Underwriter’s Laboratories vs Sauna.


Many in the U.S. who have built saunas with an electric heater have faced problems of the sauna not getting as hot as desired or the high temp limiter tripping causing the heater to shut off until it cools.

These, along with other issues, are the result of requirements from Underwriters Laboratories, UL, that conflict with what sauna is.

The biggest problem though is that in their attempts to get around the problems caused by errant UL requirements, people are negating the safety elements that UL are trying to promote and effectively making saunas much less safe than if UL had reasonable requirements that didn’t encourage people to circumvent them.

There’s a very fundamental problem here. When anything is too strict it risks creating more problems than it solves. It creates a practical problem when, for example, people completely bypass a safety measure. It creates a bigger problem when it erodes our regard for an institution. This latter is easily seen on roads in the U.S. We use stop signs in places that should have a yield right-of-way and then wonder why drivers blow through stop signs so frequently. Or roads are designed for 70 MPH, signed for 45 MPH and then we wonder why so many people drive 60. This and other stuff has resulted in little regard for U.S. traffic laws (and laws in general). That’s not good and is a key reason why we have the most dangerous roads of all developed countries.

Disclaimer: The following should be taken as my personal observations and not based on any official conversations with UL, heater manufacturers or others.

Important Note (Jan 2024): UL are expected to update UL 875 to conform to EU standards in the coming months …or years.  This is VERY GOOD NEWS.  This should eliminate the need for an air supply behind/below the heater to cool the over sensitive high limit sensor, allow proper placement of the thermostat sensor over the sitting bench and allow for temps higher than 90°c. Overall this will be a big improvement for North American sauna users and builders. More on this to come.



Following are what appear to be requirements from UL for electric sauna heaters in the U.S.:

  • Room Temp at the thermostat temp sensor is limited to 90°c (194°f) which is lower than international standard of 80-105°c and much lower than the 75-120°c that many people prefer.
  • Thermostat Temp Sensor must be placed over the heater where it provides an inaccurate reading of actual sauna temp.
  • Heaters must include a High Temp Limiter with a very low trip point that results in nuisance trips and inability to use the sauna until it has cooled which can take an hour or more.
  • Run time must be limited to 1 hr so must be manually reset frequently.
  • Rock guard on top of stones

This results in several negative and perhaps unintentional consequences for U.S. consumers:

  • Health Problems – Sauna does not get hot enough to kill mold & bacteria on the foot bench.
  • Health Problems – Poor ventilation and high CO2 for U.S. saunas
  • Consumers and sauna builders frequently circumvent UL requirements in order to have what is considered a normal sauna elsewhere. This results in increased risk and sub-optimal saunas for a number of reasons such as inability to accurately control the temperature.
  • Wasted Energy – Cold air must be brought in to the sauna to cool the High Temp Sensor so that the heater will operate.
  • Decreased Heater Selection for U.S. Consumers
  • Increased Heater Costs for U.S. Consumers. Heaters that meet the draconian UL requirements cost about 3x as much as the same heaters in the rest of the world.


What Is A Sauna

The International Sauna Association defines sauna as:

Sauna bath – Saunaing is a healthy and relaxing hot air bath, alternating between warming up and cooling off. When taking a sauna, the whole body is heated several times in a wooden-surface room with a typical temperature of about 80-105 º C, measured from a height of about 100 cm above the level of the upper sitting bench. Warming is followed by cooling in the open air or with cold water and then the process repeated as often as the bather desires.

Sauna room – The sauna is a wood-paneled room with stepped benches, a stove with stones, a room temperature of about 80-105º C measured at a height of about 100 cm above the level of the upper sitting bench, and low humidity, which is briefly increased by throwing water on the stones to create steam.

In addition, experts on sauna rightly say that ventilation for occupants – to remove exhaled CO2 as well as humidity, odors, other VOCs and Pathogens – is critical.

These requirements are difficult or impossible to meet with an electric heater and installation that is required to meet current UL guidelines.

Note: I am a proponent of UL. They have made our homes, businesses and other facilities much safer. In this case however, due to apparent ignorance about saunas, not fully thinking through the downstream consequences of their requirements, and putting consumers in a position of having to circumvent UL guidelines in order to have a working sauna, they are creating more problems than they are solving.


Some UL Requirements:

Following are what appear to be some requirements from UL for electric sauna heaters in the U.S.

1 – Room Temp – The room temp as measured at the thermostat temp sensor must not exceed 90°c (194°f).

This temp is lower than the 80-105°c range officially recommended by the International Sauna Association and much lower than the 75-120°c range preferred by many sauna users across the world and recognized by members of the International Sauna Association and Finnish Sauna Society as preferred sauna temps. 

It can be worse if the temp probe isn’t accurate (most are not). Ours reads about 3.5% higher than actual so when it thinks it is 90°c, reality is about 86.7°c.


2 – Thermostat Temp Sensor Placement – For a sauna heater with a remotely located temperature sensor. The thermostat temp sensor (for the room temp) must be placed above the heater and no more than 6” down from the ceiling. There is some conflicting information on what ‘above the heater’ means.

Since the area above the heater is much hotter than all other areas of the sauna, this placement results in an actual room temp (officially recognized as measured at 1m above the sitting bench) of much less than what the thermostat is indicating. So, while the thermostat might be indicating 194°f, the actual sauna temp (at 100cm above the sitting bench) will often be 150-175°f. The result is that it’s often impossible to reach even the lowest heat of International Sauna Association recommendations.

Outside of UL controlled territory the thermostat sensor is typically mounted 1m above the sitting bench (e.g., on a wall opposite from the heater) so that it provides an accurate reading of the temp at bathers heads – which is what we actually want to know.


3 – Heater High Temp Limiter – Heaters must include a High Temp Sensor (separate/different from the thermostat temp sensor). If the temp at this sensor exceeds a preset value then a high limit switch must trip to turn the heater off until the temp at this sensor drops below a predetermined value when it must then be manually reset by the user.

The required set point for this is extremely low, prevents the heaters from working properly, saunas from heating to appropriate temperatures and result in nuisance trips.

When this trips it becomes mostly impossible to continue using the sauna for at least an hour or three as it can take one to four hours for the heater to cool enough that it can be reset and the room then reheated to proper temps. 

This has resulted in manufacturers recommending an airflow scheme in North America that, rather than provide critical ventilation to bathers, instead cools this sensor by bringing fresh air in at the level of the heater rather than higher above the heater.

The biggest problem is that this ventilation scheme does not adequately remove CO2 nor provide fresh air to bathers resulting in poor air quality in North American saunas.

This also results in increased stratification, particularly when it’s colder outside, as this air flows down which decreases the floor temp and thus increases stratification. This can result in a foot bench temp of 5-13°c lower than without this HT cooling vent.

When it’s below about 5°f outside then ventilation (that removes CO2 and other harmful contaminants) must be reduced or turned off completely as the cooler air causes the heater to work more than the High Temp Sensor will allow.

Resetting this when it nuisance trips can also be a risk for older people who are not able to easily and safely crawl around on the floor to reach the reset button.

Important: I am not against having a sensor such as this if it provides improved safety. I think it must have a higher set point though that allows saunas to operate properly.

Note that NONE of these are a problem outside of areas controlled by UL. 


4 – One Hour Timer – A timer must limit the amount of time that the heater can be on. Users must manually turn the heater/thermostat back on once per hour.

Firstly, I don’t at all disagree with having such a timer.

This is however a nuisance and more so when users forget to turn the heater back on when the 1 hour timer is up. Many people have complained about forgetting to turn it back on when warming up so the sauna is then not ready when they expected it to be. Similarly forgetting to reset the timer when using the sauna. This is a particular PITA when the sauna is in a separate building and the user must trudge through -20°f temps and deep snow just to reset the timer. A longer duration, like perhaps 2 – 4 hours, would be better and I think should largely accomplish the safety goals that UL is aiming for.

Similarly, manufacturers could include a reminder on their app that notifies the user when the timer is about the expire and provide them with an easy way to reset the timer.  And similarly perhaps a somewhat loud beeping on the thermostat itself to provide a 15 minute warning perhaps.


5 – Rock Guard – UL requires a guard on the top of the heaters. This can reduce the amount of stone in the heater and I’m not sure what purpose it serves as it gets quite hot as well. If someone fell against the heater I’m not sure there would be any or much difference in their injuries with or without the guard. The guard can presumably prevent stones from falling but I’m not sure that’s a big problem.  Perhaps requiring that the guard be included with heaters and recommended to use but not required would be a good option.


6 – Ceiling Limited to 7’ – Tylö-Helo states in one document that this is a UL requirement. A Finnleo (owned by Tylö-Helo / Sauna360) employee has told me several times that UL states that the ceiling cannot be more than 8’ high.  From what I have been able to determine, both with my own reading of UL documents (particularly 875.42 that deals with construction and doesn’t mention anything about ceiling height), discussions with UL and discussions with a member of the UL advisory panel, this is NOT actually a UL requirement. 

For numerous reasons a 7’ ceiling is a bad idea for a sauna and an 8’ less than ideal. A height of 8.5’ is generally recommended for smaller residential saunas, and higher for larger residential saunas and commercial saunas that need a larger heat cavity.


Heat Stratification

Heat Stratification plays a critical role in a sauna and is important to this discussion. It is because of heat stratification that people find benefit in placing thermostat sensors lower than UL requires or that mold and bacteria so often grow in the foot benches of U.S. saunas rather than being killed.

Air in a sauna will be hottest nearer the ceiling and then increasingly cooler each inch below that until we reach the coolest part of the sauna at the floor.


It’s important to note that the numbers in this chart are averages. Some saunas will have somewhat less stratification and others will have somewhat more. Stratification is a very powerful force but can be slightly modified by room shape, heater configuration, airflow and wall temp.


Unintended Consequences – Health Problems

The UL requirements have created three critical health problems in U.S. saunas; Increased bacteria on the foot bench, poor ventilation resulting in high levels of CO2 and other gook, and causing burns to sauna users attempting to reset after nuisance trips.

1 – Mold & Bacteria – Saunas are ideal breeding grounds for mold and bacteria and this particularly on the foot bench. The only way that I am aware of to kill mold and bacteria in porous surfaces like the wood in a sauna is heat. Chemicals cannot penetrate deep enough in to the wood (nor do we want to use such chemicals in a sauna). Finns and Swedes rightly say that heat is critical to hygiene.

To kill mold and bacteria the foot bench must be able to be kept at 55-70°c (131- 158°f) for a period of about 20 minutes. The actual temp depends on what mold and bacteria are present with many common molds and almost all bacteria needing at least 65°c (150°f) temps for 20 minutes to kill them.

Because of heat stratification the sauna must be heated to higher temps at the ceiling to achieve the necessary good hygiene temps down at the foot bench. In a sauna with a 9’ high ceiling a temp of 194°f at 6” below the ceiling above the bench will give us 165°f at the foot bench so this should work. A sauna with an 8’ ceiling can reach only about 152°f at the foot bench – cutting it close but may do the trick. A 7’ ceiling will likely have a foot bench temp of 131°f which is not hot enough.

But it’s impossible to have a temp of even 194°f in a sauna if you follow UL guidelines. If the thermostat temp sensor is placed according to apparent UL requirements then the temp over the benches will be lower. When the temp at the thermostat sensor is at the 194°f maximum allowed by UL, the actual sauna temp above the benches might be only 180°f and so even in a sauna with a 9’ ceiling the temp at the foot bench might be only 130°f. And these BTW are the actual measurements.

It’s important to note that not all mold and bacteria are harmful to our health. Some are, some not, some only at higher dose/exposure levels and some are actually beneficial at limited dose/exposure levels. But in general we don’t want mold or bacteria in our saunas. Nor the odors that come with them.


2 – No Ventilation and High CO2 for Bathers – Like us, heater manufacturers found the UL required High Temp Limiter to be a problem.

To remedy this problem they chose to use cooler outside air to help cool the sensor. However, rather than devise a strategy to provide both critical ventilation to bathers and also cool the High Limit Sensor (as recommended in the Trumpkin Ventilation Notes), they chose to recommend only cooling the High Limit Sensor.

In their U.S. manuals they instruct installers to direct fresh air in to the sauna from behind or below the heater – to cool the High Temp Sensor. As we saw in Trumpkin’s Notes on Ventilation, little or none of this air will provide ventilation for human occupants as the majority of this air will simply flow across the floor (colder air sinks) and go up the back wall to the exhaust opening. The result is high CO2 levels for bathers as well as little or no removal of excess humidity, VOCs or pathogens. In other words, bad stale air.

This is what physics says will happen, is what VTT in Finland proved will happen in the sauna research they did in 1993, is what measured CO2 levels in our sauna indicate happens, is what Saunum found when they measured CO2 and is what anecdotal evidence indicates happens in most U.S. saunas.

While their manuals in Finland and elsewhere recommend airflow strategies that do provide ventilation to bathers, their manuals in the U.S. recommend different airflow strategies that cool the heater high temp sensor but provide little or no ventilation to bathers. Why the difference?


3 – Burns – Two people have reported being burned when they were trying to reset the High Temp Limiter after a nuisance trip. 


Unintended Consequences – Circumventing UL Requirements:

Above we discussed how heater manufacturers circumvent the High Temp Limiter by having air blow on the sensor to cool it. Because the UL requirements are so overly restrictive we see consumers doing the same, circumventing UL requirements to have sauna temps more in line with what people in other countries enjoy.

A regulation is only as effective as its adherence. If people are frequently circumventing the regulations then the regulations are either not effective or may actually be resulting in greater risk/harm. (For another perspective on this: A Wink and a Nod: Teaching Our Children To Be Criminals)

1 – Thermostat Placement – The most common is placing the thermostat lower on the wall and away from the heater. The thermostat is then in a cooler part of the room (thanks to heat stratification) and so allows the room to get hotter. So far, so good. However, this causes two further problems;

  • Misleading Temp – The thermostat controller on the wall may say 190°f but the room temp might now actually be 215°f. This is perhaps only a minor problem for the owner who knows this but could be a significant problem for guests who might be expecting a temp of 190°f but then find the actual temp to be much higher.
  • Much Greater Temp Variation – An electric heater controls the room temp by turning on and off to maintain the temp within a certain range. If you set the thermostat to 190°f for instance, it will turn on when the temp drops below 185°f and then turn off when it exceeds 195°f. The temp in the room then cycles within this range resulting in an average of about 190°f. The tighter the range, the more comfortable the sauna. If someone wants 205°f, a very commonly desired temp, they might place their thermostat sensor about 2’ lower and 12-24” to the side so that the actual room temp (higher up) is 205°f when the thermostat maxes out at 194°f. However, there is much greater temp variation up higher. So rather than the desired 200-210°f temp range they’ll have an actual of range closer to 195-230°f. That’s not good.

2 – Wet rag on the Thermostat Sensor – Some people carry a wet rag in to the sauna with them and drape it over the thermostat sensor to keep it cooler so that they can get the sauna hotter. This is such a problem in public saunas in the U.S. that they often have signs explicitly stating not to do it.

3 – Modifying the High Temp Limit Sensor – There are frequent discussions in online communities about ways to circumvent the High Temp Limiter. A common method is, similar to the thermostat temp sensor, to move it away from the heater so that it doesn’t trip. I’ve a collection of dozens of similar photos to this.

Sauna HLSensorFix

Others have recommended placing a wet rag on it or replacing the sensor with a fixed resistor that essentially fakes out the control circuit.

4 – Wood Heaters – Many people will install wood heaters rather than electric simply to get around the UL restrictions. And this when they’d otherwise install electric. I personally quite like wood heaters but they are much more dangerous than electric heaters from a fire perspective and this as much in Finland without UL restrictions as in the U.S. Saunas heated with wood also produce air pollution which may not be healthy to breath.


Unintended Consequences – Wasting Energy:

The UL mandated over temp sensor has too low of a set point for heaters to work properly so manufacturers found that they have to require a cold air inlet near the heater to help cool the heater and sensor to keep it from tripping during normal operation. Adding this unnecessary cold air to the sauna results in wasted energy heating this same air. 

If the ceiling height limit is accurate then this prevents having a large enough heat cavity to store heated air each time the door is opened resulting in loss of heat, increased colder air, discomfort for bathers and the need to re-heat the air.


Unintended Consequences – Decreased Heater Selection for U.S. Consumers:

Whilst consumers in much of the world, Europe and Asia in particular, have a wide selection of electric sauna heaters, that is not the case in areas that recognize UL including the U.S. and Canada. Manufacturers say that the restrictions that UL places on heaters are onerous and expensive to meet so many manufactures will not import any of their heaters to the U.S. and the few who do import only a very few. This results in not only much less selection from an aesthetic standpoint but critically sub-optimal saunas as in some or many cases the heater that would be most appropriate for a sauna is not available in the U.S. so a less optimal heater must be used.


Unintended Consequences – Increased Costs for U.S. Consumers:

There are two problems; increased manufacturing costs and decreased competition.

Manufacturers face two problems. First is increased costs to manufacture the heaters to begin with and second is that they must manufacture, pack and stock different heaters for UL markets than others.

There is much less competition so manufacturers are able to charge considerably more.

The result is that heaters in UL markets often cost about 3x as much as the same heaters elsewhere. A heater that’s $2400 in Minnesota will be only about $800 in Europe or Asia.


If the UL guidelines resulted in significantly safer heaters and saunas then these could simply be considered the costs of having a safer environment to live in and that is indeed the case with many UL requirements but it does not appear to be the case with electric sauna heaters and in fact quite the opposite as we have less selection and higher prices to make saunas that are less healthy.


What Should UL Requirements Be?

Finns, Swedes and others safely use electric saunas today and have for years without these overly restrictive UL requirements.

These temps are also commonly and safely used in the U.S. without problems in wood heated saunas and electric saunas in which people have circumvented UL requirements.

If the purpose of these is to keep people safe and healthy then it’s important that they be guidelines that people will follow rather than circumvent.

U.S. consumers should be able to have an electric sauna equivalent to those in other countries and that can maintain temps of 120°c or more at a point 100cm above the middle of the upper sitting bench.

1 – Allow for a sauna room temp of 120°c or higher. Some people for example prefer 94-98°c temps for their daily sauna but then perhaps once each month like to do a 115-120°c sauna. This is safe and they should be able to do this if they choose.

2 – Allow for proper placement of the Thermostat Temp Sensor over the sitting bench rather than directly over the heater. Specifying a distance from the ceiling of no more than 12” will allow it to be placed properly at 100cm over the sitting bench if the sitting bench is within 48” of the ceiling as it should be.

3 – Eliminate the High Temp Sensor or allow for a high enough temp that the sauna can heat to at least 120°c when used normally.

4 – A 4-hour time limit would allow users to warm the sauna up without problems of forgetting to reset the shorter 1-hour timer and would reduce incidences of forgetting to reset the timer while using the sauna. This is also something that manufacturers can help with, at least for WiFi controlled heaters, by sending the user an alert that the timer is about to expire and so reminding them to reset it and providing an easier way to reset than turning the heater off and then on again.


Manufacturers Interests are not necessarily aligned with consumers interests

The above is all from a consumer perspective. Heater manufacturers have an interest in this as well, but they are not necessarily aligned with consumers. 

There are two strategies that product manufacturers can pursue; Cost priority and Product priority. Both then balance costs, margins, pricing and volume to achieve their profit goals.

With Product Priority a company focuses primarily on product quality and sells that. They will usually have a higher cost basis than companies focused on cost priority and so also higher sales prices. Some may be only slightly more product priority than cost priority and some may be way on the ‘no cost is too high to achieve perfection’ end. Narvi would seem to lean towards product priority.

With Cost Priority a company is competing primarily on cost. While most will still try to produce the highest quality product they can, they will be more likely to sacrifice product features or quality to lower manufacturing costs than a company competing primarily on product quality.

Cost priority is the approach that Harvia/Almost Heaven appear to be taking in the U.S. (and Sauna360/Tylö-Helo/Finnleo/Amerec appear to lean this way) and given American consumers ignorance about saunas and well known gullibility and preference for low cost over quality this is not an inappropriate business strategy. Unethical perhaps to take advantage of Americans ignorance and gullibility but not illegal nor unexpected. 

A company pursuing product priority will be largely aligned with consumers and want the UL requirements to allow for higher quality saunas. This will allow them to better compete against lower cost alternatives and they’ll believe that better quality saunas in the market will grow the market much more as consumers will find them more enjoyable.

A company pursuing cost priority will be somewhat the opposite. For them the more restrictive UL requirements help them to keep costs down and removes competitors, particularly product priority competitors – both of which increase their profits.

On the cost side they’ll not necessarily offer a cheaper product than in Finland for instance but they will have lower liability costs and warranty costs. A heater that only heats a sauna to 160°f as these will do under UL requirements is much less likely to require replacement under warranty than one that’s heating to 205°f or higher. That can result in some significant cost savings. For these mfr’s then the UL requirements, though not good for consumers, are good for the manufacturer’s bottom line.

The stricter UL requirements reduce competition by keeping competitors out of the market. Less competition means less downward price pressure and higher sales volume.

On this latter it’s important to note that Huum have taken an end run approach by claiming that their sauna heater …is not a sauna heater. Huum, even though anecdotally proving to be inferior quality to even  Harvia, have become very popular simply because they don’t have the UL restrictions so a sauna with a Huum heater can get hotter, have accurate temperature control, not have nuisance trips, and have better ventilation. 

So, we shouldn’t be surprised if Harvia and Tylö-Helo openly or quietly fight to keep the UL restrictions in place. And try to force Huum to abide by them as well.

All may not be just UL though. Harvia through Almost Heaven and Tylö-Helo through Finnleo both manufacture (and sell) saunas and kits that would be difficult or impossible to sell in Finland or Sweden as they’d be considered too poor of designs. Not necessarily because of the thin walls but because they have too low of benches and ceilings, poor or no ventilation and various other negatives.