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A 45 year engineer clears up electric sauna ventilation

light steam graphic

Guest post series continues with Malcolm, a 45 year engineer, who clears up electric sauna ventilation. He has been using his electric heated sauna almost every day for over a year. He is enjoying his sauna 4-6 times a week. Instead of watching the “idiot TV”, Malcolm has spent his evening watching his thermal meters in his hot room.

Further, he has been able to enjoy nature between sauna rounds. Whether looking up at the stars or feeling a soft breeze as it blows through the nearby trees, sauna has become a game changer for his physical and mental wellbeing. As example, his blood flow has improved. His heat rate used to be in the mid 70s.. now it is mid 60s. Beats per minute.

Malcolm used to go to the sauna at the gym. When Covid hit, he in vested in his own backyard sauna. ‘”This thing has really made a difference.” The flow pattern is the same. Malcolm has been talking with others. And those that have adopted his venting system have had great success with Malcolm’s profile.

Malcolm has analyzed the Finnish 1992 sauna ventilation study profile, applied his knowledge and experience as a 45 year engineer to test and better understand sauna ventilation in his own backyard sauna.

Welcome Malcolm:

Hi Glenn, here is a draft of the commentary on the 1992 Finnish Electric Heated Sauna Study.

I have basically taken a 34 page research paper and condensed it into 9 pages. This helped to shorten the commentary. One of the areas that most of our fellow Sauna Dwellers don’t understand is the air flow that each particular inlet opening around the stove and exhaust opening at the back of the sauna creates. I concentrated on the results that the Study found on the various inlet air openings flow characteristics to educate our folks on how they function in an Electric heated sauna.

I’ve done some further temperature profiling of my sauna and the profile around and in front of my stove has some quite interesting temperature changes. I found several typos and misinformation in the original translation and corrected them for this article. I think the translator had English as a second language. Some of their statements were lacking the details that the Study actually provided so I expanded those so that people would have a better understanding what is actually going on there. 

There is a good amount of vagueness in the original translated study that I addressed. I’ve got a much better understanding of the processes going on in the electric heated sauna. Some of the temperature studies I have been doing have produced some quite interesting results. The old engineer’s brain is finally waking up (at least to a degree).

A few of Malcom’s temperature reading sensors

Over the past several months you have probably read or contributed an opinion on what the proper ventilation setup should be for an Electric Heated Finnish Sauna. You have probably also seen the image below in several different advertising brochures used by various Sauna kit and Electric heater manufacturers plus in comments or articles by others. 

Some folks have actually created their own version of this image, modified (can we say distorted?) in order to present their particular position on this somewhat controversial topic. In November of this past year “Sauna Times” presented a “POST “to look at some of those ventilation opinions since “just about everyone had one” and there did not seem to be an easy answer to this question. One of the areas of confusion for the Ventilation of the electric heated sauna is that it’s ventilation air flow dynamics are very different from that seen in the “Traditional Wood Stove (loaded inside the sauna)” Finnish Sauna world that nearly everyone has seen or experienced. One thing I found through one of Facebook’s “Sauna Groups” was a link to a copy of an English translation of a 1992 Finnish Research study on the “Ventilation Requirements for an Electric Heated Finnish Sauna”.  

What is being done here is to summarize the actual “Science” of that Ventilation process from the Electric Heated Sauna prospective based upon the 1992 Finnish Study of the Ventilation characteristics of an Electric Heated Finnish Sauna. The image above is from the cover of that Research Study from which the Figures and the translated and paraphrased Texts are incorporated into this discussion. A link to the actual English translation of the Study will be provided at the end of this Post.

Most North American Electric Heated Sauna Users did not know about this Study and the other resources they were dependent upon (Heater and Sauna Kit manufacturers) had other agendas, including the UL 90-degree Celius Over Temp / Heat issue, that they were more interested in addressing and not necessarily the proper Ventilation requirements of an Electric Heated Sauna. This Post will not address the UL controversy but will stick to providing the useful information on Ventilation contained within the 1992 Finnish Ventilation Study at this point.

So, let’s get to it

In my simple mind, the Finnish Sauna is just a type of Heat Treat Furnace or Oven that you load humans into and try to provide them with three characteristics. (1) not oversaturate their lungs with their own Carbon Dioxide waste products from being enclosed in a sauna with poor ventilation and limited fresh air volume changes (called Air Mixing in the study), (2) create an enjoyable Temperature distribution within the sauna environment (called Temperatures in the study) and lastly (3) provide a well distributed Ladled Steam Humidity cloud throughout the Sauna (called Air Condition in the study). Basically, everything you would expect for a good Finnish Sauna experience.

Here are some particulars about the Study’s Setup. The inlet Air Flow within the Test Sauna was maintained at 48 Kg/h (24 CFM). The inlet air temperature was 19 to 21 degrees C for most of the experiments. The internal chamber of the sauna was tightly sealed against air leaks since air leaks cause a disruption of the Sauna Ventilation flow pattern and in turn the characteristics being evaluated. The volume of the Test Sauna they used was 8 Cubic Meters heating it with a 8Kw electric, wall mounted, solid sided, stove. This stove held 25 Kg of stones as it’s Thermal mass. Other details or experimental changes are included in the Text of this Post.

Figure 1

The two drawings above show the location of the inlet air ventilation openings, labeled T1- T4 and the exhaust air (Fan assisted) ventilation openings, labeled P1 – P3 used in the Study. The thermocouple temperature measurement points were labeled as 1-15 for the Study and are also shown in the Figure 1 above. The measurements are in Millimeters with inches being hand printed next to them. We have attempted to copy and paraphrase the Data, Figures and Text from the Translation of the Study in a way that simulates the order that the Researchers followed in the Study in this Post.

The Researchers were focused on three ventilation areas of the Electric Heated Sauna environment. They were called AIR MIXING, TEMPERATURES, and AIR CONDITION. They studied the AIR MIXING (Ventilation air flow system) dynamics by looking at the flow patterns of four Inlet Air vent openings (T 1 – 4) commonly used in Saunas. Three (Fan assisted) Exhaust vent openings (P1– 3) were strategically placed in the back wall of the Test Sauna also. They are all shown in Figure 1 above. Previous studies of Gravity Convection Ventilation showed it to be inconsistent and undependability for the Electric Heated Sauna Ventilation needs. Hence each exhaust opening was equipped with an variable speed in-line fan assistance.

The TEMPERATURES properties (Temperature Distribution) in the Test Sauna were measured by 15 different Thermocouples mentioned earlier (numbered 1 – 15 above). The AIR CONDITION properties (Absolute Humidity measurements) were related to water being flashed to steam on the stones on top of the stove by a Brackish Water Diffuser system and the Absolute Humidity of the resulting steam / sauna air mixture and it’s distribution was measured using a psychrometer with thermocouples mounted on a device with a mercury meter that measured both dry and wet bulb temperatures. The Absolute Humidity was mainly measured next to Thermocouple point 7 and in some of the experiments, next to Thermocouple point 4 and at a location 150 mm above the floor.

Air Mixing Experiments

In the November post on SaunaTimes, several locations of the Inlet air vent placement were discussed. Under the stove, midway behind the stove, and a couple of different locations opposite from the stove.

The Researchers started the study by observing the flow pattern of each of the 4 Inlet Air vent openings (T1 – 4) by videotaping the smoke pattern (they used smoke bombs) coming through each individual Air inlet opening and seeing how that air flow pattern reacted to the hot air stream coming up from the stove and the existing sauna air flow field moving back towards the stove along the floor. The Researchers found that the normal hot air flow field coming up from the stove causes a very strong circulation of air in the sauna space above the stone level of the stove and continues up to the ceiling. In the sauna space below the stone level, the air movement around the stove is quite calm, unless it is disturbed by the incoming cold air flow from the inlet opening.  The four Figures below, show the flow patterns of inlet air coming into the test sauna with the different placements of the incoming air openings (T1-4). The Flow pattern Observations below were the results of the experiments conducted for the “Air Mixture” segment of the study. The Researchers found that varying the locations of the exhaust vents (P1- 3) did not affect how the air, coming in from the various inlet vent openings, would react with the existing the sauna air flow.  For this reason, the Figures below from the study, were not shown with any exhaust air vents. The inlet air opening findings were combined with the exhaust of ventilation openings (P1-3) and were then used in the Temperature and Air Condition (Humidity) research segments conducted later in the study. 

The Figures below show how the inlet supply air flow patterns stream into the sauna room when the Inlet air opening is at different heights (TI – T4). The temperature of the incoming air for each trial was approx. 20°C and the air flow was kept at 48 kg/h (24 CFM) as constants for most of the experiments.

Figure 2 Supply air opening close to the floor (T1)

The Researchers found when the inlet air opening is below the stove (T1), the inlet air, which is colder than the existing sauna air, spreads to the floor and forms a layer of cool air in the lower part of the sauna. If the exhaust air opening is also at the bottom of the floor (P1) away from the inlet vent (T1), it was found that a short-circuit flow pattern occurs, whereby the air exchange in the sauna space above the stove deteriorates and does not reach the average air exchange rate needed for the constant exhaust air flow (48 Kg/H or 24 CFM). When the exhaust air opening is near the roof (P3), air rises in a piston like flow fashion, from the lower part of the sauna floor upwards. The air around the stones on top of the stove, mixes in a very gradual way with the sauna air flow pattern in the upper part of the sauna.

Figure 3 Supply air opening behind the stove (T2)

When the intake air opening was placed behind the stove (T2), in this case too, most of the cold inlet air flow landed on the floor and did not differ substantially from the T1 case, where the intake air was brought in under the stove.

Figure 4 Supply air opening just above the stove (T3)

If the intake air opening used is just above the stove (T3), the air flow movements depend primarily on the speed and temperature of the inlet air flow (in this study 48Kg/H & 19 – 21 °C). When the air flow rate is high, some of the air stream moves over the stove and some flows under the benches and since this air is colder than the surrounding sauna air, much of it lands onto the floor. When the flow rate of the incoming air is low enough, the incoming air has time enough to mix with the hot air flow rising up from the stove and from there, flows up into the existing sauna air flow pattern in the upper part (ceiling) of the sauna where it gradually moves to the lower part of the sauna as well. In some measurements, the distribution of the incoming air flow was observed such that some went up and some went down (under the benches and on to the floor).

Figure 5 Supply air opening midway between the top of the stove and the ceiling (T4)

When the inlet air opening is located above the stove (T4), the upward air flow generated by the hot air from the stove, is so strong that the inlet air immediately mixes with the hot air flow rising up from the stove and moves rapidly up to the ceiling. The Researchers found that the T4 Inlet Air vent location (halfway between the Top of the Stove and the ceiling) provided the best flow pattern for mixing inlet Cold air with the existing Cool Sauna air from the floor and the Hot air coming up from the stove. This experiment also showed that the T4 Inlet location should be used to keep the Carbon Dioxide levels low in the sauna and will provide good mixing to meet the 3 to 8 volume changes per hour required by the Finnish Sauna regulations. The T4 location in the Study is near where the “B” Vent location was shown in the Sauna Times November “Post” drawing.

Temperatures Experiments

After determining the normal air flow path for each of the inlet air openings the Researchers examined the influences of the various Fan assisted exhaust openings (P1 – 3) on the Sauna temperature distribution created using various Air Inlet and Exhaust air opening (T1 – 4) to (P1-3) combinations. The three Figures below show their findings (Temperature Distribution) for those combinations. The inlet air temperature was around 20°C, with a constant air flow rate of 48Kg/H or 24 CFM, and the sauna floor was uninsulated concrete. Only the combination of the inlet and exhaust air openings were varied.  The vertical temperature distribution of the sauna was measured from in front of the seat board (top Bench), using Thermocouple locations 2 – 9 shown in Figure 1. The temperature of the exhaust air was found to be approximately the same as the temperature of the inside sauna air at that height of the exhaust air opening being studied.\

Figure 6.

The Figure above shows the vertical distribution of sauna air temperatures when the supply air was led into the sauna from different Inlet Air vent heights (T1 – T4) and the Fan assisted exhaust air opening was at the floor border (P1). The temperature of the incoming air was 19 – 21°C.

Figure 7. The Figure above shows the vertical distribution of sauna air temperatures when the supply air was led into the sauna from different heights (T1 – T 4) and the exhaust air opening was below the seat board (P2), where the “D” Vent location was shown in the Sauna Times November “Post” drawing.  The temperature of the incoming air was 18 – 20°C.

Figure 8

The Figure above shows the vertical distribution of sauna air temperatures, when the supply air was led into the sauna from different heights (Tl – T4) and the exhaust air opening was near the ceiling (P3). The temperature of the incoming air was 18 – 20°C.

The Researchers found that the best Temperature distribution was produced when the T4 inlet and P2 exhaust (Fan Assisted) opening locations were used.

During this segment of the study, side experiments were conducted and Temperature distributions were measured for the case where the concrete floor of the sauna was thermally insulated with 50 mm thick Styrofoam. The floor temperature was found to increase because of the presence of the insulation. The Study’s temperature distributions, when the temperature of the incoming air is around 20 °C. and the concrete floor of the sauna is thermally insulated with 50 mm thick Styrofoam, clearly showed an increase in the temperature of the lower parts of the sauna. When the floor in the sauna is thermally insulated and the supply air is led into the Sauna above the stove (T3 or T4) and the exhaust air opening is under the bench of the sauna (P2), the floor also stayed warmer than in the case with an uninsulated floor. In the tests where the inlet air temperature was 7 – 9 °C, there were no noticeable differences in the floor temperature distributions when using the T1 or T2 inlet air locations with the insulated floor. The inlet air coming in from T1 and T2 goes directly to the floor so they were unaffected by the existing Sauna air moving back on the floor towards the heater as a result of the insulation. The flow patterns of the T3 and T4 Inlet openings showed that insulation on the floor definitely increased their related floor temperatures due to their flow patterns discussed in the Air Mixing segment of the study. The Figures provided for the insulated floor experiments are not shown in this Post but may be viewed by going to the link for the Study for more detail.

Air Condition

 The purpose of the Air Condition (Humidity) sauna tests was to clarify the effect of the location of the ventilation openings on the Humidity distribution within the sauna air flow pattern in connection with the throwing of water onto the stones of the stove to create the steam cloud. In the first measurements, Humidity was monitored at the height of the Bathers’ head (70 cm above the seat board). Water was thrown at a rate of 125 g every five minutes using a repeatable procedure. This amount of water was chosen so that the Absolute Humidity at the height of the Bather’s head would mainly be in the range of 40 – 50 g/Kg of air with normal ventilation (48Kg/H). The results of the Air Condition (Humidity) sauna experiments showed that, similar to the results of the Air Mixing experiments, that both the inlet and exhaust air openings located close to the floor (T1 and P1), followed the same the short-circuit flow that causes the dispersion of steam in the sauna to be very poor and does not provide the needed steam dispersion expected by the existing Sauna air flow rate (48Kg/H). This was also the case in the steam tests, where the Humidity of the sauna air rises continuously when the exhaust air opening is near the floor (P1) and the intake air opening is either close to the floor (T1) or behind the stove (T2). As the sauna air stream continues to move, the humidity rises continuously both in the upper part of the sauna and at the footboard. The Figures and additional data describing this situation is not addressed in this Post. Go to the link for additional details. When the supply air was led into the sauna from different heights (T1 – T4) and the exhaust air opening was near the ceiling (P3), the different placements of the inlet air openings did not really cause differences in the humidity distribution at the Bather’s head. The Figure’s columns showed about the same humidity distributions regardless of the placement of the exhaust air openings, even in the case where the intake air opening is above the stove and the exhaust vent is near the ceiling (T4 & P3). Check the link for those results.

Figure 9.

When the exhaust air opening is placed under the seat board (P2) the results are shown above. This placement improves the ventilation efficiency and dispersion of the Humidity in the upper part of the sauna for all heights of the inlet opening, even when the intake air openings are still under the stove (T1) or behind it (T2) with their active short circuit effect. The vent combination of (T4 / P2) provided the best and most uniform Humidity (steam) distribution across the sauna of any of the other opening combinations.

With a separate steam test series, the Researchers wanted to find out how much the exhaust air flow of the sauna must be increased so that the Humidity steam cloud, rising in the upper part of the sauna, would have to be increased to roughly match the T4 case shown in the Humidity Figure 9 above, where the air circulation in the upper part of the sauna can be considered good. Figure 10 below shows that results.

Figure 10.

The Figure above shows the measured results of the sauna air Humidity variations using different exhaust air flows (0 – 196 kg/h) at the 70 cm seat bench height and using the worst-case scenario being at air inlet opening location T1 and exhaust opening location P1. When the ventilation was completely closed (first column air flow 0), the Humidity rose considerably. Air leaks introduced into the test sauna led to condensation of moisture on to the surfaces and absorption into the surfaces that reduced the steepness of the absolute humidity curve’s ascent. When the ventilation rate was increased (next three columns) the fluctuation in levels of Humidity decreased. When the exhaust air flow was set at 196 kg/h, (four times the so-called normal amount 48 kg/h and last column) it reached the humidity variation level that approximated the results of what Inlet Air opening T4 provided in Figure 9 using either exhaust vents P1 or P3 (worst exhaust openings). This experiment showed that the ventilation efficiency above the seat board is quite poor if the intake and exhaust air openings are close to the floor (T1 and P1) or close to the ceiling P3 even with High Air Flows. Use of very high air flow rates will seriously disrupt the needed ventilation flow pattern for an Electric Heated Sauna. Use of High air flow rates proved not to be a solution for distributing Humidity.

CONCLUSIONS

1.FRESH AIR MIXING

We like to keep the CO2 down. We want 6-8 volume changes per hour in the hot room.

An even vertical temperature distribution in the sauna requires an effective mixing of the existing sauna air and the cold air coming in from the Inlet opening. Figures 2, 3 ,4 and 5 shows the incoming air flow patterns of the four Inlet openings used in this Study. In practice, however, incoming air that is colder than the surrounding temperature always tends to “flow” downwards. In the sauna, the room air is already at normal temperature and is classified as “cold air” which flows down to the sauna floor unless it can be mixed with the hot air mass circulating inside the sauna.

In order to prevent a cold air zone from forming in the lower part of the sauna, the incoming air must be led in from above the stove, where it immediately must mix with the existing circulating air of the sauna and the hot air coming up from the stove. In this case, the effects on the sauna air also extends to the lower parts of the sauna.

The mixing of the air from the upper part of the sauna with the air from the lower part of the sauna is promoted by placing the exhaust air opening in the lower part of the sauna. This condition was found when the Inlet air opening was placed at the T4 location and the exhaust opening was placed at the P2 location under the bench.  

The Study’s experiments showed this particular combination provided the necessary mixing of the cold inlet air with the hot air stream coming up from the stove and provided the necessary dynamics to move the “cold” sauna air along the floor and into the air flow loop surrounding the stove.

2. TEMPERATURE RANGE

We like to have the temperature at our ears not hugely different than at our feet.

From the temperature measurements, it was found that there is a large variation in the temperature of the sauna air in the vertical direction. As typical values, it can be stated that the temperature at the height of the head of a person sitting on the bench is 90°C, at the height of the seat board (bench) 80 – 85 °C and at the height of the foot board approx. 60 – 65 °C.

The temperature difference between the foot board and the Bather’s head was therefore approx. 25 – 30 °C. The temperature of the air at the height of the seat board (bench) and at the border of the floor is significantly affected by the location of the inlet air opening, along with the temperature of the inlet air and the amount of ventilation the exhaust Opening provides.

Due to these aforementioned factors, the temperature at the footboard could vary from 50 to 80 °C. The highest temperature was reached when the inlet air opening was above the stove (T4) and the exhaust opening was under the bench at P2 and the floor was also thermally insulated. This last combination opening arrangement (T4 /P2) provided the most uniform vertical temperature distribution of the Study.

3. AIR MIX WITH STEAM

Fresh air mixing with steam makes better steam.

By placing the inlet air opening above the stove (T4) and the exhaust air opening between the seat board and the floor (P2), the effects of the steam distribution extended down to the lower parts of the sauna. In addition, this particular ventilation combination showed that the incoming air mixed directly with the steam and the hot air coming up from the stove and the upper part of the sauna (the breathing air of the bathers).

If both the inlet and outlet air openings (T1and P1) are located in the lower part of the sauna, a short-circuit flow occurs in the lower part, and the fresh air does not mix with the breathing air of the bathers. The Researchers also found that Carbon Dioxide buildup moves in a sauna nearly identically as to how the Humidity distributes itself in the sauna. When the supply air is led to the border of the floor (T1 opening), the temperatures of the border of the floor and the footboard remains low, even too cool in the cold season. This also has an effect on the drying of the lower parts of the sauna.

The temperature and humidity measurements performed in connection with this study, as well as the visual smoke trail observations made, support the T4 and P2 ventilation opening arrangement, where the incoming air is led into the sauna above the stove (T4) and the air is exhausted from the opening located below seat board (Bench) at (P2). The Study also confirmed that the Sauna has to have mechanical exhaust involved in it’s ventilation system.

Other Considerations

The above results are based on measurements performed in a test sauna, and Bathers’ personal observations of the issues presented in the Study have not been investigated. In practice, the tightness of saunas differs substantially from the tightness of the test sauna. As many air leaks in the normal sauna as possible should be sealed to keep from disrupting the Ventilation flow pattern of the T4 and P2 combination.

If you’re still with us and breathing fresh air, you can check this article, that adapts a 4 vent system for electric heated saunas here. In the drawing below:

  • T4 = B
  • P2 = D

A “A” and “C” are common with wood fired setups. When each of these 4 vents can be controlled by “chutes” or “sliders”, we allow ourselves, like the Clash song goes, “Complete Control… C-O-N… Control!”

electric sauna vents
4 sauna vents for an electric heated sauna that cover all the bases

Link to the original 1992 study is here.

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18 Comments

18 thoughts on “A 45 year engineer clears up electric sauna ventilation”

  1. Malcolm – Great article, I’ve been looking for an English translation of the VTT research. Your article says there is a link to the translated article, but I don’t see it above. Can you provide that?

    I also have a question about the P2 exhaust location. How is the location determined – fixed distance from the floor (27″ in the diagram), or relative to the benches (between the seat and foot benches)? In the study, the foot bench is at ~18″ and the seat at ~35″. So, in application – if the foot bench is at 36″ and the seat at 54″, would you keep the vent at 27″ or move it to somewhere between 36-54″? The experiment setup has the feet below the stones (or at least it looks that way) and seems like a lot of the custom setups I’ve seen have the foot bench at a minimum of 35″ to get the feet above the stones.

    -Todd

  2. You got it.. you get a prize if you can digest all that. I gently massaged Malcolm to strip it down and even above is a lot to take in. But he dove into this with great attention, and applied lots of his 45 year engineering experience to help tease out the essence. I’m working to try to help clear the sauna ventilation air even further: three digestible elements to help us breathe, breathe in the air, and not be afraid to care.

  3. Excellent stuff but a bit above my level to take in! Just wondering if I could get some simple advice on adding a vent to my sauna. It was built by a local company here in the UK – https://www.zoki-saunas.co.uk/. It’s a classic design just like these diagrams, about 2 x 1.5m, two level bench along the back, 6kw electric stove in the front corner. It’s really well built and insulated and I’ve never had any complaint with it – heats up in about 30-40 minutes and easily sustains 90C. Never noticed feeling any sort of ‘lack of fresh air’ and can comfortably stay in for long periods.

    However it only has one vent at head height on the side wall, opposite side of the stove. If I was to add just one vent myself, where should I add it and what actual noticeable benefit might I get? The easiest lowest risk option for me would probably be position ‘D’ – under the bench at stove height. Or would you recommend something else? Thank you!

  4. Malcolm,
    Thanks for all of your research, this is very useful. As an sauna builder with 25 years experience, I’m always interested in perfecting my work. That the cold air coming in always flows downward first, makes sense, thus for good löyly and room air, the vent above the heater seems an improvement— but as builder who wants to avoid call-backs for failed heaters, I always follow the manufacturer’s suggestions of a vent directly below and behind the heater. This is more to protect the wiring and some cases, circuitry or analogue switches inside the base of the unit. When things gets too hot the heater’s high-limit switch will trip, shutting the unit off (and prompting a call to me.) Even with this high-limit fuse, the wires may overheat and the insulation will crack, the elements will fail prematurely, and the unit can get scary hot—as evidenced by scorched walls. All of which I have seen and have been called in to correct on poor installations by others. Commercial units are extremely vulnerable to overheating, especially when the sensors are tampered with by overzealous bathers. Usually, adding the low vent will rectify the situation. All of my heater installations, with the inlet vent low behind the heater, have performed well (at least from the standpoint of non-failure of the heater and no call-backs.) My outlet vents are mostly as you specify but often dependent on the situation of the build- such as backing up against a exterior basement wall or other situation where ideal venting has to be compromised in favor what can actually be done in the space (often using ducting).

    My question to you is this: did you measure, or consider measuring, the temperature of the body of the heater, especially down low where the wiring is susceptible to overheating? Or, in any testing, did the high limit switch trip?

    Perhaps the two inlet vent option is the best one to serve both heater and bather- to keep the heater from over heating and provide good circulation?

  5. I hear you man. I love engineers. I have a few really close friends who are engineers. I’d like to hang on the bench with Malcolm but if I’d be out to dinner with him, I’d prefer he do the ordering for both of us. What makes sense to me is the verification of my hand drawn attempt, above. So, in your case, if you’re feeling good in your hot room, I’d consider leaving well enough alone.

    That said, if you’re gung ho about punching in some vents (and there’s no harm in doing so with the inclusion of “chutes” or “sliders” i’d be looking to work with “B” and “D”. Start with “D” as you note above. See if that does anything.. and it may not as D is intended to work with a little help from a mechanical friend. Hope this helps!

  6. but how im asking myself how to calculate the mechanical exhaust ventilation needs to refresh the air 3 to 8 times an hour.

  7. Good one Erwin. Our 45 yr. engineer has a device, I’ll nudge Malcolm for him to respond here.

  8. If there any folks in Minneapolis who have built electric saunas with proper mechanical downdraft ventilation, I’d like to meet and see your sauna! Is there a sauna meetup in Minneapolis? If not, perhaps we should make one.

    The electric saunas I’ve been in around town recently—the Hewing Hotel and Embrace North—all suffer from poor ventilation. Don’t get me wrong; both are enjoyable experiences—they just need better ventilation.

    I am building my own sauna at our family farm in Lindstrom. I have the ventilation designed as per this 1992 study.

    benitocanino AT protonmail DOT com

  9. Fantastic discussion on sauna ventilation and heater placement. Malcolm’s deep dive into the technicalities, enhanced by his extensive engineering experience, has clarified many aspects of optimal sauna design. It’s interesting to see how the positioning of vents not only impacts air quality but also the longevity and safety of sauna heaters. The community’s questions and Malcolm’s insights on adjusting vent locations based on bench heights are invaluable. Also, practical advice from experienced builders like Rob Licht on ensuring heater safety while optimizing airflow adds another layer of depth to the conversation. This exchange of knowledge is incredibly helpful for both DIY enthusiasts and professional builders aiming to create the perfect sauna like this https://gera-pirtis.lt/ experience. Thank you all for your contributions

  10. Right on to this Saulius! Glad you are with us. And you’re spot on about Rob: a kindred spirit of thermal goodness.

  11. Hi Ben:

    I’ll shoot you an email. With you here… and there are a couple people and examples of mechanical downdraft happening.

    What’s interesting is that there are also a couple people and examples who have developed electro hot rooms with no mechanical at all, but have adopted and adapted aged ole’ systems from the Slavic region. convective laminar flow action. It’s pretty damn cool, not just because it works, but because it works without pedantic chatter or electro fans. Gotta love the Slavs… “them are good people.” (just not Putin and those nuts).

  12. I am struggling with my sauna heater tripping the high limit before it reaches temperature. Harvia cilindro 10.5kW. Still troubleshooting but it looks like it may be related to inadequate air flow at the heater. We will be using a fan tonight to test this theory. If that is the cause, we will enlarge the vent next to the heater and add an additional vent with mechanical assist. Looks like we might want to put that additional vent under the upper bench opposite the heater? There is already a vent on the same wall as the heater near head height.
    This is the second unit. We had one that didn’t work at all, two that were damaged in shipping. Not exactly the relaxing sauna experience I was hoping for.

  13. Totally, Janet.. you deserve good heat and a chill sauna experience.

    I’d be focusing on ventilation relating to the ABCD, in the diagram above. B and D, open and A mostly closed and C all the way closed for a starting point for “tuning.” And if on the other side of this wall is a bathroom, well, you’re in luck. I’d be thinking of no mechanical. Just flip the vent on in your bathroom, close the bathroom door (to create draw) and C open with draw air out of your hot room.

    You’ll get there. Sauna isn’t a plug and play gig. Sauna is something pretty extraordinary, as you probably know. We don’t take something out of a box and hook it up. We become one with our saunas, and you will too. Keep in touch on your journey to good ventilation to solve your tripping issue.

  14. Hey Glenn,

    Would love to chat about our sauna designs — we have a 51 person aufguss performance sauna in Toronto, a 91 person performance sauna in Toronto and building a 100 person performance sauna in New York. We use forced intake below the stoves and mechanical ventilation out. In our 51 person, the stoves are tripping a few times per day and we need to turn the fan off or they trip. Can’t figure it out. Wondering if you or Malcolm are open to chatting and if the above applies to saunas of this size (ie. 800 – 1,200 sq ft).

  15. Robbie:

    With a sauna of this size and magnitude, ventilation takes on a whole new bigger mousetrap/boat/situation. As the drawing shows, “A” helps support fresh air along the lower part of the electro heater, strategically to where the over limit switch is. And “B” is the magic spot for mixing fresh air with air/steam rising. And then “D” well, that’s where we benefit from laminar flow action, the encouragement of air to circulate up, up and over the bathers, and expelling “used” air/steam out of the hot room.

    Now, when we are talking about big hot room, well, now we have to get serious about how to scale this residential venting plan. Stoves tripping is not a bummer, and all roads lead to needing to improve air movement. Obvious stuff, yes, but how we fix it is a longer discussion, with a predicted fair bit of trial and error. ABCD is the place to start.

  16. The study seems to have been conducted with an electrical sauna heater – I wonder what it would look like for a wood fired one.

    In my case, the sauna (being constructed slowly by myself) is in our backyard shed, and I prefer to avoid mechanical ventilation. My stove is near the door, with the door in the wall that would be parallel to the page on which these diagrams are printed, so to speak.

    Based on this paper I’d be very wary about placing the inlet close to the ground or behind the stove (in my situation), as the air may flow may be short circuited, going straight out under the door rather than doing much with the inside of the hot room. After all, in my case, the distance between the stove and the door crack is much closer than the distance between the stove and the opposite side of the room where the exhaust openings would be placed.

    After reading this article, intuitively, I feel that the T4 / P2 setup may work best, while keeping the door crack small (< 1 cm) to avoid sideways air flow, and with a bit P2 deeper into the hot room, away from the door. Perhaps even placing T4 a bit deeper in the room and away from the door could be helpful (and not directly above the stove, which is close to the door); though I'm not sure about that one.

    It also feels like the T4 / P2 setup may work well when there are multiple T4 and P2 openings at different depths in the sauna to allow for the ventilation to be good at any depth in the sauna (after all the study reduced it to a two dimesional problem, while in reality there's a third dimentsion to account for). Probably not going to poke more than two holes through my insulation though!

  17. Yes, for wood fired, a generous crack along hot room door, then a vent down low close to the stove are ideal for inlets.

    I have always just gone with “C”, an out vent up high, but recently I also installed “D”, below upper bench. I’m toggling between C and D for outvent to get the laminar flow action happenin’.

    Ultimately, with wood fired, venting needs no mechanical vent action. That’s the good thing there.

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