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Cob bench low temperature

 
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Let me start by stating this is my first rmh build.
It is installed on a 22m steel ship in the netherlands.

The rmh is a 15cm or 6 inch system. The stove is made from steel pressure tanks.
The chimney is double insulated 2m outside,  2 m singel skin inside.
Its a batch firebox based on peter van den berg design.

Bench is half barrel matt walker inspired
‐---‐--------------------

My problem

The bench is made of brick, granite and cob.
When i first started firing the cob was visibly drying. Steam could be seen. And if you sat on it it got warm.

But since recently the bench seems to not get warm anymore.
Yesturday i opened the cleanout at the end of the bench.
Inside the bench was cool. Lots of dropplets of water were visable on the barrels. And it was blackish inside.

I am planning on adding cob around the stove base. Insultating the pipes to ensure heat is transfered to the bench and not lost . I hope this will help. But having no real life experience with this type of bench im wondering what you guys with experience can see what im doing wrong that i do not see. Why is my bench not getting dry and warm. And what can i do to improve it
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rocket scientist
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Hi Fraser;
So, you are losing heat without cob around the transfer pipe.  Definitely cover it up.
What temperatures are you getting on your barrel?
What temperature is the exhaust pipe where it heads outside?
I know you made a few changes recently. Like removing the extra superwool around your riser.
Have you made other changes?


Where your heat enters half barrels does the pipe just dump into the bell?  
Or have you piped it further into the bench?
Some builds, the end of the bench doesn't get warm enough and to correct that they pipe the heat to the far end and let it stratify from there.

It takes a long time to build heat in brick and cob.

EDIT) And meanwhile all the steel is absorbing and the quickly radiating that heat into the room.
You could try wrapping the lower part of your barrel with superwool and see if that helps.  
 
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Hi Fraser,  
Has the fuel or method of burning changed? Is it dry and the same type of wood? Are you loading it and then restricting the air inlet so it burns slower?  
A certain amount of moisture and blackness is normal. Its only when either forms very quickly or in large amounts that is indicating that something is wrong.
To me, the moisture is forming because the dew point of the exhaust gases are too low. The blackness should only collect during start ups or temporarily when reloaded.
 
fraser stewart
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thomas rubino wrote:Hi Fraser;
So, you are losing heat without cob around the transfer pipe.  Definitely cover it up.
I plan to do this tomorrow


What temperatures are you getting on your barrel?
Around 245 degrees c. Sometimes hotter but thats average.


What temperature is the exhaust pipe where it heads outside?
48.8 degrees c.

I know you made a few changes recently. Like removing the extra superwool around your riser.
Have you made other changes?
One thing. The pipe that conects the heater to the bench is 20cm deeper.

Where your heat enters half barrels does the pipe just dump into the bell?  
No
Or have you piped it further into the bench?
Its inserted about 1 metre inside the bell/bench
Some builds, the end of the bench doesn't get warm enough and to correct that they pipe the heat to the far end and let it stratify from there.

It takes a long time to build heat in brick and cob.

EDIT) And meanwhile all the steel is absorbing and the quickly radiating that heat into the room.

I can see this being the problem. It heats up a badly insulted boat in roughly 10minuted. So all the radiant heat fom the barrel is effective but the bench wont heat. I think i understand what the problem is
You could try wrapping the lower part of your barrel with superwool and see if that helps.
I was planning to superwool the bottom and then cob over the bottom and some of the barrel/bell
 

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fraser stewart
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Gerry Parent wrote:Hi Fraser,  
Has the fuel or method of burning changed? Is it dry and the same type of wood? Are you loading it and then restricting the air inlet so it burns slower?  
A certain amount of moisture and blackness is normal. Its only when either forms very quickly or in large amounts that is indicating that something is wrong.
To me, the moisture is forming because the dew point of the exhaust gases are too low. The blackness should only collect during start ups or temporarily when reloaded.





Hey gerry. Thanks for think with me.
No different wood.
I think the dew point being too low is the reason.

I will make the base insulted and covered in cob. I hope that resolves it. Will post here when i have results.

 
fraser stewart
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So guys.

I said i would update after the changes some of you recommend.
I took the stove apart and added a long pipe to bring the heat to the end of the bench.

This is now working better and warming the end of the bench to 40 degrees c.

When the stove is on it is very comfortable heat.

However i do have my doubts about how long it stays warm. After only two hours the bench is cold again.

I think that my mass is not thick enough. I have used granite on the end of the bench and that stays warm longer. Im still waiting for the cob to fully dry before i add more.

Question for you guys. Should i add more stones or make thicker Cob layer to retain the heat?

How long is normal before a bench is dry. (All benches are different) but what are your thoughts on mine and why it might not hold heat. Any tips?
 
fraser stewart
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The grey cob is the layer of granite
The brown is just cob
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Rocket Scientist
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As you have found, mass (dense granite) is important for holding heat. I would advise to find more granite slabs or the densest stone you can get and build those into the whole bench. You could just use thicker cob if space is not so important... maybe on a 22m ship you don't have to worry about a few inches.
The next cob layer will bond better if the previous layer is not fully dry yet. It will all dry out as it is heated so you do not have to worry about trapped moisture.
 
fraser stewart
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It was my orginal plan. Weight on the ship is becoming a concern. I dont suppose one layer of granite will have alot of effect though. But when water tanks is full a noticable lean is obvious

 
Glenn Herbert
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You need mass in one form or another to store heat. Granite simply gives the mass with minimum volume. Soapstone might give the most heat storage per unit of mass. If water tanks could be incorporated (probably not drinking water), you could kill two birds with one stone.

Water has by far the highest heat storage capacity per unit of mass - 4200 compared to stone's 1000, in the first table I found.
There is a detailed table at the Engineering Toolbox.
 
fraser stewart
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With that reason i built the bench up against the radiator.
Im not sure if that is also the reason for the cooling down but i figured it would act as a mass.

What are your thoughts on this. Should i take away radiator or is it acting as a mass
 
Glenn Herbert
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The radiator is definitely acting as a mass, and if you would keep it anyway, it does not count against the added bench mass. If it warms the circulating water that is another bonus.

The Toolbox table has some interesting information. Wood, especially oak, has 2 or 3 times the specific heat of stone. Soapstone (steatite) has slightly higher specific heat than granite, but sandy clay (i.e. cob) has over half again as much capacity as granite per unit of mass. Iron has only 5/8 the specific heat of granite.

Encasing your bench in oak slabs would hold more heat per unit of mass than granite or cob, so a thinnish layer of cob to connect the flue cavity to the oak would give the most bang for the buck. I don't know if oak would conduct heat fast enough to work as a mass layer between flue and cabin.
 
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