How big a sauna heater do you need?

A heater is matched to the air it has to warm, so the first number is the room's volume: floor area times ceiling height, in cubic feet. Two rooms that seat the same four people can need different heaters if one has a taller ceiling or a wall of glass. Our size guide covers the seating side; this one is only about the kilowatts.

This is the table we size from on our own drawings. It gives a band of heater sizes for each volume, smallest to largest:

Which end of the band you land on is decided by windows. A room with no windows takes the smaller heater in its band. One window moves it to the larger. Two or more windows move it one step up our ladder of heaters, which runs 3, 4.5, 6, 8, 9, 10.5, 12, 15, 18 and 20 kW.

The heater makers publish their own method, and it works differently: instead of counting windows, it counts glass area and adds it to the volume.

HUUM's rule, as they print it, is one kilowatt per 35 cubic feet of calculated volume, where every square foot of glass or other uninsulated surface adds 3.3 cubic feet. Their own worked example: a room of 7 x 6 x 7 feet is 294 cubic feet; a 42 square foot glass wall adds 42 x 3.3 = 139 cubic feet; the heater is sized to 433 cubic feet, not 294.

Harvia says the same thing in metric: one kilowatt per cubic meter of room, and each square meter of stone, glass or similar uninsulated surface counts as if the room were 1.2 cubic meters larger. A log-walled sauna uses a factor of 1.5. Since a cubic meter is about 35.3 cubic feet, that converts to roughly 1 kW per 35 cubic feet, with each square foot of glass adding about 3.9 cubic feet. Neither asks how many windows there are.

Harvia adds two cautions we repeat to every client. The figure their calculator gives "is, in most cases, the minimum required power". And a heater that is too small "needs to be heated longer and more intensely, which reduces its life span."

We never average the two answers. Each is a real method with a real reason behind it, and a number halfway between them belongs to neither.

The case where they part company is almost always the same one. Our table asks how many windows a room has, and a frameless glass door is not a window. So on a room whose whole front is glass, the table sees no windows and gives the band minimum, while the maker's rule adds every square foot of that door to the volume.

We have had exactly this on one of our own drawings: a 451 cubic foot room where the table says 9 kW, and the glass door pushed the manufacturer's answer to 10.5-12 kW.

Harvia also warns against buying at the edge of a heater's rated range: for a 9.5 cubic meter room, a heater rated for 6-10 cubic meters "is not the best choice"; take the next model up, rated for 8-14. The same reading applies to any maker's chart. Finnleo lists its Designer at 100-425 cubic feet, Himalaya at 175-500, Laava at 390-950 and Magma at 720-2,000; a room near the top of one range is a room for the next.

Both rules assume a properly insulated room. HUUM spells out what that means: an insulation layer of 100 mm (4 in), minimum 50 mm (2 in), taped aluminum foil as the moisture barrier, at least a 10 mm (0.4 in) air gap between the foil and the wood cladding, and 100 mm in the ceiling. A room converted without that layer asks more of its heater than either rule predicts; our room conversion guide covers the shell.

The heaters on our recent estimates include the Harvia Spirit SP80E at 8 kW, Harvia 6 kW units (KIP60W and SP60E), the HUUM STEEL at 10.5 kW, Finnleo Laava heaters at 10.5 and 14.4 kW fitted as replacements, a Finnleo 4.5 kW and a Homecraft 5 kW. The electric heaters we fit are in the shop, and our equipment installation service covers sizing, supply and the hookup as one job.

Once the kilowatts are fixed, the heater's circuit follows from them, and that is licensed electrical work with its own permit and inspection in nearly every Bay Area city. We do not print breaker sizes here, because the right one depends on the heater's manual and your wire run. Our 240V guide explains the hookup, the running cost guide shows what those kilowatts cost to run, and the planner sizes the heater by both rules as you draw your own room.