Rocket water boiler

Hi everyone!  

I'm building my first RMH for the purpose of heating a big tank of water to support my radiators. There are endless options to exhange the heat from the rocket stove fumes, but given what I have in my shed, I was thinking of taking the path of a "single" water tank close to the bell of the rocket stove, as opposed to having multiple pipes. These are three concepts I'm contemplating...

Riser / bell / water tank:
Concept

1. Welding the rocket stove bell into a boiler tank, like a hat.
2. Same as no 1, but keeping the top of the bell free. Not sure why, but since this part may have to be replaced due to stress, perhaps the shape of the water tank should be more of a "doughnut"?
3. Placing the water tank on top of the bell like a kitchen water cooker. Probably less efficient, but very easy.

Also, not sure what is best in terms of the water volume (blue)? I know the diameter of the pipes circulating the water matters, but what about the water volume encapsulating the bell? Should I just maximize the surface area between water tank and bell...?

Thank you for any comments!
Henric

Of the three, options two and three seem the most viable.
Your diagram shows a bell that is roughly the size of a 55 gallon barrel.
A bigger bell could have the heat exchanging tank inside it.
Such an arrangement could give the exhaust time to stratify in the bell.
I think it would get more heat from the exhaust into the water.
Insulating the exterior of the bell should help as well.

Thanks William.

In concept 1 I was thinking of just cutting a hole in the bottom of an ackumulatortank, inserting the bell from below and welding the two parts together in a circle along the edge of the bell. Not sure it has to be super complex... I'm more concerned about losing sight of the bell's bottom area just above the riser. If the heat burns through, water will flood the riser and burn chamber. I might try it anyway...

Best regards,
Henric

Before you do anything else, have a look at Tim Barker's rocket hot water boiler.
http://permaculturenews.org/2012/11/23/rocket-stove-hot-water/

Thanks Peter!

I guess Tim Barker's choice to separate the primary water tank (the one being heated by the stove) from the secondary (coil/piping/shower) mainly is setup to get hot water for taps/shower - and to avoid boiling in the secondary system? I think this is basically the same setup I'm thinking of... just in a slightly different order... my hot tap/shower water is also taken from a copper coil, but in the accumulator tank, which also has an overflow valve. Though, I'm not sure I need another coil to heat the actual accumulator tank? I was hoping to heat it directly from the bell (please see image below). The accumulator tank also has an expansion vessel.

Image: Water boiler

Best regards,
Henric

So you plan a more complex setup than Tim's, with the bell heat exchanger separate from the storage tank. Is there a reason you don't want to have the main storage tank itself surrounding the bell?

This would work as shown, though locating the storage tank higher and making the connecting piping as large and short as practical would allow gravity circulation without a pump and make the stored water all as hot as possible.

Thanks Glenn!

Glenn Herbert wrote:So you plan a more complex setup than Tim's, with the bell heat exchanger separate from the storage tank. Is there a reason you don't want to have the main storage tank itself surrounding the bell?

[HT] My main storage tank is already in place. I'm currently heating my house by burning pellets. Though, due to the energy politics in Germany and the war in Ukraine, the pricing on pellets have sky rocketed, if you can get hold of it at all. Hence, I'm trying to enhance my system by adding a rocket stove. We have a lot of wood in Sweden 🙂

This would work as shown, though locating the storage tank higher and making the connecting piping as large and short as practical would allow gravity circulation without a pump and make the stored water all as hot as possible.

[HT] Thanks! I will try... though, my basement is only 5.10 high and I already have issues finding space for the bell. I want to build an 8 inch system and according to Peter vdB's calculations, I need a bell with an internal area of around 9 square meters... that is a very big bell given the space I have. Also, I would prefer to use a barrel or tank as bell and not use bricks. Bricks will be much heavier and more permanent.