Hot Me Up
October 18, 2010 2:17 PM   Subscribe

Can I heat a sauna with hot stones?

I'm thinking about building a sauna in a half-buried little storage space attached to the back of my house. Mrs. Crazylegs doesn't want me to put a woodstove in there because it will up our insurance rates. I know people heat sweat lodges with stones, would the same work with a sauna? Meaning, I could build a fire in a pit in the back yard, heat the stones in the pit, then carry them into the sauna with a shovel and put them in a container of some kind.

Two questions:

1 - Will this work?

2 - If so, how many stones of what size would I need, and how hot would I have to make them, to heat a space about 5x5x8 feet to sauna temperatures?

posted by crazylegs to Health & Fitness (12 answers total) 1 user marked this as a favorite
I can't imagine heating stones on the outside and carrying them into a sauna. We have an outdoor sauna that we built from a kit, heated by electricity, with a timer and thermostat on the outside. Inside is a a metal grate with a catch pan underneath. About 10-12 stones are placed on top of the grate. A bucket of water and ladle is kept next to the catch pan. When the room gets hot, we ladle water on top of the hot rocks. Voila! Steam!
posted by zagyzebra at 2:35 PM on October 18, 2010

A Native American sweat ceremony I went to was heated by rocks, but it was a full-time occupation for a helper tending a firepit outside and (very carefully) bringing the almost-red-hot rocks in. It was a major effort. There were maybe twenty (maybe lots more) rocks a little smaller than head size, with maybe a dozen people in the sweat-lodge. I can't remember the lodge dimensions; it was pretty low (think crawl in, not walk) but big enough for us all to sit round the edge and leave space for the door.
posted by anadem at 2:47 PM on October 18, 2010

Most stones will crack when you heat & cool them. You could try fire bricks placed on a rack to maximize surface area. You'd probably need a bunch, though, and they are heavy. Fire bricks are dense, so the heat is transmitted easily within them. That's why they don't crack--because there isn't as much differential expansion. Why not use an electric heater? More here.
posted by weapons-grade pandemonium at 2:47 PM on October 18, 2010 [1 favorite]

Also: your fire will probably not get hot enough to significantly heat the rocks unless you use charcoal or coal or something
posted by wayland at 2:48 PM on October 18, 2010

I'd say not hot enough.
And even if it worked, you'd hate it! Carrying all those stones...listen to zagyzebra, go electric.
posted by Namlit at 3:14 PM on October 18, 2010

Don't rule out an electric sauna stove.

I built a sauna for a friend a few years back and wood was ruled out for insurance reasons as well. He reluctantly went with an electric stove - we were both quite worried that the electric sauna stove would be inferior to wood stoves that we have experienced.

Two years in, he is very happy that he was forced to go electric. The sauna gets extremely hot - you can pour as much water on as you want, and the stove keeps pumping out steam. The nicest part is being able to flip the switch when you're done and not worry about hot coals/embers burning his sauna down.

We used a "Harvia Topclass" and I can unreservedly recommend these units (no commercial affiliation). We bought ours in Thunder Bay, but it looks like "Finn-Sauna Delight" is in Ottawa.
posted by davey_darling at 3:16 PM on October 18, 2010

Seems a long way for a shortcut if you ask me. I'm going with the other chaps who recommend electric. Less hassle, and hot rocks are... well, I wouldn't want to carry them.
posted by Biru at 3:41 PM on October 18, 2010

I don't think so - a sauna needs to be pretty darn hot. I've never seen one without a stove in it. You also need to throw water on the stones to get steam. If you aren't constantly heating the stones, I don't think they will stay hot for too long. Maybe if you made your sauna really, really small, but then you'd practically be standing on hot stones.
posted by The Light Fantastic at 3:59 PM on October 18, 2010

Heating rocks for a sweatlodge entails tending and stoking a big fire around a bunch of basalt rocks for 3-4 hours until they are red hot, then carefully carrying them into the lodge with a pitchfork. A lodge 10 ft. in diameter might use 25 rocks over the course of several hours.

In other words, what with the wood splitting and firetending and rock schlepping, it's a lot of work. I'm with the folks who say check out an electric sauna heater.
posted by ottereroticist at 4:15 PM on October 18, 2010

Pish posh on all the naysayers.

Lets say your sauna space is 5'x5'x8'. According to Wolfram Alpha, the air contained therein has a mass of 7.22 kg. Further, it tells us that the heat capacity of air is 1007 J/(kg K). So, it takes ~7300 J to raise the air in your sauna by 1 degree Centigrade (because 1K = 1C). Say the starting temp is 25C and you want to heat it to 75C, a difference of 50C. That's going to take 350,000 Joules.

Now for the other side. Granite has a thermal capacity of 280 J/(kg K). Lets say you are heating the rocks to 260C (500F). Each Kilogram of Granite heated to 260F will release (185x280) 51,800 Joules in the process of cooling to 75C (your target air temp). So, you'd need 6.75KG of stones, not very much.

(I think I have that math right)

Actually, you'd probably need a few times that, to account for heating of the walls, to allow for cooling, and to help get things up to temp quickly. You'd also need more stones if you didn't heat them to 500F. Still, quite doable.
posted by Good Brain at 4:16 PM on October 18, 2010 [1 favorite]

Still, quite doable.

Maybe in a thermodynamic sense, but there are still some major logistical issues here, as everyone else has pointed out.

And there's no advantage to using rocks over an electric heater, besides the picturesque old-timeyness of it all.
posted by magnificent frigatebird at 5:00 PM on October 18, 2010

A woodstove would probably be way too hot anyway unless it's a really big storage space. There's nothing quite like standing around naked in the snow waiting for your sauna to cool down enough to allow you back in it. Not to mention that I personally know three people who've burned down their woodstove-heated saunas. I'd go with the electric.
posted by fshgrl at 7:22 PM on October 18, 2010

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