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Size matters: how powerful stove do you need?

You will face many difficult choices when constructing a sauna. Once you have decided on an electric or wood-heated stove (or would like to install both) and know if you prefer a net stove or a conventional stove with air flowing through, it is time to calculate precisely how powerful your stove must be.

Unfortunately, it is common that people underestimate this task. We often hear a misconception that “a big stove makes a good sauna”. This is not always the case. Saunas are different and these particularities must be taken seriously. When choosing a stove, you must think about both the non-insulated surfaces and room size. More cubic meters require more power, as does heating up non-insulated surfaces (e.g. glass doors, stone or log walls) and making up for lost heat.

This article will guide you through the process of stove dimensions. We will help you choose a stove of the right size. Instead of doing the thinking yourself (see below), you can use a sauna calculator. Feel free to access the HUUM calculator here.

 

What is considered to be sufficiently insulated?

The following sauna wall and ceiling construction is considered
to be sufficiently insulated:

  • a carefully installed insulation wool layer of 100 mm (min. 50 mm);
  • the construction is damp proofed with taped aluminium paper or
    other reflective material;
  • there is a 10 mm air gap between the damp protection and
    boarding;
  • a light wooden board that is about 12–16 mm thick has been used
    for interior finishing;
  • there is an air gap of at least 5 mm on the edge of the ceiling
    panels at the upper part of the wall panelling.

 

In order to use the heater with a regular output, it would be practical
to bring the ceiling of the sauna lower (regular 2100 mm – 2300 mm,
min. height of sauna room 1900 mm) so that the volume of the sauna
room diminishes. The ceiling is insulated with an insulation layer at
least 100 mm thick and boarded according to the method described
above.
Wood should be used to cover internal surfaces; an exception maybe
made for heat resistant walls in the vicinity of the heater.

 

Calculating the volume of your steam room

The calculated volume (VC) of a steam room is calculated by adding the additional volume from the non-insulated surface area (VS) to the room cubature (VR). One square meter of non-insulated surface (S) equals 1,2–1,5 cubic meters, depending on the material. A glass door requires less power to heat up than a log wall.

 

 Non-insulated surface Corresponding index
 Glass 1,2
 Log wall or other non-insulated surfaces 1,5

Let’s walk through this process together, step by step. To calculate the real cubature of your steam room, just multiply its width (a) with its depth (b) and height (h).

VR = a × b × h

To calculate the area of the non-insulated surface (S), you must multiply the edges of each non-insulated surface.

S = a × b

The next step is to multiply the area of each non-insulated surface (S) with the specific multiplier (k). If your steam room has several non-insulated surfaces, you must repeat this equation as many times as needed and add the results together. This gives you the volume of non-insulated surfaces (VS).

VS = S × k

Finally, to find out the calculated volume (VC) of your steam room, you must add the volume of non-insulated surfaces (VS) to the real cubature of the room (VR). The number equals the power of the stove.

VC = VS + VR

Choose a stove with the most approximate power capacity.

1 calculated m3 = 1 kW

 

For example, if your steam room is 2 meters long, 1,5 meters wide and 2,3 meters high, you will get the real cubature by multiplying these numbers.

VR = 2 × 1,5 × 2,3 = 6,9 m3

Suppose that your sweat room has a 1,5 meter wide and 2,3 meters high glass surface (including a glass door). In this case, we will have to multiply these to get the non-insulated surface area.

S = 1,5 × 2,3 = 3,45 m2

Because glass surfaces have a corresponding index (k) of 1,2, we must multiply this result by 1,2 to get the volume of non-insulated surfaces (VS).

VS = 3,45 × 1,2 = 4,14 m3

Finally, you must add the volume of non-insulated surfaces (VS) to the real room cubature (VR) to end up with the calculated volume (VC) of your steam room.

VC = 6,9 + 4,14 = 11,04 m3

As you can see, the calculated volume of your steam room (11,04) is considerably larger than the real cubature (6,9). Only the former should be considered when buying a stove. Roughly 40% is added to the real cubature by non-insulated surfaces and consequently, also the stove must provide 40% more power. In this case, a 9 kW HUUM DROP stove suits your needs.

 

Wood burning stove

Once you have your calculations, you should compare them with the data offered by the stove manufacturer. Albeit size matters, it’s always good to keep in mind that a big stove does not necessarily make a good sauna. If the stove is too powerful, the room heats up quickly but leaves the stones cool. You don’t want to sit in a steam room of 100 °C (212 °F) while stones are still cold and unable to give off steam. This is particularly true with net stoves which have many stones on the stove. Be that as it may, you are better off choosing a small and less powerful stove. Heating the sauna will take longer, of course, but it will be worthwhile.

Electric stove

Before choosing a particular electric stove, you should pay close attention to the cross-section of the feeder cable. This determines the maximum voltage of your stove. If you cannot install a stove powerful enough for your steam room, opt for a large stove that has many stones. The more stones, the more energy is stored, and the more energy is stored, the more your stones will give off heat. This simple trick will compensate for insufficient power in your stove. Heating your sauna will take longer yet the sauna experience will be more satisfactory.

 

The power of your sauna heater is definitely important when you are building a sauna. There are many other aspects as well to get a great sauna experience. Ventilation is an important but often neglected part of building a sauna. Read next about correct Ventilation.

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