The short answer
A steam generator has to do two jobs: fill the room with steam, and keep up with the steam that condenses on every cool surface. Room volume describes the first job. The second depends on what the room is built from, how much glass it has, whether any walls face outside, how tall the ceiling is, whether the room is ventilated and how long it runs.
- Start with volume: internal length × width × height, measured after the linings.
- Then correct for the build-up. Heavy finishes, glass, outside walls, tall ceilings and ventilation all increase the output needed.
- Starting from cold matters. A home steam room heats from cold for every session; a commercial room may run all day. That changes how much the surfaces cost you.
- For a Harvia generator, Harvia’s tables govern. Other makers use other methods and their factors differ, so there is no “industry factor”.
- Bigger is not automatically better. Harvia publishes a minimum room volume for each model.
Volume is the starting point
Measure the room as it will be finished: the internal length, width and height after the board, membrane, bed and tile are in place. A room framed at 2.0 × 1.8 m can lose several centimetres each way once it is lined, and the ceiling may drop further if it is sloped to shed condensate.
For a vaulted or sloped ceiling, count the actual air volume, including the space under the vault. Harvia’s sizing tables are expressed in cubic metres, so that figure is what you take to them.
That number tells you how much air the generator has to bring to temperature and saturate. It says nothing yet about how much of the generator’s output the room will absorb before the air ever gets warm, and that is the part that separates a good result from a disappointing one.
What the generator actually has to heat
A steam room runs at about 40–50 °C with the air at or near saturation. The generator, outside the room, boils water and pipes steam to an outlet low in the room. A sensor cycles the generator to hold the set temperature.
When the room starts from cold, the air warms quickly, but the surfaces do not. Steam meeting a surface cooler than its dew point condenses and gives up its heat to that surface. Until the walls, ceiling, floor and benches warm up, much of the generator’s output goes into them rather than the air. Once they are warm, the generator still has to replace whatever heat the room keeps losing through the walls, the glass and any air that is exhausted.
So two rooms of identical volume can need quite different generators, because the amount of material to heat, and the rate at which it loses heat, can be very different.
The factors that change the answer, and why
| Factor | What it does | Why it matters for sizing |
|---|---|---|
| Thermal mass: stone, concrete, masonry, thick mortar beds, tile on masonry | Absorbs a large amount of heat before its surface reaches room temperature | Steam condenses on the cold surface instead of warming the air, so heat-up is slow and the generator works at full output for longer |
| Light, insulated construction: tile on insulated foam-core board, acrylic | Presents a thin layer to the steam, with insulation behind it | Warms quickly and loses little heat, so less output is needed |
| Glass: doors, screens, windows | Conducts heat readily and stays cooler than the room | Keeps condensing steam for the whole session, not only at start-up |
| Outside walls and poor insulation | Exposed to outdoor conditions or unconditioned space | The inner face stays cooler and keeps losing heat after the room is up to temperature |
| Ceiling height and shape | Steam and warm air rise and stratify | A tall room keeps the hottest, wettest layer above bathers’ heads, so it feels cool at bench level and needs more output |
| Ventilation during use | Exhausts saturated air and replaces it with cooler air | Removes heat and moisture continuously, so the same generator suits a smaller room |
| Steam pipe length | A long run loses heat on the way to the room | Less of the generator’s output reaches the room. One commercial maker counts extra pipe length as extra room volume |
All of the manufacturers we have reviewed agree on the direction of these effects. They disagree, often widely, on how large each correction should be. Ceiling guidance is an example: some makers design around a ceiling of roughly 2.4–2.5 m and add output above that, while others build height into their own tables. The selected generator’s documentation tells you how it treats each factor.
Starting from cold, or running all day
How the room is used changes how much the surfaces cost you.
A home steam room is usually switched on for a session and starts from cold each time. Every session, the generator has to warm the walls, benches and floor before the air reaches temperature. Heavy finishes and glass slow that down, which is why a home room built in stone feels slow to come up if the generator was chosen on volume alone.
A commercial room may run for many hours a day. Once its surfaces are warm, it no longer pays the start-up cost each session, but it still loses heat through glass, outside walls and ventilation, and it recovers after every door opening.
Makers read this differently. One North American manufacturer argues that the finish correction exists mainly because home rooms start from cold, and that a room in continuous commercial use needs little or none of it. Harvia’s tables for its commercial HGP220XW still give a smaller room volume for tiled stone walls than for light walls, so with a Harvia generator the wall type is always part of the selection. The selected generator’s documentation governs.
Harvia’s published figures
Harvia publishes room-volume ranges for its HGX XW and HGP220XW steam generators by three wall types, each with and without ventilation. The figures below are taken from Harvia’s official data sheets and apply only to these models.
Room volume range (m³), room not ventilated
| Model | Output | Steam output | Light wall, such as acrylic | Tile on a light wall | Tile on a stone wall |
|---|---|---|---|---|---|
| HGX45XW | 4.5 kW | 5.5 kg/h | 2.0–7.0 | 2.0–6.0 | 2.0–4.5 |
| HGX60XW | 6 kW | 7.6 kg/h | 3.5–11.0 | 3.0–9.0 | 2.0–7.5 |
| HGX90XW | 9 kW | 12 kg/h | 9.0–17.0 | 7.5–14.0 | 6.0–11.5 |
| HGX110XW | 10.8 kW | 14.6 kg/h | 15.0–21.0 | 12.0–17.0 | 10.0–14.0 |
| HGX150XW | 15 kW | 20.1 kg/h | 17.0–28.0 | 14.0–23.0 | 12.0–18.5 |
| HGP220XW | 21.6 kW | 29.2 kg/h | 30.0–42.0 | 24.0–34.0 | 20.0–28.0 |
The effect of ventilation on one model (HGX90XW, 9 kW)
| Wall type | Not ventilated | Ventilated |
|---|---|---|
| Light wall, such as acrylic | 9.0–17.0 | 6.0–12.0 |
| Tile on a light wall | 7.5–14.0 | 4.5–10.0 |
| Tile on a stone wall | 6.0–11.5 | 3.0–8.0 |
Two patterns stand out. Moving from a light wall to tiled stone cuts the largest room this 9 kW model suits by roughly a third, and adding ventilation cuts it further. In the most demanding case, the same generator suits a room less than half the size it suits in the easiest case. Note also the minimum volumes: in a light-walled, unventilated room, this model is not intended for anything under 9 m³.
Source: Harvia HGX XW data sheet (print date 17 February 2026) and HGP220XW data sheet (18 June 2025). Always use the current data sheet and installation manual for the model being specified.
Same room, different generators
Take a room 2.0 m long, 1.8 m wide and 2.3 m high: about 8.3 m³. Reading Harvia’s published ranges above, the model whose range includes that volume changes with the construction:
| Construction of the same 8.3 m³ room | Harvia model whose published range fits |
|---|---|
| Light wall, not ventilated | HGX60XW (6 kW) |
| Tile on a light wall, not ventilated | HGX60XW (6 kW) or HGX90XW (9 kW) |
| Tile on a stone wall, not ventilated | HGX90XW (9 kW) |
| Light wall, ventilated | HGX90XW (9 kW) |
| Tile on a light wall, ventilated | HGX90XW (9 kW) or HGX110XW (10.8 kW) |
| Tile on a stone wall, ventilated | HGX110XW (10.8 kW) or HGX150XW (15 kW) |
Where a room sits near the top of one model’s range and the bottom of the next, as in the second and last two rows, the choice depends on details such as glass area, outside walls, ceiling form, pipe length and how the room will be used. This is an illustration of Harvia’s tables, not a sizing method for other brands.
Other manufacturers use other methods
Harvia’s wall-type tables are one approach. Others include:
- A formula with factors. At least one European manufacturer multiplies room volume by one factor for ventilation and another for wall construction, from acrylic through to very heavy stone or concrete.
- Volume uplifts. Several North American manufacturers increase the calculated volume by a percentage or multiplier for the finish, for each glass or outside wall and for ceilings above about 2.4 m, then match the adjusted volume to a chart. Their multipliers are not the same as each other.
- Light-wall tables only. At least one manufacturer publishes figures for glass or acrylic rooms only, and directs heavier or ventilated rooms to a higher-rated unit.
Wall categories differ between brands too. One manufacturer groups porcelain tile with natural stone; another treats it like ceramic tile. Because of these differences, a generator is always sized from its own manufacturer’s method, using that manufacturer’s definitions. There is no industry factor that converts one maker’s answer into another’s.
Duty cycle: hours of use, not only room size
A generator is also chosen for how long it runs. Manufacturers separate residential and commercial units, and the difference is about duty as much as output.
| Residential steam generator | Commercial steam generator | |
|---|---|---|
| Typical use | A session or two a day, starting from cold | Many hours a day, often from opening to closing |
| Sizing emphasis | Heat-up from cold: finishes, glass and outside walls matter most | Holding temperature with ventilation, door openings and continuous losses |
| Ventilation | Separate ventilation is optional in some makers’ guidance, though Harvia recommends it | Harvia makes mechanical ventilation mandatory in public steam rooms |
| Water | Softener where the water exceeds the maker’s limits | Softened water and scheduled flushing are part of operating the room |
| Flushing | Drain and flush after each use | Interval rinsing during long sessions as well (Harvia offers 0.5–4 hour intervals with the automatic discharge valve) |
| Servicing | Per the manual and the service indicator | Harvia: a thorough service at least twice a year for community and institutional use |
| Guarantee (Harvia example) | HGX: 2 years for families | HGX and HGD: 1 year in community steam rooms, 3 months for institutions; HGP: 1 year for institutions. All conditional on water quality, installation and servicing |
A home-rated generator run for commercial hours wears faster and may sit outside its guarantee, even if its output suits the room. Some manufacturers go further and warrant residential units for residential use only. Large rooms may use a single high-output generator or several generators run from one control, which spreads the load and keeps some steam available if one unit is being flushed or serviced.
See commercial saunas and steam rooms for the wider operating picture.
Undersizing and oversizing
An undersized generator usually shows itself clearly. The room is slow to heat or never reaches temperature, and the generator runs at full output for long periods. Typical causes are a generator chosen on volume alone for a stone, glass or outside-walled room, a tall or vaulted ceiling not counted, more ventilation than the generator was sized for, or a long or poorly insulated steam pipe.
An oversized generator is not a safe margin. Harvia publishes a minimum room volume for each model, and a room below it is outside the range the generator is intended for. A larger unit also needs a larger electrical supply.
Other problems are sometimes blamed on sizing when the cause lies elsewhere. A sensor placed too close to the steam outlet, door or a vent can make a room cycle erratically, feel cold or run hot near the outlet. Spitting of hot water at the outlet usually points to dips in the steam pipe or scale, not output.
Plant location feeds back into sizing
The generator sits outside the room, and where it goes affects how much of its output reaches the steam:
- Distance. Harvia’s manuals give a maximum of 10 m of well-insulated steam pipe and recommend placing the generator as close to the room as possible. Harvia’s steam room guide prefers under 6 m, at the same level as the room. Other makers publish different limits, so the selected generator’s figure governs.
- The pipe. It should be insulated and laid to fall towards the room, with no water pockets or unnecessary bends, so condensate cannot collect.
- Ambient temperature. Harvia limits the air around its generators to 30 °C. Queensland roof spaces and closed cupboards can run hotter, so a ventilated plant cupboard or room is the safer plan.
A long or poorly insulated pipe loses heat before the steam reaches the room, so the plant location should be fixed before the generator is finally sized. Our guide to steam generator location and service access covers the plant space in detail, and steam generators and water quality covers water treatment, flushing and descaling.
Sizing from the actual build-up
Because the answer depends on construction and use, a generator cannot be chosen reliably from a floor plan. Confirming the size needs:
- the internal dimensions after linings, including the ceiling form and height
- the wall, ceiling and floor build-up, including any masonry, concrete or stone
- the finishes, including tile type and any natural stone
- glass areas and any outside walls, and the insulation behind each wall
- the ventilation approach, sealed or ventilated during use
- the intended use: hours a day, and home or commercial
- the plant location and steam-pipe route.
If any of these change during design, the generator selection should be checked again. A licensed electrician then confirms the electrical supply from the selected generator’s data. Our steam generators page and our guide to steam room design and construction cover the rest of the room.
Compliance and licensed trades
Steam generator sizing figures in this guide are taken from specific manufacturers’ documents and apply only to the products named. They are not Australian requirements. The selected generator’s current installation manual governs. Electrical work must be designed and carried out by a licensed electrician to AS/NZS 3000, and plumbing and drainage by a licensed plumber, following the manufacturer’s instructions. How a steam room is treated for waterproofing and ventilation should be confirmed with the building certifier, and commercial mechanical ventilation is designed by a mechanical engineer.
How we approach it
If you are planning a steam room, send us the drawings, the intended finishes, the hours of use and the proposed plant location. We can work through generator selection with you and your design team while the build-up can still change. Architects and builders can find more on our architects and builders page, or get in touch.