fbpx

Sauna venting: everyone has an opinion

Infinite Cedar sauna vent / chute cover slider
Guest post series continues. Venting your sauna is critical. As we look to get some fresh air thinking regarding sauna venting. And everyone seems to have an opinion. We are Pleased welcome Jeff C. back to Saunatimes! Jeff is an electrical engineer who has built his own sauna. Jeff’s thorough summary of the electric sauna stove market in the US. Part 1 is here, and part 2 is here. Welcome Jeff:

Sauna ventilation with an electric heater

I was talking to an acquaintance recently and brought up sauna. She immediately replied, “Oh I don’t like saunas, they are always so stuffy inside”. She’s right. Poorly designed saunas often are stuffy. Saunas heated with an electric heater are particularly susceptible.  It made me wonder how many people were missing out on sauna due to a previous bad experience with poor ventilation.

My sauna (electric heater) was stuffy at times despite what I thought was a pretty good vent system. It turns out I was wrong and had made some mistakes. After Glenn wrote about how the Finns were fanatical about ventilation, it inspired me to get things fixed. Through research, and trial and error, I’ve got things working much better now. I thought I’d pass along some of the things I’ve learned.

A few key points to understanding venting your sauna

  • An electric-heated sauna will not pull in air with the same volume as a wood-fired sauna does (i.e. to feed the fire). Therefore, properly functioning ventilation is critical to avoid stuffiness.
  • An electric-heated sauna must rely on either convection-driven flow (heated air rising) or mechanical means (a fan) to draw fresh air into the sauna.
  • Good convection flow is not guaranteed and is easily disrupted with an inefficient design. Factors include poor vent location, improper vent sizing, obstructions, and intake/exhaust pressure differentials.
  • Intake/exhaust vent placement can drastically affect the floor-to-ceiling temperature profile of the sauna (i.e. a temperature gradient).

So, with those thoughts in mind, here are some examples of different venting schemes along with some observations on how they affect airflow and temperatures.

The simple low height to high height convection flow

Source: Tylo Helo website

This system will provide excellent ventilation, provided that the intake and exhaust each vent from/to the same space (very important and discussed more below) . Cool air is pulled in the low vent by the vacuum created by hot air rising, which then exhausts out the high vent on the opposite side of the room. This system is nearly foolproof.

However, this system has one big drawback however. This system creates a very large temperature gradient as shown in the graphic (this is from Tylo but comparable to my experience). Imagine sitting on the top bench with your head up around 85 deg C (185 F). Your feet down on the lower bench are only at 45 deg C (113 F). It’s downright cold. Someone laying on that lower bench may not even break a sweat. This might be desirable if you have small kids in the hot room, but otherwise it’s not what I would call a kick-ass sauna.

The low height to mid height convection flow

Source: SuperiorSaunas website

You’ll see this setup recommended by many sauna manufacturers (the graphic is from Superior Saunas). This seems like a good approach in theory, but it does have some drawbacks. The idea is that air is forced through a more roundabout path before it exhausts the sauna. This results in a dramatically reduced temperature gradient. The lower bench only being around 20-25 F cooler than the upper bench (unlike above where it was around 50 F cooler).

This is the scheme I originally used in my sauna and it worked okay, but not great. Stale air would pool in the upper corners of the ceiling making it stuffy for those on the upper bench. Sometimes air flow would seem to just stop for a bit.  The problem is that convection can be easily impeded or interrupted. Restrictions from the roundabout path can cause airflow to slow with air pressure changes and obstructions akin to too many bends in an air duct. Also, heavy blasts of loyly would become a little uncomfortable sitting on the upper bench, as the humid air would stagnate at the ceiling.

My solution was to combine both high and mid-level exhausts, along with fixing some vent location problems. I will discuss in more detail below, after reviewing some other options.

The exhaust air “chimney”

This is a clever approach used by some higher end sauna manufacturers and is worthy of consideration. The idea is that the airflow uses the same roundabout path as in the previous scheme. However air flow is increased by convection within the “chimney” on the right side of the drawing (shown by the blue air in the wall). Hot air rising within the chimney creates a vacuum at the exhaust vent pulling in air despite its lower location.

I did not try this approach. But it should have a small temperature gradient and reasonable airflow. However, I would still be concerned about airflow restrictions and would add an adjustable exhaust vent at the top of the chimney. This would help vent pooled air at the ceiling, just in case it was needed.  If not enough hot air gets to the exhaust opening you won’t get a that required draft in the chimney.

Mechanical ventilation

If none of these are sufficient, or if the goal is to minimize the temperature profile, a fan can be added so that the sauna doesn’t need to rely on convection for good airflow. Note that this pertains to an electric-heated sauna only.  Mechanical ventilation can disrupt the natural drafting of a wood-fired stove and is not recommended (unless you really know what you are doing).

 
Here’s an example of a mechanically-ventilated sauna from Tylo. Compare the temperature gradient to that of the first example at the top of the post. Note that the intake on the left is now moved above the heater and is also located above the exhaust near the floor on the right. (The vent at the top right is for venting the sauna afterwards and is closed during sauna’ing).

Without a fan, this arrangement will not flow (or maybe will flow backwards) due to the poor convection and would be extremely stuffy. With a fan airflow is guaranteed with a minimal temperature gradient. Here’s another example from a Finnish sauna heater manufacturer.

This is a very similar scheme as shown in the Tylo example. The temperature gradient is excellent. Note that here the intake has been moved even closer to the ceiling. This helps ensure good air movement across the top of the sauna, and minimal pooling of stale air in the upper corners.

So it would seem that mechanical ventilation is the best way to vent an electric-heated sauna. Well maybe. Do you want to listen to a bathroom fan running the entire time during a sauna? I don’t. I have a vent fan installed in my sauna that I run after I’m done. This removes humidity and helps preserve the wood. Before I got my convection ventilation fixed I would run it occasionally during sauna if it got stuffy inside. I found the fan noise extremely distracting even though it was a supposedly “quiet” fan.

I could not imagine having a noisy fan running continuously every time I took a sauna. Perhaps an extremely high-end fan with some sort of noise insulation located at the far end of an exhaust duct might be acceptable. I would recommend anyone considering this type of arrangement to experiment first to ensure the noise is acceptable, before locking yourself into this configuration. If you want to go back to convection later you’ll need to add additional vents after the fact to get the airflow moving properly.

Configuration for venting my sauna

After some trial and error I ended up with a relatively simple convection system. It works very well and provides considerable flexibility. My system uses a single always open intake near the floor next to the heater, and two adjustable exhaust vents, one about 6” below the ceiling and one below the upper bench. All are vented directly outdoors to ensure good convection flow, and minimal air pressure differentials. By varying the opening between the two exhausts I can dial in the my preferred balance between airflow and the temperature profile.

Usually I sauna with the lower exhaust vent open about twice as much as the upper vent (shown above). This maintains a good temperature profile while preventing stale air from pooling up around the ceiling. If my kids are in the sauna I’ll open the upper vent up which dramatically cools the lower bench and floor by about 20 deg C while the upper bench stays pretty hot. It’s pretty amazing what a difference it makes. If I want to quickly break a sweat I’ll close the top vent and sit on the upper bench and it warms up a few degrees. It’s a simple and inexpensive system that works well. If it ever does get stuffy (say after a too intense loyly blast) I simply open the top vent all the way for a minute to quickly clear things out.

A couple of other lessons learned venting your sauna

One thing that can’t be overstressed is that the intake and exhaust must vent to the same place. Have either both indoors or both outdoors but don’t mix them. I made this mistake as I used a gap under my door (to the changing room) for the intake and an exhaust vent to the outside. I thought that since the changing room was vented to the outside it would still flow. It did most of the time but on windy days it would completely stop flowing due to the indoor/outdoor pressure differential. It would actually flow backwards sometimes as I could feel cold air coming in the exhaust vent. Remember, with an electric heater you don’t have the strong intake pull from a fire, so it’s really easy to interrupt the convection flow. I fixed it by adding a dedicated intake to the outdoors next to the heater about 6” above the floor.

Another lesson learned is to include generous vent sizes. For a typical family sauna make the intake at least 25 sq inches, 30 sq inches is better. Grills will also impede airflow so leave a grill off the inside (but use a screened soffit vent cover on the outside to keep the critters out). My intake is wide open (not adjustable) as I regulate flow using the exhausts only. However, I’m in a mild climate and I don’t know how well this works if it’s 10 below outside. If you choose the two vent exhaust system like I use make sure both of them are big enough so if only one is open it can match the intake size. Bottom line is you can always close the vents if they are there, but can’t open them if they aren’t.

Good luck and if others have different schemes that work well with electric heaters please post them.

EDITOR’S NOTE: As you can see, sauna ventilation is subjective. Different sauna manufacturers offer different recommendations. One of the great things about sauna, as we know, is that every sauna has its own soul. Good ventilation is critical for good sauna. How we build ventilation into our saunas may in fact be “optimized” through listening to the soul of our own sauna, and installing vents accordingly. (ie, start using your sauna, and then put in vents where you think your sauna may be whispering to you).

Jeff applied trial and error, and has achieved awesome venting in his sauna hot room. However you go, as Jeff mentions, be sure to install vent covers or “chutes.”  Then, you’ll have total control of fresh air, allowing our saunas to breathe for optimal sauna experience.

Infinite Cedar sauna vent / chute cover slider
Hand crafted sauna vent cover, or “chute” which allows for total control for venting your sauna

Other Posts You May Like

29 thoughts on “Sauna venting: everyone has an opinion”

  1. Hi Walker – It only works if the lower vent is open much more than the upper vent (at least twice as much or more). That minimizes the temp gradient from about half way up the sauna to the ceiling. Air flows across the ceiling and back down to the lower exhaust. It is still much warmer at the lower exhaust than the intake so the flow continues.

    If you open the top vent too much with the lower vent open you’ll get a larger temp gradient and flow will reverse where the lower exhaust will act as an intake and flow toward the upper exhaust (not good). At that point it’s just better to close the bottom vent if you want the top vent open a bunch (for whatever reason).

    I originally had only the lower vent and flow would stall pretty often. By adding the top vent open a small amount it adds a boost (so to speak) as some air continuously flows across the ceiling. This seems to help pull along air destined for the lower exhaust.

    I’m an EE not a fluid dynamics guy so I’m sure I’m not explaining the mechanism exactly right. I can say it does work and I can feel the difference. My inexact measurements include feeling for cold air coming in the intake (good), cold air coming in the exhaust (bad), and my wife’s complaining about stuffiness (bad).

  2. Thanks Jeff. Yeah, I can’t see how the lower exhaust vent would do anything positive really. There’s nothing that I can see to induce outflow. Even if its outlet is on the leeward side of your house relative to the supply (so lower pressure than the side of the house where the supply vent is) it’d likely not produce much flow and most or all of that would be fresh air flowing straight across the floor from the supply to the exhaust without providing any air exchange. So you’d not get the temp gradient… but also not fresh air.

    With the upper open the lower exhaust would seem likely to act as a supply unless you have a back flow in there somewhere. If it’s acting as a supply it’d likely create a worse temp gradient (and possibly less ventilation).

    Maybe there’s something but I can’t see it.

    BTW, enjoyed your heater posts. Great info.

  3. There are a couple of things that can reduce the temp gradient problem when using the upper vent.

    1) Make sure that the supply air is entrained in the rising hot air from the heater rather than going around it. This will also provide better ventilation for bathers.

    2) Have your foot bench (lower bench) at least level with and ideally a few inches above the top of the rocks.

  4. This is almost exactly the physical arrangement I ended up with for my sauna. The only difference is that my in/out venting is to the same inside space. No outside connection. I haven’t tried your 1/4 upper and 1/2 lower exhaust setup, but it makes sense and I’m excited to give it a go. I’ve been using a 12v computer fan in my upper exhaust vent, Noctua brand which is nearly completely silent. I think it’s the 90cm version, and a nice bonus is that it’s already supplied in matching brown color tones. https://noctua.at/en/products/fan
    Walter, I think the reason the air doesn’t travel directly from the intake across to the lower exhaust is the big updraft created by the stove.

  5. Great comments and questions guys, and I’ll be honest in that I can’t fully describe how the air flows but am making some assumptions based on physics and air temp measurements. Reasonably certain Mike is right in that the strong updraft from the heater (provided the intake is close) impedes air from travelling across directly to the lower vent. Some does travel this path I’m sure but I don’t think it’s significant. That’s why I show the curved arrows in my sketch. Unfortunately this is all trial and error, it would be nice to model this but I don’t have the right software. I might hang some tissue strips within the sauna and at the vents to gauge airflow. At some point though this turns into a science project and sauna looses it’s mystique.

    Since both my exhausts vent outdoors and are aligned vertically, I am going to try adding a “chimney” on the outside wall rather than having them vent straight outdoors. Probably just box it in with some 1x lumber with a screen at the top. This should help prevent the lower vent from pulling in cold air with the top vent open.

  6. Hey Glen, my name is Jack. I live in Missoula Mt. I’m a bit fan of the podcast as well as a long time lover of sauna. I grew up saunaing in northeastern South Dakota on a lake cabin of my Scandinavian families. I’ve missed it ever since being it west my adult life. Now I’m changing that and am committing to building a mobile sauna! It’s on a 6×10 trailer. I was wondering what ur option is on framing it out with 1 3/4’’ pieces?

  7. Hi Jack:

    Glad you are digging Sauna Talk. Regarding framing out your 6×10 trailer, we gotta keep weight in mind as primary constraint. Secondly, is weight differential..(keep stove over the axle as ever close as possible).

    There are generally two build methods for mobile sauna building.
    1. Knees up – this involves purchasing a base trailer, and framing up from there. We recommend (highly) integrating metal studs and framework within the base frame. Then use tapping screws to fir out the metal studs with 1×2 nailers, et voila, you’re off to the races.
    2. box trailer – this involves purchasing an off the shelf ice house or box utility trailer. In this case, the framework, or shell is in place. Here, you’ll fir it out like above, installing nailers for paneling, etc.

    Now, you’ll be getting into insulative decisions and for that, there’s lots to consider. Keep in mind “for ever product available, there is equal and opposite opinions of the product.” Especially from free range organic ethos (I say this with compliment). Yet at some point, we accept the service temp. ratings issued by the mfr. as well as their trade council (The cigarette salesmen saying that, well, you know.). More here.

  8. Mike, it’s possible, especially in a warm climate like that, but air can be extremely stubborn and does what it wants. In colder climates during winter it’s pretty much impossible. This is why in Europe with under bench exhausts the supply will always be above the heater because that is the only way to get it to rise and even at that some chunk of it finds its way down to the floor and so provides no ventilation benefit to bathers. A low supply to low exhaust will pretty much always flow across the floor.

    One item on my agenda is to play with introducing the supply air in to the middle of the rocks (Helo Himalaya) to see how that works as well as a few different configurations of micro-vents above the heater to introduce the supply air more gradually and hopefully have more of it benefit bathers to reduce CO2 levels.

    That Jeff is seeing such temp stratification tells me that the supply air is going around the heater. If the supply air is getting entrained in the rising plum from the heater there will be minimal stratification above the top of the rocks and a bit more below but still not as much as Jeff makes it sound like he has.

    Mine for instance is a low supply w/ a convection exhaust in the ceiling. My temps are 90°c @ 1m above the bench, 86°c at sitting bench and 78°c at the foot bench. You don’t really notice stratification so much as that your feet just aren’t really hot.

    The big issue is that there is nothing that I can see to induce outward airflow in his lower exhaust vent. In a heated space like this only about the upper 1/2 has positive pressure to atmosphere. Generally the higher you go the more pressure (but other things can vary that). The midpoint will tend towards atmospheric pressure (so zero flow in or out) and below the midpoint will be negative to atmosphere so will want to pull air in. That’s why below the bench exhaust vents always have a blower.

    All that said, there are lots of surprise exceptions to expectations but they never violate physics.

  9. Jeff, smoke can be a huge help with experimentation like you’re doing. Regin makes a number of products that generally work well. My concern is in not polluting a good sauna w/ chemicals. I’ve no idea if that’d be a real issue or if they have products that would not do that. Dry Ice (CO2) can also provide some information though you have to keep in mind that it naturally sinks and dissipates fast.

    Have you thought about doing some type of venturi in your chimney? That could help to get a bit of flow from your lower vent.

  10. Good ideas Walker.

    If anyone wants to try and model venting Wiki has a pretty good summary of what’s called “buoyancy-driven ventilation” (i.e. hot air rising).

    https://en.wikipedia.org/wiki/Passive_ventilation

    Scroll down about half way and you’ll find the equation. It’s an approximation at best as it doesn’t include effects from obstructions (e.g. turbulence).

    Some sauna manufacturers recommend 6 air changes per hour but this seems excessive to me, particularly with an outdoor sauna in a cold climate as it causes significant heat loss. Maybe 2-4 changes per hour is more reasonable.

    As an engineer I’m hell-bent on stripping all joy out of sauna and reducing it to nothing more than hard facts and numbers! Haha.

  11. Walker wrote, “A low supply to low exhaust will pretty much always flow across the floor.”

    Disagree with this as do many sauna manufacturers who recommend exactly this configuration (as shown in some of the pictures I posted above). An intake close to the heater that immediately heats the incoming air is the difference and what forces the roundabout path across the ceiling (in my opinion).

    Bottom line is that without empirical measurements or a detailed model this is all speculation. That’s the whole point of trial and error, experiment and use what seems to work for you.

  12. Been playing around with the ventilation equation I referenced above and it starts to make sense why air doesn’t flow directly from the intake to the lower exhaust vent (provided it is immediately warmed by the heater). As expected, it’s because the buoyancy the air experiences when rapidly warmed. The equation includes the term for gravitational acceleration which is fast and has a per second squared component (i.e. it accelerates). The air shoots up much faster than it can cross the room.

    You can visualize this by picturing an inflated ball held underwater. Try throwing the ball horizontally and it will shoot to the surface before it’s gone very far sideways. Same idea at work here.

    This assumes that 1) the intake is right next to the heater, right beneath it would be optimum and 2) the exhaust is not right above the intake but on the other side of the room.

  13. Hi Jeff, The only sauna manufacturer I have ever seen recommend it is Tylö-Helo / Finnleo / Amerec and for them only in North America. I have also taken them to task on it because it’s a bad and potentially dangerous scheme that doesn’t work. And I’ve tested it just to be sure there wasn’t some oddity about sauna that causes it to work (there’s not that I can find).

    In every country other than the U.S. Tylö-Helo, Harvia and others, recommend either a passive/convection or powered system of low supply and high exhaust that can work moderately well or a powered system of high supply and low exhaust. https://www.tylohelo.com/sauna-room-ventilation.

    The reason is that it is impossible to get sufficient convection exhaust flow with an exhaust vent that low. Think back to your thermo classes at uni. Convection exhaust requires pressure inside at the exhaust vent to be somewhat significantly greater than pressure outside (wherever the exhaust vent outlet is). This pressure difference must be great enough to overcome all static pressure and stack effect (hot air rises) in the supply and exhaust systems (ducts, etc.).

    In your superior saunas example… Keep two principles in mind; hot air rises (and sort of cold air sinks), air (a gas) flows from high pressure to low pressure (gases equalize their partial pressure within any space resulting in the gas mixture equalizing its pressure within the space). With no vents or openings in the sauna there will be pressure stratification with higher pressure at the top (hot air rises) and lower at the floor. As soon as you place an opening near the floor then the floor pressure will equalize to atmospheric pressure and pressure stratification will go from atmospheric at the floor to something greater at the ceiling. E.G., pressure increases as you go higher in the sauna.

    If you open a vent at 2′ above the floor then you are doing so at a point where the pressure is barely above atmospheric. If the supply vent and exhaust vent are each simple 5″ round ducts, straight through the wall and of perhaps 8″ length then you MIGHT have enough pressure to overcome static pressure losses and have some airflow in the lower vent and out your 2′ high vent. Perhaps 0.3 CFM of airflow (and you need much more than that).

    All of the pressure that you need for convection is in the space above the vent. And it stays there. Think about an upside down glass of air pushed down in to a bathtub of water. The air in that glass that doesn’t flow down and out is the air and pressure in the space above the highest functioning vent opening in a sauna.

    Now move the vent to 3′ above the floor. You’ll have a tiny bit more pressure and so a tiny bit more airflow out.

    Now ADD a vent at the ceiling. You now have three vents; floor, 3′, and 8′ (for convection flow it doesn’t matter where these are – they can all be on the same wall or three different walls in corners or the middle of the walls). The neutral pressure plane is now at about 4′. Above this is positive pressure to atmosphere and below it is negative pressure to atmosphere. The higher you go above the NPP the greater the positive pressure and the greater the potential airflow OUT of any opening will be. Likewise, the lower you go below NPP the greater the negative pressure to atmosphere and the greater the potential airflow IN any opening will be.

    So, making some assumptions about room size, shape, temp @ ceiling (100°c for easy math), vent sizes (all are 5″ round vents of 8″ length straight through the wall) then likely airflows are perhaps 11.8 CFM IN the floor vent, 0.2 CFM IN the 3′ high vent (it’s below NPP but only by a little) and 12 CFM OUT the upper vent.

    Now, if you move the 3′ high vent down to 2′ it will have greater airflow IN as it is farther below NPP and so at a point of greater negative pressure to atmosphere – its value has increased.

    This is for buildings but the same thing happens in sauna: https://www.buildingscience.com/documents/insights/bsi-075-how-do-buildings-stack-up

    BTW, analysis doesn’t strip the joy out of it at all! The analysis itself is satisfying and the result should be a much better experience for everyone for years to come. FWIW, I’ve 6 temp probes, 3 CO2 sensors, and 1 hygrometer all connected to data loggers along w/ a manometer & pitot tube and vaned airflow meter. I must be more joyless than you? 🙂

  14. Jeff, I forgot to add, if you have some explanation for how that superior saunas ventilation can work I’d love to hear it. This is pretty fascinating stuff. From a physics standpoint and in my testing I can’t get to it.

  15. Walker – Good stuff and I’m working on a model to better quantify this. Just running the straight equations does show significant airflow to both the upper and lower vents even if doesn’t tell us anything about the path taken.

    Maybe I’m misreading what you wrote above but our numbers to the lower vent are off by two orders of magnitude from each other, so one (or maybe both) of us are making a big calculation error. I’m using the buoyancy airflow equation I’ve linked above, which is backed up by several other sources.

    If you work through the equations even with only a 2.5 foot rise there is a significant pressure differential between the intake and exhaust. Assuming a 2.5 foot height increase going from 60 F (outdoors) to 150 F (indoors) a 4″ by 4″ vent will flow 33 cfm resulting in 6-7 changes per hour for a typical family sauna. This assumes moderate flow restrictions (discharge coefficient of 0.5). I’ll admit that the discharge coefficient is the big unknown here, particularly if the air must travel a circuitous path.

    So unless I’m making a huge calculation error (possible but I don’t think so) there will definitely be airflow to the lower vent even if it does pool at the ceiling for a bit until it cools enough to be forced down to the lower vent.

    Another important point it that the upper vent is (in my sauna) at 6 feet above the lower vent whereas the lower vent is 2.5 feet higher. Thus air to the upper vent will be travelling faster than that to the lower vent (due to gravitational acceleration) as shown in the equations. Not surprisingly this is why the lower vent must be open much more than the upper vent to have comparable airflow (which I found by trial and error). Assuming comparable path restrictions, airflow to the upper vent moves about 1.8x faster than that to the lower vent using my vent heights. Airspeed is driven by height differential and temp increase from outdoors to indoors.

    So all of this backs up what I’ve found by trial and error but I don’t understand why your numbers show such a low volume moving through the lower vent. We’ve got a huge disconnect somewhere between our calculations.

    BTW, Looks like MIT has a pretty cool natural ventilation model you can download for free that might be helpful. I’m going to try this when I get a chance.

    http://coolvent.mit.edu/

  16. You are using the equation under “Estimating buoyancy-driven ventilation”?

    That equation assumes that the lower vent is at floor level and the upper vent is at the ceiling (E.G., it does not include the overall dimensions of the enclosure nor the locations of the two vents relative to the floor and ceiling of the enclosure) so in your calcs would work somewhat OK for a 2.5′ high sauna. That’s a tad small for my tastes.

    The flow rate of an exhaust vent not located at the ceiling is dependent on its position relative to the neutral pressure plane and the ceiling (E.G, what is the pressure at this level of the room) as well as the flow rate across the neutral pressure plane (so you have to know the flows of every supply below NPP and every exhaust above NPP which, IIRC, is an iterative series of iterative equations.

    My guess is that the MIT program does this for you (it’s actually not complicated, just laborious). It should, I think, ask for room dimensions, room starting temp, room heat source (amount offered + location IIRC), and then for each vent; the exterior temp, exterior pressure, vent static pressure, vent size, maybe vent shape (round is more efficient than rectangular though this is sort of covered w/ static pressure), and height in the room.

    This would give you a very basic idea. Another factor that s/b included are room/route static pressure (e.g,, obstructions to airflow within the room such as sauna benches) but my guess is that they always assume an open space. Ideally they should give you a chart indicating a range of flows across a range of exterior pressures as these can make a very significant difference.

    Keep in mind that each vent has varying value based on its location relative to NPP. A very slight change (size or open aperture, static pressure, etc.) to the vent farthest away from NPP can have a massive effect while huge changes to vents near NPP can be negligible (or about zero if @ NPP).

  17. Jeff, I played w/ CoolVent a bit. Great find. We’re 100% Mac now so took a bit to get windows up on an old Mac.

    It seems to work accurately for most structures but not for a sauna. My attempts at modeling a sauna consistently showed 18CFM of airflow in and out (and 2 levels even though I’d said 1) no matter where vents were located. Strange. So I tried modeling my own house and got good numbers. Also good numbers w/ my studio and a fictitious high rise building. Have you tried it? Results?

    I may try simplifying my studio down towards a sauna to see if there’s one element that’s throwing it off.

  18. Nice talking with you (and Eero and Blake) today Glen. I was recently reading about Wim Hoff so it was interesting to read your blog about meeting him. I have a cold plunge pool outside my sauna that is a refreshing dip after the hot room. Sometime we get enough snow to roll around in but not too often here in southern Connecticut.

    I signed up for your newsletter too. Ive only briefly perused your website but it looks like great info!

  19. I’ve been running this by a few folks and there is some interesting debate about it. From a purely mathematical standpoint it is possible that the basic buoyancy equation may, contrary to what I said above, hold up for two vents regardless of ceiling height. So a supply vent at the floor and an exhaust at 2.5′ will flow approximately the same with a 2.5′ high ceiling or 8′ high ceiling.

    Even a small vent/hole closer to the 8′ ceiling will change things dramatically though so if you have both a 2.5′ and 8′ vent then the 8′ needs to be well sealed for the 2.5′ high vent function well.

    What numerous people have pointed out though is how insufficient any math/computer modeling is for something like this in real life. There are too many things that can alter how everything works in reality, especially for a convection system that is susceptible to even very minor changes. That said, I’m still interesting in exploring the models.

  20. Would be grateful for advice on ventilation in my particular case.

    I am having an 8’x12′ shed built (in Brunswick, Maine) to accommodate a sauna plus change room configured much like one of those in the Saunatimes 3D Warehouse sketch. Particularly the one where the wood-burning stove stands at the intersection of the 12′ exterior wall and the 8′ interior wall separating the sauna from the change room.

    I expect the stove to be one of the Harvia SL models — so the fire chamber is loaded not from the sauna room but rather from the change room.

    What would be good placement of vents in this case?

    – Does the lower vent in the sauna room serve exactly the same purpose as in the standard case, notwithstanding the fire chamber being loaded from another room?

    – Is a gap under the interior door a good substitute for a separate low-placed 4″x10″ vent on the interior wall abutting the stove?

    – Is a high place on the other end of the same interior wall a good place for the upper vent?

    – Given that the fire chamber is loaded from the change room, would I do well to install ventilation from the exterior to the change room, in addition to vents from the sauna to the change room? And if so, where?

    Many thanks!

  21. i use the second method (middle vent) and seems to work well. my electric heater is in the corner of the hot room, on the wall shared with the changing room, with an intake vent down low under the heater. for exhaust, i wanted better circulation than simply placing a vent just a few away from the intake so i built a horizontal wooden ‘duct’ underneath the upper bench that connects to the changing room. i have a damper under the bench at the opposite corner of the hot room so intake air is drawn up through the heater and travels across the ceiling to the opposite side of the hot room and then through the duct back to the hot room. you can definitely feel more air being sucked in the more the damper is open.

  22. Stephen, I think start with what your stove manufacturer says.

    You need air for two purposes; combustion and occupant health (keep CO2 levels down). Does your stove pull combustion air from the changing room, the sauna or both? How much does it need? Where on the stove does it pull combustion air from?

  23. Hello,

    Thanks so much for this information. Definitely, a lot to think about. I am in the process of converting my shed into a sauna. I have a small changing room and then the 7″ x 5″ sauna. I was going to put a vent under the electric heater into the changing room and then an exhaust vent to the outside but you have my rethinking that. Separate from all that however is do you have a cheap recommendation for the actual vent itself. Something that I could purchase from Home Depot for example. Thanks for your information on this.

  24. Yes, Marc! Your ears are ringing. Launching a post on this right now! (crazy how the sauna world whispers and listens). You’ll have link to the exact product that you can put into your cart whilst navigating the Home Depot aisle.

  25. Bought your book. Thanks…great stuff!

    Going to embark on a build soon. 8×12 shed build with 7.5x7x7 hot room.

    Door will go from small changing room into sauna with appropriate gap at bottom of sauna door for air intake. Stove will be near door with benches on far wall.

    Where to put vent?
    -Interior wall back into changing room, high up on opposite side of door?
    Or
    Far wall above benches to outside?

    Sauna will be in NW Montana…so can get cold in winter.

  26. Erik:

    I’d go with an exterior vent to the far wall above benches to the outside. But as you can read above, everyone has an opinion, even (especially) those in the business. Please read my comment to Liz towards the bottom of this post.

    This is my stronger suggestion for you, as to where to put your vents. Happy sauna to you!

  27. I just posted a comment on the other newer sauna ventilation post where I discussed a now vanished product from Finland called Saunair. that name exists on a current australian site, but does not resemble the one i am referencing . if I am going to try incorporating Saunair concepts, – 50C sauna temp switched low volume outside sourced airflow, ceiling room inlet positioned in front of bathers faces on top laude,- use of a gaming computer cooling can seems to make a lot of sense. these move much less air than conventional bathroom fans. And they can be extremely quiet. such fans can be purchased form a number of websites as well as brick & mortar stores such as microworld. sauna with fresh air ventilation of this sort should reverse the common convection airflow and release overpressure exhaust air thru a gap at the entry door bottom. this should reduce stratification and temp gradient from bathers head to legs.

Leave a Comment

Blog Categories

Latest Sauna Talk Episode

Kick Ass Saunas

Stay in the

Authentic sauna loop

Receive Monthly Updates on the Latest in Authentic Sauna!