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A hot question (about a sauna

  1. Jul 23, 2014 #1

    so I have this interesting theoretical question. Consider scenario:

    A hot sauna. The heat source is a number of stones which take heat from a chimney which takes heat from a burning wood. Periodically you pour some water on the stones to create some steam. This, of course, cools down the stones, as the water is colder than the stones. Ideally, you want the sauna heat to last as long as possible and still make some steam.

    Now the question: what would be better - pouring down the water on one single stone at a time, or spreading it across all of them? And by better I mean keeping the sauna heat as long as possible.

    I'll leave my own thoughts out for now, I really want some scientific approach to this :)

  2. jcsd
  3. Jul 23, 2014 #2

    Simon Bridge

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    Welcome to PF;
    technically your thoughts will be important here anyway because it will tell us how best to talk to you.

    The sauna will stay hot as long as energy leaves slower than it arrives.

    Concentrating on the "steam" generation, you are removing heat from the stones in stages and adding it continuously.

    The heat from the stones gets to the sauna by convection (of moist air), and radiation. The effect of pouring water on the stones is to warm the sauna - adding hot vapor (note: not steam) to the air, while cooling the stones.

    Heat leaves the sauna through the walls and up the chimney pretty much continuously.

    The design of a sauna is (usually) such that putting water on the stones does not influence the temperature of the sauna whether the water is sprinkled over all the stones or dumped on just one, though it is probably more efficient (more water goes to vapor) to spread the load. So, even though the temperature of the stones changes, the temperature of the sauna stays roughly even.

    Apart from that, there are too many unspecified variables to make a sensible comparison.
  4. Jul 23, 2014 #3


    Staff: Mentor

    Do you mean temperature as sensed by a thermometer, or the perception of temperature by the human body? There is a very big difference.

    Putting water on the stones raises the humidity which makes it "feel" much hotter to the people, even if the real temperature drops.

    Sauna is typically very dry (low humidity) heat. That's why you can tolerate it for long times. Water is thrown on at the very end when it is time to exit. Throw a lot of water on very hot stones and the exit will be rapid.
  5. Jul 23, 2014 #4
    Let me clarify the question a bit:

    Let's assume that no heat is being added to the stones anymore, as in all the wood has burned off. So the stones will not receive any additional energy anymore. The goal is to prolong the useful time of sauna as long as possible (useful as in at least of X temperature), while still pouring on some water ever Y minutes. So in that scenario - should you pour it on one place, or spred it out, or it doesnt matter at all?
  6. Jul 23, 2014 #5

    Simon Bridge

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    The rate heat leaves the sauna depends on the conduction through the walls.
    You now have a finite heat supply - that stored in the stones.
    You need to know how the heat gets from the stones to the sauna.
    If you go too fast, the sauna gets hotter, increasing the conduction rate through the walls.
    Ideally you want to transfer heat from the stones to the sauna as fast as the heat is being lost.

    You also seem to be running your sauna as a steam room.

    It does not look like it will matter much, in this description, how you pour the water - if you dump too much in one spot, it is inefficient though: not all the water goes to vapor.

    How are you thinking about it?
    What about anorlunda's questions?
  7. Jul 24, 2014 #6

    I was talking about the perceived temperature.

    Once the burning stops, only limited time is left. So we pour on water to make it feel "hotter". Naturally, we want to be able to do this as long as possible. So we got into an argument - should we spread it out, or pour it in one place at a time.

    My opinion is that it doesn't really matter..
  8. Jul 24, 2014 #7

    Simon Bridge

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    OK - so you really just want to know how long before the stones stop being useful.
    It most likely won't matter, I concur - you can totally cool off one stone at a time or do them all at once by increments, you are still using up the same amount of energy at the same rate.
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