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Test HUUM DROP 9 kW heater : the fast lane to soft steam

Saunologia tested the combination HUUM DROP 9 kW electric sauna heater equipped with the upgrade kit and HUUM's UKU wi-fi controller. During a month full of steam, we found DROP a capable heater with a pleasant design all over the product, from the control panel to the mobile app. The heater is effective, and the mobile app is pleasant to use. The upgrade kit felt like a warranted solution that should clearly increase the heater’s service interval. I also got a chance to try the larger HUUM models HIVE and HIVE Mini at HUUM factory saunas, enjoying plenty of steam.


Estonian sauna equipment manufacturer HUUM was founded only ten years ago, in 2013. Based in the southeastern town of Tartu, HUUM began operations with an electric heater DROP and a wood-burning heater HIVE Wood. They have grown the selection since with several electric heaters, but DROP remains their best-known model, thanks to its award-winning unique design. For its tenth birthday, DROP received an upgrade kit, which made it a lucrative topic for a Saunologia test.

For testing purposes, the renewed DROP 9kW was installed in the Saunologia HQ sauna, which has already hosted a handful of heaters earlier this year. The sauna has about nine cubic meters, making it a good fit with a 9 kW heater. In the following, I will share the experiences from heater installation to experiences of löyly (as we call sauna steam in Finland).

Experiences with HUUM DROP: hardware installation

DROP is a wall-mounted heater with special care in aesthetics. It only takes six screws to secure to a wall, but if you want it to look nice, you should prepare the cabling so that it goes through the wall and surfaces at the bottom of the heater. This way it won’t show poking out below the sleek heater. My installation was not the prettiest possible and I used additional 12 mm (1/2") plywood board between the heater and the wall to ensure firm installation.

The heater can be installed pretty low in the room, only 10 cm (4” from) from the floor. This helps to achieve a desirable, relatively flat vertical temperature distribution.

DROP is available in Europe in 4, 6, and 9 kW models, in the North America 4.5, 6, 7.5, and 9 kW variations.

The HUUM heater system (for EU) includes several components besides the heater.

  • A control / power unit (called the main module),
  • a room temperature sensor,
  • the physical UKU control panel, and
  • a door safety switch.

And then some cabling. Installation of this equipment will take some time, depending on how you plan to guide all the necessary wiring. This is no different from other brands, and the HUUM power unit is big enough to allow the electrician to work comfortably. The downside is that the power unit is somewhat bulky and requires some space to hide. The device should be installed outside the hot room, although it is IP-rated (IP65).  Main module also supports controlling lights (max. 100 W) and a vaporizer, neither of which are offered under the HUUM label nor were tested in this trial.

HUUM system diagram
HUUM wiring diagram is designed with care. The smooth looks do not hide the fact that the system with all its parts requires lot of cabling.

Notice that before the main module box gets wholly hidden away, you should record the Sauna ID number and the code number you will need to connect to the UKU wireless controller! For some reason, this information is not emphasized in the user manuals.

UKU installation

UKU control panel is available in different flavors. I tried the standard black. It should also be installed above a cable hole in the wall to preserve the wireless look. The distance between the controller and the power unit can be quite long. The installation package includes reasonably long cables for the panel and temperature sensor, but the door sensor cable must be extended, increasing the work.

Altogether, all electrical works by a professional technician should take a maximum of two to three hours if the cabling preparations are done in advance. Once the heater is wired, observe a test heating for a maximum of five minutes without stones. This will burn away any protective grease, so the bad smell shouldn't bother you later.

Stone installation

DROP is a small heater with a nominal 55 kg (120 lbs) stone volume. I managed to fill it up with 48 kilos. I think I could have gotten in some extra pounds if I had tried hard to insert very small stones here and there, but I honestly gave up because if was too tight.

The other challenge is that the outer mesh shell structure has big gaps and even getting the giant stones to stay inside the mesh requires care. Otherwise, stones will fall to the floor, potentially creating dents or chips on the floor or hitting your toes.

The original design is really tightly packed with heating elements whereas the mesh is spacious.

Also, the space between heating elements is minimal, and trying to fit irregular-sized natural stones in there is a downright awful task. From the photos, you can see some larger gaps on the side.

There's a hole in this stone installation, which I'm not proud of.

Stone installation has changed recently as in Fall 2023 DROP introduced the upgrade kit. The kit includes a retainer that protects the heating elements by clearing space between them. This also means that the stone volume has gotten slightly smaller, and you don’t need to fit tiny stones between the elements, which saves time. I received the test unit without the upgrade and got to experience retrofitting the retainer.

New retainer unit truly revolutionizes the stone space.

With the retainer, the trouble of fitting the space between elements disappears. Because the volume is small overall, the small challenge of fitting stones does not take too long, maybe thirty minutes at most.

User manuals

HUUM has put an effort into keeping the user manuals nicely laid out, showing attention to overall design. The graphics are clear and crisp, text is well-readable. As usual, the manuals are short but should be enough to help even the uninitiated get their sauna running. You can download one freely and see for yourself.

HUUM UKU remote controller usability

HUUM was the first company to introduce an app-controlled remote control within a control panel in 2014. Uku device is a small, wired panel that includes a single moving input device: a wheel with a push function. The principle is familiar from lighting controllers but works very differently. In Uku, pushing the wheel in a normal operating mode allows to turn the heater on and off. Turning the dial sets the temperature. All other functions are foremost accessible through the app.

The controller is available in several variations, which all work the same. Wood, glass, or various colors of plastic finish of UKU are available for an extra charge.

UKU in various formats. All work the same. The glassy ones come with a glare that speaks ill of my photographic skills.

Getting started with UKU requires a bit of patience. Nowadays, UKU is usually connected to a Wi-Fi network. Only 2.4 GHz connection is supported. Also a 4G/LTE and an entirely local model are available. The local controller can be upgraded to Wi-Fi, and the 4G model comes with an annual subscription fee (29 € in the EU).

To connect to Wi-Fi, one must utilize UKU’s setup mode in which the wi-fi network and password are set. This caused me slight confusion as I didn’t immediately realize my initial success. Luckily, you need to succeed only once, and the Wi-Fi will continue to talk with the local access point (your Wi-Fi router that is). Once this is done, you will need the Sauna ID and code numbers found in the power unit to connect with the app.

Is it ready now? Setting the wifi password to UKU is the hardest part of the setup.

The usability of the controller is good enough. It is saved by the simplicity of operation even though the wheel’s responsiveness is not instant. Other family members managed to operate it without guidance, so that's a good sign. My feeling is that HUUM has designed the app to be the primary controller, so let’s turn our attention to it.

HUUM app

HUUM also pioneered heater apps. Their app for iOS and Android has been available for about ten years already. This long lifespan shows favorably in the app. The works smoothly and the user interface has not been crowded with unnecessary functions. You could say the clean design language extends seamlessly between the physical and digital boundary.

Clean and simple user interface of the HUUM app (Android 12).

The app works as well as you can imagine and gives good access to the basic functions. These are remote operation, scheduling, and status. App uses notifications sparingly but does inform the user when the sauna has reached its target temperature. For remote operation, the most significant difference to competition is the integration of the safety check within the app.

App statistics can be useful. For instance, you can calculate your power consumption from here (month by month).

The most interesting unique feature of the app is analytics. HUUM app incorporates historical data about sauna heating, which is unique and valuable to individuals like me. The statistics have several views to track your sauna usage. The app tracks how often the heater was used, for how long, the average set temperature, and how quickly the sauna heated up. The total heating time also allows you to estimate how much electricity was used if you know your heater's power rating, which is unknown to the app. As a big friend of data, I regard this feature very highly.

HUUM remote operation safety feature

HUUM heater system incorporates a door switch as a safety device, but the logic is slightly different than in, say Harvia’s system. While the door must be closed, the final user confirmation in HUUM happens within the app, not at the physical control panel. Opening the door only leads to a temporary pause in heating without forcing the user to inspect the sauna.

Careful now! The user takes the responsibility, even if the door is close, you must be sure it's safe inside.

This seems to be a somewhat potentially hazardous interpretation of the safety mechanism required in remote operation by the electrical standard ISO EN 60335-2-53. While standards do leave some room for interpretation, all other manufacturers have implemented this differently.

users should always check the sauna regularly and preferably keep it clear of any moving materials to ensure safe remote operation

All I can say for sure is that regardless of the safety mechanism, users should always check the sauna regularly and preferably keep it clear of any moving materials to ensure safe remote operation. Sauna-related fires can be dangerous to the whole building and fatal to inhabitants.

Experiences of HUUM DROP heat and steam

DROP is a heater that heats up the sauna very quickly. This is especially true when it is equipped with the retainer from the upgrade kit. During my tests, the average heating was one degree Celsius per minute. After installing the upgrade kit, the heating sped up even a bit more.  The heater warms up mainly from the top, the sides are clearly cooler. This means that it can be safely placed in a tight space. The specified safety distance of 100 mm seems justified.

The steam potential of DROP is reasonable given its small size. The combination of moderate stone volume and how they heat up means there is a clear sweet spot for creating löyly or “sauna steam” on top of the heater. The best technique for getting the most steam is to spread the water equally on all stones on the top. Trying to pour a lot of water into a single location will inevitably make the water run through the heater to the floor because the lower half of the heater is relatively cool and won’t be able to vaporize water before it runs through.

HUUM Drop infrared image from the side

A small amount of löyly can also be extracted from the sides of the heater, but a steady hand will allow you to get more mellow steam from the top. Other family members didn't have anything negative to say about the steam, bespeaking of its pleasant nature.

Drop infrared image from the top

In my test sauna of about 9 m3, the 9 kW heater seemed to be a good match, and I didn’t run out of steam. However, the number of stones may be insufficient for those who want to combine high temperature and humidity. But this is probably an issue for very few, and most users will be happy with its steam potential.

One minor detail about the heater is that it is very silent in switching the heating on and off. During use, the heater makes no noise. On the other hand, the lack of noise was accompanied by the constant smell of a new heater that somehow remained throughout the twenty heating rounds. It usually dissipates quickly after a couple of heating times, so this was unusual behavior.

Disassembly and safety observations

In the Saunologia test report of a similar product from Harvia, I noted that North American sauna fans had faced issues with the DROP heater. Some felt the heating elements had aged prematurely or the stones had disintegrated suddenly.

As I noted, I didn’t have the DROP upgrade package at first. I installed the package after some ten sessions, halfway through the test period. By this time, the heating elements had started bending, and I had to make a real effort to get the retainer in place. However, once the retainer was in place, and the went through some ten more sessions, the elements seemed to have straightened themselves – or at least no further pending presumed. Thus, I believe HUUM has taken a long step to prevent premature element failures with the upgrade kit.

So, after my test period of 20+ heating sessions, I didn’t observe any signs that would indicate element failure or overheating around the heater. For instance, the plastic sticker facing the wall behind the heater was intact. But I definitely recommend getting the upgrade package if you already have the heater, it will most certainly help in the longer run.

I should note that I didn’t intentionally and repeatedly try to push the heater to its limits. I didn't intend to do a safety or durability test, but simulate use and behavior in a regular domestic environment. My sessions were short, just an hour of bathing on average, and the temperatures were moderate. Two times I heated the space to 90+ C degrees, but this didn’t bring about any surprises.

Summary and recommendations

HUUM DROP is a solid and stylish sauna heater choice. Heater’s design seems to fit light interiors the best, and although it can be placed close to wooden surfaces, it looks better with some room around it. In my relatively small and very dark sauna, I didn’t feel the heater was a perfect fit with the other interior elements. One contributing factor was that I intentionally installed the heater as low as possible and ended up viewing it mainly from a bird’s eye perspective. While this was an evident engineering choice, the heater looks better if installed a bit higher.

The full package of DROP 9 kW and black plastic UKU Wi-Fi costs over 1,200€. The safety railing adds another 170€. HUUM’s design comes with a price, and the heater is not cheap in the EU market. The closest comparison is the Harvia Spirit heater, which in the EU goes for under 1,000€. In North America, the HUUM package will cost about double in dollars (2,000-2,500 USD) whereas pricing for Harvia is still unknown at the time of the writing.

Is DROP a better heater than Harvia Spirit, which appeared almost ten years later to challenge DROP with its rounded design?

After HUUM introduced the DROP upgrade kit in August 2023, heaters now seem equal in terms of build quality. The looks of the heater are slightly different, as well as their mobile apps. HUUM’s advantage lies in the app, which is very easy and responsive. Heaters’ performance is quite similar and doesn’t set them apart. So in the end, you need to decide based on your budget and appreciation for their looks. To me HUUM has the slight upper hand in all three areas: the heater, the app, and the control unit when it comes to design. Harvia’s safety mechanism is probably more fool-safe (but cumbersome), which might be important for some.

I believe the upgrade kit with its retainer has taken HUUM a long step to the right direction of improving heating elements’ longevity. While my one-month intensive testing period was too short to reveal latent problems, I could observe the notable difference with and without the retainer. I think there is a very good reason why Harvia is using a similar structure in Spirit as well as in their best-selling tower heater Cilindro. HUUM has also released a retainer unit called the air tunnel for their HIVE heater models.

A parade of HUUM electric heaters at Tartu headquarters

Bonus round:
a short meeting with HUUM HIVE and HIVE Mini

While DROP is a relatively small wall-mounted heater, HUUM product line also includes models with a larger footprint and stone volume. These larger models, such as HIVE and its smaller sibling HIVE Mini, were unfortunately too large to fit in my test sauna. However, upon visiting HUUM headquarters in Tartu, I got a chance to try them out.

I tried out two saunas with HIVE Mini 9 kW, which can hold up to 160 kg (340 lbs.) of stones. The amount of stones directly increases the potential for löyly. This holds here, too, even though HIVE does not heat up uniformly from top to bottom. This is a beneficial feature because the heater can now be placed on a regular floor without heat shielding.

HUUM HIVE in a concept sauna

The short experiences with HIVEs were positively sweaty. They provided enough steam in the small cabins (under 10 m3, 350 cu ft). As seen in the infrared image, the heater surface was not super-hot, allowing you to extract just the desired amount of steam when you used a steady hand. HIVE models can also be fitted with an air tunnel. This seems to be a generally wise solution as the you gain more in heating efficiency than you lose in steam potential.

Mini Hive bottom is cool, which allows it to sit on a wooden floor without issues.

The last moment before my departure from Tartu, I also tried the larger HIVE model (18 kW). This heater didn’t leave questions about its ability to deliver quality steam. This was expected given its almost 240 kg (530 lbs) stone mass.

The memories are warm, and the HIVE heaters seem like a justified option to consider when you have the suitable space for it. Suitable would be in my opinion slightly more than what you can see in the attached photos from HUUM example saunas.

This article was done in collaboration with HUUM. Thanks to Teet, Roman, and Siim for hosting me at the factory. 



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