How to stack sauna stones in an electric heater

Stack loosely. That's Harvia's advice. Tight stacking cuts airflow, and stones that lean on the elements can bend them.¹

Stacking steps differ by heater. Treat what follows as a checklist of general points, and follow your own manual for your model.¹

What you need

• The heater's manual, with its stone size and amount.
• Enough stones of the type and size the manual names.
• A bucket and water to wash the dust off new stones.
• Work gloves, if your manual suggests them.

Before you start

The heater must be cool and turned off before you begin.²

Wash the dust off the stones before they go in. Harvia says to wash new stones and good ones from an earlier load, then place them loosely enough that air can circulate.³

The steps

1. Check the manual for the stone size and amount. Harvia says to always check them in the manual that came with the heater.¹
2. Put the stones in one at a time. Harvia's list says not to drop them into the heater.¹
3. Arrange them evenly and sparsely, so air can flow between the stones.¹
4. Place elongated stones vertically.¹
5. Keep stones off the top of the heater frame, and don't build a high pile on top.¹

Extra points for electric heaters

Harvia lists these for electric heaters.¹

• Don't wedge stones between the heating elements.
• Pile the stones so they support each other instead of putting their weight on the elements.
• Support the elements with stones so that the elements don't bend.

Size and layers

HUUM suggests a denser top layer made of slightly smaller stones.⁴

For other makers, the answer depends on the heater. Harvia notes that the type and shape of the heater affect how the stones should be piled, and sends you to the manual.¹

What one maker's test found

Harvia ran its own test in a 10 m3 sauna with a 7 kW heater. It compared new and old stones, each loosely laid and tightly stacked. The loose stacks held 80 kg of stones and the tight stacks held 90 kg, so the amount of stone differed as well as the stacking.³

With new, loosely laid stones, the elements reached a maximum of 580 degrees Celsius. With old, tightly stacked stones they reached 680 degrees. With old, loosely laid stones the maximum was 640 degrees. The 580 and 680 figures differ in stone age as well as stacking.³

After an hour, the room was around 70 degrees with new loose stones and around 60 degrees with new tight ones. Those are Harvia's figures for one heater and one room, not a promise for yours.³

After stacking

Harvia says new stones may give the sauna a natural stone scent on the first heating. It suggests heating the sauna and throwing enough water to douse all the stones, so the scent goes. Your manual may say something different.¹

Don't overfill

Harvia says overloading the heater with too many stones can prevent good airflow and cause the elements to overheat.²

If the stones are packed too tight in the compartment, they will block the heater's air circulation.³

The right amount is covered on the page about how many stones a heater needs. Stone sizes are on the size page.

When it's a hard-wired heater

Restacking stones doesn't mean touching the wiring. If a heater needs electrical work, call a licensed electrician.