air vents for radiators - which to choose automatic or manual

I am asking for advice from people experienced in installations and who do not want to get points for an answer that does not provide anything.

Regularly 2-3 times a season, I have to vent the radiators in two different buildings that do not have ventilation risers. The installation was built on thin modern sheet radiators (steel in one building, aluminum in the other, but it probably doesn't matter). These are probably the minerals that remained on the surface after the water evaporated.

1
is it possible that air is being sucked into the system, since there is overpressure in it? water is always topped up slowly to limit the amount of air during filling.

2
what do you recommend with air vents? manual or automatic? possibly which models and manufacturers? for my part, I prefer to bleed the installation once a season than to replace the automatic air vent every few years. it also minimizes the possibility of failure.

it is possible that the installation is designed incorrectly .. but I am not an expert in this industry.

it is certainly visible, for example, in the radiators that have a slope once towards the supply and the other time towards the vent. Unfortunately, not all radiators were able to correct the position so that the side with the vent was higher.

some are covered and additionally the thermostatic heads are set to 4 or 5 on a 6-position scale. what you write about underpressure when switching on actually makes sense.

only that if there is a vacuum and the amount of water is not enough, the automatic air vent will open ..?

vacuum + opening = suction

therefore, only manual operation excludes the possibility of "sucking" additional air into the system.

Rysiek, of course I have a closed system.

after the season I will change the vents.

we'll wait, maybe someone else will comment on the above interesting issue?

http://aqua-solar.pl/technika-grzewcza/643-od...ejnika-1-2-z-zamkn-higoskopijnym-caleffi.html
This is an automatic air vent, but may leak a little at first when fitted as it is not float but with viscose. And if necessary, it has a knob that can be easily closed by hand without tools and then works like a regular manual.

Pour the inhibitor into the system, maybe the link is done (the type of heaters matters)

Andrzej,
thanks for the suggestions. Currently, some of the vents are caleffi .. closing and opening are realized by cellulose plates, which change thickness under the influence of moisture - a solution not well thought out in my opinion .... I described the effect after several years of use.

Mirrzo,
thanks for the response! can you expand the term link? is it about combining copper with aluminum?