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Stove sizing – how to choose the right stove for your sauna
The right stove heats the sauna and its stones to suit how you use it. Besides volume, the choice is affected by glass surfaces, structures, how you use the sauna and the benches.
In brief
Calculate the sauna volume (width × depth × height) and add one cubic metre for each square metre to be taken into account. Then choose the stove whose volume range the result falls within: Iivari 9–18 m³, Gösta 18–35 m³ or Original 20–40 m³. Choose the structure, Kaski, Punos or Havu, after the model.
Selection route
Choosing a stove follows a clear sequence. Each step narrows the options, and the installation requirements are checked last.
- Calculate the calculated volume of the sauna.
- Find the stove volume range that matches that volume.
- Consider how you use the sauna and the steam you prefer: structure and stone mass.
- Plan the benches and air circulation.
- Check the installation requirements.
Sauna volume and structures
Room volume is the space inside the sauna room. The calculated volume is the figure the stove is sized by. It also takes into account how much heat the structures absorb or let out.
Base volume
Base volume = inside width × inside depth × inside height. Measure the inside dimensions in metres. The space beneath the benches is part of the volume. A sloping ceiling can be divided into sections, with each section's volume calculated and the results added together.
Why the structures matter
The stove heats the air as well as the stones and the sauna structures. Glass walls, glass doors and uninsulated stone, brick and concrete surfaces affect the heating need. The mass has to be heated, and some of the heat leaves the sauna through the structures or the ventilation. These are different effects, but both increase the need for a larger stove.
JUUP's calculation method
JUUP's stove calculator uses the same method as this guide:
- Typical sauna: base volume + 1 m³ for each square metre to be taken into account.
- Bare solid log sauna: base volume × 1.5 + the allowance for any other surfaces to be taken into account. Log walls are not added again as a surface allowance.
- An insulated, panelled log building is not automatically treated as a bare solid log sauna.
- A standard tiled floor is not automatically treated as an uninsulated stone wall.
This is an indicative sizing. Manufacturers' coefficients may differ, so the figures are not directly comparable. Extensive external glazing and unusual structures are assessed separately.
Calculation examples
| Sauna | Base volume | Addition | Calculated volume | Model range |
|---|---|---|---|---|
| Insulated sauna 2 × 2 × 2.4 m | 9.6 m³ | Glass 2 m² → +2 m³ | 11.6 m³ | Iivari |
| Insulated sauna 2.5 × 3 × 2.4 m | 18 m³ | Glass 6 m² → +6 m³ | 24 m³ | Gösta or Original |
| Solid log sauna 2.5 × 2.5 × 2.4 m | 15 m³ | 15 × 1.5 + glass 2 m² (+2 m³) | 24.5 m³ | Gösta or Original |
If the result falls within the range of two models, compare both and consider how you use the sauna. You can also check your own figures in the stove calculator.
Output, stove size and stone mass
When choosing a stove, it helps to separate three things that are often confused:
- Heating output describes how quickly heat is produced or transferred.
- Stones store the energy needed to turn löyly water into steam.
- The sauna volume stated by the manufacturer describes the size of sauna the model is intended for.
The rule of thumb of about 1 kW/m³ for electric stoves cannot be applied as it is to wood-burning stoves. For a wood-burning stove, the heat depends on the firewood, the size of the logs, the draught of the chimney and the structure of the stove. Output figures stated in different ways should not be compared directly. The most reliable starting point is the sauna volume stated by the manufacturer.
A stove that is too small
A stove that is too small can heat the sauna slowly or leave it short of the target temperature, and the stones cool down during löyly. Continuous heating that is too strong also puts a load on the stove.
A stove that is too large
Oversizing is not the goal. With an electric stove in particular, the air can heat up before the stones have heated sufficiently. Always respect the minimum volume of the model too. Near the upper limit, assess how you use the sauna and the starting temperature. A larger model is suitable only if the sauna also falls within its operating range.
A generous stone mass
A generous stone mass stores heat, but it also needs energy to heat up. Extra stones do not increase the output of the stove or the permitted sauna volume. Stack the stones according to the stove instructions.
The feel of the löyly depends on the temperature of the stones, the amount of water, where the water is poured and how quickly it turns to steam. A large stone mass alone does not guarantee a soft löyly, and a room thermometer alone does not show whether the stones are ready for löyly.
Upper bench about 110–120 cm from the ceiling
The distance between the seating surface of the upper bench and the ceiling is recommended to be about 110–120 cm.
Benches are planned from the ceiling downwards, because the warmest air collects at the top. A fixed height from the floor alone is not enough, since the interior height of a sauna varies.
| Inside height | Upper bench (seating surface) |
|---|---|
| 220 cm | about 100–110 cm from the floor |
| 240 cm | about 120–130 cm from the floor |
| 260 cm | about 140–150 cm from the floor |
The lower bench is often about 45 cm below the seating level of the upper bench. The final dimensions take into account the users, ergonomics, a safe way up and the shape of the ceiling.
The distance between the upper bench and the ceiling is a different matter from the safety clearance between the stove and the ceiling. Bench guidance does not replace the safety clearances in the installation instructions.
The löyly law
According to a traditional design principle, a bather's feet should ideally be at or above the top surface of the sauna stones. Then the feet are also in the warm air and the löyly zone.
The reason is simple: warm air rises and cooler air stays low. That is why the head can be hot while the feet are cold.
The löyly law is a design principle, not a limit below which heat or moisture cannot reach. Air movement, the stove, ventilation, the heating time and the shape of the sauna all have an effect.
Combine the principle with the 110–120 cm distance to the ceiling. If a tall stove and the benches do not fit into the same overall plan, the plan is assessed separately. The seating level should not be raised so close to the ceiling that there is no room to sit upright.
JUUP's natural air circulation
JUUP's structure guides the cooler air in the sauna room to the lower part of the stove, where it warms and rises back into the sauna. Working air circulation can reduce the temperature difference between the upper and lower parts of the sauna and ease the stratification of heat.
This is natural convection: air warms on the surfaces outside the firebox and along the air circulation routes in the structure. The effect also depends on the structure of the sauna, the benches and the ventilation.
Air circulation does not remove the need for good bench sizing. The height of the lower bench and the distance of the upper bench to the ceiling still determine the temperature at which you sit. Even temperatures do not by themselves show how the moisture of the löyly is distributed.
The mixing of air in a sauna has been studied in a VTT publication from 1992 that examines sauna temperatures and ventilation. That study concerned mechanical exhaust ventilation, not the performance of JUUP stoves.
You can read more about the double stone chamber and air circulation on the Löyly page.
Ventilation and operating conditions
Keep three things apart:
- Air circulation: air moves inside the sauna.
- Ventilation: replacement air comes in and used air leaves.
- Combustion air: the air the stove needs for burning.
Air circulation does not replace ventilation. There is no single vent placement instruction that suits every sauna. Follow the installation instructions for the model and the ventilation plan for the site.
When the calculator is not enough
In these situations an overall assessment is needed in addition to the calculator:
- a cottage sauna heated from cold
- extensive external glazing
- a high or sloping ceiling
- a sauna in a loft
- long periods of use and professional use
Installation requirements
Once you have chosen a model, check the safety clearances, the chimney connection and the ventilation in the installation instructions for that model. The instructions are on the Instructions page.
Choosing a JUUP model
Choose the stove model by the sauna volume first, then the structure. A Havu Iivari with its larger stone mass does not replace a Gösta in a sauna that falls outside the Iivari range.
| Model | Sauna volume | Description | More information |
|---|---|---|---|
| JUUP Iivari | 9–18 m³ | A compact wood-burning stove for small and medium-sized saunas. Dual stone chamber structure. | View model |
| JUUP Gösta | 18–35 m³ | For larger saunas, with a dual stone chamber structure and a generous stone mass. | View model |
| JUUP Original | 20–40 m³ | A deep stone bowl and the original JUUP structure. No dual stone chamber and no mesh structure. | View model |
Kaski, Punos and Havu
Iivari and Gösta are available in three structures. The structure affects the appearance, the stone mass and the character of the löyly, not the volume range of the model.
Kaski
Fully clad, calmly simplified structure. Water is poured into the stone bowl for löyly.
Punos
The upper part has a mesh structure, so more stones are visible. Water is poured into the stone bowl for löyly.
Havu
The mesh version with the largest stone mass. The character of the löyly varies with where the water is poured.
The Original has its own structure: a deep stone bowl and the original JUUP structure. It has no Kaski, Punos or Havu options.
The character of the löyly also depends on where the water is poured. With Havu, water poured into the inner stone bowl usually gives a softer and more humid löyly, while water poured onto the outer stones gives a hotter and sharper one. Exact stone quantities are on the model pages.
When comparing stoves from different manufacturers
- Compare the sauna volumes stated by manufacturers, not just their outputs.
- Ask how glass, stone and log surfaces are taken into account in the calculation.
- Ask what a stated output means for a wood-burning stove and which volume it is intended for.
- Read the instructions on stone mass and stone stacking, and check their limits.
- Check the safety clearances and the chimney connection for your own sauna.
Frequently asked questions
What size stove does a 12 m³ sauna need?
It depends on whether 12 m³ is the room volume or the calculated volume. If the insulated sauna has 4 m² of glass, the calculated volume using JUUP's method is 12 m³ + 4 m³ = 16 m³. That falls within the Iivari range (9–18 m³) but close to its upper limit, so consider how you use the sauna and the starting temperature. Without glass the volume is 12 m³, which also falls within the Iivari range. The Gösta range starts at 18 m³, so it does not suit this sauna.
How does a glass wall or a solid log structure affect sizing?
In a typical sauna, add 1 m³ for each square metre of surface to be taken into account, such as a glass wall or glass door. In a bare solid log sauna, multiply the base volume by 1.5 and add the allowance for any other surfaces to be taken into account. An insulated, panelled log building is not automatically treated as a bare solid log sauna.
Can a stove be too powerful?
Yes. Oversizing is not the goal. With an electric stove in particular, the air can heat up before the stones have heated sufficiently. With a wood-burning stove, a larger model is only suitable if the sauna also falls within its operating range.
Does a larger stone mass make up for a higher output?
No. A generous stone mass stores heat, but it also needs energy to heat up. Extra stones do not increase the output of the stove or the permitted sauna volume. Stack the stones according to the stove instructions.
How far should the upper bench be from the ceiling?
The distance between the seating surface of the upper bench and the ceiling is recommended to be about 110–120 cm. Benches are planned from the ceiling downwards, because the warmest air collects at the top.
What is the löyly law?
The löyly law is a traditional design principle: a bather's feet should ideally be at or above the top surface of the sauna stones, so that the feet are also in the warm air and löyly zone.
Why do my feet feel cold?
Warm air rises and cooler air stays low, so your head can be hot while your feet are cool. Factors include the height of the benches, the position of the stove and stones, ventilation and the heating time.
Can I ignore bench sizing when I have a JUUP stove?
No. JUUP's air circulation can reduce the stratification of heat, but it does not remove the need for good bench sizing.
Contact and links
If the sizing remains unclear, send the following to us or to your nearest dealer:
- the inside dimensions of the sauna
- the glass surfaces
- the wall structures
- whether the sauna is used in winter
- a photo or drawing of the benches and the stove location
See also: Iivari, Gösta, Original, stove calculator, Löyly and Instructions.
Sources
This guide is based on JUUP's current range, the stove calculator and the valid product instructions, as well as the following sources:
- Bench design: Finnish Sauna Society, Lauteiden suunnittelu – lyhyt oppimäärä (in Finnish).
- Mixing of air: VTT 1992, Saunan lämpötilat ja ilmanvaihto (in Finnish). The study concerned mechanical exhaust ventilation, not the performance of JUUP stoves.
