Combustion air upgrade, wood-fired sauna

I want to share an innovation I came up with that provides fresh combustion air to the wood stove of my sauna. I use clothes dryer ducting to do this. It's relatively simple but it makes for huge improvement in the overall efficiency of the heater. I don't remember how I came up with this. Not that there's much to it but I'd like to think it was an original idea. Maybe it was but usually these sort of things come from elsewhere and I just don't remember.

I was forced to recently refurbish the air intake. Doing so inspired me to write about it so I could share it with fellow sauna enthusiasts. 

What we see here are the remnants of the old clothes dryer aluminum ducting that I used as the fresh air ducting. There's maybe something related to the heat and/or the caustic chemistry of wood ash that caused the aluminum to lose it's malleability. The aluminum skin became wispy and started to crack and fall apart. It lasted about 15 years but now needed to be replaced.

First, some basic wood-fired sauna theory: There are two ways you can situate a wood burning stove for a sauna. One has the feed door on the outside of the sauna space. This might be completely outside or in the antechamber/changing room. The wood stove goes through a wall. The advantage of this is that it keeps smoke outside the sweat space and none of the heat generated by the wood stove is wasted by the fire pulling it into the stove.

There are disadvantages to this though. For one it's a little more of a hassle to manage the burn of the fire since you have to go outside to feed or check on the fire. Second, the wall needs to be built with some consideration so the heat of the stove doesn't, ahem, burn everything down. This is usually pretty straightforward and done with sheet metal or masonry. Third--and which to me is a deal breaker--is that you do not get the primordial, soul nurturing of seeing and listening to the fire from inside the hot dark sauna space.

The other more common method is to have the stove entirely inside the sauna. One of the big draw backs of doing this is that the fire and the "chimney effect" of the fire and stovepipe draw a lot of air out of the sauna space. Also if the fire is dampered, or somehow the chimney becomes blocked, smoke can get in the sauna space. Even worse, in some rare circumstances poisonous carbon monoxide can fill the sauna space. 

Wood stoves consume a lot of air. I remember on certain particularly cold nights growing up in Alaska watching a thick cloud of "fog" roll in from the door and snake it's way along the floor into the firebox of the wood stove. It was eerie and mesmerizing. Sometimes it would be a layer 18 inches or so thick. It would completely obscure the floor. This experience gave me a strong visual impression of the enormous volume of air that gets pulled though the sauna into the stove to feed a fire. That's a lot of cold air getting pulled in from the outside into what should be a hot space!

When I built my urban wood-fired sauna I had this experience in mind. I improvised a method for getting fresh outside air for the wood stove directly to the fire. This would help improve the efficiency of the wood burner: more heat in the intended space and less of it wasted going up the stove pipe. It also provided some assurance that we had fresh air to breathe.

What I came up with is simple and it works surprisingly well. Essentially it is a simple flexible aluminum clothes dryer duct routed so that it terminates right in front of the wood stove door. 

Let the photos demonstrate:
Here's the vent on the outside. (Forgive the un-aesthetic T1-11 and plastic vent cover. No one ever sees this wall!) There were louvers on the vent cover which I broke off. The louvers were there to help prevent cold air from being drawn toward the interior. Nope. We want that! The grill stays on to keep the critters out.

This photo is taken under a bench near the wood stove. The wood stove is to the right just on the opposite side to the cementitious tile backer board. In the recent refurbishment, I replaced both the vent cover and the aluminum wall ducting. Here (above) is the new vent coming in through the exterior wall on the left. The new ducting is pushed through the backer board on the right.

Why is it always so damn hard to slip the ducting on over the vent pipes!

A cementitious cider block is placed near the ducting to protect it from being crushed by fire wood which will soon be piled back under the bench.

This photo shows the ducting going through the backer board (on the left), through another cinder block and terminating right near the door to the wood stove.

It may not be pretty but it works great! My sauna uses an old 1970s-era home heating wood stove. It's welded plated steel so it could potentially handle drilling and welding an air intake but with my method, I didn't have to do this. The ducting was quick, inexpensive and very effective.

For a variety of reasons, I typically do not damper the stove. In fact I often leave the door cracked open just a little to help concentrate the air movement into the stove. This acts like a black smith's air bellows, blowing air onto the fire to really feed it. I want the fire to burn fast and hot. I'm not trying to keep the fire burning over night and since this is an urban sauna I want to minimize bothering my neighbors with smoke. Also a hot oxygen-rich fire reduces the impact on the overall regional air quality--something I care about.

Even though the entry door to the sauna is not particularly air tight, there's still enough negative air pressure for gobs of air to get pulled into the fire directly through the air duct. I know this because when the fire is going well I can put my hand in front of the duct and feel the rush of cool oxygen-rich air being drawn through the duct directly into the stove. The flow is similar to lightly blowing on your hand from about 8 inches away. It's really noticeable!

In summary, while my system is super rudimentary it is an important feature and it works great. In my experience, I believe some sort of provision for a combustion air intake should be made for all wood-fired saunas where the stove is on the inside. It does a lot to improve the heating efficiency and it helps keep the occupants safe.

 Happiness is a sauna with wood under the benches.

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