Cracked or crumbling sauna stones: what it means

Some wear is normal. Harvia says all stones will break down over time, and that stones disintegrate slowly but gradually in use because of the large changes in temperature.¹²

What matters is how much, and what you do about it.

Signs to look for

• Pieces of stone on the floor under the heater.³
• Stones that crumble or are discolored.¹
• Stones that crack when tapped together and sound shallow.²

What you need

• The heater switched off and cool.
• Gloves, a bucket and a soft brush.
• Replacement stones in the type and size your manual names.

Why heat does this

Heat cycles stress rock. Wikipedia's thermal shock page defines the term as thermal stress from a sudden change in temperature.⁴

Harvia ties the wear to the same thing: large variation in temperature.²

How to check

1. Make sure the heater is cool and turned off.¹
2. Take the stones out. Harvia says this also lets you look at the elements.¹
3. Tap stones together. If they crack and sound shallow, Harvia says it's time to change them.²
4. Discard stones that crumble or are discolored.¹
5. Clear any stone debris from the bottom of the heater.²

Why it's worth doing

Harvia says broken stones can shift into smaller cavities and get packed too tightly. That restricts airflow and can make the elements heat up too much.¹

Harvia also says poor, crumbling stones can break the heating elements of an electric heater. Then the heater needs a professional, or at worst a replacement.³

What to do with the old stones

Harvia says to recycle them according to the sorting instructions in your area, or use them in your garden if you have one.¹

Stones that pass the tap test can go back. Harvia says to wash them and let them dry before restacking.¹

Grit matters too. Harvia says grit from broken stones can damage the heater, so tapping the stones together is an important step.¹

Stones made to last longer

Heat-treated stones exist. Harvia says heat treatment gives longer service life.² For the type and size to buy, your heater's manual is the place to look.

How often

Harvia recommends checking at least once a year, and more often if you notice stone residue or small pieces on the floor.¹

Other makers give their own schedules. Look in your manual.

When there's damage to the heater

If the elements look discolored, bent or crusted, stop and read the manual. Hard-wired heaters need a licensed electrician for repairs. Don't open the wiring yourself.

If you're replacing stones, the amount is on the stone weight page, and the layout is in the stacking notes. Rocks from outside are covered in garden and river rocks.