Sauna rocks are the part of a traditional heater that actually stores and releases the heat, and they are consumable. Get the stone, the stacking and the replacement interval right and a modest heater performs well. Get them wrong and a good heater underperforms in ways that look like a fault.
Which stone
Sauna rocks are dense igneous stone, usually peridotite, olivine or a similar volcanic rock, chosen because they take repeated heating and sudden cooling without splitting. Never use river stones, granite from the yard or anything porous: trapped moisture turns to steam inside the stone and it can break apart violently in a hot heater.
How to stack them
Rinse off the dust, then place them by hand rather than tipping them in. Larger stones at the bottom, smaller above, with gaps throughout so air moves between them and around the elements. A tightly packed load chokes the airflow, overheats the elements and shortens their life. The stones should cover the elements without resting hard against them.
When they are finished
Stones crumble with cycling. The signs are dust in the tray, stones that have split or rounded, and a heater that no longer gives much steam from a ladle. Most cabins in regular use want the load restacked once a year and replaced every few years, sooner for heavy use. It is the cheapest thing you can do to bring a tired heater back.
How much stone
Heaters state a stone capacity in pounds or kilos, and running one underfilled is common because bags are sold in fixed sizes. An underfilled heater heats the air quickly and gives poor steam, which is often mistaken for the heater being too small. Fill it to the stated capacity before concluding you bought the wrong unit.
Questions people ask about sauna rocks
Can I use ordinary rocks?
No. Porous or river stone can trap water and break apart when heated. Use stone sold for sauna use, which is dense volcanic rock.
How often should sauna rocks be replaced?
Restack annually and replace every few years in regular use, sooner if you see dust and split stones or the steam has fallen off.
Does stacking really matter?
Yes. Tight packing chokes airflow, overheats the elements and reduces steam. Place them by hand with gaps throughout.