Outdoor wood burner furnace

Hello Permies,
Long time lurker, rare poster.

I saw an interesting article today that Finland built the world's largest sand battery to store energy.  It reminded me of something my Dad built back in the 80's.  I did a little searching and could not find it by the name I remembered it called, but it was Nordic/Scandinavian and it was pronounced "hah-suh".  Doing a few web searches, I tried many spellings (hassa, hasse, huhsa, etc.) but did find an old Mother Earth News article that didn't use the name, but described it completely.  https://www.motherearthnews.com/sustainable-living/renewable-energy/outdoor-wood-burning-furnaces-zmaz77zbon/.  I rarely post, but didn't find anything like this in the forum and thought it was worth sharing as people are looking for better ways to manage their energy.

The basic principle was this:

1. obtain a metal oil tank and cut it in half for the burn chamber
2. have a house nearby with hot water/baseboard heat (ours was about 20' away)
3. build a small 10x10x8' building out of concrete blocks around the burn chamber
4. add a door and smokestack to the burn chamber, with air inlet to control combustion
5. connect the water from the house baseboard to a copper coil (125'-200' iirc) and locate that above the burn chamber
6. fill in everything with sand
7. slap a roof on it

We heated our house from spare wood for years with that.  Aside from the burn chamber melting, which was then replaced with 1/2" plate steel with railroad rail reinforcements, it ran pretty well for almost 15 years before we moved.  Sadly, the new owners removed it.  

If I were going to do something like this today, I would maybe combine it with a rocket mass heater in such a way to use less wood and be more efficient with the heat transfer and smoke output.  I have considered doing something similar for heating a greenhouse, but smoke always gives me and my wife headaches so it isn't the right fit for us.


Roger

Thank you for sharing about this.

That article did not show a picture of the furnace so I was wondering if the furnace looks like any of these:

https://permies.com/t/192002/Batch-Box-Outdoor-Wood-Heater

https://permies.com/t/150655/firing-DIY-outdoor-wood-furnace

Years ago, here on the forum, I read about building a rocket stove outside and the piping the heat into the house.  

Roger -

Not to dissuade you from your plans, but the Richard Hill stick wood furnace might be a good point of comparison for something like this.  He used a forced draft downdrafter burn chamber (this was from the bad old days of the oil crunch in the late 70s, so long before rocket stove J tubes and the like).  He had a refractory tunnel downstream, with injection of pre-heated secondary combustion air, which was then passed through an atmospheric fire tube boiler, then a draft inducer fan and up the stack.  He heated up about 500 gallons of water (two old fuel oil tanks) per firing, then circulated the heated water for dozens of hours afterward.  In coastal Maine, he fired the boiler every second or third day to heat a rental house.

This is the best quality scan of Hill's report available, to my knowledge:
https://web.archive.org/web/20260122141201/http://www.hotandcold.tv/resources/woodfiredfurnace.pdf

Virginia Tech did some efficiency and emissions testing of this furnace under various conditions, and it was in the neighborhood of 80% thermally efficient, depending on the moisture content of the wood.  If I can find a link to their report, I'll add it on edit.

Hill found that he could cleanly burn even green wood (except green spruce, which was too wet to sustain combustion without a mix of other feed stock).  But, he gave up quite a bit in thermal efficiency, since the heater was boiling so much water contained in the fuel, which then escaped up the stack along with the latent heat of vaporization.

A couple of versions of Hill's wood furnace were commercialized in the US and Canada, and the HS Tarn Garn design was similar.

Not the same as yours, also not a rocket stove (it was too early for that), but an interesting approach to clean and efficient wood combustion, nonetheless.

I have seen documentation of the sand battery style of stove from your youth, perhaps in TMEN, (I even vaguely recall a name similar to the one you mentioned), but I wasn't having any luck finding it by that name, either.  But I'll look a bit more.

Kevin

Roger -

The design you and I were recollecting is the HAHSA (Heat And Heat Storage Apparatus), invented/design by Kenneth Darby of Falls, PA.

Here is a link to a contemporaneous CSM article:
https://www.csmonitor.com/1981/0304/030458.html

Somewhat different from the TMEN article to which you linked - your article's description was closer to the Richard Hill design, I think.  Dick Hill also designed a thermal mass heater (maybe for a local Audubon Society chapter? - fuzzy memory...) which is profiled in "The Book of Masonry Stoves" by David Lyle.

The HAHSA company still exists as Darby Industries (look under the heading "The little business that could...", subhead "A change in direction"), even though their focus has changed (apparently many times):
https://darbyindustries.com/about-us/newman-darby/

I don't know if the plans for the HAHSA are still available from them.

But, a rockety burner with a mass of sand or pebbles (see Paul's pebble bed thermal mass design) seems doable.  The batch box burner designs might be a better fit than the J-tube style for this type of heater.

Separating the burner unit from your residence may decrease the fire risk for conventional combustion (i.e. not very efficient), but for any clean combustor (whether Hill's downdrafter with secondary air, or a rockety burner), I doubt the decrease in risk is worth the loss in efficiency.  Though your homeowner's insurance company may disagree.

I suspect that a true masonry heater should be a lot easier to insure than a Hill replica, IF your insurance agent is fussy.  Mine, representing a major company with fancy television advertisements, recently quoted me $60 a year to add a wood burner to my home owner's policy, but she insisted on a separate/dedicated flue for the wood stove.  I plan on adding a manufactured insulated double wall flue anyway (Selkirk brand is locally available), so that's fine by me.  She apparently doesn't care what kind of stove I use; I seem to be much more concerned about clean and efficient combustion than is she.  But, many people here heat primarily with wood, and many more have wood heat as a backup or just for ambience, so my request for a quote to add the wood burner rider was no doubt run-of-the-mill for her.

On edit: I attached a HAHSA advertisement I found online, vintage 1981.