This diagram shows a cross section of an antivac valve. When there’s no pressure in the boiler, the valve seat will be in the open position (left image). When the boiler is powered, steam pressure forces the valve seat to close (right image). In this way, the antivac valve’s function is the reverse of the safety valve’s.
If you hear a little fizz or hiss as your machine begins to heat up, don’t be concerned (though you should always keep an eye on your espresso machine’s pressure gauge). This sound is most likely created by the antivac valve, which is usually positioned next to the safety valve at the top of your boiler. The valve’s function is to allow air to enter the boiler after the machine is switched off, preventing a vacuum from building up, which could damage the boiler.
The valve’s other function is to allow the same air to escape from the boiler the next morning, as the machine heats up. When the water approaches boiling point, steam will begin to displace the air in the boiler. Once there is enough steam inside the boiler, the additional steam pressure will push up against the valve seat, forcing the valve to close, allowing steam pressure to increase in the boiler. When the desired pressure is achieved, the pressurestat will switch off the heating element(s) and you will be able to start using the machine to prepare coffee.