The best solution I've found a few years ago is one Venta LW 45 for every 30 m² of space. That's enough to run them on the lowest speed while maintaining acceptable humidity and CO₂ levels.

Higher speeds are too noisy. Smaller machines evaporate less.

For sub-zero outside temperatures, it's necessary to add at least 5 g of water to each cubic metre of air coming from outside.

The recommended ventilation rate of 30 m³/h per person requires to evaporate 4 liters of water per day.

That’s a lot of refilling. You might want to look into a whole house humidifier, I added an aprilaire 700 evaporative to my hvac ducts, it costs a few hundred $. Plumbed in, automatic. So much less screwing around

Chinese I’ve mentioned blew Ventra out of the water. I’ve been using humidifiers for the last 15 years and switched to smartme around 5 or something like that (liked the idea of auto speed and was tired of aged squickness after many years of its predecessor).

Haven’t used the first generation. Had a couple of the second (they notoriously had water level sensor issue that could be fixed just enabling “drying mode” that always ran for 8h after the sensor thought there were no water).

Third gen is the charm.

Cheap, effective (pump, double-bottom for rounded instead of flat tank — uses evry last drop), quiter, 5L tank, less creak after a few years.

tldr; I only wish it lasted whole 24 hours when it is -5C and lower outside, but I guess that requires 7-8L.

Also, having a few helps with the noise (I have total of three in my apartment).

Which venta are you referencing here?