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More chatter about wood burning sauna stoves, gasification, chimney smoke, hot room efficiency

sauna stove in hot room

In follow up to a post about installing a wood burning sauna stove with a feed from inside the hot room or outside, here’s some more ranting:

I am building a wood sauna stove to place in a barrel sauna. Half will be inside and half out. The insulated chimney will be placed outside top between the end where the feeder door is and the sauna wall. What do you recommend in inches should the chimney be placed away from the sauna wall?

Thank you,
Tom

Tom:  This is a bit of uncharted waters for me.  I always build sauna with stove feed from the hot room.  The method of feeding the sauna stove fire box from the outside or from the changing room is a traditional way to go.  Traditional in that all old school sauna stoves were built inefficiently.  They burned a shit load of wood and most of the heat went up the chimney in the form of smoke.  As up to 70% of the BTUs in a stick of firewood is contained within the burning gases (ie smoke), old school wood burning sauna stoves required constant feeding.
sauna stove in hot room
A DIY sauna build on Prince Edward Island, Canada. Outside sauna stove feed.

Technology to the rescue

GOOD NEWS:  Today, thanks to technology, and the tenacious tinkering of third generation Finns, most wood burning sauna stoves are built much more efficiently.  How do we know?  Just go outside and look up at the chimney of a quality built wood burning sauna stove in action.  After you light a wood burning sauna stove, with dry kindling or a Nate’s Firestarter, you will see smoke coming out of the chimney.  But after 5-10 minutes, you won’t see any smoke.  “Did the fire go out?”  No!  What you are NOT seeing is smoke.  But if you look carefully you will see the waifing of heat.  
What’s going on here?  
Because of gasification, all the smoke is burning within the heat chamber.  This is the significant reason why a guy can take a sauna using only an armful of firewood.  This is the significant reason why anal retentive sauna envious neighbors are not going to freak out when you make the commitment to build your very own AUTHENTIC sauna in your backyard. In cities, a 1′ side yard setback requirement is building code for external structures.  This goes for pretty much any urban building planning rule book, but call innocently – without tipping your sauna building hat – for the ruling in your area.
DSC_3518
A sauna build with an inside feed wood burning sauna stove

Stove pipes require non combustable material around them

So, as far as non combustable material adjacent to chimney?  If you are running your chimney to the outside and feeding your stove fro the outside, you will need that entire wall to be block, ie. non combustable material.  Also, keep in mind, whatever percent of your stove and stove pipe surface area is NOT going to be in your hot room will reduce the efficiency of your heating by that exact percent.   In different words, stoves that feed from the hot room capture all the heat potential of the stove.  

What materials are conductors and what are insulators?

AND here’s something else:
  • Cement is a heat conductor.
  • Wood is a heat insulator.
Conductors take away heat.  This is why your bare feet freeze when standing on a cement patio between sauna rounds.  Insulators hold heat.  This is why your bare feet want to be on a wood deck between sauna rounds.  More data here.
Your cement block wall is a conductor of heat, not an insulator.  This means your block wall is going to suck the heat out of the hot room like crazy until the cement block is hot.  Then, your cement block is going to want to cool your hot room because the other side of the block is cold as hell.  Hate to be such a downer, but all these are important considerations.
 
That’s a lot to chew on, Tom, but above is what I know and hope it helps, thanks,g.

 

7 thoughts on “More chatter about wood burning sauna stoves, gasification, chimney smoke, hot room efficiency”

  1. So envious of the beautiful sauna stoves people use in the upper Midwest. For those on an extreme budget (and hopefully far from fire code or insurance inspectors), here is a youtube video on how I build my sauna stoves. I guess you would call these rustic rather than authentic – but they work great (six pieces firewood for two-hour sauna) and are pretty much free if you have old door hinges, and can find someone with a foam insulation business who will give you an empty heavy-duty 55 gallon drum: https://www.youtube.com/watch?v=FqsPocboV6o (Sorry about the wind noise – it dies down later.)

  2. Have any of you used a rocket heater to make a more efficient and cleaner use of wood in your sauna? Rocket heaters reportedly require a quarter to a tenth of the wood that high efficiency wood stoves use to heat the same space. Rocket stoves have an L or J shaped feed & burn chamber made from insulated fire brick, combined with an insulated heat riser that develops enough heat to burn all gasses before releasing the heat inside of a 55 gallon drum. They burn smaller dimension wood, and radiate lots of heat into the room raising temperatures quickly.

    The back of the barrel (facing away from the benches) could be surrounded by rocks to capture heat needed to generate steam. The exhaust drawn from the base of the barrel can carry heat through a 20′ run of 6″ exhaust pipe transferring heat into a gravel or clay subfloor wherever you want it, including under the adjacent changing room and sauna room’s benches. Surrounding these exhaust tubes with clay or gravel works like a heat battery that radiates lower levels of heat into the space following the burn. Flooring with decking over those spaces would allow that heat to return to the room for several hours if the clay or gravel has been insulated from the ground. By using a rocket stove in combination with this heat battery results in a very cool exhaust by the time it reaches the vertical portion of the stove pipe. If the barrel is not located too close to flammable materials, the risk of burning down your sauna is minimized. These stoves can be built very inexpensively with easily acquired skills, or you can spend thousands on prefab units. Many DIY videos show how to mix mortar and set bricks in a simple pattern for the base of the J, methods for creating a heat riser, how to insulate beneath and set the vent pipes, dig and trowel clay cob or gravel to simply use gravel, and where to connect your vent pipes, ash clean outs, and the vertical stove pipe that creates some of the draw. If you do everything required by code that you would typically do around a wood stove, you should be able to get inspector approval.

  3. Have you ever used an old fashioned cast iron wood stove for a sauna. I have one All wood building still struggles to get past 125 degrees. Room is 7 by 7 by 6 .5. Fully insulated.

  4. Paul:

    I’m a big fan of using a sauna stove vs. other ideas. New sauna stoves work, create loyly (steam), burn wood efficiently, and create the awesome experience outlined here on saunatimes and throughout the land.

  5. In response to David Farin, I’ve thought this very thing! I’ve been doing my research and am opting for building a RMH in a wood framed ( with slip chip infill) sauna. Got a model built and holes dug. Now to nail down physics of Burn chamber, horizontal run and exit height. I’ll be documenting this in hopes of supporting others to DIY a sauna with grace, efficiency and ease.

  6. My 5×7 sauna now exists as a shell. The stove is a fab steel 12 x 14″ with a 14 x 16 stainless overlay for rocks / steam. Stove pipe is 4″ exit but I have 8″ dia class A section for exterior. Question…will the stove draw properly with the transition to large chimney?
    Note: I am building using materials on hand and donated. 8″ pipe was donated.

  7. John: Your question is taking me to the dark side. Though I applaud the resourcefulness, I can’t speak for a home fabricated stove’s capabilities.
    You could go forward as best you can, and if the stove ends up being lame, you could start saving some coin for a better sauna stove. Just a thought.
    Take pics, hope it works out!

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