Design problems with water heater

Hello everybody;

My man and I are working on a Rocket water heater so we can create a shower. However we keep bumping into the same problem with every design we tried so far. Our water doesn't generate enough heat and therefore stays cold.

The design of the rocket stove mass heater works perfectly. We have downdraft and no emissions.

We currently are using 15m of copper pipeline strapped around the heat riser (inner pipe). Around this inner pipe is another pipe slightly larger filled with lava stones for isolation. A large barrel is covering this entire construction.

I will include a drawing of the design we have at the moment. At the drawing the ratios aren't quite correct but in real life we used the instructions written in the "rocket mass heaters book".

Does anyone like to share their opinions/ideas with us how to make this work? We can't figure it out. Thanks in advance!

Is this a matter of time? Should we preserve the water we are heating in a tank and send it back over and over again trough the heater till it's warm enough?
Obviously we have to foresee a ventilation point so the tank doesn't explode cause of the steam when it get's too hot.

I understand an instant hot water system is very dangerous to build and should not be tried at home (like this http://www.ecofilms.com.au/rocket-stove-water-heater/ )

Is there a way in the system we're using now to create enough instant warm water? The water runs trough the copper lines at the moment, we are not storing it. So there is no danger in blowing ourselves up.

You have to expose your 15m of copper pipeline directly to the heat, not strapp it around the inner pipe.
As you have it the heat can only be transfered, where the pipeline has direct contact to the inner pipe.
That's a pretty small contact area, even in the best case.
The by far largest surface part of your pipe is surrounded by insulation.

Also you should reverse input and output.
A counter flow with the input at the top is more effective.

A barrel as in the "rocket stove water heater" at ecofilms is perfectly safe if it is not pressurized.
Without excess pressure in the barrel the water in the coil cannot overheat.
Simply use another tube with a siphon as a simple valve.
Small amounts of steam can condense in the tube and flow back to the barrel.
The tube can also be used to replace water lost by evaporation.
In the worst case the barrel will just evaporate empty.

Hello Karol,

Thanks for the reply!
I have been thinking about putting the copper directly into the pipe. However there are a few things to say about.
Since you need full heat to have secondary burning in the pipe, I am afraid to lose temperature with the water inside.
Copper is very expensive. After informing several plumbers to ask if copper could stand longterm direct heating they all said no, so it would be a constant cost.
After spending days of watching and thinking about this system, I think I have found my problem. I don't have enough mass around the water to sustain contant heating. After 2 min of burning the stove and than running the water trough the tube, I generate steam and hot water for about 1 minute and than it cools down. I have decreased water pressure, installed a smaller tube (in diameter) but with no results. A conventional gas heater has his copper surrounded by water (very good conductor) wich is used to have more mass around the copper, so it wouldn't cool down that much. (and is used for the central heating system).
What I need is a good conductor around my tube, but that would go directly into the stove principals since it needs a very good insulator.
So now my question:
Is there something that has enough insulating qualities for the stove principals, so I can generate enough heat. But still can give me enough conducting mass around my tube?

The specifics of the copper tube we are using at the moment are; 10 mm inner diameter, 1 mm wall, 15 meters long.

Thanks!
Pieter (Lola's man)

Pieter,
Please forgive me for butchering your drawing above. I have been thinking about what you want to accomplish and this seemed the best way to explain what I'm thinking.
The insulation on the riser is to preserve heat in that part for best combustion. The barrel needs to "harvest" heat in order for the draft to work. Therefore it seems to me to be the best place to pull heat off is at the barrel. To do this (and heat water) I am thinking the barrel needs to be surrounded with the water tank. The top needs to be open to vent and to allow filling with "lost" water. The only possible problem I've thought of is the water might do too good of a job cooling the gasses and an upright flue would not work. In this case the exhaust would heed to be horizontal or even slightly downward. So far, this is just a thought experiment, so..........

Lola Bachar wrote:
A conventional gas heater has his copper surrounded by water (very good conductor) wich is used to have more mass around the copper, so it wouldn't cool down that much. (and is used for the central heating system).

Actually water is a rather bad conductor, but it can carry heat around by moving.

After 2 min of burning the stove and than running the water trough the tube, I generate steam and hot water for about 1 minute and than it cools down.

Likely due to the bigger expansion coefficient of copper.
The copper coil looses contact to the inner pipe while heating up.

What I need is a good conductor around my tube, but that would go directly into the stove principals since it needs a very good insulator.

You need insulation for the burn chamber and the lowest part of the riser.
Inside the barrel insulation for the raiser is not so important.
For one the temperature difference there is not so big secondly the heat will stay inside the barrel anyway.
You want to extract a big deal of heat there, which makes the insulation some kind of a joke.

Alumina for example can conduct more heat than steel.
Replace the insulation with a mixture of above 90% alumina and clay.
This mixture should conduct heat at least 20 times better than water.

If you want a look at a really simple open boiler, check out "handy farm devices" by rolfe cobleigh, page 99. It is a steel pipe laid in a fire, one end plugged by a wood peg, the other fed into the barrel full of water. The pipe is laid in an open fire, converts the water to steam which heats the barrel. Not safe but very effective at making heat.

A mechanic/mason named Peewee I met showed me several of his handbuilt fireplaces. Although they did not encompass the principles of secondary burn gases, they were very effective heaters, and simple in design. Built with Rumford geometry but a bit bigger, the angled back-plate heat-reflecting part below the throat was made of 3/4 plate steel. A squirrel fan pushed air through a duct behind the plate, producing air at up to 400f.

I am not very knowledgeable about rocket stoves, but the principles of a proper burn chamber that consumes the volatiles is a very old one. Thermal mass masonry stoves (cob, stone, brick, adobe) can achieve very high efficiencies that approach gasification burner efficiency. Some very effective designs are centuries old.

My guess is that what is lacking in your stove design is a linking contact between the inner pipe and your copper coil. The above recommendation as regards clay will solve your problem - the clay will not just conduct the heat better through proper transfer, it will build up and then hold enough heat for your water for a period of time. This will also improve the efficiency of your stove as a heater, releasing heat slowly in the winter and keeping your home cooler during short summertime burns. Lava rock may gain in efficiency as an insulator if you fill the spaces with perlite. Although many types of clay may not be 90% alumina they should work ok anyway.

In effect, you have a batch heater. The tubing you describe will hold a gallon of water. Once you start to use the water it moves through the tubing much faster than the heat can transfer to heat the incoming water.
Let's apply some math...
Low-flow shower heads are designed to provide 2.5 gallons of water per minute. A 10 minute shower will consume 25 gallons of water.
The task before you is to heat 25 gallons of water from around 60 degrees coming out of the ground, to 100 degrees. 25 gallons of water weighs in at about 200 pounds. The temperature difference is 40 degrees x 200 pounds = 8000 BTU.

If you want that water heated as you use it, it must be raised by 800 BTU/minute. A pound of dry oak firewood will produce 9400 BTU. If you burned a pound of oak , and all of that energy was transferred through the pipe and into the copper tubing, you'd have it made. But it doesn't work that way. You are also heating the combustion gasses, the mass of the rocket stove, the pipe, the tubing, the insulation, and the room. If 10% of the energy went into heating the water, you'd have to burn 10 pounds of fuel in 10 minutes. That's a ROARING fire.

Instant hot water might not be the way to go.
Consider the idea of heating a tank.

Any tank will do: bathtub, 55 gallon drum, even a water heater. Run plumbing from the tank to the copper coil, back to the tank. Put in a circulation pump to keep the water moving continuously. A suitable pump can be had, I've used fountain pumps in the 150 GPM range, cost about 30 bucks, uses 8 watts of electricity. Add some insulation to the exiting hot water and the storage tank for better results. While the fire is burning, water is pumped continuously and will raise the temperature of the tank. I've done this to heat 50 gallons of water in a greenhouse using the sun and black tubing on the ground, and did this for years, often reaching temperatures over 140 degrees. This would be scalding hot for a shower.

If the hot tank is above the shower head, it will gravity feed. If the tank is below the shower head, you could use the pump that is already in place, just add some plumbing.

Water heating with a rocket stove: I found these videos of different styles which seem to work.
http://www.youtube.com/watch?v=194QU0C5hpc&list=FLJRynazMzQnzgMC4hdAomKw&index=57&feature=plpp_video
http://www.youtube.com/watch?v=YTnr8ua54Uw&list=FLJRynazMzQnzgMC4hdAomKw&index=22&feature=plpp_video

There was another guy from Poland who lives in Bristol, England. He had got to the point of having on on-demand, mains water fed hot water from his rocket stove but he recently took all of his videos off-line. I think the heat exchanger was a thin steel water jacket (about the diameter of a length of scaffold pipe) connected to the water mains, this he put in a separate chamber, next to but not in the heat riser (which would cool the rocket down). He was making great advances, shame he has now gone off-line...

Good luck!

have you also considerd alternative water heating that doesnt use wood burning to accomplish the task?
IE: compost pile, solar. hamster wheel? Im kidding about the solar. thso options have proven themselves to work very well and have significant less cost and headache to deal with.

Hello everybody,
Sorry for my absence but I've been away for work. What a nice surprise to come back and see many of you took the time to come to a solution. Now i need to read, inform and learn about the ideas because there is a lot of input. I'll let you know when i have a new design and lets talk about that later!
Thanks!!!

Just an Idea

karol kerl wrote:You have to expose your 15m of copper pipeline directly to the heat, not strapp it around the inner pipe.
As you have it the heat can only be transfered, where the pipeline has direct contact to the inner pipe.
That's a pretty small contact area, even in the best case.
The by far largest surface part of your pipe is surrounded by insulation.

A note of caution. There will be more than enough heat around the heat riser to boil substantial quantities of water, albeit at the expense of efficient combustion though probably not at the rate needed for a shower. The insulation will get hot next to the riser, and as well as conducted heat there will be plenty of radiated heat at the temperatures involved. As an example it takes a burner of 30kW or so to heat 9 litres a minute to hot water temperatures. That's a lot to demand from a rocket stove.

As suggested by Joe Braxton, it would be better to take heat from the barrel, and high enough temperatures would be available round the bottom of the barrel. This would give the best combustion efficiency and maintain the gas pumping of the rocket heater. It means you need a buffer store though to collect the heat which you will use later for the shower.