I Sauna rocks: how do they work?

  • Thread starter rugerts
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I've looked at many posts related to saunas, and all have been about what happens when you pour water on the sauna rocks and so forth so I believe that question has already been answered.

I've struggled to find, however, what physical mechanism governs the heating of saunas? Then, more specifically, I'm curious as to what purpose do the rocks serve and by what properties/laws/principles do they achieve this?
*FYI: I sort of fell down this rabbit hole after I witnessed an argument at a local gym about whether or not pouring water on the rocks of a sauna causes damage or not to the (electric) sauna. Feel free to comment on this situation as well, but it is not the crux of my question. The conclusion I came to for this question is that it shouldn't matter as most water will almost immediately evaporate. If poured in excessive amounts, the water may damage the system depending on whether or not the sauna is properly waterproofed. Correct me if I'm wrong.*
Thanks for your time all.
 

Klystron

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Your answer sounds correct.
My health club manager explains that sauna is implemented as 'dry heat'. A room lined with cedar and redwood (in California) planks is heated by an electric heater. Porous lava rocks (from Hawaii) are placed above the heating element for decoration and to protect patrons from accidentally contacting the heat elements. Pouring water on the hot rocks leads to 3 problems:
  1. releases steam defeating the 'dry heat' concept.
  2. the rocks may crack or even explode presenting danger to patrons.
  3. excess water may short circuit the electric heater or even electrocute a patron.
Many clubs and spas have steam rooms a/o jacuzzi for 'wet heat'.
 

russ_watters

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You can simply google this question and get answers from sauna manufacturers: they are indeed made to have water poured on to them. Don''t pour a ton of water on, though, as it won't instantly vaporize. Just sprinkle it on.
 
104
5
Your answer sounds correct.
My health club manager explains that sauna is implemented as 'dry heat'. A room lined with cedar and redwood (in California) planks is heated by an electric heater. Porous lava rocks (from Hawaii) are placed above the heating element for decoration and to protect patrons from accidentally contacting the heat elements. Pouring water on the hot rocks leads to 3 problems:
  1. releases steam defeating the 'dry heat' concept.
  2. the rocks may crack or even explode presenting danger to patrons.
  3. excess water may short circuit the electric heater or even electrocute a patron.
Many clubs and spas have steam rooms a/o jacuzzi for 'wet heat'.
That’s interesting that you say the rocks are decorative. That is not the first I’ve heard that claim. The only issue I have with that is I’ve read that they do appear to serve some purpose (if I’m not mistaken, they even serve a purpose in this electric scenario) From what I’ve seen, the rocks function as spreading the heat out somehow.

If this is true (that the rocks aid in heating the room), my main question lies in that. How does this happen that rocks help in some way increase the room’s overall temp. and what from physics explains this?
 
104
5
You can simply google this question and get answers from sauna manufacturers: they are indeed made to have water poured on to them. Don''t pour a ton of water on, though, as it won't instantly vaporize. Just sprinkle it on.
Thanks for your reply. Do you have any more insights as to the other questions I’ve posed? Namely, how do rocks (if they even do) help increase the temperature of the room?
 
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Lava rocks don't increase room temperature. Many types of electrical heaters are considerably directional (consider a portable radiative space heater). One reason lava rocks are used is to serve as a diffuser to spread heat output evenly over a large area.
 

BvU

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I'd like to read a comment from a genuine Finlandian. My guess is the stones have a historical origin: as a buffer of heat so you don't extinguish the fire by pouring water on.

The idea that sauna heat should be dry is preposterous. There is a cycle that begins dry but at some point the subjective heat experienced is turned up by humidifying the air: ladling on water.
 
Another Finn chiming in here...

The rocks in a Finnish-style sauna are indeed there for the purpose of having water poured on them. Not a lot of water at once -- a cup or two is plenty -- but some. This water is supposed to evaporate rapidly on contact with the rocks, and thereby transfer heat into the air in the room (and further onto the occupants, as it condenses on their skin). While a proper Finnish sauna is indeed supposed to be relatively dry (and, in particular, never so humid that there's permanent steam in the air), it's not supposed to be completely bone dry.

The main function of the rocks is heat storage: they are heated gradually by the heating elements (or by the fire, in a wood-fired sauna) below them, and release this heat rapidly when they're doused with water. Without the rocks there to act as heat buffers, even a small amount of water thrown directly on the heating elements would rapidly cool them below the boiling point of water. Not only would this subject the heating elements to excessive thermal stress, but it also defeats the purpose of throwing water on the stove to begin with: any heat absorbed by water that doesn't boil will just end up flowing uselessly down the drain. The rough surface of the rocks also helps here, as it increases the contact area between the rocks and the water, and thus the rate of heat transfer.

While I can't say with 100% certainty that there aren't any "saunas" anywhere in the world where the rocks are indeed purely decorative, and not safe to pour water on, I've never seen one. I kind of suspect that trying to sell such a sauna stove here would probably violate product safety laws anyway, since it could not be safely used for its apparent and customary purpose.
 
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Non Finn here, but relating a story I heard while in a sauna in Helsinki.

Finland was always careful to do business both with the west and with the USSR. Business negotiations almost always include lavish dinners, alcohol and of course sauna. Near the end of the sauna time a Finn would pour water on the rocks. The humidity soared. The discomfort with 90C heat soaredl People rushed out. The last man leaving could claim to the be the toughest, and would dominate the next day's negotiation. The strategy worked fine with Americans, but less well with Russians. The Russian negotiators were afraid of being sent to the Gulag if they failed so they tolerated the torture until they were red like lobsters.

So I would say that the real purpose of the water on the rocks is (in Spanish) "¿Quién es más macho?"

Probably not true, but still a fun story to tell in the sauna just before you pour the water on the rocks. :biggrin:
 

Klystron

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I'd like to read a comment from a genuine Finlandian. My guess is the stones have a historical origin: as a buffer of heat so you don't extinguish the fire by pouring water on.

The idea that sauna heat should be dry is preposterous. There is a cycle that begins dry but at some point the subjective heat experienced is turned up by humidifying the air: ladling on water.
Fire? Genuine Finlandian? Must authentic spa instructions be written in Suomi? I was merely parroting English instructions posted where I swim indoors in the winter.
My apologies to international readers for the narrow scope of my reply, limited to two companies managing 'health clubs' in California and Nevada. The quotes around 'dry heat' were meant to emphasize the colloquial, if not contradictory, nature of the term as used in contemporary settings.
 
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My apologies to international readers for the narrow scope of my reply, limited to two companies managing 'health clubs' in California and Nevada.
Those companies have a motive to keep it dry.

When water is put on the stones making the sauna humid, it will not be dry again and ready for re-use until the following day. That is destructive of the business model of spas.
 

Janus

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Non Finn here, but relating a story I heard while in a sauna in Helsinki.

Finland was always careful to do business both with the west and with the USSR. Business negotiations almost always include lavish dinners, alcohol and of course sauna. Near the end of the sauna time a Finn would pour water on the rocks. The humidity soared. The discomfort with 90C heat soaredl People rushed out. The last man leaving could claim to the be the toughest, and would dominate the next day's negotiation. The strategy worked fine with Americans, but less well with Russians. The Russian negotiators were afraid of being sent to the Gulag if they failed so they tolerated the torture until they were red like lobsters.

So I would say that the real purpose of the water on the rocks is (in Spanish) "¿Quién es más macho?"

Probably not true, but still a fun story to tell in the sauna just before you pour the water on the rocks. :biggrin:
Finnish-American here, who grew up in Northern Minnesota. We had a sauna built into a corner of our detached garage. Wood-fired stove type. One of my favorite things to do was to sit on the top bench, toss a fairly good sized amount of water on the stones and soak in the wall of moist heat that would hit me.

I'd have to echo what has been said above. The stones are there to store heat, and thus transfer that heat to the water evaporating it, without a severe drop in temp. There is a lot of latent heat involved in evaporating water, some 2,360 joules/gram. The specific heat of granite is ~0.79 joules/gram per degree Kelvin in comparison.

You need a fair mass of stones to provide the required energy without too sharp a temperature change.

The other thing the stones provide is lots of surface area to facilitate the transfer of the heat to the water. A thin layer of water spread over the large surface area as provided in a pile of stones, will evaporate much faster than a thicker layer poured over a flat surface with the same footprint.
 

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