How a sauna heats: the heater, the stones and löyly

The short answer

An electric sauna heater warms the room mainly by moving air. Cool air is drawn in low through the heater, warmed by the elements and the stones, and rises to the ceiling. It spreads across the ceiling, cools, sinks and returns to the heater. That loop runs the whole time the sauna is on.

The stones are part of the heater, not decoration. They store heat, shield the surroundings from the glowing elements and, when you throw water on them, turn it instantly into steam. That burst of steam is löyly, the Finnish word at the heart of the sauna.

Three things follow from this, and they shape how a good sauna is designed:

• The air layers. It is always hottest under the ceiling and coolest at the floor, so the benches sit high and the ceiling stays low.
• The heater must breathe. Air has to flow through the heater and its stones. Badly packed stones or a blocked heater heat poorly and can overheat.
• The heater is sized to the room, including its glass and hard surfaces, not just its floor area.

The heater: a loop of moving air

Inside an electric sauna heater are resistance elements, much like the elements in an oven. When they are switched on they get very hot, and they heat the air passing over them and the stones packed around them.

Hot air is lighter than cool air, so it rises out of the top of the heater towards the ceiling. Cooler air near the floor is drawn in to replace it. The heated air spreads under the ceiling, gives up some of its heat to the room, the people and the timber, then sinks and finds its way back to the heater. Heater manuals describe exactly this: fresh air enters, is warmed by the heater, rises and circulates through the room.

That is why manufacturers care so much about air getting through the heater. Harvia asks for the centre of the stone load to be piled loosely so air can flow through it, with the stones supporting each other rather than wedged between the elements. Another maker warns that packing the stones too tightly slows heat-up and overheats the elements, and a third that lint blocking a heater’s intake openings limits the convection and can lead to excessive temperatures. A heater that cannot breathe heats the room slowly, works its elements harder and can trip its overheat protector.

The room also needs fresh air. Ventilation in a sauna is not only about comfort: the supply air feeds the heater’s loop. How and where that air enters depends on the heater, which is why ventilation is designed with the heater manufacturer’s instructions in hand. Our ventilation guide explains the principles.

Radiant heat as well

Moving air is not the whole story. The glowing elements radiate heat, the hot stones radiate heat, and once the room has warmed through, the timber linings radiate gently too. The Finnish Sauna Society describes the heat a bather receives as a combination of radiation from surfaces, convection from moving air and the exchange of moisture with the skin.

The stones moderate the radiant part. Harvia’s manuals, like those of other makers, instruct installers to pack a dense outer layer of stones against the heater’s mesh so the elements cannot radiate directly onto nearby timber. That is one reason open-basket heaters need generous safety clearances, and why the stones must always be loaded as the manual shows.

What the stones do

The stones have three jobs, and all three are practical:

1. They store heat. A heater’s stones weigh from about 20 kg in a small wall heater to around 90 kg in a tower heater such as the Harvia Cilindro, and more than 200 kg in the largest models. Once hot, that mass keeps giving off heat, and it is the reserve the heater draws on every time water is thrown.
2. They shield the surroundings. A properly packed outer layer stops the elements radiating straight onto benches and walls.
3. They make löyly. Hot stones flash water into steam the moment it lands.

They also shape the character of the heat. Harvia describes heaters with a large stone mass as giving softer löyly, and smaller wall-mounted heaters as suiting people who like it dry and hot. “Soft” and “dry” are not technical terms with numbers attached, but they describe something real: a big, hot stone mass can turn more water into steam, more evenly, and recovers more steadily between throws.

The stone store is not always built the same way. In most home heaters the elements run among the stones. Some heavy-duty commercial heaters keep the stones in a separate store above or beside the elements, which keeps the airflow steadier and makes the elements easier to service. The principle is the same: the stones hold the heat that turns water into löyly.

Stones wear out. Heating, cooling and water slowly crack and crumble them, and fragments settle and block the airflow. Our sauna stones guide explains the stone types, loading and when to check and replace them.

Why the air settles in layers

Because hot air rises, every sauna has layers: hottest under the ceiling, coolest near the floor. The difference is large. In a 2026 study of four Finnish saunas in normal use, the average air temperature at sitting height was about 51 °C while at 1.73 m it was about 69 °C. Tests published by the Finnish site Saunologia found a difference of around 36 °C between the ceiling and foot level with a closed-body heater, and much less with a tall, open-mesh design.

Good sauna design works with this rather than against it:

• The ceiling stays modest, so the hottest air stays close to the bathers. Heater manuals give minimum ceiling heights for each model, often around 1,900 to 2,100 mm.
• The upper bench sits high. Harvia recommends about 1,100 to 1,200 mm between the upper bench and the ceiling, and Finnish guidance gives 1,000 to 1,200 mm.
• Feet stay above the stones. A long-standing Finnish rule of thumb, attributed to the ethnologist Sakari Pälsi and restated by the Finnish Sauna Society, places the bather’s feet above the level of the heater stones so the heat feels even from head to toe. It comes from naturally ventilated saunas, and heater design and ventilation can change the picture, so it is best treated as a design principle rather than a fixed number.

Our home sauna planning guide covers bench depths, tiers and room sizes in more detail.

Löyly: what happens when water meets hot stones

Löyly is the steam that rises when water is thrown onto hot sauna stones. UNESCO’s description of Finnish sauna culture defines it in just those terms, and to Finns the word also carries the spirit of the sauna itself.

Physically, three things happen in quick succession.

1. The stones turn the water into steam

Turning water into steam takes a great deal of energy. Heating a litre of water from 20 °C to boiling takes about 330 kJ; turning that boiling water into steam takes about 2,260 kJ more. That energy comes out of the stones. This is why stones need to be properly hot before you throw water, and why they need time to recover between throws. If they are not hot enough, water trickles through instead of flashing off.

2. The moisture in the air jumps, the temperature barely moves

A ladle of water changes the room’s humidity far more than its temperature. In a worked example by the Finnish HVAC engineer Unto Hakkarainen, one throw in a 20 m³ room at 80 °C lifts the air temperature by only about six degrees, while the moisture in the air roughly quadruples. Because steam is lighter than hot air, it rises to the ceiling first and then rolls down over the benches as a wave.

3. The steam condenses on your skin

The wave of heat you feel is mainly the steam condensing on your skin, which is cooler than the air around it. As water vapour condenses, it releases the energy it absorbed from the stones, straight onto you. A peer-reviewed study by Zech and colleagues in 2015 used labelled water to show that a large share of the water running off bathers after löyly was condensed steam, not sweat. The effect lasts only while your skin is below the air’s dew point, typically a few minutes, and it is gentler when water is added little and often.

How much water, and how often

Little and often is the rule, and manufacturers express it in different ways:

• Harvia limits the ladle. Its manuals set a maximum of 0.2 litres per throw. More than that does not all evaporate, and boiling water can splash out of the heater. Never throw water while someone is close to the heater.
• Another manufacturer limits by room size: about 100 ml per cubic metre of room, then around ten minutes for the stones to reheat.

These are different methods, not one rule, so follow the manual for your own heater. Either way, the reason is the same: each throw draws heat out of the stones, and stones that have not recovered let water trickle through instead of flashing into steam.

Where you throw matters too. On Harvia’s tower heaters such as the Cilindro, water thrown at the front of the stone column gives softer löyly, and water thrown on top gives sharper löyly.

Use clean tap water; manufacturers warn against seawater, chlorinated pool or spa water and water high in iron or lime, which damage stones and heaters. Scents should be ones made for saunas, diluted as their maker directs, and only where the heater manual allows.

Temperature, humidity and heat-up time

How hot a sauna “is” depends on where you measure. The control’s sensor sits high, in the hottest air near the ceiling, so the bench is cooler than the number on the panel.

Between throws, a traditional sauna is dry. The Finnish Sauna Society’s reference range works out at roughly 6 to 21 per cent relative humidity at those temperatures, rising briefly with each throw.

Heat-up time depends on the heater, the room and how cold it starts. Harvia gives about an hour for its Cilindro in a correctly sized, insulated room. Its general guidance is that a heater runs at full power for the first 30 to 40 minutes, then at roughly half power once the room is up to temperature. Another maker gives 45 to 90 minutes. Glass, tile and masonry, a cold outdoor room or an undersized heater all lengthen it.

Why heater sizing matters

Because a sauna heats by warming its air and its surfaces, the heater has to suit the room’s volume and construction, not just its floor area.

• Volume first. Each heater model is approved for a range of room volumes, from a minimum to a maximum, set out in its manual.
• Hard surfaces add volume. Glass, tile, brick and concrete absorb heat and lose it faster than insulated timber, so manufacturers add an allowance for each square metre. The allowance differs by brand: Harvia adds 1.2 m³ per square metre, and other makers use from 1 to 1.5 m³. Log walls can count for more again.
• Too small or too large both fail. A heater that is too small never reaches temperature. One that is too large reaches the set temperature before its stones are fully hot, so the löyly is poor.

The same 10 m³ room with 4 m² of glass works out at about 14.8 m³ by Harvia’s rule, and anywhere from 14 to 16 m³ by other makers’ rules, which can change the heater model. We size to the rule published for the heater being specified. Our heater sizing guide covers sizing, power supply and controls.

Where the sensor goes, and why

The control needs to know the temperature of the hottest air, so its sensor goes high, usually above or beside the heater near the ceiling. Harvia’s control manual, for example, places the sensor of a wall heater on the wall above it, on its centre line, 100 mm below the ceiling. It also needs to be well away from a fresh-air vent: Harvia keeps it at least 1,000 mm from an undirected supply vent (500 mm if the vent blows away from the sensor), and other makers set their own distances, because a stream of cool air makes the sensor read low and the heater overrun.

Not every heater follows the same rule. One maker whose heaters circulate the air with a fan says its sensor must not go above the heater at all. That is why the sensor is always positioned to the specified heater’s manual, never to a generic drawing. The sensor box often holds the overheat protector too, so its position is a safety matter as well as a comfort one. Our sauna controls guide explains why.

Infrared and hybrid rooms work differently

An infrared sauna has no stones and no löyly. Its emitters warm the people in the room directly, so the air stays cooler, typically around 40 to 65 °C. A hybrid sauna puts a stone heater and infrared emitters in the same room so it can be used either way. Our comparison guide sets the three side by side, and you can read more about hybrid saunas and infrared saunas.