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

light steam graphic

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.
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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.

 

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