Extracting heat from the RMH exhaust stream to heat water

This isn't a plan for consuming heated water (washing etc.) just using it for additional thermal mass heating when a cob mass is small or in addition to a cob bench/mass for additional space heating with water. I just want to heat my aquaponics water and green house.

I start with a few assumptions. 1-Water is a better thermal mass than cob. 2- depending on the point of heat extraction from the RMH system exhaust stream, downstream heat will be reduced. 3- Any closed system runs the risk of overpressure ans 4- Extracting heat for water at any point inside the barrel or near the J tube is DANGEROUS for even the experienced and would also likely affect burn efficiency.

I think the earliest point in the system to safely and easily extract heat to heat water in a tank for additional radiant heat (just a barrel/s of water, or an aquaponics fish tank) is the outside bottom of the barrel. With my barrel temps at the top around 600-650deg, the bottom 6-8in doesn't get much over 200. So not much risk of steam pressure. I wonder if pulling heat off at this point would aid the cooling of gasses inside the barrel and thus enhance the "pump" action of the exhaust gas flow. It would reduce heat going in to the initial mass (cobbed duct or other cobbed mass bench etc) but wouldn't be lost, just redirected.
Here (in no particular order) are my top options so far. I sometimes tend to over engineer being a gadget guru, but the simpler and cheaper the better for reliability and safety is the desired result.

All of these are open water systems.
Option 1: Wrap bottom of barrel (or higher?) with copper tubing (3/8 or 1/2 or?) In at the bottom, out at the top of the coil, and circulate via thermosyphoning or a low flow pump on an open system to transfer the water to the water mass tank/s. At $83 for a 1/2"x50ft coil from Lowes it's somewhat costly. Note- that length of pipe would only hold ~a half gallon of water.
Option 2: Place pipe (PVC,PEX,CPVC,??) in the cob thermal mass relatively close to the the exhaust duct (bench, cob covered duct, etc) and pump/circulate it to the additional water mass tank/s. Kind of the opposite or radiant floor heating with PEX pipe. This would necessarily take heat out of the cob mass (the larger the mass the less effect), but it would be transferred to a water mass. Adjusting for temps of the cob mass the length/volume/flow rate of piping used would determine heat output into tank/s. The circulating pump (12v or 110) should cycle with temps in the storage tank/s. Manually or with sensors.
Option 3: Using an automotive radiator (since I have lots of them) in some proximity to the barrel and possible enclosing it in a sheet metal box then circulating the water to and from the tank/s. I have seen this done fairly nicely, but it can become rather involved designing such a system to prevent overheating. After some experimenting with a small car radiator just suspended over the top of the RMH barrel I had very little heat gain with continuous flow of my pump. It might work better with a larger radiator and or careful positioning over the barrel (or maybe on the side of it) and a well regulated pump flow.
Options 1 and 3 would only be employed during active operation of the RMH.

I know solar water heating is an option depending on climate (although it's doable even in cold locations with sun) but I will likely only use it in fall and spring or as a supplement to wood fired water heating.


Let the thought experiments begin A little real world operational experience would be nice too.

John Adamz : Must of your Fellow members are self-inoculated with a healthy fear of Trying to extract high temperature water off of any part of the Rocket burner or burner base.

You have clearly stated your intention to have a Non-pressurized or 'open water' system, and I am just as clearly stating that I understand this, and that furthermore my discussion
will only relate to the Possible Pit-falls of designing such a system !

I can not be happy with any water heating system that flirts with water temps in the 200*F range ! If the potential for such high temperatures is there then you must plan for IT !
Any working fluid ( or solid ) that can potentially reach 200*F carries with it the potential to cause scald injuries, this is why domestic hot water heaters come from the factory
pre-set at 140*F, which can still be too hot for some people, esp. children and the elderly ! We have a case self-reported, (In the wood stove forum )of exactly that scenario with
water in that temperature range causing extensive 2nd * burns to hands and one arm to above the elbow, because this injuries locale involves the hands and major joints it should
always be seen by a medical professional. Tho the heating plant was different, the results will be the same.

Here, we had adult #1 who had to take adult #2 to the hospital but not before that solid fuel heating plant was shut down to prevent damage to it or children left at home alone
during this emergency !

Not very long ago I started a new thread to ask about thermo-syphoning in general, and any charts or grafts available to help implement the system and was directed to '' The
Engineers toolbox ", While I never did follow up specifically to locate this useful information, I am assured by a reliable source that it is there !

I can tell you that to get a thermo-syphoning action you will need 3/4 inch or bigger piping and a storage tank at some elevated point ! Trying to flow the amount of water that you
would need to flow at the bottom of the barrel even with a powerful pump would be nearly impossible, the resistance to flow and the extreme temperatures present at this location
still make the production of a steam bubble possible in this size piping, as water flashes to steam it expands instantly to 1700 times the volume of itself !

Remember our operating conditions, with a well made Rocket Mass Heater RMH, after about a 1/2 hour of operation your highly radiant and well insulated combustion chamber is
now so hot It glows with a red heat, and any wood dropped into the Feed tube seems to spontaneously burst into flame ! Now accept the idea of a power failure ! Now you must
have a system that will flow water by gravity alone in reverse fast enough to prevent a steam bubble, protect the coils from damage, and do so from a large enough water supply
to give you time to fish the fire wood out of the combustion zone and then force additional air trough the system to cool it down.

What caused the failure was it electrical with you working in the dark?, was it a dead pump ?

If you are not there to shut down the system until you are sure it is safely drained to allow you to restart your RMH, Who is your 2nd trained operator (or did you provide for a
second back up heating source ?!

Where did that hot water drain to ?

I have been told by our Fellow Members from Great Britton that even though solid fuel furnaces are common and Any Fossil fuel fired forced-air furnace is a rarity, that the sale of
Any solid fuel fired water heater must be an unsealed type, Further I have been told that sometimes late a night the pipes will start banging away and the head of house will simply
get out of bed and draw off a bathtub full of water !

Mostly this deals with location problems with Option 1 I do want to touch on mixing valves which should be installed after the storage tank to reduce the operating temperature of the
water for safe temps for plants and people, In a 'Too Hot Water' scenario the Anti scald Mixing valve will limit the amount of hot water that can be drained out of the system and can
result in at least a "Partial Pressurization of the system Especially if it were installed directly after the water heating equipment as I actually have seen done !

As a common Plumbers practice that has become codified and enshrined in the building codes in some locations it is a requirement to install all valves with the hand wheel below the
valve, the reason for this practice was if the valve was installed backwards and the brass screw holding a washer in place backed out, some water would still flow through the system.
If the valve was installed hand wheel up, and the valve was opened loss of the screw would still hold the 'Faucet' washer down against its seat ! I was probably 10 years into the 'NO
Heat calls' and service work before I found my 1st backwards installed valve, sans 'faucet' washer screw, since then I have found two more installed Backwards! It can happen !

Two final thoughts, there is probably no better place to find someone who can eyeball a supplementary hot water system and tell if it is plumbed correctly, Than a well trained and
experienced solar hot water installer, even with an unpressurized System, (whatever that is !) I would myself ask for a safety check from him! Second, when a Hot water handling
pump that takes its working energy off of the heat of the Fluid pumped is developed it will see full adaption in the solar community ! For the good of the Craft !

Think like Fire, Flow like a Gas! Don't be a Marshmallow ! As always, your comments and questions are solicited and welcome ! PYRO - Logically Big AL !

I definitely need to heat water in my GH.
I have a 750 gallon catch basin that I am planning to use as a fish tank, BUT keeping the plants in the GH alive trumps any aquaponics idea.

My plans are to coil up some tubing and put it inside the barrel.

My water supply (the catch basin) is below the level of the burn barrel.

Using a 12 volt auxiliary water pump from a Mercedes to push water UP through the coil and back out into the FT.

Totally open system. The pump needs to be primed, as it sits above the water level, and when it's not running, water flows backwards through it.

I've tried the copper tubing wrapped around the outside of the barrel, but it was a waste of time and money.

And I have maybe 3 days to put something together.

Changed my plans,,,
1) coiled up tubing - copper too expensive - steel to hard to bend

So, I thought the best way to heat water is to weld a piece of steel to the top of the barrel (propane tank in my case) and two nipples on either side.

The heat stream from the heat riser will heat the water in the area between the newly welded piece and the top of the tank.

This will be an open system, i.e. the effluent will be without restrictions.

Marty Spehar wrote:Changed my plans,,,
1) coiled up tubing - copper too expensive - steel to hard to bend

So, I thought the best way to heat water is to weld a piece of steel to the top of the barrel (propane tank in my case) and two nipples on either side.

The heat stream from the heat riser will heat the water in the area between the newly welded piece and the top of the tank.

This will be an open system, i.e. the effluent will be without restrictions.

Marty, I'm not quite sure I understand what you're saying. What volume of water are you heating on top of the barrel? Are you using a propane bottle or just a plate welded on top of the RMH barrel and pumping water thru it. 2 nipples- cold in low, hot out high on tank?

My estimate (and the piece isn't put together yet) is about 80 cubic inches giver or take. Flow rate is approx 100 GPH and thats a guess as well.

Just a plate welded on the top of the barrel (my barrel is an old 100lb propane tank that I turned upside down with the bottom cut out, and now the top too) with two niples cold in and hot (hopefully out)

The water source is a 750 gallon catch basin that is buried in the ground.

Depending upon how hot I got to get the water to keep the greenhouse warm over night is the determining factor of aquaponics.

Marty, that approach of putting a sealed top on with two small openings concerns me. With barrel top temperatures in the 500 and up vicinity, a small volume of water is going to run a real risk of flashing to steam and creating pressure in the cap. Depending on the material you use coming off those nipples on the cap, you may be at risk of temperatures exceeding what the material can handle, as well as pressure limits.

Peter Ellis wrote:Marty, that approach of putting a sealed top on with two small openings concerns me. With barrel top temperatures in the 500 and up vicinity, a small volume of water is going to run a real risk of flashing to steam and creating pressure in the cap. Depending on the material you use coming off those nipples on the cap, you may be at risk of temperatures exceeding what the material can handle, as well as pressure limits.

I do not think the barrel temp will ever be able to get that hot with cold water running through it.

1/2 inch black pipe with no resistance i.e. free flowing out the exit pipe.

I think, that I'll be lucky to be able to increase the water temp 20F on an ongoing basis.

Got the water heater manifold back, it holds about 1/2 gallon or roughly 100 cubic inches.

Now to measure the flow rate and temp increase of the water and maybe I'll be able to figure out how many BTU's I'm capturing,,,

Threw everything together last night and did a quick burn, no water, wanted the cob inside the barrel and cob by the opening to dry out.
I did a water test today with a burn, temp increase of water was negligible being about 1/2 degree.
The test lasted about 30 minutes, and I still have damp (or even wet) cob inside the barrel.
The flow seems to be greater than 3 gallons/minute, the fittings on top are 1/2 inch black pipe, and there is a 3/4 inch hose that returns the water almost all the way to the water level,
so it is helping siphon the water out.
Tonight will be the test of how hot it can get the water, need to dry the cob out as it's going to get very very cold in the next few days.

When you talk about cob inside the barrel do you mean around the base below the barrel? Maybe you could slow down the flow rate with a valve at the discharge end. Also if the in and out are going into the same space at the same level (even at opposite ends/sides) that is a factor in the amount of heat transfer you're getting into the water.
Jim LaFrom gave a link in my other thread about greenhouse water heating that you should check out Death of the rocket heater :)

cob was put inside the barrel up to the level of the exhaust port.
I needed to raise the height of the burn barrel to add a clean out.
The water is forced around the perimeter of the tank by way of metal welded inside tank.


Like this


o(o

Did a little better burn (longer and hotter) and water temp increased by nearly 2 degrees F. 52.4 in, 54.2 out.
Haven't measured flow rate yet, but I'm guessing 3 gal / minute

@ 3 gal/min thats about 24 lbs/min at 2 degrees thats about 48 BTU/min or nearly 3,000 BTU/hr or about 1/2 degree temp increase in my 750 gal Fish tank.

Compared to the Solar pool water heater at about 3F/hour

I also shut down the fire, and turned the pump off and left for a few hours.

I'm giving this thread a bump to see if we can't get some more ideas going on this topic.
For starters I'm thinking of a system that uses a pocket rocket as an immersion heater, and a heat exchanger of PEX pipe circulation from/to water barrels for mass. This video gave me the idea. Aquaponics immersion water heater