By the internal volume first, then adjusted for what the surfaces are made of. Steam has to raise and hold the temperature of every surface inside, not just the air, so a heavier lining absorbs far more before conditions stabilise.

Stone, thick tile and large glass areas all draw heat away and push the required output up; an acrylic cabin of the same volume needs noticeably less. Exposed external walls and a cold adjacent room raise it again.

Undersizing shows as a cabin that takes a long time to become usable and never quite gets there. Oversizing wastes energy and cycles the generator hard, so the calculation is worth doing rather than choosing by cabin footprint.

So condensate runs to the walls rather than falling on the occupant. Steam condenses on every cool surface, and the ceiling is the largest one directly overhead.

A flat ceiling collects droplets until they drop, which is uncomfortable and, in a public installation, a genuine complaint. A modest fall of a few degrees is enough to move the water sideways to the wall, where it joins the wash-down and reaches the drain.

This is one of the details that separates a purpose-built steam cabin from a shower enclosure with a generator added, and it is very hard to retrofit.

A continuous seal around every opening, and a door that closes against a stop on all four sides rather than draining into a gap at the bottom. A standard shower enclosure deliberately leaves ventilation gaps; a steam cabin cannot.

Glazing is set into gasketed channels, and any service penetration for lighting, controls or the steam head is sealed as a vapour barrier rather than just a water seal.

The consequence of a poor seal is not only lost performance. Vapour escaping into the surrounding bathroom condenses on cold surfaces there, which over time damages finishes well beyond the cabin.

Outside the cabin, in a ventilated space within a set pipe run of the steam head, commonly in an adjacent cupboard, a void or a plant area.

Distance matters because steam condenses in the pipe, so the run is kept short, insulated and falling back toward the generator or forward to the head, never sagging in between where condensate can collect and cause water hammer.

It also needs service access, a drain for the flush cycle and a power supply. Siting it somewhere that satisfies the pipe run but not the maintenance access is a frequent and expensive oversight.

In hard water areas it usually does. The generator boils water continuously, and everything dissolved in that water stays behind as scale on the heating element and the tank walls.

Scale insulates the element, which reduces output and shortens its life, and it eventually interferes with the level sensing. Most generators include an automatic drain and flush cycle to limit accumulation.

Where hardness is high, softening or scale-inhibiting treatment on the feed is cheaper than the descaling regime it replaces, and it is far cheaper than the element failures it prevents.

Temperature setpoint, session duration and a way to stop immediately from inside. Session timers matter for safety as much as convenience, since a steam room is not somewhere to fall asleep.

Controls are sealed for the environment and usually mounted so they can be reached from a seated position. Many systems place the main control outside and a simple stop control inside.

Additional functions such as aroma dosing, chromotherapy lighting and audio are common, and each adds a penetration through the vapour seal that has to be detailed properly.

Rarely with good results. The seal, the sloped ceiling, the bench, the drainage of condensate and the pipe run to a generator are all things a standard enclosure was built without.

A conversion typically leaks vapour into the bathroom, condenses on the flat ceiling and drips, and takes far longer to reach temperature than the generator rating suggests it should.

Where the room allows it, a purpose-made cabin or a properly detailed built-in steam room is the better route. The generator is the cheap part of the installation; the enclosure is what makes it work.