ᐅ Heating system gurgles despite having an automatic quick air vent

Our heating system makes loud gurgling noises several times a day, even though we have an automatic air vent at the highest point on the upper floor (its black cap is loosely turned).

Does anyone have an idea what the problem might be and how to fix it?

1) You can’t manually bleed automatic air vents, right?

2) Is the problem that there is air in the heating system that isn’t escaping through the automatic air vent (because it might be faulty)? Or is the problem that the automatic air vent works but in certain situations sucks air in? In that case, I would need to turn off the heating system and, after some time, close the black cap before turning the system back on.

Depending on how your heating system is installed, the air might not be pushed all the way to the fan. The air stays in place while the water gurgles past. Can you pinpoint where the noise is coming from?

wiltshire schrieb:

Can you locate the noise?

I hear the gurgling sound most clearly at the two distribution boxes on the ground floor and first floor, at the upper of the two horizontal pipes. The two automatic air vents are also installed on these horizontal pipes.

However, the gurgling does not occur continuously, only a few times a day.

I tried closing the two black caps, but that had no effect.

Could the two automatic air vents be clogged, preventing air from escaping?

Would it help to replace the automatic air vents with manual air vents?

Is the heating system located in the basement, and how many floors are there above it up to the air vent?
10 m (33 feet) of height corresponds to 1 bar of pressure.
In our old house, when the pressure dropped below 1.5 bar (at about 12 m / 39 feet height), the heating system also started to make gurgling noises.

Nida35a schrieb:

Is the heating system located in the basement, and how many floors are above it up to the air vent?
10 m (33 feet) of height corresponds to 1 bar of pressure.
In our old house, when the pressure dropped below 1.5 bar (at about 12 m / 39 feet of height), the heating would start to gurgle.

It is a single-family house (basement + ground floor + upper floor) with the heating system in the basement.

The upper floor’s floor level is 5.60 m (18 feet) above the basement floor. The pressure is 1.8 bar.

Do the two manifolds have no air vent, like radiators do?