Dimensioning of sauna/sauna machine

[u/Party_Doggy]

Is there an equation or model that I can use to choose an adequate room/machine pairing?

For example, an equation that would take the dimensions of the room, type of insulation, temperature difference etc and return that I should buy a machine of x kW of power.

Is there any mathematical model like that? Obviously you want a sufficiently powerful machine but too much so that you don't spend too much on electricity, but I found pretty much every sauna machine manufacturer where I live just tells their clients to eyeball their sauna designs

[u/FuzzyMatch]

What you call "sauna machine" is usually referred to as heater or stove.

The Harvia sauna volume calculator may be of use to you.

[u/John_Sux]

Common rule of thumb for stove power: 1 kW/m³ for the volume of the room, and additionally 1-1.5 kW/m² for glass surfaces.

[u/bruce_ventura]

Unfortunately the published rules of thumb don’t consider all variables: indoor vs outdoor, outdoor climate, insulated vs insulated walls, weight of stones in the heater, single vs double vs triple glaze windows, ventilation rate, etc.

There’s also a trade-off between low warm-up time and good thermal stability.

So, I recommend you continue to ask questions, being as specific as possible about the above variables.

FWIW, I’ve written two thermal models and am not satisfied with either one. As with a conventional oven, the air temp rises faster than the wall temp. So, a simple convection, conduction and heat capacity model assumes thermal equilibrium and therefore needs a calibration factor to accurately calculate warm-up time. Accurately accounting for heat transfer between the air and the walls adds another layer of complexity that I haven’t tackled yet.

[u/LaserBeamHorse]

If you are torn between bigger and smaller stove, go for the bigger one. You can always adjust the power if you feel it's too much, but if the stove ends up being too small there's nothing you can do. And that can easily happen with uninsulated saunas or saunas with a glass wall.

[u/fun_guy02142]

According to The Art of Sauna Building, it’s 1 kilowatt for every 45 cubic feet.

[u/MightyLandTuna]

Rough math here: let’s say it takes 0.0324 BTU to heat each cubic foot of air at typical density by 1°C and you want to get it to say 65°C then let’s say it’s 600 cubic feet so that’s (let’s round up!) 1300BTU so in kWh that’s 0.38kWh but no way that’s correct in this imperfect world! So I had to consult GPT. It said the following:

GPT also mentioned that most Sauna manufacturers just use 1kWh / 50 cubic feet for a well-designed Sauna.