Sauna air change rate
Sauna air change rate is a specified airflow divided by a specified effective sauna-room volume. It is normally expressed as air changes per hour (ACH), with the unit h−1. If volumetric flow Q is in cubic metres per hour and volume V is in cubic metres, the nominal rate is n = Q/V. A reported value is incomplete unless it states which flow is represented, which volume is used, and the operating and reference conditions of the measurement.[1]
Numerator and denominator
Outdoor-air ACH counts controlled air originating outdoors. Supply ACH counts all air delivered to the room and can include recirculated air. Extract ACH is derived from air removed at the room boundary; infiltration ACH concerns unintended leakage. A tracer-derived rate represents exchange inferred from concentration behaviour within its test boundary. These quantities can differ because of leakage, recirculation and imbalance. Dividing a fan's nameplate capacity by room volume does not establish any of them, because installed resistance determines the fan's actual operating point.
The complete Sauna ventilation route includes a Sauna air inlet, a Sauna air outlet and, in some systems, transfer through a door gap. A value measured at one terminal does not automatically characterise the other side of the mass balance. For example, an extract flow may be replaced partly through a planned inlet and partly through cracks, while a central supply terminal may deliver a mixture of outdoor and recirculated air.
The denominator also requires definition. Sauna volume normally follows the room's internal geometry; benches and the heater are not arbitrarily deducted. A poorly mixed region may require an effective-zone interpretation in a tracer analysis rather than an undocumented reduction of the gross volume. Open doors and connected spaces can make a single-zone boundary invalid.[2]
Calculation and physical meaning
If flow is measured in cubic metres per second, multiplying it by 3,600 converts it to cubic metres per hour before division by volume. A measured flow of 30 m³/h for a declared 10 m³ zone gives 3 h−1. Similarly, 10 litres per second equals 36 m³/h and gives 3 h−1 for a 12 m³ zone. These examples demonstrate units only; they are not recommended sauna rates.
A nominal six ACH corresponds to a volume-to-flow ratio of ten minutes. It does not mean that all original air disappears after ten minutes. Under ideal complete mixing, the original concentration decays progressively. Actual rooms can contain short-circuit paths, stratified layers and slowly exchanged regions. ACH consequently gives no map of airflow distribution, draught or temperatures at individual seats.[1]
Direct sauna studies illustrate this distinction. A 1992 VTT experiment in one mechanically exhausted, electrically heated room found that changing the opening arrangement altered vertical temperature and mixing.[3] Fan, Holmberg and Heikkinen subsequently modelled two inlet positions in the same approximately two-metre-cube room and obtained different temperature and velocity fields under the tested conditions.[4] A common nominal rate can therefore coexist with different occupant-level environments.
Variation in operation
Natural sauna ventilation varies with wind, indoor–outdoor temperature difference, chimney draught and opening state. Mechanical sauna ventilation also changes when fans or controls change state, filters load, dampers move or doors open. “Design ACH” describes a target under stated assumptions; a commissioned ACH is a measured result for recorded conditions. Neither is an immutable property of a sauna.
Warm-up, bathing, water application and post-use drying are different test states. An open door may temporarily dominate the much smaller designed flows. Moisture removal depends not only on air quantity but also on dry-air mass flow, the humidity difference between entering and leaving streams, stored water and surface temperature. A higher rate may assist post-use ventilation, but no checked source gives a single rate or operating duration for every construction.
Measurement methods
Terminal and duct methods determine flow across a defined opening. ISO 16956:2015 covers field airflow measurement through ducts, diffusers, suction openings and exhaust openings in steadily operating systems.[5] ASHRAE Standard 41.2-2026 addresses air-velocity and airflow measurement, including density effects, and Standard 111-2024 includes field methods for room air-change rates, room pressurisation and outdoor ventilation.[6] A point velocity is not a volume flow unless it is integrated using an appropriate method.
Air volume changes with temperature and density even when dry-air mass flow is conserved. Equal mass flows at a cool inlet and hot outlet can occupy different actual volumes. Supply–extract comparisons must therefore state whether flows are reported at local or reference conditions and apply consistent density treatment.
ISO 12569:2017 describes concentration-decay, continuous-dose and constant-concentration tracer-gas methods for a qualifying single zone. Their validity depends on mixing, effective-zone, exhaust-sampling and flow-stability conditions. Strong sauna stratification may require multiple sampling locations. A tracer result incorporates exchange through all paths in the test boundary and need not equal one terminal's flow.
An enclosure pressurisation test answers a different question. ISO 9972:2015 measures air permeability under imposed pressure and expressly does not evaluate individual components.[7] Its n50 result is not operational ACH. Converting leakage at 50 Pa to natural exchange requires a model and site-specific pressure and weather data; there is no universal conversion factor.
Published sauna values
Harvia's current general guidance and a named heater manual state that sauna-room air should change six times per hour.[8] Saunum specifies at least six changes per hour for its Professional climate unit.[9] Agreement between two manufacturers is not independent scientific consensus, a medical threshold or a worldwide legal rule.
A 2002 VTT study of Helsinki public and apartment-building sauna facilities proposed a preliminary supply-and-extract flow of 3–4 dm³/s per square metre for the facilities studied.[10] Converting that area-based figure to ACH requires a declared room height or volume and does not remove the study's age, small sample and facility limitations.
Finland's Decree 1009/2017 expresses general outdoor-air provisions primarily per person and floor area, permits more flow for special uses and requires measured commissioning within its applicable projects.[11] EN 16798-3:2025 concerns non-residential mechanical systems and does not provide a universal sauna ACH in its public scope.[12]
Interpretation and reporting
ACH is an engineering descriptor, not an exposure finding. Carbon-dioxide concentration depends on occupancy, generation, outdoor concentration, mixing and time, and is not an overall indicator of Indoor air quality in a sauna.[13] Volatile organic compounds in a sauna, carbon dioxide, carbon monoxide and combustion particles have different sources and removal processes. An ACH value cannot certify combustion safety.[14]
A reproducible result states the airflow boundary, volume convention, units, averaging period, door, heater, water-use and occupancy states, fan and damper settings, sensor locations, density reference and uncertainty. Air-quality monitoring can supplement that record but does not replace it. Heat recovery can alter the energy consequence of a flow without changing the need to define its measurement boundary. No independent evidence checked for this article establishes one universal ACH for all heater types, uses, climates and jurisdictions.
Comparisons should also distinguish instantaneous, interval-averaged and scheduled values. A system that operates at a high flow only during drying can have the same daily mean as a lower continuous flow while producing a different occupied condition. Likewise, reporting a maximum commissioned setting does not show the rate normally used. Where flow changes with control signals or wind, a range or time series can be more informative than one decimal value. The significant figures should reflect measurement uncertainty: converting an approximate fan rating and approximate volume into a highly precise ACH creates spurious accuracy. These reporting choices are part of the result rather than optional background information.
See also
References
- ↑ 1.0 1.1 ASHRAE, “Ventilation and Infiltration”, chapter 16, 2025 ASHRAE Handbook—Fundamentals, 2025, official online chapter, accessed 5 September 2026.
- ↑ International Organization for Standardization, ISO 12569:2017, Thermal performance of buildings and materials — Determination of specific airflow rate in buildings — Tracer gas dilution method, edition 3, confirmed 2024, official record, accessed 5 September 2026.
- ↑ Erkki Äikäs and Rolf Holmberg, Saunan lämpötilat ja ilmanvaihto [Temperature and ventilation of the Finnish sauna], VTT Research Notes 1431, VTT, Espoo, 1992, 40 pp., ISBN 951-38-4325-4, official record, accessed 5 September 2026.
- ↑ Youchen Fan, Rolf Holmberg and Jorma Heikkinen, “CFD simulation on the air flow in a sauna”, Building Research & Information, volume 22, number 6, 1994, pp. 307–312. doi:10.1080/09613219408727409.
- ↑ International Organization for Standardization, ISO 16956:2015, Thermal performance in the built environment — Determination of air flow rate in building applications by field measuring methods, edition 1, confirmed 2025, official record, accessed 5 September 2026.
- ↑ ASHRAE, Standard 41.2-2026, Standard Methods for Air Velocity and Airflow Measurement, 2026, and Standard 111-2024, Measurement, Testing, Adjusting and Balancing of Building HVAC Systems, 2024, official purposes and scopes, accessed 5 September 2026.
- ↑ International Organization for Standardization, ISO 9972:2015, Thermal performance of buildings — Determination of air permeability of buildings — Fan pressurization method, edition 3, confirmed July 2026, official record, accessed 5 September 2026.
- ↑ Harvia, “Ventilation in the sauna”, updated 29 January 2026, official support page, and PC110EE/PC110HEE Installation and Use Instructions, official PDF, accessed 5 September 2026.
- ↑ Saunum, Professional Installation Manual, current hosted edition, section “Sauna room ventilation”, official PDF, accessed 5 September 2026.
- ↑ Mikko Saari et al., Terveen saunan tekijät [Elements of healthy sauna], VTT Research Notes 2144, VTT, Espoo, June 2002, 111 pp., ISBN 951-38-5899-5 and 951-38-6049-3, official record, accessed 5 September 2026.
- ↑ Finland, Ministry of the Environment, Decree 1009/2017 on the Indoor Climate and Ventilation of New Buildings, issued 20 December 2017, sections 1, 8–10 and 27, Finlex record, accessed 5 September 2026.
- ↑ European Committee for Standardization, EN 16798-3:2025, Energy performance of buildings — Ventilation for buildings — Part 3: For non-residential buildings, 2025, official EVS adoption record, accessed 5 September 2026.
- ↑ ASHRAE, Position Document on Indoor Carbon Dioxide, revised 12 February 2025, official PDF, accessed 5 September 2026.
- ↑ Tina Chen, “Rapid review: Environmental health risks and safety considerations in saunas”, National Collaborating Centre for Environmental Health, 16 January 2026, official evidence brief, accessed 5 September 2026.