Sauna stones: types, sizes, loading and replacement

The short answer

• Stones are a working part of the heater. Harvia’s manuals describe them as significant to the heater’s safety, not just its appearance.
• Use only the rock type, shape and size your heater specifies. Most Harvia electric heaters take 5–10 cm stones; some larger and wood-burning heaters take 10–15 cm. The manual for your heater governs.
• Wash them, then load them loosely. Air must rise through the stone mass. Never wedge stones between or against the elements; stones should support each other.
• Stones wear out. Heat and water crack and crumble them, and they settle. Crumbled or packed stones choke the airflow, which makes the heater run hot, can damage the elements and is excluded from manufacturers’ guarantees.
• Re-stack and replace them on the manufacturer’s schedule. Harvia asks for at least once a year, more often with frequent use. Commercial saunas need far more frequent attention.

Why stones matter

It is easy to think of the stones as decoration on top of a heater. In fact they do six jobs at once.

1. They store heat. A stone load of about 20 kg to more than 200 kg, depending on the heater, holds a reserve of heat. Harvia’s help centre advises stones that are heavy, dark and reasonably large for this reason. Why it matters: that reserve is what turns each ladle of water into steam. Without it, water would simply run off.
2. They vaporise water. Turning water into steam takes a great deal of energy, and it comes out of the stones. Hot stones flash a ladle of water into vapour almost instantly; stones that are not hot enough let it trickle through. Our guide to how a sauna heats explains the physics.
3. They control humidity, the löyly. As a sauna heats, the air dries out. Water thrown on the stones raises the humidity again, and the wave of steam that rolls down over the benches is the löyly. Why it matters: the stones are the only way to adjust humidity in a traditional sauna, so their condition decides how good the löyly is.
4. They let air through the heater. An electric heater warms the room mainly by drawing cool air in at the bottom and sending hot air out through the stones. Loosely laid stones let that air flow; tightly packed or crumbled stones block it.
5. They shield the surroundings. A dense outer layer of stones stops the glowing elements radiating directly onto nearby timber. Harvia warns that a bare element can endanger combustible materials even outside the usual safety distances.
6. They support and protect the elements. Stones hold the elements upright. Loose stones keep them cooler: in a test Harvia published, element temperatures rose from about 580 °C with new, loosely laid stones to about 640 °C with old, loosely laid stones and about 680 °C with old, tightly stacked ones.

The right rock types

Good heater stone is dense, dark and tough enough to survive being heated to several hundred degrees and cooled by water hundreds of times. Harvia’s stone instructions approve four rock types:

• olivine diabase, the classic Finnish heater stone;
• peridotite;
• olivine;
• vulcanite, where the heater’s manual lists it (not every model does).

Other heater makers name the same family of rocks, with olivine and olivine diabase the most common. Whatever the rock, the manual for your heater has the final say, because some models list fewer types than others.

Stones never to use

• Stones from a garden, beach or river. They are not a tested type, can crack and crumble in the heat, and Harvia warns they may contain unwanted substances.
• Ceramic “stones” and soft soapstone, unless your heater’s manual specifically approves them. Harvia’s help centre says they do not store enough heat and can lead to overheating and broken elements. Several other manufacturers prohibit ceramic or artificial stones outright.
• Light, porous or crumbly rock, which stores little heat and breaks down quickly.
• Decorative stones as the main load. They belong on the surface only, in the quantity the manufacturer allows (see below).

Why it matters: the heater’s output, element temperatures and safety distances were tested with its specified stones. Different stones change all three, and Harvia excludes damage caused by non-recommended stones from its guarantee.

Harvia’s stone types

Harvia, the Finnish heater maker, sells several families of heater stones:

The item codes help when ordering replacements: quote the code together with your heater model.

Which size does my heater need?

Stone size is set by the heater’s design, not by preference. Larger stones leave larger air gaps and suit heaters built for them; smaller stones suit heaters with closer element spacing. These are the figures in Harvia’s current manuals:

Three cautions:

• Weight is a guide, not a target. Harvia notes that the right amount varies with the stone type, because different stones have different densities. Fill to the coverage the manual shows.
• Small pieces don’t belong in the heater. Harvia’s manuals say very small stones should not be put into the heater at all. One European maker sets a floor of 35 mm across, because smaller pieces fill the air gaps and overheat the elements. For wood stoves, Harvia’s help centre says to remove stones smaller than 10 cm.
• Manuals change between model versions and markets. The manual supplied with your heater is the one to follow.

Angular or rounded?

No manufacturer we have found publishes measured data comparing the steam from each shape, so treat claims about “softer” or “sharper” steam as descriptions rather than proven properties. What the manufacturers do say is more practical: where each shape goes.

The outer-layer rule appears in Harvia’s current stone instructions, and another heater brand in the Harvia group uses the same wording. Why it matters: a rounded stone touches an element at a single point and can roll, so it neither supports the element nor stays put as the load settles. Angular stones lock together around the elements and hold them upright.

The Harvia Spirit and rounded stones

The Spirit is one of Harvia’s most distinctive heaters: an open, sculptural frame that leaves the stones on show. It is designed around rounded stones. Harvia’s Spirit product pages recommend rounded olivine diabase, 5–10 cm: four 15 kg boxes of its rounded stones (R-991), around 60 kg in all. Earlier Spirit manuals allow rounded stones only; the newest manual also lists split-face stones but still recommends rounded stones over the air channel. The manual supplied with your heater is the one to follow.

Harvia’s Spirit instructions build the load from the largest stones at the bottom and sides, place flat stones tight against the outer frame so the elements are hidden, pile the middle loosely and finish with rounded stones over the air channel, where their shape keeps the airflow open.

Decorative top-layer stones

White and red decorative stones are for appearance. Harvia’s rules for its own decorative stones:

• top layer only, never as the working load underneath;
• one 10 kg box per heater at most;
• placed loosely, so air still flows, and not touching the elements;
• they may crumble or discolour in use, and Harvia says their properties do not fully match normal heater stones;
• damage caused by decorative stones is not covered by Harvia’s guarantee.

Why it matters: decorative stone is chosen for colour, not for heat storage or resistance to thermal shock. In the top layer it adds a finish; deeper in the heater it would break down and block the airflow.

Small or large stone mass?

The stone load is fixed by the heater you choose, so this is really a question of which heater suits the room and how it will be used. Our heater guide covers the choice.

Two things don’t change with size. Adding stones beyond what the manual specifies does not improve the löyly; Harvia warns that too many stones restrict the airflow. And heat-storage heaters such as Harvia’s Forte are a separate design, with a closed stone chamber and their own loading and replacement rules.

Washing stones

New stones carry dust from quarrying and transport. Harvia asks for the dust to be rinsed off before loading. One maker specifies plain water and a brush, with no cleaning additives.

Why it matters: dust left on the stones burns off as a smell in the first sessions, stains the floor and the timber around the heater, and the fine grit settles into the air gaps. Let washed stones drain before loading.

When you re-stack, the stones you keep can be rinsed the same way to remove grit before they go back in.

How to load sauna stones

Loading is something owners can do themselves, following the heater’s manual. The principles that run through Harvia’s instructions, and that other manufacturers share:

1. Make sure the heater is off, isolated as the manual describes, and completely cool. Stones hold heat for hours.
2. Wear protective gloves. Harvia warns that metal parts can have sharp edges; another maker specifies cut-resistant gloves.
3. Wash the stones and sort them by size.
4. Start with the largest stones at the bottom and work up.
5. Place stones one by one. Don’t drop or tip them in; it can damage or bend the elements.
6. Never wedge stones between or against the elements, and don’t let stones rest their weight on them. Stones should support each other, and the elements should stay vertical. If a stone won’t fit between elements without forcing it, leave it out.
7. Build a denser outer layer against the frame or grid where the manual shows one, overlapping so the elements are hidden, and keep the middle loose so air can flow. Take care at corners, where elements sit close to the grid.
8. Cover the elements fully. Harvia’s manuals require that the elements are not visible beyond the stones; other makers say not visible from any angle. Some designs, such as Harvia’s Virta, separate the elements from the stones by construction; follow the instructions for your design.
9. Don’t build a high pile on top. Too many stones restrict the airflow, and stones may block the heater’s air paths.
10. Put nothing else in the stone space that could change the amount or direction of air through the heater.
11. Keep spares, so broken stones can be replaced and settled levels topped up.

Why loose loading matters

The heater depends on air rising freely through the stones. When stones are packed tight, or when crumbled fragments fill the gaps, hot air can’t escape fast enough and heat builds up around the elements. Manufacturers describe what follows:

• The heater overheats. Elements run hotter (the 580 °C to 680 °C difference in Harvia’s test), and the heater’s overheat protector can trip. Harvia lists crumbled or compacted stones first among the causes of an overheat cut-out. One European maker gives packed or too-small stones as a fire-risk warning.
• The elements fail sooner. Harvia’s help centre links tight or unsuitable stones to shortened element life and says crumbling stones can break elements. Stones touching the elements suffer the greatest thermal shock and break down first.
• The sauna heats more slowly. In Harvia’s test, a 10 m³ room reached about 70 °C in an hour with new, loosely laid stones, but only about 60 °C with tightly stacked ones.
• The surroundings suffer. Nearby timber can overheat and darken, and hot chips falling from the heater are a fire risk.
• The guarantee may not apply. Harvia’s guarantees exclude damage from decorative or non-recommended stones, and several of its heater manuals exclude faults caused by disintegrated or too-small stones blocking ventilation. Other makers exclude element damage caused by wrong stones or poor loading.

Recognising stones that are breaking down

Stones are a consumable part of the heater. Every session heats them, and every ladle of water cools their surface suddenly. Strong or wrongly diluted fragrances speed this up, according to one maker. Over time the cycling opens cracks until stones split and crumble.

Look and listen for:

• cracking noises while the heater warms; Harvia notes that occasional bangs are usually stones cracking;
• crumbling, flaking or discoloured stones, including stones that have turned pale or yellowish;
• fragments and grit collecting at the bottom of the heater or on the floor beneath it;
• a dull, hollow sound when two stones are tapped together (sound stones ring);
• settling, so the stone level drops and elements start to show;
• white deposits on the stones, which can come from hard water; one maker says they reduce the stones’ ability to store heat;
• performance changes: the room takes longer to heat, water thrown on the stones cools them quickly, or the overheat cut-out trips;
• darkening timber near the heater, which can point to stone, clearance or element problems.

Re-stacking and replacing stones

What re-stacking involves

1. Switch the heater off and let it cool completely.
2. Lift every stone out. Don’t just top up the surface; the fragments collect at the bottom.
3. Remove the fragments and grit from the bottom of the stone space, and clear any that have fallen on the floor beneath the heater.
4. Check each stone. Discard any that are cracked, crumbling, flaking or noticeably smaller than the heater’s specified size.
5. Look at the elements without touching their connections. If an element is bent, damaged or no longer upright, don’t reload; have it checked first.
6. Rinse the stones you are keeping and let them drain.
7. Reload as described above, making up the quantity with new stones of the specified type and size.
8. Check again over the following weeks as the new stones settle.

Re-stacking is owner maintenance under Harvia’s instructions. If you would rather not do it, or you find damage, our service team inspects, restacks and replaces stones to the heater manual’s specification.

How often: Harvia first

Harvia publishes no separate interval in months or operating hours for commercial use. Its commercial manuals use the same “at least yearly, more often with frequent use” wording.

How other manufacturers set it: examples only

Other heater makers publish their own figures for their own heaters. They are shown here only to illustrate how much intervals tighten with heavy use. They are not Harvia’s guidance and not a BSC rule.

A commercial heater can be heated, cooled and have water thrown on it many times more often than a home heater, so its stones break down far faster. For gyms, hotels, clubs and other heavily used saunas, stone inspection belongs in the planned maintenance schedule, set by hours of use and the heater’s manual, alongside the other checks in our commercial guide.

Water on the stones

• Little at a time. Harvia’s current manuals set a maximum ladle volume of 0.2 litres. Too much at once doesn’t all evaporate and can splash boiling water.
• Aim at the stones, and never throw water when someone is near the heater.
• Let the stones recover between ladles. If water starts running through rather than flashing off, the stones need time to reheat.
• Use clean household water. Harvia publishes limits for minerals such as iron, manganese and hardness, and prohibits chlorinated water and seawater. If you are on tank or bore water, check it against the limits in your heater’s manual. In Queensland, where many homes have pools, never use pool or spa water.
• Scents only as directed. Use fragrances made for sauna water, diluted as their maker directs. Neat essences and herbs on the stones can ignite.

Our first-use guide covers the first heats and your first session, and the care guide sets stones in the context of the whole maintenance routine.