Sizing a sauna heater
A traditional sauna heater warms the air and a load of stones to a room temperature of about 160 to 195 °F (70 to 90 °C). The power you need depends mainly on the volume of the room, and on how much heat its walls absorb or let escape.
The rule of thumb
Manufacturers commonly recommend about 1 kW per cubic meter (about 35 cubic feet) of sauna volume, for a room with well-insulated wooden walls. Surfaces that soak up or lose more heat count extra: for each square meter (10.8 sq ft) of glass, stone, brick, tile or uninsulated wall, add about 1.2 m³ (42 cubic feet) to the volume.
power (kW) ≈ room volume (m³) + 1.2 × area of glass or uninsulated walls (m²)
Worked example
A sauna 6 ft long, 5 ft wide and 7 ft high, with a 10 sq ft glass door:
| Result | Value |
|---|---|
| Room volume | 210 cu ft (5.9 m³) |
| Effective volume with the glass | 249.4 cu ft |
| Heater power needed | about 7.1 kW |
| Current at 240 V single-phase | 29.4 A |
| Current per phase at 208 V three-phase | 19.6 A |
You would choose the next size up in the manufacturer’s range, such as an 8 kW model, checking that the room fits within its stated minimum and maximum volumes.
Why size matters
- Too small: the sauna heats slowly, may never reach a comfortable temperature and the heater runs constantly.
- Too large: the room heats fast but the thermostat cycles on and off often; the stones may not store enough heat for good steam (löyly) when you throw water.
- A well-sized heater brings the room up to temperature in about 30 to 60 minutes.
Electrical supply
Sauna heaters draw high currents and need a dedicated circuit. In North America, residential heaters usually run on 240 V single-phase; larger models may use 208 V three-phase in commercial buildings. Wire size, breaker and any required contactor must follow the manufacturer’s instructions and the National Electrical Code, and the installation should be done by a licensed electrician.
Tips
- Insulate walls and ceiling well and use a vapor barrier: it lets a smaller heater do the job and saves energy.
- Keep ceiling height to about 7 ft: hot air rises, and extra height means extra volume to heat.
- Wood-burning heaters follow similar volume guidelines but need proper chimney clearances.
- Infrared saunas work differently, heating the body directly; this rule does not apply to them.