Test HUUM HIVE Flow – A fiery heater revolution
Saunologia got the opportunity to perform an extensive test with the new HUUM HIVE FLOW Mini wood-burning heater. The test was full of surprises, as should be given the radically different nature of the new heater. My overall feeling is paradoxical perplexion. On one hand, this heater is an excellent device that can deliver extremely pleasant löyly and heat up the sauna room very evenly. On the other hand, it is demanding to use and slow to heat up, possibly demanding adjustments to the sauna room setup. Read the full article to understand why!
What is HUUM HIVE Flow and HIVE Flow Mini: specs on paper
In the Fall of 2024, Estonian sauna equipment manufacturer HUUM released a totally new wood-burning heater called HIVE Flow. After several years of intensive research and development, HUUM’s chief engineer Anti Reiman eventually cracked the feasibility problem in heater design, producing a totally new kind of heater. The resulting unique heater is among the few that meet the new, very demanding standards for wood-burning stoves in Germany.
Called BImSchV level or version 2, the new German requirements came into effect on 1st January 2025 and call for very strict emissions levels from various wood-burning equipment, including heaters. The major change in requirements concerns the addition of particulate matter (fine particles) threshold in addition to new levels of carbon monoxide and efficiency. The new limits are more strict in comparison to both the old BImSchV standard as well the pan-European standard (15281) that governs emissions in wood-burning sauna heaters (< 30 kW).
The HIVE Flow comes in two models: standard Flow and Flow MINI. Additionally, both models have a variation identified by “LS,” which means the heater is a “through-the-wall” model, fed from an adjacent room. Both models have the unmistakable HUUM look with their rounded barrel-shaped carefully constructed around the totally rectangular center body.
They have interesting specifications not only for emissions. Both models have 75 % efficiency, but only 9,8 kW and 8,5 kW nominal heating power (Flow and Flow Mini, respectively). The more powerful is rated 8 to 18 m3 (283 to 636 cu ft), and Mini for 6 to 14 m3 (212 to 494 cu ft). Their heating power is transferred to 150 or 105 kilograms of stones around the heater. Furthermore, they have extremely low stated maximum flue gas temperatures (385 and 343 Celsius; 725 and 650 Fahrenheit), meaning they can be installed in a T400 category steel flue. There is a flue connector at the top of the heater at the top and from the back.
The power ratings are particularly fascinating because they are ridiculously small! Normally, wood-burning heaters have a nominal power rating of 16 kW or more and also consume much more firewood. This naturally evokes curiosity as to how this can be made to work and what are the consequences!
The test setting
The test took place at Saunologia headquarters, during which I replaced my reference heater, Narvi NC16, with HIVE Flow Mini. I have a two-story brick chimney with a spacious flue. I installed the heater using the side flue connector, which forced me to utilize two angled adapters to compensate for the nearly 15 cm (6”) difference in connection height.
The sauna room is small, 9 m3 (315 cu ft), and mostly paneled wood with underlying 30 mm PIR insulation. It uses mechanical downdraft ventilation, natural incoming flow in the floor level directly from the outside, and a gap underneath the bathroom door. During heating, mostly dry birch was used as firewood, measured using a luggage scale to ensure instructions were followed precisely.
I used the heater for ten distinct times over almost three months before concluding the experiment. I felt a lot of repetition was needed to master the heater as it was unusually demanding to use.
Installation
Setting up began by preheating outdoors. This was a must, and I recommend doing it twice. I didn’t, and the heater still smelled bad on the second heater that happend during the first indoor run.
The preheating already showed up the character of the heater because it seemed that the fire would never die out! Conventional heaters burn even the first batch fast and fierce (once you get the fire going), but not HIVE Flow Mini. This means its behavior matches exactly the nominal heating power rating that predicts minimal firewood consumption.
Another unexpected turn in the installation was that in order to use the side flue connector, the user must swap a connection cover plate from the top to the side and make a new opening in the metal grate! After first marking the measures with masking tape, I used a reciprocating saw with a metal blade to make the cuts. No cutting instructions are provided; the opening must be made according to the flue connector to be used.
Then comes the actual installation. This requires disassembling the metal grate into two parts, the lower of which will be the first to be filled with stones. Then goes the top part, along with the flue connector. I must admit that using the top connector would make the installation much easier.
Putting in about 100 kg (220 lbs) of stones takes time. The grate has so much space between the cross bars that the stones will fall through if you don’t watch out for it. Putting in the stones and assembling the grate took about an hour and a half.
Altogether, the installation can easily take a day, especially if you do two rounds of preheating, as I suggest, and let the heater cool between the rounds. There’s a lot of waiting in between, but you should reserve the time for it.
Experiences during the first heating
My first attempt to use HIVE Flow indoors was a bold failure. I eventually got to use it, but before that, I had disabled the smoke alarm and aired my downstairs extensively.
Learn from my mistakes
It became apparent that you can’t operate the new heater in the old fashion. Although I had carefully measured the weight of dry firewood, I didn’t follow the wood stacking instructions in detail. So, what followed was a smoky mess.
For one thing, I quickly learned that opening the firebox door must be done with care and swiftness. Unless you start by inching the door open a tiny bit and then quickly add all new firewood at once, you will get a huge puff of smoke into your sauna room. If you follow the instructions, you usually only get a minor puff back, but I never learned to keep the sauna room air clear of smoke. This never happened with my reference heater.
The front part of the heater, where the firebox extends from the rounded mass, emits noticeable radiant heat. From the infrared images, you can tell it gets easily to above 300 C (570 F; or more) So much that I thought better to cover up the wooden trim seen in the photo above with a aluminum profile for the duration of the test to avoid overheating and damaging the wood.
Another lesson was that the fire must be started from the top. If you try to ignite the relatively gigantic first batch from below or in the middle, it just won’t burn. And what I just told you about the smoke risks of opening the door is that you can’t go messing with fire once it starts or trying to get going.
The final lesson was that I should have preheated the unit outdoors twice. In addition to smoke, there was an acrid smell during the first two indoor rounds, which luckily disappeared after the following rounds.
Experiences with HIVE Flow Mini: the bright side
After the initial shocks and airing the sauna room, I got to experience the heater for real. This happened some two hours later. After all the trouble it had given me, I was in for a positive surprise.
The heater created a very consistently warm sauna with a pleasantly minimal head-to-toe temperature difference. This is pretty much as good as one could get in my sauna with the heater that’s 80 cm tall (31”). The heater and its stones were also quite uniformly hot, not excessively so, meaning that plenty of löyly could be had.
And indeed, there was! Besides great heating, HIVE Flow also delivered excellent steam! A pleasant, slowly building heat that eventually extended as low as one could hope (toe level).
It was good before it eventually ran out. I discovered that the heater doesn’t really store heat that much, but you have to keep the fire going to maintain a good temperature for the stones.
Excellent steam, heater doesn’t get too hot
Although the heat is slowly coming, the heater maintains the sauna room temperature much longer than my reference heater despite not consuming more firewood. This guarantees that the sauna room will be efficiently dried without any other user action needed.
Experiences with HIVE Flow Mini: The Dark Side
Multiple irksome features come with the new heater as I quickly discovered. First, you must unlearn many of your previous habits of operating a heater. HIVE Flow doesn’t work like any old heater; you need to use it as instructed, or it won’t perform as expected. This includes paying attention to the correct arrangement of firewood within the first batch, using right sized batches afterward, being very careful with the hearth door opening, observing the safety distances, and keeping the flue damper open most of the time. Let me explain in more detail.
First, the heater’s operation requires relatively little wood to burn simultaneously. In fact, the air intake, particularly the ash grate, is designed to prevent the burning of a large quantity of firewood. This is in line with the heater’s surprisingly small nominal power ratings, which are directly correlated with the amount of wood consumed over time.
The hearth door and putting more firewood is tricky. I don’t mean that you might incidentally burn your fingers, but there seems to be a massive potential for a back puff of smoke into the sauna room.
The unusually slow burning operation of the heater has few surprising consequences. Because the consumption of firewood is limited, the exhaust effect created by burning wood is also restricted. If you reduce the amount of firewood by half, the ventilation effect by air flowing through the heater is also halved. I noticed this in my experiment by seeing elevations in CO2 concentrations. The rise was not substantial, some 300 or 400 ppm above the normal, but I argue this is also noticeable because of the puffs of smoke, reducing the air quality. With a regular heater, the smoke would dissipate fast because the heater would cycle the sauna air in about ten minutes. Now, it takes longer.
The other challenging part is that the fire dies out very slowly. After the last batch of wood was added, there was still fire almost two hours later. Coals glow red even longer. This means that you will need to keep the flu damper open most of the time, which then cools off the sauna in the end. On the other hand, this effect is compensated by the extremely long period when the heater still provides room heating – even though it is no longer usable for sauna as the stones in the outer layer lose their heat.
While this feature may only be an inconvenience to most, some people living in locations with a high fire hazard may be put off by it. Having to watch out for the fire hours after the sauna session can be an issue.
The silver lining of the slow burn and fire dying out is that this also gives an extended grace period for rekindling the fire, whereas normally, it would die out much faster. This shows in the elevate temperature of the sauna room still many hours after the active heating. The effect of how the stones lose their energy quickly means that you will notice the need to maintain a steady burning constantly. Letting the fire diminish will quickly reduce the stone surface temperature and löyly potential.
Summary and Recommendations
The performance of HIVE Flow is so controversial that it makes it impossible to recommend it without conditions. This product may be a disappointment for anyone with substantial experience with traditional heaters. For those just getting used to wood-burning saunas, this might be a natural step to a new normal and an excellent first-step into the world of gentle flames and smooth löyly.
The heater is best suitable for a sauna cabin in a moderate climate, where the ambient temperature never falls very low. I recommend the LS version that extends through the wall and is fed from the adjacent room. This would circumvent the inconvenient features of the device and emphasize its strengths and the need to arrange the ventilation differently. Located in a dedicated cabin would effectively vanish most fire safety considerations and slow combustion issues. HIVE FLOW would be a great solution to ensure the cabin dries out after use.
Extraordinary feeling of heat after an extraordinary wait
As I know most of my readers won’t fit this ideal user situation, the rest of you must judge whether HIVE FLOW’s special characteristics seem suitable for your needs. If I were to judge purely on the outcome of the steam (löyly/leil) experience, I would have no regrets about recommending this to all fans of steam. If someone wants to experience a very hor and dry sauna, then the match is not good. Then again, if you’re living in Germany or otherwise conscious of your environmental effect, the narrow selection of BImSchV level 2 heaters might be a major factor in choosing a new HUUM.
The new technology comes with a pretty steep price tag. The new HUUM HIVE Flow models currently cost around and above 1,500€ in the EU. This is about double of a similar stove from another manufacturer. In the US, price is around $2,000 which is not so exceptional.
Manufacturer website: https://huum.eu/product/hive-flow-wood-burning-stove
This article was produced as a part of commercial collaboration between Saunologia and Huum.
Special thanks for Jarkko Tissari, Research Director at University of Eastern Finland, and folks at HUUM: Teet, Roman, and Anti.
