What sauna stones do in a heater

A sauna heater has two jobs for its stones. They hold heat, and they give the water you pour somewhere to turn into steam. That's the whole idea.¹

How the stones fit into an electric heater

An electric heater doesn't heat the room directly with open flame. Electric heating elements warm the stones, and the stones stay hot.¹

The heater has a thermostat and a timer too.¹

Wikipedia also says steam is generated by pouring water on the hot stones.¹

The two jobs

Holding heat. Harvia says stone quality, size and placement can have a significant effect on heat retention and steam generation. It also calls stones one of the most important parts of the sauna.²

Making steam. HUUM puts it simply: you regulate the heat and steam by throwing water on the stones.³

That's why the maker's choice of stone isn't random. HUUM says the shape, size and composition of the stones, and how they're placed, all make a difference.³

What you'll want on hand before you open the heater

• Your heater's manual. It's the first place to look for stone type, size and amount.
• The model name from the heater's label, in case you need to ask the maker.
• A cool, switched-off heater, if you're going to touch anything inside.

None of this needs a source except the manual advice, and the manual is the source. Harvia says the same thing from its side: always check the right stone size and amount in the manual that came with the heater.²

Why placement matters as much as the rock

Stones aren't just a pile. Harvia warns that if they're packed too tight into the stone compartment, they will block the air circulation of the heater.⁴

Air has to move through the stones to carry heat out into the room. Harvia says tightly packed stones block air circulation, so the heater needs more time and energy to heat the sauna.⁴

For how to place them, see how to stack sauna stones. For sizes, see choosing the right stone size.

What changes as stones age

Harvia describes the stones as playing a very central role in every sauna session. It also says they should be checked every year. Old, crumbling stones are covered on the page about cracked or crumbling stones.⁴

Rock type is its own topic. Sauna stone types compared covers the names you'll see on product pages.

What this page doesn't cover

It doesn't say which heater to buy, and it doesn't make claims about how a sauna session will feel. It also doesn't cover hard-wiring a heater. That's work for a licensed electrician, and the heater's own manual sets the rules.