# Hints & Tips No 3



## Grumpy (Jan 21, 2011)

When water is heated it expands, to allow for this coffee machines have a anti Vac valve in the top of the boiler, this is weighted to allow air to escape and close when the boiler is producing steam. In reverse the anti Vac valve opens to allow air to enter the boiler when cooling.

These Anti Vac Valves live in the closed position and will eventually stick in the closed position giving the impression the machine is up to temperature when its only compressed air, so always heat your machine with the steam valve open until steam is starting to escape and then close it.

This problem is one of the causes of soured boilers, you think you have full steam pressure and start to foam your milk and as soon as you open the valve the pressure drops instantly and as soon as this happens it will suck milk back into the boiler.


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## Glenn (Jun 14, 2008)

A sufficient purge of the steam wand will also indicate the pressure. You should have a steady stream of clear steam when working well.


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## Grumpy (Jan 21, 2011)

Your quite right Glenn, a steam wand should be purged before to remove water and of course after to remove milk, but it happens that people don't always follow the rules.


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