Wood burning stove in boat

Postby ericmark » Sat Oct 17, 2009 9:59 am

Fitting a wood burning stove in a narrow boat with back boiler with radiator feed under gunnels and return under floor so should thermo siphon. However it at the moment has an Eberspächer Hydronic D4 which is still wanted for back-up heating and domestic hot water.

The original system has only a very small pressurised header tank and to ensure what ever goes wrong no damage is done to the back boiler the idea was to fit a larger header tank likely out of a car or truck but still trying to work out size required.

Unlike a standard house there is no mains water so any idea of dumping hot water and refilling with cold is out so only option is to allow it to boil if there is a system failure and there must be enough water in header tank so fire will run out of fuel before header tank runs out of water.

Facts wood will give approx 15 MJ/Kg and the Latent Heat of Vaporization for water is 2260 KJ/Kg.

So assuming fire box will take one Kg of wood then the maximum required will be 14 pints unless some one can see a flaw in my calculations?

However not all the heat will go to water so I was considering around one gallon will be maximum required. But a car header tank only holds a pint or two so not really big enough.

Second is how much steam will be produced under fault conditions and can a car radiator cap handle the steam from a 27Kw stove? Maybe two or more header tanks will be required?

I am sure we are not the first to use wood burning stoves on a narrow boat but it seems others rely on pumps and I am considering anything mechanical will at some point break down.

While on the boat if things go wrong one can always rake out the fire and put it in the cut but the worry is if something goes wrong while not with the boat i.e. wind change and the damped down fire starts to burn more rapid. Between flaring up and running out of fuel I want the boiler to stay full of water.

Any ideas as to how much the heat output can change and how much steam will be produced and will it all get through the vent on standard car type header tank?

Also how to use twin heat exchangers / calorifiers and store domestic hot water or availability of triple source units? Or combining engine cooling water system with back-boiler cooling water system with either motorised or manual valves.

Any thoughts please?
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Simply Build It

Postby Richard.Harrison » Fri Oct 30, 2009 8:22 pm


Sadly I am not an expert that can answer all your problems, but am currently dealing with the exact same issue. Maybe we can compare notes.

My set up differs from yours slightly, but essenially we have the same concern about redundecy in the system to deal with extreme events such as something failing. Which in my experience happens at the most unfortunate moment possible (-: I would rather take a belt and braces approach so when something goes wrong in the middle of knowhere there is no need for mass panic!

My setup differs in the following way... I got a good deal buying someones failed attempt at a new build, some things good but not the plumbing )-: There is a squirrel stove and back boiler going down to 3 radiators along a 69' semi trad, entirely dependant on a single electric pump. Let me see now... what could possibly go wrong here???

So I consulted a boat saftey advisor which seemed like a good start. They recomended as there is no gravity circulation in my system at all that I should have a small radiator that would be gravity fed to disperse heat in the event of pump failure. This seems sound as long as the heat produced is managable by a small auxillary rad.

When trying to calculate how much heat a fire will produce and how much redundancy is required in the system; I tend to take best bet and double it, so many unknown factors can throw out calculations so wildly, just ply safe. Heat output of different fuels is probably the most noticable.

So lets just avoid that confusion altogether. I am currently looking at two pumps (on seperate fuses) with a seperately fused relay to changeover upon failure of either. The entire system (plus maybe a calorifier) is more than capable of taking anything the fire can kick out, so lets just make sure that runs.

If we get really belt and braces, lets add another relay (wired opposing pump1) which triggers an audio or visual alram. Add a manual overide switch so that you can test pump2.

I havn't purchased anything yet as a little unsure of my ideas (hence I am here). What do you think?

In your case... I wonder... I know you can get propellor pumps that allow gravity fed flow to pass through, but can be operated either when quicker heat is required or gravity fails.

I wonder if a pressure safety valve, very near the back boiler could activate a normally passive pump when pressure exceeds X bars?

I know if my system goes wrong, no amount of pooring water in the header tank will help, so having a bigger one will only buy you time. A fully loaded fire of good fuel will last alot longer than your water will. A pressure release valve will just scald canoeists and release yet more water from the pressurised system. Keeping the water flowing and dispersing through the radiators as intended has to be the best solution.
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Postby Richard.Harrison » Fri Oct 30, 2009 8:27 pm

Just had an after thought... fitting an idle pump that allows gravity fed flow through, will inevitibly cause a reduction in flow. However, an overide switch on the pump would allow you to get the place nice & warm quickly if you get in late one night!
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Postby ericmark » Sun Nov 01, 2009 4:30 pm

I do wonder how one can ensure a fail safe. In every other system the power producer can be switched off. So first worst case scenario is water fails to circulate and it boils in the boiler this will lead to either an explosion or at least failure of the boiler if it can't take the heat.
To use the latent heat of evaporation to get rid of all the energy would need a large outlet for the steam likely bigger than the 1/4 inch pipe leading the steam away from a radiator cap so some circulation will be required.
What I have failed to find is any information as to how fast thermo-siphon will dissipate heat and of course number of bends heights of radiators will all alter the speed at which heat can be removed.
Using a pump seems a big problem as there are so many things that may stop it working from pump failure, sensor failure to power failure. And two independent batteries and two independent pumps would be required.
However pumps not used will tend to fail so some means of swapping which pump runs first would also be required.
Very quickly the cost of failure prevention becomes more than the cost of failure and each time I return to thermo-siphon as only safe method once one includes cost.
How ever much I want to plan it all first I can see no way unless someone has already done it and has some figures.
The charging is just as important as flat battery means no pump and we have also been talking about that. For example Google "ericmark.talktalk.net/narrow2.GIF" and yes I would love to talk more about the options and of course some form of control panel so we have some warning of over heat or low battery.
Problem here is all posts take so long to be vetted if you can think of any quicker method and way to swap drawing I am all ears.
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Postby Richard.Harrison » Tue Nov 03, 2009 10:00 pm

You are quite right, a power failure would be catastrophic. I have chosen to ignore that for now, as I have adequate batteries, solar and wind generators to power the builge and other pumps in all but a nuclear winter. Any dual pump solution would have to feature entirely seperate circuits and fuses to source.

I think an external pressure valve is a necessity to prevent explosion, then the worst that can happen is the boiler is ruined. Unless of course the stove can get hot enough to set fire to things in its vacinity (a boat safety examiner would never allow that to be the case). In which case the cost of failure maybe less than cost of prevention, however it is not just inconvenient to replace in terms of shear weight, but even more inconvenient to wait to get it replaced.

Pumps have finite life spans and require power, so relying on them alone is not an option. In my case it is impractical to install enough gravity fed radiators to disperse a full load of heat. My calculations were based on the fact that it takes 540 calories to boil a gram of water.

Anthracite = 36 MJ / kilo
1 joule = 0.2390 calories
Grate capacity = 5 kilos

So energy in fire = 5 * 36,000,000 * 0.2390 = 43,020,000 calories
Deduct radiator output capacity / inefficiency in system
Will boil = 43,020,000 / 540,000 = 79.6 litres = 17.54 gals

Hmmm, seems a lot... but lots of unknowns here such as obstructions to the free flow of the water as you pointed out, variant in fuel, not all heat will heat the water as some is dispersed through stove to room.

Another factor is when would you ever leave a raging fire unattended? A fire that is restricted through airflow to only smouldering overnight or whilst out for the day, would probably only require a minimal gravity solution. I suppose a strong wind might draw the fire a bit depending on flu and chimney. The converse scenario is you are sitting by a raging fire when your pump packs up, and you need to damp fire and disperse heat quickly.

I have more than enough non gravity fed radiators and calorifer to disperse a full load of heat and not much room to install gravity fed rads; so my focus is going to remain on pumps and safety valve to keep that water flowing, with a minimal gravity system as contingency.

In regards to the size of header tank and rads. I do not think we are going to achieve any exact science with this, but the more facts we know will at least provide a more eductaced guess. A more advanced system such as a railway steam engine, would have pressure valves, temperature gagues and a competant crew who new how to optimise it all properly!
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Postby ericmark » Tue Nov 03, 2009 10:44 pm

Email telling me of my own post arrived Tue, 03 Nov 2009 22:00 which must be about a day after I had seen post up on site. So if you have any ideas as to quicker methods I am all ears. I am sure if you google my call sign GW7MGW you will find me.

I have not visited the boat in a few weeks they have only been married just over a month so did not want to visit too much. He has tiled floor in but as to pipe work unknown but he will not hang around it's getting cold.

I still think it will Thermo syphon and I still like idea of car header tank what are your thoughts.
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Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.

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