How to pick the size
- Measure the inside of the hot room: length × width × height gives the volume in m³ (1 m³ ≈ 35.3 ft³).
- Add the cold surfaces. Glass, tile, stone and concrete soak up heat. Harvia's manual, which most planners follow, adds 1.2 m³ for every m² of such surface (a glass door counts) and multiplies the volume of a solid log sauna by 1.5. The result is the room's effective volume.
- Pick a heater whose maker's range includes it, ideally near the middle. At the top of its range a heater works hard and heats slowly; at the bottom it cycles on and off.
The heater size calculator does all three steps and lists the heaters that fit.
Power supply
Small heaters (up to about 3.6 kW) run on a single-phase household circuit; most heaters from 6 kW up are connected to three-phase 400 V in continental Europe and to a 240 V circuit in North America. Each heater page shows the current it draws on your supply and the fuse or breaker the maker states. A licensed electrician connects the heater and sizes the circuit from its manual; the electrical requirements tool tells you what to ask for.
Stones and steam
Heaters range from compact wall units with about 10–25 kg of stones to towers holding 100–200 kg. A larger stone mass takes longer to heat, then stores heat and gives a softer, longer steam when you throw water (löyly). Use the stone size the manual specifies and re-stack the stones at least once a year, replacing crumbled ones.
Controls
Some heaters have their thermostat and timer built in; others need a separate control unit, often with Wi-Fi and an app. Remote start is only allowed with the safety devices the maker specifies, such as a door sensor.