# Thoughts Please?



## ||Downhome|| (Jan 12, 2009)

This is something I have wanted to do for a while.

For those unfamiliar with these various wood heat systems it may not make any sense.

My thoughts thus far combine Russian masonry furnace,rocket mass heater, HASA (Heat and heat storage Apparatus)

The fire box will be standard brick lined with fire brick. the front sloping down at a angle, grate at the bottom. Air tight door on top.

Combustion air entering under grate, and like a rocket mass or RMF, no dampering of the fire, just a hot complete burn.

And like a RMH the flue gas through a heat riser and secondary combustion chamber, located between two fuel oil tanks filled with themo fluid.

Around these a brick surround with air chases for the flue gas, like a RMF.

The empty space between filled with sand like the HASA.

Then the whole thing heavily insulated and housed in a building.

I'm on the fence as to whether or not to run tube from the tanks through the sand.

I am also kicking around Ideas to insulate the mass from the ground.

On thought was a concrete made from portland and perlite, reinforced with steel mesh and rebar, covered with the foil barrier then another conventional pad on top to carry the weight.

If all the stars align and I get complete combustion, and get even 90% of the heat in the thermal mass, I have to think they way I plan on insulating it should hold heat very well. 

I would love to build a small one for proof of concept but justify the expense vs just going for it.

rambling off....


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## stanb999 (Jan 30, 2005)

||Downhome|| said:


> On thought was a concrete made from portland and perlite, reinforced with steel mesh and rebar, covered with the foil barrier then another conventional pad on top to carry the weight.
> 
> 
> 
> rambling off....


The only thing I can add is it's difficult to use portland in high temp applications. Generally the concrete cracks at temps above the boiling point. This is due to the fact that concrete never "dries". It cures. So it contains most of the water it did at the time of mixing. 

There are refractory cements and hi temp plasters that could do the trick tho.


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## Harry Chickpea (Dec 19, 2008)

Sand is an insulator. The entrapped air spaces and lack of continuous connection are in essence the definition of one type of insulation.

Complete combustion with minimal loss of heat is a holy grail. Catalytics are good at that because they don't introduce more cool air into the mix. Secondary burns need to be at high temps, and air can be preheated with waste heat, but the simplicity is what makes it a strong contender. Catalytics require some care.

A highly insulated firebox, complete combustion, and THEN extracting heat from the exhaust is most efficient. The common medium that stores heat most effectively is water.

Once you start getting into real efficiency, you have to rely on forced draft. The draft created by rising hot gasses simply means you haven't extracted enough heat and they are less dense.

A super efficient system might be designed with wood being dumped in a retort from the top. A grate on the bottom would allow gases to pass through the bed of coals for more complete combustion, then having the gasses go upwards through a catalyst, then through a double wall tube where the jacket between the two tubes is water. The (heated) water (which rises) would be pulled off at 2/3 the way up the stack. A separate water loop would handle the lower temperature gasses above that. Near the top, a divider/mixer such as seen in gas water heaters would help push the heat near the water jacket. An exhaust fan on top of the stack would pull the exhaust through the system while operating at a low temperature. Those gasses would then be channeled down and form an air jacket of low energy heat around the wood scheduled to be burned, to help drive off the moisture. Wood that is uncured (wet) only has about 1/3 the available BTUs as cured (dry) wood. The missing BTUs are what is required to turn the water in the wood into steam and water vapor.


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## ||Downhome|| (Jan 12, 2009)

Stan the cement parts to insulate the whole apparatus from the ground.

The most high heat would be in the "retort" as harry called it, and the heat riser and secondary. 

Something I left off was I want to use a clay slurry rather then mortar, I can get a gray clay just around the corner. 

I figure it not only would seal the brick but if used in the chase help protect it as well.

Harry the Fuel oil tanks hold 550 gallons of water.

So would you think it better to use gravel rather then the sand.

I can't think of a inexpensive way to design the whole thing using just water as the mass.

Remember I have 550 gallons of water, plus the Brick flue (I have to do the math on how much mass) plus the mass of what ever fills the center around the fuel oil tanks and secondary chamber. 


I'll see if I cant do some sketches to give a clearer Idea of what I propose.

I also have some brick I've been playing with well thinking of how to build the flue.

I'll snap a pic.


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## stanb999 (Jan 30, 2005)

||Downhome|| said:


> Stan the cement parts to insulate the whole apparatus from the ground.
> 
> The most high heat would be in the "retort" as harry called it, and the heat riser and secondary.
> 
> ...



The 550 gallons will likely be too much water. They will take a very long time to heat to a high enough temperature to really give back the heat so you will most always be fighting to get it hot. I'll demonstrate it.

1 btu = 1 pound of water raised 1 degree.
water weighs 8.3 pounds per gallon +/- a little with temp but close enough.

550 x 8.3 = 4565 btu's per degree. So say you want the water to be 110 and it's 60. That's approx. 274000 btu's 

Wood contains 8000 btu's per pound dry. Good dry cord wood is about 90% of that or 7200 btu's per pound. 

Say your burn is 75% efficient which would be very high...
7200 x .75 is 5400

you will need to burn about 50 pounds of wood to get the 270000 btu's into it. That's without heating the masonry, chimney, and sand surround. I could see your having do double this amount easily.

Now before you say it... Yep, that's the point. Huge storage!  

If it get's cold it will take days to heat up so be sure to take this into account in it's design.


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## Harry Chickpea (Dec 19, 2008)

The hot water will sit on top of the cold water if it is designed right (cold water to be heated drawn from the bottom and returned at the top).


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