A lukewarm stone gathers no heat

Guest post series continues. Please welcome Jeff Pyzyk to SaunaTimes. Jeff began doing “real sauna” over 25 years ago at his lake home which he built 17 years ago. He then built his wood burner at home going on seven years ago.  He is owner of three saunas. He saunas everyday and has done so for years how. He may prefer his wood fired sauna steps out his backdoor outside of Milwaukee, WI, yet is known to have magical experiences in and out of his electric heated sauna by the shores of his cabin in Northern Wisconsin. His third sauna is a tent sauna and is ready for deployment between either of these other two. Welcome Jeff:

Do more stones mean better steam?

There is a recent trend and interest in sauna stoves with a large mass of rocks. They look really cool and give the impression that they produce a huge amount of löyly (the Finnish word for steam). These stoves may come in the form of a “rock tower” or “hive” or “drop” where the electrical elements (or in the case of a wood-burning stove, the firebox – and even a portion of the chimney) are buried in a mass of stones. The question is: do more stones mean more and better löyly? 

Empirical research (by those who sauna daily) says it ain’t necessarily so!

Factors to consider:

• Do stones placed along the sides of the stove even get hot enough to convert water to steam?
• To make good löyly, stones need to be superheated, ie 350f. (177c.) or higher!
• Löyly  moves upward, not laterally.  So what good are stones on the sides of a stove?
• At what point do more stones act as an “insulator” becoming further and further removed  from the heat source and thereby no longer able to conduct heat?

But radiant heat is bad right?

Whether heat is radiating from heated rocks, as in a savusauna, or from the heated brick walls of a stove, as in a banya, or from heated firebrick surrounded by metal walls, as in a Kuuma or a Narvi AK stove, it is mainly radiant heat. Radiant heat is required to get any sauna up to temperature. For some, that may mean 160F and for others it may mean 220°f.

Factors to consider:

• Rock tower stoves can be wimpy when it comes to heat-up time. They sometimes take longer to get up to operating temperature, and tend not to hold temperature over time compared to traditional Finnish-American made stoves such as Nippa and Kuuma.  The wood-fired rock tower stoves that I have experienced are much more finicky when it comes to size of firewood and dryness of wood, and require much more fire tending.
• If you like a “low and wet” sauna, rock tower stoves may be a good option for you.  I call this kind of sauna environment “St. Louis in the summertime”. To me, it’s miserably wet and requires constant fire tending and addiction to pouring water on the rocks to make löyly as the only means of experiencing appreciable heat.  A low and wet environment is the result of insufficient lateral radiant heat.
• “High and dry” is the type of ambient environment I prefer, meaning a higher ambient temperature coupled with a lower ambient humidity.  Humidity rises when löyly is created.  A high and dry environment requires a powerful stove, with high thermal mass that generates good radiant heat (from the fire and into the room – heating all the mass, not just the steel of the stove, of course).

Bench Height and the Law of Löyly

Lack of lateral radiant heat results in a much steeper heat gradient from ceiling to floor.  The “Law of  löyly” is meant to save mankind from the sin of having cold feet in the sauna.  Thus the “law” specifies that the lower bench must be above the top of the sauna stove.  The “law” is meant to place the bather’s entire body within the convective heat pocket. 

Factors to consider:

• A heavy stove having high thermal mass, coupled with a stone or masonry backsplash will create a “wall of heat”.  Like Phil Spector’s “wall of sound” created in the 1960’s, the “wall of heat” incorporates and overlays all the elements of heat transmission including conduction (fire to steel and stone), radiation (steel and stone to backsplash, benches, walls, and bathers), and convection (air currents carrying heat from stones upward throughout the sauna, and löyly !)
• Violations of the “Law of  löyly” are much less egregious when your sauna creates a wall of heat because the temperature gradient from ceiling to floor is not as dramatic.
• Just because the sides of a sauna stove are covered with stones doesn’t mean it will create a wall of heat – it is actually likely to be quite to the contrary. If the stones aren’t superheated they will not radiate heat and will not generate löyly when confronted with water.

Comments from others:

Diminishing negative return

Whether wood fired or electric, at any moment, there are only so many BTU’s pushing heat into sauna rocks. With a shit ton of rock, could we be falling off the ledge?

Comments from a Finn: (fewer words)

“Over marketed. Rocks on the edges are just warm. I don’t get claimed horizontal heat. Raising heat makes sense.  
Besides stove style was common in Russia way before xxxxxx. (brand name withheld). I have seen with my own eyes. I suspect xxxxxx’s are least efficient stoves. “

Kimmo Ratio, Sauna Sherpa

Comments from a Brit (Note: British diplomacy):

I think its convenience versus heat/steam quality. More stones, better steam/heat but longer heat up, fewer stones faster heat up but sharper heater and less pleasant steam.

I think it depends on your use case and desired experience as to what is right for you, but my opinion is the sweet spot for most users is somewhere in the 60-100 kg (130-220lb) range. Good every day heat up time, but still a good steam/heat.

If you have a busy life and want sauna because of the health benefits of daily sauna and cold plunge, fewer stones are going to speed up heat up time, if sauna is part of your weekend disconnect, to enjoy as an experience, a decent wood burner with a good heat and steam will be right.

In most places you cannot justify firing up a savusauna every day for you to have your sauna session (other than Finland). Thus, there is certainly a place for use-case specific recommendation bands for the lämpömassa of your kiuas.

Jake Newport, Finnmark Sauna, UK