How Heater Classes Are Estimated

Every profile lists an approximate heater class in kilowatts (for electric) or as a stove with a stone-capacity preference (for wood). Here is the reasoning behind those numbers — and why they are deliberately a band, not a single value.

Start with volume

The dominant factor is the room’s air volume. A widely used starting point for a well-insulated sauna is roughly 1 kW of heater per cubic metre of interior volume. A 7 m³ room, then, begins around 7 kW. The archive lists each profile’s estimated internal volume so you can see where its class comes from.

Adjust for glazing

Glass is the single biggest reason to size up. A glazed wall loses far more heat than an insulated one and effectively adds to the volume the heater must serve. This is why the archive’s glass and heavily glazed profiles sit at the top of the range or above it, while minimally glazed, high-performance rooms sit comfortably at the bottom.

Adjust for uninsulated surfaces and cold exposure

Any large uninsulated or high-mass surface — a masonry wall, a big cold slab, a log wall — draws heat and pushes the class up. So does cold-climate exposure: a sauna that must warm from deep sub-zero temperatures, or that sits in wind, needs more margin than one in a mild backyard.

Stones and the quality of the heat

For wood stoves, the archive notes a stone-capacity preference rather than a kW figure. More stones store more heat and give a softer, longer-lasting steam (löyly), at the cost of a longer warm-up. A small stone load heats fast but gives a sharper, shorter steam. This is a comfort choice as much as a power one.

Why bigger is not always better

An oversized heater can drive the air temperature up before the stones and surfaces are properly hot, producing a harsh, dry heat and a room that feels hot but bathes poorly. Right-sizing — not over-sizing — is the goal.

From a band to a decision

The archive’s ranges are there to help you compare profiles and understand the trade-offs, not to buy a heater. Once you have chosen a configuration and finalised its glazing, insulation and site, use the heater manufacturer’s sizing tool and confirm the electrical or fire-code requirements with a qualified professional. Final heater selection must follow that documentation.