How sauna works: the three types of heat transfer

light steam graphic

All sauna enthusiasts can feel the three types of heat transfer​: radiation, conduction, and convection​. So, here is how sauna works: the three types of heat transfer. As we sit on the bench, we don’t have to know how sauna works in order to feel them in action.

More importantly, when we have a resonating understanding of how heat transfers, we can think about what makes for a good sauna vs. a approximation of good sauna. So, let’s analyze the three types of heat transfer in context to sauna.

Radiation

An example of radiation is the warmth your feel on your face when you go outside on a sunny day. This is most evident in the winter when the air is cold but you still feel that warmth. This is radiative heat transfer from the sun to your skin. In your sauna: If you feel that same kind of heat coming off the sauna heater, you are feeling heat transferred from radiation.

Conduction

An example of conduction is the heat you feel if you pick up a hot cast iron pan. In your sauna: If you bend over to pick something up ​on the hot room floor ​and bump your ass ​onto the ​metal ​sauna stove, conductive heat transfer will quickly raise the temperature of your skin to the point cells are damaged. You will feel it immediately, and see a mark the next day. Another important example of conduction in the sauna is when you throw water on the rocks. That water quickly turns into steam by conduction from the rock to the water. What happens next, when natural air currents carry the steam to the bather is convection.

Convection

An example of convection is the heat you feel from a forced air furnace. Heat has transferred from the furnace through the ducts in the room. In your sauna: If you toss water on the rocks and recruit an Aufgussmeister to swing a towel around, you’re feeling convection. Swinging the towel around in a hot sauna is going to enhance convective heat transfer whether there is steam or not. Steam enhances it though.

Three types of heat transfer: (photo: https://bleschsciencelabs.weebly.com/chapter-12-energy.html)

Question: Which type of heat transfer is best for sauna?

Answer: all of the above, working together. To understand how sauna works, consider the three types of heat transfer working together.

Why?

Radiation from a good heat source transfers to Conduction of rocks and mass (lämpömassa), which reaches us through Convection by good air flow in our hot rooms. This triple play helps “good heat” get right to our bones. Heat transferring by the mass motion of molecules is what penetrates our bodies. Infrared cabins heat only through radiation. Picking up hot sauna rocks heat only through conduction. Air movement of bad heat don’t do nothin.

Good sauna is all about all three methods of heat transfer working together.

Bass, drums, guitar.

We can have some solos, or dub it once and awhile, but ultimately, all three have to be rockin’ for good heat to be happening.

How sauna works: the three types of heat transfer

Summary​ (in order of goodness)​:

  1. Infrared cabins: No convection or conduction, only radiation.
  2. Toaster oven sauna heaters: too much radiation, too few rocks, lame conduction.
  3. Barrel saunas: Walls and feet are cold. Not enough convection. Can also suffer from #2 above.
  4. Sauna stoves with lots of hot rocks: (not wimpy tin can stoves that don’t heat all the rocks). Good! lots of conduction.
  5. Savusaunas: Great! lämpömassa everywhere (stones, walls, benches, buttox region, entire body & bones).

So, next time you settle yourself onto a sauna bench, consider partaking in a zen heat transfer consciousness exercise. Take a slow, deep Wim Hof breath sit back and become one with the hot room. Feel how you are engaging with the heat. Are you feeling the positive vibration of three types of heat transfer? Are you feeling the vibe in a good sauna or are you feeling such things as the ill effects of:

Well, I have great news for you. Click on any of the three internal links above, and you will begin your sauna transformation towards The Holy Trinity of Good Sauna.

This is how sauna works: the three types of heat transfer.

Guest input: Please welcome L. Grant Whittle, Technical Director of a company that steam cures epoxy liner pipes, Grant is in the process of researching and building his own sauna. He is applying his knowledge of heat, heat transfer and physics of good heat towards good sauna building principles. Enter Grant:

Transfer of energy through convection is more efficient with steam. The moisture penetrates our bodies better and carries the energy deeper into our bodies than dry hot air alone can. The hygroscopic nature of sauna wood cladding also contributes to proper sauna energy transfer. Steam transfers energy at a more consistent and comfortable temperature; the phase change of the water molecules actually buffers the temperature. The rocks may be at 500+ F, but at ambient pressures, the steam released is always in the air at a more comfortable 210 to 212 F. 

Steam MUST be pressurized to ever get above 212 F. In most locations, steam will quickly cool to 210 F at ambient pressure, regardless of how hot the rocks are. So the stove and the walls of a sauna can “superheat” from the radiant heat off of the stove. But the steam should moderate the convective temperature of the “air” (which is in contact with your skin) to no hotter than 210 F, even when the walls of a banya are radiating heat at 240 F and the stove of the banya is radiating at 1000 F!

Wet steam vs. dry steam

The more energy being carried (from hotter rocks) the less “wet” the steam will be and the further the energy will carry without “falling out” through condensation. Drier steam has a different feel on the skin and heats deeper than wetter steam (which carries less energy in the water molecules and which immediately condensates and rolls off your body).

In contrast, radiant heat and conducted heat are NOT moderated and “feel” harsher to the body. Moments in front of the stove feeling the harsh radiant heat can be enjoyable, but are not sustainable. Convected heat, especially with the temperature moderation of steam, is what makes loyly feel so perfectly “good” during a sauna session. This perfect temperature “balance” throughout the hot room and into the body simply cannot be replicated in a sauna with dry rocks heating dry air. Dry air doesn’t transfer the heat into the body as efficiently, and the temperature is far less “balanced.”

The German concept of Aufguss, uses rapid air & steam movement to briefly give a burst of “hotter” temperature sensation to the whole body through more rapid convective energy transfer. So even convective air and steam flow has to be “balanced” to be a sustainable, enjoyable heat session. Air and steam movement in a sauna has to be carefully balanced for both air quality and convection quality. Too much airflow can make convection feel harsher. Too little airflow, and air quality suffers. 

Editor’s note:

I’d like to thank senior engineer Tom for his thorough review, edit, and input to this article. For a yahoo like me, understanding the three methods of heat transfer has taken a lot of head scratching (on the sauna bench). For over 40 years, I have done my best to avoid bad heat, and help create really good heat. I know it when I feel it, but having a friend like Tom is wonderful in many ways, including that he has helped me understand the how. And the why. Good heat makes all the difference.

5 thoughts on “How sauna works: the three types of heat transfer”

  1. Hello Glenn,
    Last night was my 23rd Sauna. I am still learning about how much wood to use, how long between rounds, how much water to present to the rocks, etc., etc. But one thing I do know, the Lampomassa is indescribable! I appreciate the article, ‘How Sauna Works’, because I could actually start to discern some of the different “heats” as I sat from 1st round thru 3rd, 4th and even 5th rounds. Just didn’t understand the ‘science’ behind it all.
    Truly, there is nothing like GOOD HEAT!

    Thank you for your dedication to your website, it has helped me tremendously.

    Robert Smith

  2. Fourth type of heat transfer – löyly

    When water is poured on rocks we get löyly. “Latent heat″ is energy transferred in a process without change of the body’s temperature, for example, in a phase change and in this case steam condensates (to water) which releases (vapor’s latent energy absorbed during evaporation is released) energy to skin (”negative vaporization”). Relative humidity on skin raises close to 100% (possible as skin temp is approx 40˚C) which reduces perspiration thus increases sense on heat even more”.

  3. “but at ambient pressures, the steam released is always in the air at a more comfortable 210 to 212 F.”

    “But the steam should moderate the convective temperature of the “air” (which is in contact with your skin) to no hotter than 210 F”

    I’m struggling a bit with what you’re saying here. These are true at saturation (or phase equilibrium) but not at moisture content below saturation. You can for instance have an air temp of 115°c (239°f) and a relative humidity of perhaps 25%.

  4. Hi Glenn,
    I’ve got a few questions regarding IKI 6 kW electric sauna heaters:
    1. Currently available documentation is oriented to European customers because electric diagrams use 3-phase 220 V power line (that why they use 5 wire cable). Is it possible to get installation instructions adapted to US standards (1-phase 240 V)?
    2. Does the mechanical timer in the models with built-in controllers produce noise or it is practically not audible?
    3. According to the documentation ‘Finlandia’ control panel can be installed outside of the sauna (which is my case) but how it will behave with freezing temperature in the winter? Probably it should be mounted inside of some kind temperature controlled enclosure.

  5. Hi Vlad,

    1. Regarding installation instructions for US standards, installation manuals from different manufacturers do have this detailed. UL certified electric heaters include, Tylo/Helo, Harvia, Iki. This post here is an awesome resource.

    2. others who own mechanical timer electric heaters can answer about the noise.

    3. Yes, for sure, mounted in a climate control box is key. You may also want to type “changing room” in the search bar above. Depending upon climate, an outdoor sauna with electric heater may be “a little engine that can’t.”

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