How many stones
Every heater has a stone capacity set by its maker, from about 10 kg in compact wall heaters to well over 100 kg in tower heaters. Fill to that figure: too few stones leave the heating elements exposed and give harsh, short steam; tightly packed or too small stones block the air flow around the elements.
| Output | Models | Stones | Typical | Most stones |
|---|---|---|---|---|
| 2–4 kW | 38 | 18 lb–176 lb | 24 lb | SAWO Heaterking DRFT3-35 |
| 4.1–6.9 kW | 86 | 15 lb–331 lb | 110 lb | HUUM HIVE Mini 6 |
| 7–9.9 kW | 96 | 18 lb–353 lb | 110 lb | SAWO Heaterking DRFT6-80 |
| 10–13.9 kW | 56 | 33 lb–551 lb | 198 lb | HUUM HIVE 12 |
| 14–20 kW | 39 | 55 lb–794 lb | 243 lb | SAWO Heaterking DRFT12-150 |
Maximum stone load as each maker states it, across 423 electric heaters in our catalog. Fill to the maker's figure with the stone size the manual specifies.
Each heater's page lists its stone capacity, for example under sauna heaters.
Which stones
- Use stones sold for saunas. Most are hard igneous rocks such as olivine diabase, peridotite or vulcanite; Narvi, for example, recommends olivine diabase or vulcanite of 10–15 cm for its wood stoves.
- Never use stones from the garden or a beach. Unknown stones can crack and split when heated, and may hold moisture.
- Size: follow the manual. Electric heaters usually take smaller stones (many manuals say 5–10 cm), wood stoves larger ones.
Rinse and stack
- Rinse the stones to remove dust.
- Put larger stones at the bottom and smaller ones on top.
- Stack loosely so air can flow, without wedging stones between or against the heating elements.
- Cover the elements completely, but don't pile stones high above the heater's top.
When to re-stack and replace
Stones crumble with heating and water. Most makers ask you to re-stack them at least once a year, more often with daily use, and to replace any that have cracked or crumbled. Crumbled stones block the air flow, which overheats the elements and shortens their life.