Thermal syphoning with Copper tubbing wrapped around Bell/Barrel

Hi all, first time poster here. I would like to construct a small bath house that would we heated by a RMH (waterproofed with Tadelakt). Main problem with this idea is that I would like the RMH to act as a heater for the space/people and the water. I am not certain if the bell of the RMH would be sufficiently hot enough to warm water by setting up a copper tube around the exterior of the bell to create a thermal syphoning system. I have also read that the top of the bell does not get hot enough to quickly heat water. The copper tubes would connect to an open-top metal container located inside of the bath house; so there would be no water pressure issues.  Does anyone have any advice/experience with using this system? I am also open to different ideas...I mainly wanted to set the system up with a RMH for it's wood burning efficiency and heat output. Maybe a Rocket Stove would be an easier idea but I am worried it would not heat the space well as it is small. Also to note I am reading 'The Rocket Mass Heater Builder's Guide' by Wisner so I have a basic understanding already of the RMH concept.

Hi Michelle;
Welcome to Permies.
I'll start by saying be very very careful.
A bare barrel RMH will be at or above the boiling point for most of its surface.
A several inch layer of cob on the barrel would help.
Your copper pipe will need to be large... like 1" or so.  Your picture looks to use 1" .

Were you hoping to pump that hot water for a shower?
A large pot simply set on top of the barrel would supply plenty of dipping water.

Hi Thomas, I am not sure how to directly reply to your post, so I hope you will be notified about my response.
First off, thank you for your help! I would like to respond to your questions and ask you some back for clarification; if that is ok.

thomas rubino wrote:
Were you hoping to pump that hot water for a shower?

We would like to live as much as possible without electricity. So the idea of installing a pump for us is out of the design. I was thinking I would have the copper tubes coiled around the barrel and attached to a large inox metal container. This container would hold the water which will circulate and heat up via the copper tubes. On the side, we would keep a bucket of cooler water so that we can mix the hot and cold water to the temperature we like. Following we would take a smaller metal pot to pour water on ourselves for the bath.

thomas rubino wrote:
A bare barrel RMH will be at or above the boiling point for most of its surface.
A large pot simply set on top of the barrel would supply plenty of dipping water.

It seems from your response that I would be able to collect hot water from both the copper tubing thermal syphoning system around the barrel and also from setting a pot of hot water on the barrel for boiling? Both would work efficiently? Would you predict that the water would come out steaming hot?

thomas rubino wrote:
Your copper pipe will need to be large... like 1" or so.  Your picture looks to use 1" .

I am a novice at this idea of thermal syphoning..how do you determine the size of the tube? For example, how would I know that a 1" copper tubing would be needed for a RMH's barrel diameter?

I am also curious if I should be careful about how small the bath house would be. Would you say that having too small of a bath house could be dangerous?

Thank you for your help!

The riser of a RMH needs to be insulated because it needs to be hot and stay hot so that the expanding gasses push the exhaust out as opposed to a normal fire where the rising smoke pulls the exhaust gasses up the chimney.

I think you need to put the coil around the part of the exit pipe close to the rise but not around the riser itself. You typically have some sort of insulated hot water tank that is positioned above the heating coil so the hot side water can rise up into the top of the tank, and the cool water at the bottom of the tank can sink down into the coil inlet.

You will also need to attach a pressure relief valve o the tank in case the water boils. When I was a child we had what is called a "wet back" attached to our coal burning stove which heated the house, and on cold days you could hear the water boiling in the pipes and the hot water cylinder.

Remember that water absorbs an incredible amount of heat energy. I don't know if a RMH is the right design for this application because it won't be running long enough to get 100 gallons of water up to shower temperature.

I don't think there is any clear chart of pipe size for applications like this.  Even if you find charts likely you won't have the technical data to use them.

The danger everyone is concerned about even in an open system is if the water in any section of the line flashes fully to steam.  The momentum and the friction of the water ahead and behind limits the escape and the steam bubble gets high enough pressure to rupture the pipe spraying shards of metal  and super heated steam and boiling water into the room with the people.  Potentially injuring or killing by physical damaged from the shards of pipe, by the blast damage  from the explosion and by burns from the super heated steam and hot water.

The reasoning for larger pipe being safer is basically 2 fold.  1.  If there is enough water in any section of pipe you are not adding heat fast enough to flash it all to steam in any section of the pipe.  Thus instead of behaving as an explosive force it behaves more like a coffee pot peculator.  2.  Larger diameter means less drag so the water pressure can't climb as high when the steam flashes.  Plus the water velocity is lower because instantly adding say 5 cubic inches of steam to a 1 inch pipe  will create 4 times the velocity of adding it to a 2 inch pipe..  Lower velocity and lower drag equals less pressure and less risk.

I have no idea where the real risk points are.  Larger stove, longer heater coil(ie more wraps), greater riser height, stove heat output and a whole host of other factors will decide what is safe and what is dangerous0  General online consensus of what I have read seems to be that 1" is the absolute minimum safe size with some say it is 1 1/2" and some saying 2"

You are grabbing the heat at the right place.  Suggest keeping return pipe to the stove as short as possible and the lift head getting back to the tank as short as possible.  And while you may run a smaller pipe from the cold side to the bottom of the coil keep the pipe back to the tank larger with no constrictions. The bigger your storage tank the lower the chance of over heating everything.  The taller the storage tank the better it can stratify also reducing risk as you are pulling colder water into the coil.

My solution to this was to put a pot of water on top of the bell,  then I made a coil small enough to fit inside the pan,   I then filled the pan with water  with the coil in it  ( this gives great heat transfer to the pipe because it is surrounded by hot water.


Then I insulated the pan and coil.       My mistake was using 1/2 inch line I should use larger line then insulated it ......


Anyhow my next plan is to put this outside my bathroom window and then heat the water in my bath tub via a pump back to the tub,  then I will add cold water to make the water just right for a bath, or a shower using the pump to pump the water from the tub the shower...

Hi Michelle;
In all truth using copper pipe around the barrel is a risky thing. Smaller 1/2" copper pipe is readily available but the potential to flash to steam is  high.
The  larger size copper tubing will be expensive.
A large pot on top of your barrel is the safest / cheapest way to heat bathing water.

Bigger is better in your bath house. A RMH can get quite toasty.  At times you might use it as a sauna.


A cable pully system can be arranged to lift a 5 gallon bucket of just right temperature water with a spin on showerhead.

Hi all,

Thank you very much for your advice.
With the safety concern I have opted out of the coil design. I am considering instead to design the stove in the way that the video shows here:

Fast forward to  3:21 - 3:32 minutes

I could not attach photos, so sorry that you have to look at the video.

Has anyone used this system before? Is it a safer alternative to the coil idea?

Also, I am curious, do you have to buy a stove with the interior pipe already installed or can you weld one in yourself? Anyone done this before?

P.S.: In the mean time I will settle with the stock pot idea until I find safer alternatives

You have conflicting design objectives, which may be hard to work around.

You want to heat the water to a comfortable temperature, without also making the room unbearably hot for people. The balance between those demands will be difficult to meet without a fairly sophisticated system, which defeats the point of the design. I would probably try to keep the two systems separate - RMH bench for space heating and a smaller fire for water heating.

I agree with Michael, two separate systems may be the path.

I use thermo syphoning in my home.
I have a wood combustion stove with a 6 inch flue.
I have wrapped a whole coil of 3/4 inch pipe around the flue just where the flue exits the heater.
I use a special copper storage tank built for wood stove 'wet backs' and solar hot water from panels.
They have more inlets and outlets at specific heights.
The tank is vented to the atmosphere and the hot water is gravity fed around the house where its needed.
It sits above the ceiling of the bathroom and holds about 200L.

I use larger pipes to connect to the shower etc.

Sometimes the water boils within the pipe if the fire is vigorous, but after 38 years it is still working safely.

Hi John, curious about the thermosiphoning system you have set up

John C Daley wrote:
I have wrapped a whole coil of 3/4 inch pipe around the flue just where the flue exits the heater.

Previous comments on this post have warned that setting a coil around the flue is dangerous...risking that a steam explosion can happen in the house.

What makes you feel secure to set up a coil around your flue? Are there conditions for which you can set up a coil system safely for thermosiphoning?

Another option is a flash boiler. It's used by home brewers where they intentionally flash the water over into steam which is then piped to an injection point that heats the wort without scorching it.

They are just like those instant hot water heaters except they have more contact time between the heat and the water, and they have more heat of course.
https://www.homebrewtalk.com/threads/flash-boiler.153116/

My pipe starts at the bottom which is the top of the firebox, and is coiled around towards the top in close rings.
Cool water from the tank enters the bottom of the rings from a low tank outlet, as the water is heated, it rises.
Hot water from the top of the rings syphons back to the tank at an entry point which is towards the top of the storage tank.

I do hear the water boiling inside the tube from time to time, but since its open ended into a tank which is also vented there are no problems.

Its worked for 38 years or so.

Others may be surmising explosions will occur, but there is no technical reason why they would and certainly no empirical evidence either.
And the price for a full roll of pipe is not expensive compared with the amount of hot water I have drawn over the years.
Sometimes quality is cheaper!