a 4'' Rocket Burner,and use an exhaust fan to push the hot exhaust gases through
one tank ( why would you want to remove the flue baffle ?) and see what happens while you research the shape of the manifold !

Other than attempting to use a Rocket burner at a remote location you may be on to something, please do keep us up to date on your project with pictures !
For the good of the Craft Big AL !

Clarifications -
1) The proposed location is at a home (either where I live, or a friend's house who has an aquaponics greenhouse.)
2) One great advantage to an RMH is that you don't have to keep a fire running all the time. The heat from the short (but intense) burn is stored in the mass to be used throughout the day.
3) The reason I chose a 6 inch rocket was that I saw somewhere that smaller rockets perform poorly. I need to get the book to understand more fully.
4) I had considered using dampers on the cooler exit (bottom) of each water heater to be manually adjusted according to approximate tank temp in case one tank seemed to be getting most of the heat. In my mind this is a once a day adjustment until an approximate equal distribution is seen. I planned on connecting them all in parallel to provide the same total flue pipe cross-section area as a 6 inch pipe. Using a single tank would limit the storage somewhat, but would make things MUCH easier. Even using two tanks in series with an exhaust fan would be simpler than trying to fine-tune a manifold. I need to research vent fans that can handle some heat.
5) The flue baffle looks like it might restrict flow - I could be wrong. I am going by product pictures on retail sites.
6) I think you are correct about needing a shed. Unfortunately that just significantly increased the total cost.

Tim Willams : O.K. It is a little clearer now, and a few more options open up!

Regarding #2) We usually say that the RMH sheds 40% of its heat by radiating it off of the Barrel creating the 'push me - pull you' effect that lets the gases flow
horizontally through approx 30' of Cob Thermal Mass, in your case I would suspect that you could get at least a flip 60% radiated to a series of barrels and then
20 feet or a little more for additional heat energy storage in your thermal mass.

Here is the image that comes to my mind Connected to one side of a Greenhouse a separate room for your project, this would be a low humidity room, air could
be drawn through from outdoors to keep your Rocket burner wood supply dry and provide some heat with air exchange, also reducing humidity in green house
as needed.

The discharge from your tanks enters directly into a cob thermal bench that enters the green house, providing raised grow beds for seedling starting,* and can
further but up against or go underneath your aquaponic tanks, racks of additional plants stored above the Aqua tanks hold much of the heat within the tanks
and reduce water loss.

vermiculture and aquaponic's are a natural follow through from there. The fish poop feeds the Worms the worm poop feeds the plants, you get the plants, veggy
garbage is run through a thrift store blender for both the worms and fish, what worms the fish don't eat you sell for bait, and you keep the fish (or not), at least
two new income streams !

Tho a little pricey by having a deep settling tank you provide a water connection point for a pump to flow return water to a 3/4'' I.d. Copper line wrapped around
the outside of one of the water tanks where it picks up heat off of the tanks surface and returns heated water via insulated pie to keep fish warm and happy AND
temper your greenhouse !

This would deal with many of the remote location issues, solve a major problem with trying to put a Rocket Mass Heater and its wood supply into a very high
humidity environment, and still give you the thermal mass where you want it, allowing you raised grow beds (great for your back ) more protection for your
plants and fish and an attendant to watch the greenhouse and your Advanced rocket heater! Building your Rocket 2 phase Mass Heater onto an end of a Green
house will save you some on initial costs, especially if your buddy can clearly see the gain !

I was unable to visualize any use of thermal mass without a barrel, now I understand and I think your plan is doable, how well it will work is to be determined!

Basically you are correct the way you figured the Constant Cross Sectional Areas. I mentioned laminar flow, right at the very surface of the pipe you are trying to
flow a gas (or a liquid) through you have an area of zero flow, the hottest gases flow through the center of the pipe at the highest speed, the Flue Baffle breaks
that up! The smaller pipes haven g more surface area to Cross sectional Area are effected by Laminar flow more so than larger pipe !

Because building a custom manifold will be a challenge for even a HE.V.A.C. ( Heating, Ventilation, and Air Conditioning ) Engineer, given what you want to do with
your flow, I was suggesting a smaller than 6'' system to match up with one tankFor experimentation , expecting that you would then need a fan and a vertical
chimney! basically you are just moving the problem from the top of your tanks to the bottom by moving the dampers there.

Your way holds promise and should be investigated ! Generally I am opposed to the use of booster fans but in this case it is worth investigating Let me do a little
digging on my end ! For The Craft ! Big Al

Late Note : Check out Kevin Prata's Forum Thread Chimney Auto-draft in the 'Rocket Stoves Forum' A.L.

Tim Williams : Sending you a P.M. Big AL !

Light Bulb just lit up -

A) I understood that a hot riser was needed to induce the draft for a rocket stove, but did not understand that the barrel was part of the system - so the cooling effect in the barrel (compared to the hot riser) creates the suction needed to pull air through the whole system. That means if I don't use a barrel, I will need a fan to artificially pull the air through. In that case I don't need a riser either, and can simply mount the first water heater tank directly above the fire pit. The fan speed will also determine how hot the fire burns (Which may be a concern - I don't want to melt the gas HW heater!). If I knew for certain the exhaust gases would be well cooled by the time they exit the chimney, I could use a cheap duct in-line fan (<$30), but the chimney auto-draft fan has variable speed that could be useful. Has anyone measured the CFM flow from a properly working rocket stove? I also would want to add a thermostat and/or timer so the fan cuts off after the fire goes out so we don't end up cooling all that hot water.

B) With aquaponics, we never allow copper to come in contact with the fish water - the water is well aerated and would oxidize the copper, poisoning the fish and plants! Instead, I would pump the hot anti-freeze water from the heater through plastic (or stainless steel) tubing submerged in the water. I have also seen a working RMH with stainless tubing inside the barrel used to heat the fish water directly. Since the pump is pushing cool water, an ordinary pond pump can be used, rather than the heat-tolerant pump I would have to use. The only hazard is the fish water tends to have a fair amount of crud that I worry would cook inside the stainless tubing and clog it. So far the system I saw has not had that problem. His "fish house" is adjacent to the greenhouse, and he can move warm air from the fish-house into the greenhouse. I still may want to consider a similar setup.

C) I forgot to mention - I have no welding skills. The Mother-Earth article I saw that explains how to modify a gas HW heater to use wood fuel suggests cutting out the flue vent and installing a larger diameter flue. That's not within my capabilities. I wonder what that would cost at a welding shop?

So - Now I will compare two designs for cost and effectiveness -

I) The first design uses a gas HW heater over a rocket fire pit with a chimney auto-draft fan, and HW circulating pump to hydronic radiators in the greenhouse (with controls for the fan and pump) all inside a small shed next to the greenhouse.

II) The second option is to build a regular RMH in an insulated shed adjacent to the greenhouse using stainless tubing in the barrel to heat the fish water, and circulating fans to move warm air from the RMH shed into the greenhouse.

My instinct is the second option is more likely to work well.

Tim Williams : In the middle of posting this I found a new Tread extension from you, Pleas e Excuse some repetition of message, and please go to " Questions
about rocket stove water heater '' Posted in the rocket stove forum and read my reply !

If you have not noticed I sent you a private message (Purple Mooseage ) you can open it by visiting the Permies tool box at the Top right of this page !
Just click on it to open !

We are talking past each other, Your original plans call for a Heat Riser to start the draw, Good!!! What we don't know is whether the Exhaust Gases rapid cooling,
becoming Denser and Heavier will have the same effect as The Rapidly cooling gases flowing down and out of the barrel on a regular Rocket Mass Heater will work
the same ! I see no reason why not ! This was one of the resins I originally suggested trying a reduced capacity RMH, with a Single Tank !

High and low cut off switches like found clipped onto heat exchangers and the 'bonnet' of Fossil Fuel Fired Forced-Air Furnaces like for a Trailer home are like $14.
each

The Chinese are flooding the market with cheep stainless shell piping, the problem is the middlemen who mark it up like it was made here in the U.S.A. !

Instead of an inline fan I found you a way to '' Boost '' the flow from before the Feed Tube ! The more it runs the cooler it gets !

There is a clear ratio of Quantity of Wood (every type has its own average BTUs ) and Amount of air that must be feed to it measured in Cubic Feet, but My tiny brain
will not let me go there!

I don't understand your proposed flow pattern perhaps some sketches ?!!

However- This is where I Tell you that as A minimum you should plan on wrapping the S.S. piping around a section of thick walled steel buried in the cob ! As Water
will Flash to steam expanding 17,000 Xs when heated to 212dF this we call Boom->Squish! Think Boston Marathon Bombing with more Full thickness, full body
Scald Burns !

This is also why I was SO interested in your contraflow use of hot water tanks, this is a safer and easier to operate system !

If you want, we can break the system down step by step!

To build the second system as you would have it is very dangerous, and the learning curve to create a safe version is a go back to school and bone up- then live
with a Solar heat system, then pay a Hydronic Engineer to design you a safe system, then learn how to operate it !

Under C) You might find an Oil fired Domestic Hot water heater that depending on capacity could be 5'', 6'', or 7''. they usually come with the combustion chamber
lined with fire brick or molded refractory material !

If you are saying to yourself, wow what got into him, that was a fast Dr. Jeckel / Mr. Hyde, transformation, watch the Myth Busters video again - THATS QUICK !

For the Good Of the Craft ! Big AL !

I did get the P.M. and I read the thread on long feeding tubes. Thanks!

With regards to "Boom - Squish" - The RMH I have seen with stainless tubing inside the barrel is an open venting system. The fish water is pumped through the stainless tubing and dumped back in the fish tank (or possibly a mixer tank before entering the fish tank). Pressure cannot build up. He experimented some before building his system for his fish tank, so he likely limits the length of tubing to keep from overheating the water. The problem I see with this approach is the heating only occurs while the fire is burning - i.e. any thermal storage is in the fish water itself. It is not healthy for the fish to raise the water temp too rapidly, and you have to compensate for any overnight temp drop during the morning burn. If I tried this design, I might still want to use a separate insulated tank that I could heat quickly (without endangering the fish), and use the heat as needed to reheat the fish water.

With regards to the "Downdraft through water heater" idea as possibly performing the same function as the upturned barrel - I see what you are talking about. Also using the fan to push cold air does simplify things greatly. As far as the problem with a steel feeding tube that overheats - If you used ceramic pipe (I thought these were used as chimney flue liner - but I can't find it on Home depot site) would that solve the problem, or would the wood in the feed tube ignite too far up? Originally George sealed his feeding tube so there was no oxygen, thus no burn, but wood gas and smoke built up and was released when the feed tube was opened, so he used his fan to blow fresh air into the feed tube door to keep the gas flushed out. He uses a heat exchanger on the outside of the lower portion of his feed pipe to keep it cool enough to prevent the fire from creeping up the feed tube. I guess the air flow also keeps flames moving away from the feed door. If I had an inlet fan strong enough to push the air through the smaller tank flue pipe, I might need an airtight door for ash removal as well as one on the feeder door, since the fire pit area will be slightly pressurized.

The oil-fired water heater is a great idea - but I have never seen this type of water heater in north Alabama, so finding one used will be highly unlikely.

Tim Williams ;Like the quote from A. Lincoln With the S.S. Tubing inside the barrel or wrapped around the outside of the barrel it ill be almost safe some of the time
and mostly safe some of the time but never safe all of the time. Example, You get a warning that there is going to be a late season blizzard, and cram your extend
Feed Tube full and turn your fan to high, in preparation for the chance of several days without power, the lord helps themselves who help them selves, and ounce of
prevention, vs a pound of cure ! Because you did not stint on materials within 5 - 10 minutes of starting your fire, the inside of your burn tunnel is glowing a Cherry
Red, and the the last few pieces that auto loaded seemed to burst into flame spontaneously, You are off in the Green house checking on something else and monitor-
ing the temperatures of various parts of your plumbing when you loose power, with no water circulation and the rocket stove running any where between 1400 dF,
and 2400 dF the water in the coils within or on the Barrel must be Drained now, before they can flash to steam! Boy that flashlight in your pocket was handy wasn't
it! opening the bleed valve of the holding tank will let water flow down and out through the coils keeping them from making steam bubbles that will try to rise against
the flow of water stoping Flow and making a steam explosion more likely, In the mean time you have to disassemble your Extended Feed Tube, and remove all of
the burning wood within in a race to save the stainless piping from Warping due to the increased torching effect of uneven heating ! All this with welders gloves tongs
and a bucket and scoops that were right there to hand ! and the Burn tunnel is still Cherry Red.

Now that you have all that done you can consider how to deal with no power. Because you placed your S.S. tubing in/on the barrel you can not re-fire your Rocket
without distorting /destroying the tubing, and you still have to restore water circulation, if possible, do you restore the Rocket Mass Heater to thermal mass only and
destroy your tubing or do you shut your RMH down in the middle of the storm !

Different Example, you are in town getting supplies does your buddy know what to do ?!!

Let me here back from you, perhaps a link to the other Aquaponic site, If i never have to meet the guy I can be brutal as any thing For the craft ! Don't be a Marshmallow !
big AL

Tim Williams as I was finishing up the last thread I was called away to a Ambulance call, and was in a hurry ! And It shows, still it'll do ! Big AL!

P.S. All doors and clean outs should be air tight ! The only exception would be in the case of a Regular Feed Tube made correctly, not out of metal,
that should never need any heat scavenging unit to keep it cool ! A. L.

Satamax Antone : Very interesting thanks for sharing ! All water heating ties place in a ' Bell ' after the Rocket stove rather than coils around the Rocket Burner,
or very early in a Cob Thermal Mass ! Big AL !

Alan, you mentioned -
"All doors and clean outs should be air tight !"

I found some nice rectangular and some round clay flue liner pipes online. I haven't yet checked local stores. The rectangular would work really well for ash clean out access. How would you make an airtight door on the end of either of the clay pipes?

Tim Williams : The short answer is Cob! Because of my familiarity with it- I am recommending that as your next progression in getting ready for your 1st Rocket
Mass Heater RMH Build you goto-> rocket stoves.com to Download a PDF Copy $18ooUS of Ianto Evans' Great book " Rocket Mass Heaters " with
over 100,000 RMHs made world wide this is " The Book " most used, and 95% of all new builds (that worked ) were made following " The Book ''!

This will allow you to pick up the language commonly used to identify parts, their size, shape, and Orientation to each other and learn the Constant Cross Sectional
Areas of the 6'' systems, and the 8'' systems,* and allow you to use the same vocabulary when coming to These forums to ask questions and share your findings !

For the craft! Think like Fire, Flow like Gas, Don't be the Marshmallow! As always your questions and comments are solicited and Welcome! PYRO - Logically BIG AL

* the little bit of math is 5th grade stuff, not a problem ! A. L.

Tim, don't play with water if you're not a plumber. Please remember that supersaturated steam can expand 400 times, which means any pipe can act as a loaded gun. They use to proppel locomotives, and steam turbines with this kind of stuff. And when it explodes, it's realy realy dangerous.

Max : I have been using a figure I Think I got from Erica Wisner with a hot water to steam expansion of 17,000 times in a micro second !

Tim : Locating a single oil fired hot water heater with dead controls, or replaced when a system in a restaurant or convalescent home or convenience store was
up-graded should take you to the same places you would have to go to to find 4 very similar gas fired hot water heaters, a heating contractor or junk dealer !

I worked up a preliminary design of the most likely plan, and came up with a preliminary materials cost using materials from Home Depot & Amazon. Bottom line is it is too costly for my friend's small hobby greenhouse (200 square feet), but I will keep it in mind as I consider larger greenhouses. Here's a description of the design -

A small 6ft X 8 ft shed ($218 at Home Depot) will be used to house the RMH. The shed has a plywood floor over a frame. I would start with gravel on the ground, then lay the floor supports, but place concrete blocks directly on the gravel (between floor supports) to support the fire pit and HW tank. After finishing the construction of the RMH, I would add plywood for the remaining floor area, and then build the shed around the finished RMH.

I envision the fire pit built inside a 2 ft X 4 ft X 1 ft tall wooden box lined with foil. The fire pit is built with fire bricks and cob, with a 6" clay flue liner as feed tube, and a rectangular flue liner used for ash cleanout. The ash cleanout needs a door (TBD). The riser is made from 4" stove pipe inside an 8" stovepipe with clay and perlite mix sandwiched between them. The riser is long enough to reach the height of the top of a gas HW heater. At the top of the riser, the exhaust gas is routed into the top of the HW heater vent opening using two 90 degree elbows and a reducer to fit the 3" gas HW heater vent. Stone wool insulation is used to cover the riser and stove pipe sections. The "air inlet" opening at bottom of the gas HW heater (with burner removed) is connected to more stove pipe elbows to the final stove pipe that exits the shed.

Since the gas HW heater vent pipe is 3" I.D., a fan will likely be needed to keep sufficient airflow through the RMH combustion chamber. The simplest solution is to use a stovepipe auto-draft fan that fits a 4" stovepipe ($154 on Amazon) on the last section of stove pipe. An alternate (and possibly cheaper) idea is to put an airtight door on the feeder tube with a fan that blows air through the feeder tube.

A small reservoir water tank is placed above the height of the top of the gas HW heater for expansion and venting. This system is NOT pressurized (for safety). It is filled with non-toxic anti-freeze solution that will be pumped through radiators for heat transfer. Two 1/25 HP Grundfos HW circulation pumps are used - One to pump water through hydronic baseboard radiators in the greenhouse, and the other to pump HW through plastic (Pex) pipe submerged in the fish tank to heat the fish tank water.

My preliminary materials cost is over $1200, with the most expensive pieces being the shed, stovepipe fan, circulation pumps, then stove pipes and fittings and stone wool insulation.

Caution, i gonna get harsh!

Tim, with zero hands on experience.

Please, stop wasting your time with lenghty thinking and posting on the web. Get yer hands dirty.

First of all, a few hours project, make a cooking rocket. They're nice and pish easy to build, they serve a purpose. And will teach you things. How about this? Shovel and a bit of digging. Find some clay and tin cans. Straw and whatnot. And you're sorted.

Then, may be you could consider making your first mockup rocket heater core. Then add a barrel, and soo on. It will take the course of several months before you're up to par.

By then, you might think about home heating. And stop pipe dreaming.

On the contrary - This has not been a waste of time at all! I have learned a lot. and have been able to understand what is not practical, and what might be practical in the future, all without "getting my hands dirty", and without spending a dime (which, unfortunately, is really important at the moment, since I don't have many dimes). These forums are a great way to pick the brains of people with real practical experience. Thank you both!

Since the RMH water heater design is not feasible for my present small project, I came up with an alternate solar design. The materials cost is a more manageable $250. It won't heat the greenhouse, but it could save approximately $400 per winter in aquaponics water heating. If I get a chance to build it and test it, I will post the results on the aquaponics forum.

Cheers!

Tim : Good Luck : see you in the Funny papers! For the Crafts! Big AL

Tim, sorry for habing been harsh. But nevertheless, go and try a cooking rocket. Tin can, ashes, clay, straw etc. All that is available for free. Then, you might get the feel for rockets.

http://www.youtube.com/results?search_query=tin+can+rocket+stove&sm=1