What exactly does the anti-vacuum valve do?

This is what a typical defect looks like. What does this valve do, and why is it one of the most common defects in machines? You can find out here...

The anti-vacuum valve serves the following purpose:

Assuming the kettle is hot, it will be about 70-80% full of hot water, and very hot steam will have formed above it, which we need for frothing cappuccinos and lattes. Everything's fine.

Now we switch off the machine. The boiler will then cool down, along with the hot steam. What happens is that the cold air requires much less space than the extremely hot steam, creating a vacuum in the boiler that would prevent the machine from functioning. That's what the valve is for. It opens when the pressure drops, allowing air from outside the boiler to flow into it as it cools. When we switch the machine back on and heat it up, the renewed pressure closes the valve, sealing the boiler, and everything works perfectly until it cools down again. So far, so good. However, the valve is constantly exposed to hot and cold steam, which dissolves a huge amount of minerals and limescale, depositing them on the valve. Depending on the water used, this can happen after just two years, when so much limescale or mineral residue builds up on the valve that the rubber no longer seals properly. Then the valve no longer closes. You can see exactly that in the video. The solution? We replace the valve because we have to honor the warranty. As a private individual, you can also descale it, as the problem is solely due to the deposits. Simple, yet a constant nuisance. This valve is the most frequently replaced component in our high-quality machines.

WARNING! For anyone tinkering with this themselves: The boiler operates under high pressure, and the steam/water reaches temperatures of 120-130 degrees Celsius. Such hot steam and boiling water will cause severe burns upon contact with skin, or worse, with the eyes. So think carefully about what you're doing. Some people have already had boiling water splash into their faces because something wasn't assembled correctly; this can lead to lifelong scarring or permanent eye damage. So be careful!