Mini Wood Stoves

[Guest]

Off the keyboard of Geoffrey Chia

Follow us on Twitter @doomstead666

Friend us on Facebook

Published on the Doomstead Diner on January 29, 2016

Discuss this article at the Doomsteading Table inside the Diner

It's not Rocket Surgery: choosing an indoor mini wood stove

Geoffrey Chia, Jan/Feb 2016

 

As of 25 January 2016 at the time of commencement of writing this article, the cold spell affecting East Asia killed more than 65 people in the subtropical island of Taiwan. The temperature in Beijing dropped as low as minus 40 degrees C, however Northern China is used to dealing with heavy snow dumps and subzero temperatures in winter and hence coped better compared with Taiwan. The last time snow was seen in Taipei, the capital of Taiwan, was more than a century ago. The fact that Taiwan is a relatively small subtropical island, which we would expect to be dominated by the moderating effect of the South China Sea, makes this rare event seemingly even more bizarre. That is, until you understand the physics of how coastal and island provinces on the leeward (downwind) side of continents can often be dominated by continental rather than maritime weather, which I mentioned in my previous article "Location, location, location", where I compared Vancouver Island with Nova Scotia.

It is no coincidence that a record breaking monumental snowstorm also affected the East coast of the USA, the leeward side of the North American continent, at the same time as the cold weather affected East Asia. In general, at those latitudes; the windward (Western) sides of the continents, which are more dominated by maritime influences, are less prone to extremes of heat and cold than the Eastern sides.

Climate scientists tell us the peculiar combination nowadays during Northern winters of high temperatures in the Arctic regions and low temperatures extending to low latitudes, is due to the disruption of the circumpolar jet stream as a consequence of climate change.

One issue I did not mention in my "Location" article was the beneficial warming effect of the Gulf Stream on North Western Europe. The demise of the Gulf Stream appears to be a foregone conclusion as climate change progresses, following which North Western Europe can expect even worse cold spells in winter, even as the rest of the world heats up.

My "Location" article focused on avoiding heatwaves and managing fresh water supply. It did not discuss coping with cold weather (which well prepared humans can survive better than heatwaves). In choosing our future location, it is far better to settle where there will be low risk of heatwaves (despite the occasional cold snaps), rather than to settle where there will be low risk of cold snaps (but have high risk of terrible summer heatwaves). Most locations in our future world will face the latter situation in due course.

This article will focus on one item which can keep you comfortable in cold weather, especially when faced with the inevitable fossil fuel shortages in the near future: the indoor wood heater/stove, with special attention to the secondary combustion biomass (SCBM) heater/stove. It is not a comprehensive article but will concentrate on models which can be used in a caravan or tiny house, my particular area of interest.

The other important role of this biomass heater/stove will of course be for cooking. It will become an essential appliance when we face future shortages of kerosene and LPG.

Before we proceed, it is vital to mention the first three principles in dealing with cold weather: insulation, insulation, insulation (just as the first three principles in managing electricity use are efficiency, efficiency, efficiency). The well insulated tiny house, warmed by two people and a dog, may not require any additional heating because of the small interior volume of air.

Furthermore, in my particular tiny house design http://www.resilience.org/resource-detail/2544932-building-a-tiny-house, I incorporated an indoor steel water tank (under the lounge seats) which will provide thermal mass superior to (and lighter than) concrete.

Good insulation, small interior volume and thermal mass (heated by daytime passive solar influx) alone may enable you to go without active heating for most of the year, depending on where you live.

On the other hand, for adequate ventilation and to avoid condensation, it is necessary to allow some fresh (cold) air in and stale (warm) air out of the tiny house. In the case of the hermetically sealed modern Scandinavian dwelling, this is achieved (with minimal temperature drop) by a heat recovery ventilation system. For a less airtight dwelling in a less cold climate, sufficient ventilation may occur through "natural" leaks in your house (eg air coming in via the gap under the front door and out via a poorly sealed upper window), which will of course cause a drop in internal temperature. If you are installing a wood stove in your tiny house for future cooking anyway, this stove can double up as your heater and there will be no need for an electrically powered heat recovery ventilator. In this article I use the terms biomass and wood interchangeably, because 99.9% of the time, most of us use wood for our (non fossil fuel) stoves.

My criteria for the ideal biomass stove / heater for a tiny house are as follows:

1. 
   

   
   Must have exhaust flue / chimney – absolutely essential requirement
   

   
   


2. 
   

   
   Minimum use of fuel
   

   
   


3. 
   

   
   Minimum emissions
   

   
   


4. 
   

   
   Small and light
   

   
   


5. 
   

   
   Little need for constant tending
   

   
   


6. 
   

   
   Ability to monitor fuel and flame
   

   
   


7. 
   

   
   Able to source air intake from exterior
   

   
   


8. 
   

   
   Affordability
   

   
   


9. 
   

   
   Other issues eg aesthetics
   

   
   

Criteria 2 and 3 can be summed up in one word: efficiency. The least efficient, most hazardous, most polluting and most wasteful heat source is an open fire. Next worst is the open brick fireplace. The standard cast-iron combustion box with flue is a good deal better but still woefully inefficient. Furthermore even a "small" cast-iron stove can easily weigh 150kg, which is quite unsuitable for a tiny house on wheels.

Even though the technology has been around for decades, we have failed to widely embrace the secondary combustion biomass (SCBM) stove, of which the "rocket stove" is the commonest design. Standard fires only burn the primary solid material of the biomass, releasing secondary combustible materials such as soot, hydrocarbon gases and carbon monoxide into the atmosphere, which can cause acute irritation of the airways and eyes, even poisoning or asphyxiation. If inhaled chronically over decades it can lead to emphysema and even lung cancer. These remain terrible problems in Third World countries. A stove designed to burn both primary biomass as well as the secondary emissions is far more efficient, causes far less pollution and is thus far healthier than traditional wood stoves. It needs far less fuel to do the same work (eg just a quarter of the wood normally used for a conventional stove), hence requires less back breaking physical effort chopping and carrying wood, hence also protects against deforestation. Additionally such a stove can reach much hotter temperatures. There have been great initiatives to introduce SCBM stoves (such as the InStove) for use in the developing world, if only for health reasons.

If you google "rocket stove", the vast majority you find will be solely for outdoor use and will lack exhaust flues. Even those with exhaust flues may not be certified for indoor use (the commercial forces of our fossil fuel economy have long suppressed this market). Hence most "permies" these days are pioneering this option without official sanction.

Development of the "rocket mass heater" actually preceded invention of the rocket stove. This mass heater is a different beast, in that the hot metal components are enveloped in big heavy slabs of cob (or similar earthen or concreted thermal mass) to retain the heat for slow and steady release even after the fire goes out. The cob can be shaped in the form a comfortable warm bench. If used indoors, this mass heater is only suitable for a large, fixed, ground based dwelling, not a tiny home on wheels. Furthermore many rocket mass heaters may not incorporate a stove in their design.

With regard to SCBM stoves with exhaust flues/chimneys (which may or may not have been officially approved for indoor use) my websearches revealed the following types:

WEBSITE	http://www.instove.org/60-100-liter-cookstove	http://www.rocketheater.com/	http://www.bobcatrocketstove.com/ Unfortunately no longer in production
Size and weight	Probably too big for most boats/caravans	Probably too big for most boats/caravans	Small (perhaps too small) and light
Need for constant tending	+++ horizontal fuel port needs constant feeding	++ vertical fuel port, wood self feeds by gravity as it burns down	+++ small horizontal firebox needs regular feeding
Ease to monitor flame and fuel	Easy	Easy	Need to open door periodically
Ability to source external air intake	Maybe, but need to custom configure	No	?probably yes
Cost in US dollars	$850 for 60litre, $995 for 100litre	?$1500 (contact dealer)	Unavailable (?previously $400)
Remarks	Mainly designed as a stove to cook for large numbers of people, cooking slots only fit custom sized pots	Mainly designed as a heater. Sides of secondary combustion chamber can get very hot & pose risk of burns	Add-on small water tank can provide thermal mass and hot water supply
Aesthetics	Utilitarian	Industrial	Odd looking, may appeal to some

————–

WEBSITE	Silverfire Hunter Stove	Kimberley Stove/Heater
Size and weight	http://www.silverfire.us/hunter-chimney-gasifier-stove	https://www.unforgettablefirellc.com/
Need for constant tending	Small and light	Ideal size for boat/caravan
Ease to monitor flame and fuel	++ "batch feed" wood vertically	can burn slowly overnight without tending
Ability to source external air intake	Need to remove pot to view status of wood and flame	The only SCBM stove with viewing window
Cost in US dollars	No	Yes
Remarks	$220.00	$3750 – 3995
Aesthetics	When starting, large flames with smoke leap out of central combustion chamber. Pots and pans get coated with soot from primary combustion	By far the most expensive but also the best made
	Looks like a biscuit tin	Classy

The above are mere impressions obtained from web searches, not based on any practical experiences of mine. Practical reviews from the manufacturers and customers can be found from amazon.com or youtube. This article is intended to spark (pun intended) interest in this topic so that the reader can do their own research and make their own decisions. One issue I have not yet looked into is the maintenance required for each stove type. Every setup will require periodic cleaning of the chimney.

Based on size and availability, the only two SCBM contenders for use in a tiny house on wheels are the Silverfire Hunter and the Kimberley stoves. Although the Kimberley is far superior in every way it is also hugely more expensive, however you generally get what you pay for. It is false economy to buy a cheap stove if your house ends up burning down.

The Silverfire Hunter is described as a "toplift updraft" or TLUD gasifier. It generates bare flames and smoke out of the central cavity at startup (secondary combustion occurs later in a ring around this). This issue may be a dealbreaker for the indoor user. During cooking, the base of the pot/pan is in direct contact with the primary flame, causing soot deposition. An optional cast iron disc cover is available to minimise this, although it will also reduce heat transfer. One reviewer wrote that it needs frequent ash removal which requires it be disconnected from the flue, taken outdoors and turned upside down.

For those still keen on mini wood stoves of standard design, the following review websites are helpful:

http://www.tinywoodstove.com/small-stove-reviews/

http://www.waldeneffect.org/blog/Smallest_wood_stoves/

As mentioned previously a small standard cast iron stove can easily weigh 150kg, however the tiniest models may weigh less than 25kg.

For tiny house purposes, two particular models seem especially suitable (mainly because I greatly value the ability to source external air intake):

Jotul 602: apparently more than a million of these have been made, hence one would expect all the bugs have been ironed out by now. H25.25” x W12.6” x D21.25” , costs around $900. Weighs 73kg, able to configure for external air intake. Apparently 75% efficient (?capable of some secondary combustion)

The Salamander Hobbit stove: http://salamanderstoves.com/the-hobbit-stove/

Size: 302mm wide, 272mm deep and 465mm high, the low emission version costs £525 pounds sterling.

Weighs 60kg, able to configure for external air intake. It is designed to enable some secondary combustion, although not as efficient as the purpose designed SCBM stoves.

 

CONCLUSION:

As in all things, your choice will depend on how you weigh up the various advantages versus disadvantages, as well as your individual circumstances. Many amateurs have cobbled together home-made rocket stoves for outdoor use, however few sane people will hazard home-made stoves for indoor use.

 

G. Chia Jan/Feb 2016

 

 

 

 

 

 

 

 

 

 

• Tweet

[Eddie]

That Liberator sounds very good. A little pricey at $1900 and change, delivered...but probably worth it in the long run.

• Tweet

[Eddie]

I have a kitchen wood stove, an older  model European that isn't made anymore, that I found in brand new condition on CL a few years ago. Now that I have the tractor to move it, maybe I'll finally get it installed. it weighs a LOT with its fire bricks. I need to remove the big kitchen window to install it, but I have to do that anyway, because it shattered from the summer heat, or somebody cracked the outer pane with a rock, not sure.

Looks about like this one:



Better for cooking, but not as efficient as a rocket stove.

I wonder if people encase those Liberators in rocks or cob for thermal mass. They appear to have been designed with that in mind.

• Tweet

[Eddie]

The missus wants a tiny house. I suggested a Monolithic dome instead. It might be the year to get that started.

• Tweet