# inground outdoor wood furnace



## rmanbike (Jan 24, 2014)

I would like to heat my insulated basement, about 1000sqft. in which about 1/2 is daylight.  Their is a block retaining wall connected to one end of the basement. The ground is sloped behind the retaining wall, 8ft on down.  Thinking of digging out behind the retaining wall and installing a wood broiler with access by cutting out for the door in the retaining wall. Stove would be about 4 ft underground. Would be about 3 ft from the basement exterior wall. The stove would be a octagon shape wrapped with copper tubing, stove pipe would exit from the back up through the ground. Venting controlled at the door. I would fill in around the stove with sand then gravel then earth. The area is well drained. 
Anyone tried a similar setup or advice?


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## Clarkbug (Jan 25, 2014)

I havent heard of an idea like this before, unless you were doing it to roast a pig or something.  

I think that your stove is going to bleed a lot of heat into the ground if its sand then gravel.  

If you are just looking to heat 1000 SF?  Why not just install a wood stove in that space?  Sounds like you have exterior access to install a chimney, and the stove would be easier to control, and less likely to make your home owners insurance ask questions.


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## rmanbike (Jan 25, 2014)

I dont have any wood burning stoves or fireplaces in my home now, thats one reason my homeowners insurance is inexpensive and easy to find. That said, a outside wood furnace is my only real option as I do not desire any smoke in the house or drafts from a flue. 
From what I have witnessed from all other outside wood furnaces is not enough mass for controlling the heat. Typical outdoor furnaces, you have the fire box, then water jacket then some insulation. The fire doesnt burn hot enough because of the water jacket and not enough insulation, plus stainless steel is not a good conductor of heat. The best conductive material is copper. Sand and earth holds heat. Heating the earth or outside air? What difference does it make? Heating the earth allows you to put a small winter green house over the furnace. 
So I trying to understand why someone else hasnt thought about this before?


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## Boil&Toil (Jan 25, 2014)

Has been done decades ago; HASHA or HAHSA - not terribly efficient burner setup. Little current information on a quick web search;  Hard to repair (get a shovel...)

My recollection is that, compared to what you described, those are at least keeping the sand insulated and dry, not just digging a hole in a bank where water will suck away much of your heat - but they are also limited by the fact that sand/gravel is a poor heat storage medium as compared to water on both a volume and weight basis. But if you can find more information and use a better burner setup to make more heat and less creosote, that sort of thing might suit your preconceptions better, while working better than what you have described.

A better job (based on most people's experience, and the preference to keep fire out of the house) is a good wood boiler (inside type, gasifier preferred) in a well/super-insulated shed connected to adequate heat storage (water) placed in the basement, and preferably radiant heat loops in the basement floor.


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## Coal Reaper (Jan 25, 2014)

If your access door is in the basement you will still have a draft in the house and potential for smoke spillage into basement. I have seen an indoor boiler placed in a block room that was built into the side of the hill. Room was not insulated and tanks where 20' away in the house. Room stayed warmish and boiler was a model quite well insulated. Idk what would happen if they were away for a period of time during below freezing temps.


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## rmanbike (Jan 25, 2014)

I should have mentioned the boiler and door would definitely be outside. A short run for the pipes as all piping would be covered (dig for access) but would be deep enough to avoid any frozen pipes. Their again I could drain the system if not at home. And would only have access to the front for loading, fan and ash tray. Pump would be in the basement. If I could get it to burn hot enough their would be not as much smoke. But not sure as this as it would terminate at about ground level.


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## Boil&Toil (Jan 25, 2014)

rmanbike said:


> But not sure as this as it would terminate at about ground level.



If you want to fail, you can certainly arrange for it to be a fiasco that fails. If you have some trace of desire for it to succeed, you'll put up enough chimney to be of some use, and not smoke the expletive-deleted out of your house from smoke emerging at ground level beside it.


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## Coal Reaper (Jan 25, 2014)

Yeah. Def gonna want your chimney at least even with the roofline. 
And how much is your insurance really going up if install is inside?  Kinda like saving pennies while missing out on saving dollars. 
Pls explain octagone stove wrapped in copper tubing. Is this to be a homemade job?


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## flyingcow (Jan 25, 2014)

Also, curious where are you located?


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## Vizsla (Jan 25, 2014)

No part of an open loop zone should be above the water level of the boiler. This a first I've heard a plan like this. Good luck and please do plenty of research, it can only help your design


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## maple1 (Jan 25, 2014)

Since you did ask for advice, mine would be don't.


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## rmanbike (Jan 25, 2014)

I very much appreciate all the good advice. Im in the foothills of North Carolina, have land with plenty of firewood. From what I remember in mechanics school is anytime air is passed over a opening it creates a low pressure over the hole, called venturi effect I think. This is and that hot air rises causes the smoke to exit out this pipe coming out and going up. So im sure any thing that blocks the air or wind from blowing past this pipe will effect this draw. 2 ways to look at this; put in a power vent or as you say place the pipe higher or away from any obstacles that block this flow of air. The hexagon shape comes from a manufacturer that builds outdoor boilers= more sides more areas for heat transfer. I was thinking of wrapping this hexagon fire box with a thin aluminum mess then wrapping the copper tubbing around this, creating a more circular design for better efficiency. The water would circulate and be a closed system under  pressure(pressure relief value required). 
My insurance company said they would insure me only at a much higher rate, if I had a wood stove inside the house and I have heard others insurers canceling their policy if they have an interior wood stove. In this area it happens on a regular basics, house fires from wood stoves or fireplaces. But this is more typical of homes 60 years older or more. 
From my research sand is a pretty good at storing heat but not too much to interfere with the transfer of heat from the firebox to the copper tubing.  
Learning more as I get into this!


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## rmanbike (Jan 25, 2014)

I have read some interesting articles using sand as heat storage:
*"What Makes Sand Good for Storage?*
The following examples illustrate the differences between
sand-bed storage and water-storage systems. Let us assume that
the model building is a well-insulated, single-story building with
a 2,000-square-foot (186-square-meter) footprint. Using our rule
of thumb to size the collector array (2,000 square feet x 0.2), we
get a 400-square-foot (37-square-meter) array. If using water as a

heat-storage medium, the water-storage
tank must have a capacity of 400–500
gallons (1.5–1.9 kiloliters). This tank
would weigh between 1.67 tons and
2.08 tons (1.51–1.89 metric tons). If
using a sand bed as a heat-storage medium,
the sand bed must contain 2 cubic
feet (0.06 cubic meter) of sand for every
square foot (0.09 square meter) of the building’s
footprint. A cubic foot of dry, compacted
sand weighs approximately 100 pounds (45.4
kilograms), so for every square foot of the building
footprint the sand bed would have 200 pounds (90.7 kg) of
storage, resulting in 200 tons (181.4 metric tons) of storage for a
2,000-square-foot building. You can see that the sand-bed storage
system has 100 times the weight of the water-storage system.
Heat dissipates quickly in a water tank because water is uniform
and has no spaces between water molecules. Sand, on the other
hand, has spaces between the granules, so heat transfer is much
slower. However, heat can travel in a concentrated path rather
quickly through a sand bed toward cold."


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## maple1 (Jan 25, 2014)

rmanbike said:


> I have read some interesting articles using sand as heat storage:
> *"What Makes Sand Good for Storage?*
> The following examples illustrate the differences between
> sand-bed storage and water-storage systems. Let us assume that
> ...



So after reading that you still want sand?


Just don't.

This tablet stinks for typing but I'm not moving from the couch - all kinds of wrong in this idea.


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## Fred61 (Jan 25, 2014)

I think you should go for it. Sounds like you have it pretty much figured out and laws of physics have been pushed aside. You feel you have invented a better mouse trap and you're not going to let the cumulative knowledge of these guys deter you. Several have popped up here looking for advice. Some are willing to take the knowledge that others here have learned either by experience or by research while others ask for advice after they are well into the project which turns out to be a folly.

I've done a few things twice in my life and I've learned to take the advice from those in the know.


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## Coal Reaper (Jan 25, 2014)

Sounds a bit like you are trying to re-invent a rocket mass heater with a coil. If a recall there are issues transferring heat from the sand to water. So how does your insurance company feel about this as apposed to a wood stove?  And wrapping the boiler in whatever?  Oh and the long dimensions of a hexagon have less perimeter than that of same overall dimensioned square/rectangle. 
The way i see it the only way to make this underground boiler setup work well enough to not be a total waste is with a well insulated unit and a chimney  that clears everything that you dont want to get washed with smoke.   Idk where you are going with venturi, but the concern is smoke getting into your house through window seals or eaves if you dump it at ground level next to the house. Of course the better way to make this work is with a well insulated shed like most do.


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## rmanbike (Jan 26, 2014)

Coal Reaper said:


> washed with smoke



The opposite of this, I think its called Gasification! I have seen a number of outdoor boilers that create quite a bit of smoke. Not something I would want around my house. The key is in your design to reduce 90% of this smoke! 

Their has been several studies on heat transfer in sand, much has to do with the fact that over 70% of metal casting is done with sand. I think the article "What makes sand good for storage" speaks for itself. 

A better mouse trap is always around the corner, we just not made it to the corner yet, so how can we see it now!


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## Coal Reaper (Jan 26, 2014)

You are asking for advise and then not answering the questions to then points that we raise. On the surface your idea idea is flawed beyond reasoning. If you are going to make a move on this, please document and take pictures for us all. I you are going to take one bit of advice away from the knowledge base here, perform a small scale test before you endanger your home and family. 
You might be able to cope with visable smoke for 10% of the time but the other 90% still has CO and other gases.


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## maple1 (Jan 26, 2014)

rmanbike said:


> The opposite of this, I think its called Gasification! I have seen a number of outdoor boilers that create quite a bit of smoke. Not something I would want around my house. The key is in your design to reduce 90% of this smoke!
> 
> Their has been several studies on heat transfer in sand, much has to do with the fact that over 70% of metal casting is done with sand. I think the article "What makes sand good for storage" speaks for itself.
> 
> A better mouse trap is always around the corner, we just not made it to the corner yet, so how can we see it now!


 
Just read the last sentence of that sand item you posted above:

*"However, heat can travel in a concentrated path rather quickly through a sand bed toward cold." *

What does that mean to you?

And the venturi effect will indeed pull smoke (and other bad things) out of your pipe - but as soon as it gets out of the pipe, it will go horizontal right along with the across-pipe venturi-inducing air flow.

Yes, please take lots of pictures if you proceed & fully document progress & results.

Out...


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