Sauna Stones: Why Quality Matters and When to Replace Them

Quick answer: Good sauna stones are dense volcanic rock, like olivine diabase or vulcanite, made to be heated and splashed with water thousands of times. Cheap or random stones crack, crumble and pack down. That blocks airflow, makes your sauna slower to heat and can shorten the life of your heater's elements. Check your stones once a year and swap out any that crumble.

What do sauna stones actually do?

Your heater warms the stones. The stones store that heat and slowly give it back to the room. When you pour water on them, they turn it into steam. Finns call this steam löyly (say "LOO-loo").

So the stones do three jobs:

• Store heat so the room stays warm and steady.
• Make steam when you add water.
• Let air flow through the heater so it heats evenly and the elements don't run too hot.

What makes a good sauna stone?

A good sauna stone is heavy, hard and built for heat. Look for these things:

• The right rock. Olivine diabase is the most common sauna stone. Vulcanite is a darker volcanic stone that is also very tough. Both handle heat well.
• Dense and heavy. Heavy stones hold more heat, so you get a steadier temperature and softer steam.
• Tough when hot. Good stones can go from hot to wet and back again without cracking. This is called thermal shock resistance.
• The right size. Most home electric heaters use stones about 5 to 10 cm across. Always check your heater's manual, as some heaters need a set size or shape. For example, Harvia says some models, like the Spirit, need round stones.
• Low dust. Good stones shed very little grit. Grit can fall onto the elements and cause damage.

What's wrong with cheap or random stones?

Not every rock is made for a sauna. Stones from a river, beach or garden can hold water inside them. Some types of rock, like sandstone or limestone, are soft and break down quickly in high heat. When they crack, the pieces settle and pack tightly together.

Tightly packed stones are a problem. Air can't move through the heater, so:

• The sauna takes longer to heat up.
• The heating elements run hotter than they should.
• The heater uses more power to do the same job.

Harvia tested this in a 10 m³ sauna with a 7 kW heater. With new, loosely stacked stones, the room reached about 70°C after one hour. With tightly stacked stones it only reached about 60°C. The hottest element temperature also rose from 580°C with new, loose stones to 680°C with old, tightly packed stones (Harvia). That's a big difference from something as simple as stones.

Olivine diabase vs vulcanite: which should I pick?

Both are great choices. Here's the simple version:

• Olivine diabase: The classic grey sauna stone. Reliable, long lasting and good value. A great pick for most home saunas.
• Vulcanite: A dark, volcanic stone that looks almost black when wet. It is very hard and resists crumbling, so it suits people who use their sauna a lot or want a premium look.

At HealthHQ we stock both. Our Traditional Sauna Heater Stones (20 kg) are a quality, everyday option for most home heaters. Our Harvia Black Vulcanite Sauna Heater Stones come from Finland and are a premium upgrade. You can see them all in our sauna stones range.

How often should I replace my sauna stones?

Harvia's guide is simple: if you use your sauna about twice a week, change the stones once a year (Harvia support). If you use it more often, check them more often.

Signs your stones need replacing:

• Bits of stone or grit on the floor under the heater.
• Stones that crumble, crack or have changed colour.
• Your sauna takes longer to heat than it used to.

A handy test from Harvia: knock two stones together. If they crumble or break, they need replacing (Harvia).

How do I stack sauna stones the right way?

1. Turn off the heater and let it cool completely.
2. Rinse new stones to wash off dust, then let them dry.
3. Put bigger stones at the bottom and smaller ones on top.
4. Stack loosely. Leave gaps so air can flow. Don't jam stones between the elements.
5. Cover the elements fully, but don't pile stones high above the heater.
6. Use the amount your manual says. Too many stones blocks airflow. Too few gives you weaker steam.

The first time you heat new stones, there may be a slight smell. That's normal and soon goes away.

Key takeaways

• Use proper sauna stones, like olivine diabase or vulcanite. Never use river, beach or garden rocks.
• Good stones heat your sauna faster, give softer steam and help protect your heater.
• Check your stones every year. Replace any that crumble or crack.
• Stack them loosely so air can flow, and follow your heater's manual.

Frequently asked questions

Can I use any rocks in my sauna?

No. Use stones made for saunas. Ordinary rocks can hold water inside, crack in the heat and damage your heater.

How many kilograms of stones does my heater need?

It depends on the heater. Small wall heaters may need about 20 kg, while large tower heaters can need 80 kg or more. Your heater's manual lists the exact amount.

Do I need to wash sauna stones?

Yes. Rinse new stones before use. When you check them each year, take them out, wash off the dust, throw away any broken ones and restack them loosely.

Are vulcanite stones better than olivine diabase?

Both work very well. Vulcanite is harder and looks darker, so it's a good premium choice. Olivine diabase is the reliable classic and great value.

Can I use sauna stones in a wood-fired stove?

Yes, but check your stove's manual. Some stoves need larger stones, often 10 to 15 cm, and some stones are only recommended as a top layer.

Not sure which stones suit your heater? Send us your heater model and we'll point you to the right size and amount. Call us on 03 662 9381 or email info@healthhq.co.nz. You might also like our guide to choosing the right size sauna heater.

Sources: Harvia, The easy guide to looking after your sauna stones; Harvia, Changing sauna stones makes the sauna heat up faster; Harvia Support, How often should the heater stones be changed.