Sauna stones: choosing, stacking and replacing them
The stones are half of your heater. They store the heat and turn water into löyly. Old, crumbled stones make harsh steam, slow the heater down and can overheat its elements. Here is how to pick the right ones and when to change them.
When are your stones due?
Based on Harvia's guide of about once a year at two saunas a week, which works out to roughly 100 heatings. Check at least once a year either way.
Why the stones matter
- They make the löyly. A good stone mass turns a ladle of water into a soft wave of steam. Too little stone, or stones that have cooled, give a sharp hiss and a sting.
- They protect the heater. Stones break down over time from being heated and cooled again and again. Broken pieces slip into the gaps, pack the heater too tight and block the air flowing through it. Harvia warns that this can overheat the elements, slow the heating and in the worst case start a fire.
- They hide the elements. A heating element that shows between the stones can ignite things near it, even outside the safety distances.
Which stones to use
| Stone | What it is like | Notes |
|---|---|---|
| Olivine diabase | The classic Finnish heater stone. Grey-green, split, rough surface. | A safe default for most electric heaters and wood stoves. |
| Peridotite | Dark, heavy and durable, stores heat well. | Used in the Finnish Sauna Society's saunas in Helsinki. |
| Olivine | Often sold as rounded stones. | Some open stone heaters ask for round stones; check the manual. |
| Vulcanite and other specialty stones | Black volcanic stones and other looks. | Fine when sold as heater stones and allowed by your heater's manual. |
| Ceramic stones | Manufactured, light and even-shaped. | Allowed in some stoves, such as Narvi's heat-storing Aito, but forbidden in others: Harvia's Cilindro manual rules out light, porous ceramic stones. Follow your manual. |
Size and amount
Use the stone size your heater's manual gives. Most wall-mounted electric heaters take split stones of about 5–10 cm; some open stone heaters take smaller round stones, such as 3–5 cm for HUUM's Steel. The manual also gives the weight. Too many stones block the air flow and overheat the elements; too few give poor löyly and can leave elements exposed.
Small wall heaters take around 20–25 kg, tower heaters like Harvia's Cilindro around 90 kg, and heat-storing stoves 100 kg or more. The accessories guide lists stone loads for heaters in the calculator.
When to change them
Harvia recommends checking the stones at least once a year, and for a family sauna used about twice a week, changing them once a year. A Finnish Sauna Society heating master suggests about every year and a half when the electric heater runs once a week. More use, sooner. Signs that it is time:
- Gravel or crumbs on the floor under the heater.
- Stones that crumble, crack or have changed colour.
- The sauna takes longer to heat than it used to.
- The löyly feels harsh and dies away quickly.
- An element shows between the stones.
How to change and stack them
- Switch the heater off at the breaker and let it cool completely.
- Take all the stones out and vacuum the crumbs from the stone space and the floor. Check the elements and frame: discolouration or limescale is a sign to have the heater checked.
- Sort the old stones. Keep only whole, hard ones. Knock two together: a dull or crumbly stone goes. Old stones make good garden edging.
- Wash the new stones under running water to remove the stone dust, and let them dry. Wear gloves when you handle them.
- Lay them one by one, never pour them in. Start with the largest stones at the bottom and sides, wedge smaller ones to hold them, and leave room for air to flow. Do not wedge stones between the elements.
- Check that no element shows, and do not pile a tall heap of stones on top. The manual shows how full the stone space should be.
- Air the sauna well the first time. New stones can give off a little dust and smell as they warm up.
Questions
Whole, hard ones, yes. Wash them and mix them with new stones. Throw out anything that crumbles or has split.
Constant heating and cooling, and the shock of cold water, wear every stone down. Cracking quickly is a sign of the wrong stone type, or stones that were not made for heaters.
There is no need. Washing off the dust is enough.
No. Use stones sold for sauna heaters, of the type and size your heater's manual names.
Sources: Harvia, The easy guide to looking after your sauna stones and how often to change the stones; Harvia Cilindro and HUUM Steel heater manuals; Taloustaito, interview with Finnish Sauna Society heating master Ari-Pekka Paavola (2018).