Why garden and river rocks are a bad idea in a sauna heater

Don't put garden or river rocks in a heater. Makers sell stones for heaters for a reason, and your manual names what its heater can take.

What you need to know first

• Your heater's manual and the stone types it names.
• That a rock's look says nothing about how it behaves in heat.
• That this page reports what makers say. It isn't a rock test.

What Harvia says

Harvia says many stone types are unsuitable for a sauna because they contain dangerous toxins, or they don't withstand large temperature fluctuations.¹

It says finding stones from nature that are safe to use isn't a simple task, particularly if you aren't trained to recognize stone type and quality.¹

Harvia's page doesn't name river rocks or garden rocks. It says stones from nature in general are hard to judge, and it has its own stones tested and quality controlled.¹

Why temperature swings matter

An electric heater's stones go from room temperature to very hot, and then take a splash of water. Wikipedia describes thermal shock as a type of thermal stress caused by a sudden change in temperature.²

That's the general idea. A rock that can't take big swings can split. Harvia says some stones don't withstand large temperature fluctuations.¹

Heater stones are quarried and checked

Harvia says sauna stones must be quarried and selected carefully to pass quality control.³

HUUM says its olivine diabase stones are enduring and their thermal capacity is high.⁴

What the testing looks like

Harvia says its stones are carefully selected by experts and tested by the Geological Survey of Finland. It says its quality control confirms the stones don't release any emissions under intense heat.¹³

Harvia also says stones that don't meet its quality standards are repurposed for construction and other industries. So even among quarried rock, some doesn't make the cut.³

A rock from the garden has had none of that. You'd be the only one checking it, and Harvia says that's not a simple task.¹

But I like the round look

Round stones are sold for heaters. Harvia's rounded olivine diabase stones resemble nature's rounded stones from the great rapids. They're a quarried stone made for the job.³

HUUM recommends round stones too, for milder steam. Its range is round olivine diabase.⁴

What to do instead

1. Find the stone type and size in your manual.
2. Buy stones sold for sauna heaters in that type and size.
3. If you've already used garden stones, switch them out with stones your manual allows, when the heater is off and cool.

For the names you'll see, read stone types compared. If you're seeing damage, this page on cracked stones is next.

Warranty terms differ by maker. Read yours before you load anything the manual doesn't list.