RMH principle use to heat water

Hello !

I'm new here. I discovered your wonderful forums about permaculture and RMH today. It's impressive and very cool.

I'm from Belgium (Brussel, Europe), so excuse my poor english.

I'm working in central heating : hot water distribution, under light pression (1.5bars), into radiators or ground heating (pipes in the ground).
The water is usually heated in a boiler, with gas or fuel. Lately we've started to use wood boiler.

Like this scheme http://www.maison.com/upload/cm/adm/Image/construction/chauffage/schema-combi.png

I think I've understood you don't like very much the idea of warming water with the RMH. But due to my little experience with wood boiler and to the wonderful efficiency of your RMH I can't resist to have a closer look and see what is possible.

My questions are these :

- I'd like to get more technical datas about the RMH, do I have to wait for the 4 DVD (coming in June or later), is there anything else I could get ?

- Have you any idea of the exhaust gas temperature along the system (in the chamber, along the exhaust pipe in the mass, at the output) ?

- Could the mass be reduced and replaced by water pipes (which would heat a 1000L buffer tank) without decreasing the efficiency of the RMH ?

- Could the exhaust pipe of the RMH be placed on a existing chimney or the draft would be to high and would disturb the RMH combustion ?


thanks for your sharing,

Damien


PS : to the moderator... Sorry I've put the topic in the wrong forum

Hello Damien, welcome to permies. Your english is very good!

Here are some youtubes that Paul Wheaton did with Ernie and Erica Wisner.

They are very knowlegable about the rockets. Check them out here. http://www.ernieanderica.info/
I think the idea of heating water with rockets is interesting to a lot of us but the caution that you are seeing and hearing is that someone with very little experience with heated water, and steam especially, will blow themselves up.
If you have experience and come up with a good , safe design, please share.

Hi Wyomiles,

Thanks for the links. I've been reading and watching a lot more about it tonight.

I would need more information to be able to make a small testing model.

I'm looking for detailed like the length of the exhaust pipe, the measure of the burning chamber or of the space between it and the top of the barrel.

But as I want to adapt it for hot water heating, I think that I need more general info than a building plan for one specific model.
The "fire science" post of Erika Wisner is very instructive.
The "fire science" dvd of "wood burning stoves 2.0" should probably be very complete (they talking about math ) but it's only coming out later this year.

As I understand, the burning chamber should be left a the higher temperature possible (in order to burn all the gas left).
The water exchanger should be out of the exhaust gas, to prevent corrosion.

Here a splendid drawing of a first idea. In blue the water exchanger. Red the mass in the combustion chamber. Yellow insulation.

What I would need are :

- General infos about the measure of a RMH
Do you know if I could find this in the book (rocket mass heater) sold on Erika & Ernie site, or in one of their building plan, or any where else ?

- Infos about the differents temperatures along a RMH system. From combustion chamber to exhaust pipe.
I suppose I've to take a lot of the heat but not to much to prevent disturbing the draft.


A first thing I could share about the problem of boiling water. I use two things :

- one good "pressure valve" which would open at 2.5 bars. It would prevent any explosion of the system but not boiling water (which could damage the RMH just by movement and high dilatation of the exchanger pipes)

- one double (for security) "temperature valve", this one open when the temperature of the exchanger goes up to 95°C (water boil at a bit more than 105°C at 1.5 bars pressure)
at the entrance of the exchanger is a "pressure valve" which let cold water flow through if the pressure goes under 1.00 bars.
So, if you have a problem with the system (electrical failure of the pomp, to much heat in the RMH when your buffer tank is already very hot) the "temperature valve open" and release hot water (outside) : the pressure drop.
When under 1.00, the second pressure valve let cold water come in the exchanger and cold it.

I've been experienced this home once and at my clients (6 times) and it works perfectly. Even if it is quite impressive.... imagine a 250kg boiler starting to boil...just a bit.

Finally, do you know if any one as already make measure of the quality of the air at the exit of the exhaust pipe ( temperature, CO measure, CO2 measure, O2 measure, NOX measure) ?

Damien

Hello Damien,
Thanks for joining the forums, and for your patience.

I would definitely search the 'rocket stoves' forum specifically for 'hot water,' and also for the charming phrase 'boom squish' which is how Paul christened the idea of do-it-yourself hot water.

There are some examples of past projects, including several successful ones with good safeguards.

The people who did most of the work for our Montana workshop were Ernie (Wisner, my husband) and Caleb Larson (alternative hot-water installer with cool toys like monitors and regulators).

The system itself was built around a design nearly identical to our Cabin 8" heater plans. The hot-water jacket was placed around the exhaust similar to the horizontal position shown in your drawing. At this point, the exhaust itself may be a few hundred degrees F (similar to woodstove exhaust or a little cooler), but the jacket didn't get above 90 F for the first couple of days while the mud was drying. Low temperatures suitable for radiant-heated floors, and relatively safe to experiment with. Caleb provided a spare commercial water tank with safety valves, overflow and pressure regulators, and all the trimmings. Even so, we did spring one leak and have to go back and fix things up.

The other option I often suggest is an open pot or tank directly above the barrel, like the top position shown in your drawing - but with a large reservoir instead of pipes.
One of the other forum threads shows an Australian hot-water heater where they built the vertical insulated firebox and then suspended a whole (open) barrel above the fire. Then ran the coil through this larger barrel. The large barrel can come to a complete boil (it's open to atmosphere) without damaging anything except maybe splashing the fire. The pressurized pipes running through the large hot water reservoir are protected from boiling as long as there is water in the big tank. It makes something like an on-demand heater, at a skill level that many DIYers (do it yourself-ers) could handle. I believe Geoff Lawton was involved with that design.

I would be very cautious about placing water pipes around the heat riser, the vertical position you've shown near the fire. Embedding pipes in the insulation like that can cause the water to heat too fast, and the fire to cool. The temperatures in this area can easily exceed 1000 F depending on insulation values and length of burn. (Fire itself exceeds 2000 F routinely - over 1000 C - within the vertical portion of the fire chamber.) Ernie has done one prototype of this kind, and he would not teach or repeat this design because one mistake in the size or orientation of the pipes could cause serious injury or death. It is better to heat a little slower.

There is one other prototype with a location you did not show - pipes on the outside of the barrel. It's shown in a thread about a rocket mass heater for a yurt. There is a lovely lady taking a bath with it, too.

Hope to see you on the rocket stove forums shortly.

Yours,
Erica Wisner

p.s. outside of the barrel, surface temperatures of the metal while exposed to air range from about 700 F (500 C?) near the top, down to about 300 F (150 C) near the bottom, at full burn. Plus or minus a hundred degrees based on fuel, chimney draft, etc.
If you wrap water pipes with no insulation, these temperatures may drop; if there is insulation to focus heat on the pipes, temperatures may rise. It can be hot enough to be dangerous, but not as extreme as inside the barrel or firebox.

Also the fire may not draw properly if the barrel gets too hot. The barrel must shed heat quickly enough to make the exhaust draft downward. If the barrel does not cool the exhaust, then the exhaust still wants to rise up, and it becomes trapped at the top of the barrel. So I avoid insulation around the barrel.

Damien Jehaes wrote:Hi Wyomiles,

Thanks for the links. I've been reading and watching a lot more about it tonight.

I would need more information to be able to make a small testing model.

I'm looking for detailed like the length of the exhaust pipe, the measure of the burning chamber or of the space between it and the top of the barrel.

But as I want to adapt it for hot water heating, I think that I need more general info than a building plan for one specific model.
The "fire science" post of Erika Wisner is very instructive.
The "fire science" dvd of "wood burning stoves 2.0" should probably be very complete (they talking about math ) but it's only coming out later this year.

As I understand, the burning chamber should be left a the higher temperature possible (in order to burn all the gas left).
The water exchanger should be out of the exhaust gas, to prevent corrosion.
...

Finally, do you know if any one as already make measure of the quality of the air at the exit of the exhaust pipe ( temperature, CO measure, CO2 measure, O2 measure, NOX measure) ?

Damien

We have someone who visits the forums at Proboards.com who did some temperature and exhaust quality tests, but it turns out he built a very different heater (metal firebox) and the results probably do not apply.

We are hoping to do exhaust emissions tests on some of our best designs this year, either with university equipment or through the independent labs for EPA-recognized results.
If you know of anyone who has tools to measure the exhaust emissions - especially the gases you mention plus particulate carbon - I would be happy to give them a set of plans in order to get some preliminary test reports before I pay for independent lab testing.

Temperature of exhaust pipes we have in some detail for several systems. We vary the length of the exhaust depending on the heating load. Typical is 20 to 35 feet for an 8" ID (200 mm) system.
Where the exhaust exits the barrel the gas itself may be 600 F (300 C), but the material of the pipes may be more like 300 F (150 C) due to laminar flow. Maybe a little hotter; easily within tolerances for sheet metal.
It goes through the pipes, losing heat more or less quickly depending on the angle and conductivity of the surrounding material. After about 5 or 8 feet (1.5-2.5 meters) it has lost a significant amount of heat. From this point you can comfortably touch the metal of most of the cleanouts - under 150 F (65 C).
Normally we allow the exhaust to exit while it is still over 100 F (40 C) up the chimney. We achieve this by building a longer or shorter mass; bringing the exhaust back past the barrel to add a little more heat; and by balancing the chimney draft and firebox draft according to experience. This is easier in a heater that is used all winter. The warmth of the thermal mass (70-90 F at the surface; 20-30 C) helps establish draft quickly for the next firing. If a water jacket cools down quickly, it could cool the exhaust and create a 'cold plug' that might slow down the stoves. Draft priming might be wise. There is a bypass option in our Daybed 6" plans and our draft greenhouse plans, that is helpful for cold starts. It might also be a way to regulate heat to a water jacket around the exhaust.
The original designer (Ianto Evans) sometimes used a horizontal exhaust at close to room temperature (60-70 F or the temperature of the mass; 20 C or so). But he built unusual buildings that could tolerate and support the very low draft of such a system.

I can get more specific with temperature performance for our Cabin 8" heater under normal operating conditions, but the details of the masonry mass won't help you a great deal if you are changing the performance with a highly conductive water mass. Caleb might be able to give you more details from his monitoring of the prototype in Montana.

Please let me know if you have any good sources for preliminary exhaust testing. I will think about what else I can offer for such help.

Yours,
Erica W

pp.s. I just noticed you said you want to build a scale model.
Please note: A small (reduced size) scale model will not work properly. Laminar flow means you get much slower flow and lower heat from smaller channels.
I suggest building a system that is known to work - at least 6" ID (150 mm) pipes for the exhaust, and all other openings a similar cross-sectional flow area. Expect the temperatures to increase dramatically with increases in size. The temperature ranges I quoted you are for an 8" ID (200 mm) system. Inside the downdraft barrel can be much larger (2-3 times the cross-sectional area is common) until it reaches the exhaust pipes. Just no corrugations or increases and decreases in size beyond this point.

Also, I do not remember whether the initial water heater readings were 90 F for the tank, or for the water jacket around the exhaust itself. Of course the directly heated pipes could become much hotter than the main tank - I think it just took time to bring the whole tank up to operating temperatures.

Hello Erica,

Thanks for all these helpful information. I'm, during a few month, very busy for my work so I'll work back on it at the end of this summer.
But I've in my possession a small machine to measure t°C, CO, O² and CO² levels. So if you know someone around may place ( Belgium) which would have made one of your models I would be please to make you some first measures

So let me know.

regards,

Damien