FinSteam Gas Sauna Heater and Gravity (Natural) Ventilation

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FinSteam Gas Sauna Heater and gravity ventilation

The importance of ventilation in a gas-heated sauna

Ventilation in a gas-heated sauna is particularly important for both safety and sauna bathing comfort. The FinSteam Botnia81 gas sauna heater does not require a chimney, and its combustion gases (mainly carbon dioxide and water vapour) circulate within the sauna’s air flow.

This means that ventilation must ensure both a sufficient supply of oxygen for the gas sauna heater flame and bathers, and the removal of combustion products. Properly designed gravity ventilation can effectively replace a chimney, provided it is implemented correctly.

With correctly installed ventilation, a sauna equipped with a gas sauna heater remains safe, fresh, oxygen-rich, and dries quickly after use, preventing musty odours and moisture damage.

FinSteam sauna heaters are also equipped with a safety system (ODS) that automatically shuts off the gas supply and flame if the oxygen level in the sauna room air drops too low. However, this safety system should never be relied on alone. Properly designed supply and exhaust air openings ensure that oxygen does not become depleted and that the ODS does not shut down the heater unnecessarily.

We strongly emphasise that FinSteam gas sauna heaters must only be used in saunas with ventilation implemented according to our instructions.

Differences between heater types

It is useful to understand the differences between various sauna heater types:

Wood-burning sauna heater
A wood-burning heater has a chimney that acts as an efficient “extractor,” drawing air out of the sauna for combustion. For example, a wood-burning heater may consume approximately 8 m³ of air when burning 1 kg of wood. With 10 kg of wood, about 20 m³ of sauna air could theoretically be exchanged four times during heating. This natural draft helps ventilation, although it does not alone guarantee perfect air exchange.

Electric sauna heater
An electric heater does not consume oxygen during heating, so ventilation relies entirely on separate ventilation openings or even mechanical ventilation.

Gas sauna heater
A gas sauna heater is a hybrid solution: like a wood-burning heater, it consumes oxygen during combustion, but without a chimney the combustion gases remain in the sauna. In other words, a gas sauna heater does not create active draft to the outside; instead, it pushes hot air into the sauna and generates internal thermal air currents.

It is important to note that FinSteam gas sauna heater consumes oxygen cleanly during combustion and produces moisture (H₂O) and carbon dioxide (CO₂) as combustion products, which are not toxic.

For this reason, it is essential that sufficient fresh supply air can flow into the sauna and that the heated circulating air can exit through a correctly designed exhaust air vent.

If gravity ventilation is too weak, the sauna air can quickly become enriched with excessive carbon dioxide and moisture. In such conditions, incomplete combustion could produce carbon monoxide (CO). However, the ODS safety system of FinSteam heaters will shut down the heater before the sauna becomes life-threatening.

Principles of gravity (natural) ventilation

Gravity, or natural, ventilation is based on temperature differences and convection – the circulation created by temperature differences, where warm air rises upward and cooler replacement air flows in from below.

The effectiveness of ventilation is influenced by many factors, including:

Temperature difference between outdoor and indoor air
Difference in air humidity
Height difference between ventilation openings
Size, shape, placement, and distance of vents from the heater
Wind direction and strength

Together, these factors create the pressure differences and airflows required for gravity ventilation.

1. Temperature difference between outdoor and indoor air

Effect: The greater the temperature difference, the stronger the natural ventilation.

The main driving force of gravity ventilation is convection: warm air is lighter than cold air and rises upward. If the sauna temperature is, for example, +80 °C and the outdoor temperature is +5 °C, a clear pressure difference is created, drawing cool replacement air in and exhausting hot air through an outlet vent near the ceiling. The temperature difference affects both airflow speed and air volume, as warm air expands and tends to occupy a larger space.

Practical example: In winter conditions, gravity ventilation works more effectively than in summer heat because the temperature difference between indoor and outdoor air is greater. This can result in stronger draft at the exhaust opening without any mechanical assistance.

Very strong gravity ventilation caused by a large temperature difference may also affect how the sauna heats up in cold weather. In such cases, slightly reducing (but never completely closing) the exhaust airflow during heating can help the sauna warm up better. During bathing, however, care must be taken to ensure that exhaust airflow is not too restricted, so there is sufficient oxygen for clean gas combustion and for bathers to breathe.

2. Difference in air humidity

Effect: Moist air is lighter than dry air and therefore rises more easily.

Although humidity is not as strong a driver as temperature, it still affects air density, as moisture reduces air density. During bathing, sauna air becomes very humid. This moist air rises upward and exits, reinforcing gravity-driven exhaust airflow. A humidity difference relative to outdoor air enhances exhaust flow.

Practical example: After throwing water on the stones, humid air rises to the ceiling, circulates according to airflow patterns, and exits more quickly through a well-positioned exhaust opening. If the exhaust opening is too small or too low, moist air will not exit efficiently, airflow inside the sauna will be insufficient, and hot air may stagnate, making breathing heavy and uncomfortable.

3. Height difference between ventilation openings

Effect: A greater height difference strengthens and improves air circulation.

The effectiveness of gravity ventilation increases with the vertical distance between the supply and exhaust openings. The supply air vent should be as low as possible, and the exhaust air vent as high as possible.

The effect of height difference is mathematically proportional to the pressure difference created by thermal expansion of air. For example, a 2-metre height difference in a sauna with a 60 °C temperature difference to outdoor air can generate several pascals of draft force, sufficient to move tens of litres of air per second through a correctly sized and positioned exhaust vent.

Practical recommendation: Place the centre of the supply air opening approximately 25–40 cm above the floor, and the centre of the exhaust air opening less than 25 cm below the ceiling. This creates at least a 1.5 m height difference, sufficient to generate effective gravity airflow.

4. Size, design, and distance of vents from the heater

Size

Effect: A vent that is too small restricts airflow; a larger vent is not a problem if it is adjustable.

Gravity ventilation works only if air has sufficient space to flow. For FinSteam gas sauna heaters, the supply air opening area should be at least 350 cm² (e.g. a rectangular 200 × 175 mm opening or a round vent of approximately Ø210 mm), and the exhaust air opening at least 200 cm² (e.g. 200 × 100 mm rectangular or approximately Ø160 mm round).

If a vent is, for example, only Ø100 mm (≈78 cm²), air exchange will be insufficient. Also consider the airflow resistance caused by grilles, louvers, and filters, which can reduce the effective free opening area by up to half.

Remember that many ventilation vents, including those in the FinSteam sauna tent, are adjustable. Since gravity ventilation conditions vary (e.g. frost vs. summer heat, calm vs. windy weather), the ability to adjust vents as needed is important.

Keep vents at least slightly open at all times so that a small amount of replacement air always enters the sauna, keeping it fresh even between uses. Most importantly, vents must never be permanently blocked. A sauna is not a fully airtight space, nor should it be made one.

Design

Supply and exhaust openings can be rectangular, square, or round. For rectangular openings, ensure they are wider than they are tall, with a maximum aspect ratio of 1:2.5 (width no more than 2.5 times the height). A recommended ratio is between 1:1 and 1:2.

If vents are connected to ventilation ducts, remember that gently angled, unobstructed ducts improve airflow, while sharp bends and narrow restrictions hinder it. The vent and associated duct should be as straight and smooth as possible. A gentle 45° bend is far better than a 90° turn. Smooth, round ducts create less friction than rough or square ones.

Location and distance from the heater

The supply air opening should be located near the heater so that incoming fresh air mixes with the air heated by the heater and supports clean gas combustion. This allows oxygen-rich air to spread warmly throughout the sauna without creating cold drafts.

A good practical guideline when using a FinSteam gas sauna heater is to place the supply air vent as close to the heater as possible, with the lower edge of the opening at the level of the gas burner or slightly above it. This allows the heater to utilise oxygen for combustion and immediately warm the incoming air, preventing cool air from circulating along the floor.

This arrangement also combines the benefits of low-level fresh air supply with the improved efficiency created by height difference between ventilation openings.

Some sauna construction guidelines suggest that supply air should not be placed very low or close to the firebox to prevent all fresh, oxygen-rich air from being consumed by combustion. This is not correct, as a gas burner consumes only the amount of oxygen chemically required for combustion. There will be sufficient oxygen for other purposes if the supply air opening is large enough.

The exhaust air opening in a gas-heated sauna should be placed as high as possible, since warm, humid air accumulates near the ceiling. The best location is on the wall opposite the heater, close to the ceiling. Alternatively, a ceiling-mounted exhaust vent can be used, also positioned diagonally opposite the heater. The key point is that the exhaust opening must be clearly higher than the supply opening to allow temperature and height differences to generate gravity-driven draft.

Placing supply and exhaust openings on opposite sides of the sauna also ensures that air heated by the heater circulates throughout the entire steam room, including the bench seating area. Adequate oxygen improves bathing conditions: the steam feels softer and breathing is easier when oxygen does not run out.

5. Wind direction and strength

Effect: Wind can either enhance or disturb ventilation, depending on its strength, direction, and the protection of outdoor vents.

Gravity ventilation works best in relatively calm conditions, but wind can also act as an enhancer. If the outdoor exhaust opening is on the leeward (low-pressure) side, wind can help draw exhaust air out more efficiently.

Conversely, if the supply air opening is on the windward side, wind can force air into the sauna quite strongly. This can be beneficial or problematic, depending on the situation.

If a ventilation opening is directly exposed to wind without protection, several issues may occur:

Exhaust airflow may be blocked if wind prevents air from exiting
Supply air may enter with excessive pressure, bringing in too much cold air and making heating more difficult
In the worst case, airflow may reverse direction, causing exhaust air to blow back into the sauna

Practical solution: In fixed sauna buildings, use wind-protected outdoor grilles and canopies, and place outdoor vents on the sheltered side of the building when possible. In tent saunas, position or orient the sauna to minimise wind impact. If wind conditions are too strong or stormy, it is better not to heat a tent sauna that day.

Summary of gravity ventilation

Gravity ventilation is sensitive to these factors, and they interact as a combination. For example, a large temperature difference is of little help if there is no height difference between vents. Well-placed vents will not help if they are too small or if strong wind pushes directly against them. It is essential to consider all these factors together to ensure reliable sauna ventilation in all conditions.

Special considerations with a FinSteam gas sauna heater

Using a gas sauna heater introduces specific ventilation requirements. As noted, all combustion gases remain in the room air, so sufficient ventilation is the only way to remove them. The FinSteam gas sauna heater is designed to be safe for indoor saunas, provided ventilation is implemented according to instructions.

In practice, this means following the principles described above: correctly sized vents placed low and high, kept open during bathing. A gas sauna heater must never be installed in a completely airtight, unventilated space. Additionally, the gas cylinder must always be placed outside the sauna, in a well-ventilated location. This ensures that any potential valve leak or hose failure does not direct gas directly into the small steam room.

From a safety perspective, additional reassurance may also be considered: carbon monoxide and carbon dioxide alarms (CO and CO₂ detectors) are not mandatory but can provide peace of mind in a gas-heated sauna. A properly functioning sauna equipped with a gas sauna heater is, however, proven to be safe when ventilation is adequate.

FinSteam heaters are CE type-approved in this respect and have an EU type-examination certificate in accordance with the EU Gas Appliances Regulation (EU/2016/426).

Ventilating the sauna after sauna bathing

Finally, a reminder: always ventilate the sauna thoroughly after use.

After bathing, leave ventilation openings fully open and, if necessary, leave the door slightly ajar so that both moisture and any remaining combustion gases can be vented out. Gravity ventilation continues to operate after bathing, especially if the heater is still warm and drawing air. Proper ventilation dries the benches and structures, preventing mould and harmful microbial growth.

In summary:
The FinSteam gas sauna heater offers a chimney-free sauna experience, but it requires carefully designed gravity ventilation. Ensure sufficiently large and correctly positioned supply and exhaust openings so that your sauna is safe, fresh, and pleasant. A well-ventilated sauna rewards its users: steam remains soft and oxygen-rich, and the sauna itself lasts longer, staying cleaner and drier.

With correctly implemented ventilation, a FinSteam gas-heated sauna is not only a unique opportunity but also excellent in use, combining the warmth of a real flame and the atmosphere and steam of a traditional wood-burning sauna with modern ease and safely.